Monday, 27 January 2020

Magnitude 5.4 Earthquake beneath the South Taranaki Bight, New Zealand.

The GeoNet project, which monitors quakes in New Zealand, recorded a Magnitude 5.4 Earthquake at a depth of 64 km, about beneath the South Taranaki Bight, between North and South islands, New Zealand, at about 11.45 pm  New Zealand Daylight Time (about 10.45 am GMT) on Saturday 25 January 2020. There are no reports of any damage or casualties associated with this event, but it was felt  by over 26 500 people from Christchurch up to Auckland, according to GeoNet. 

The approximate location of the 25 January 2020 South Taranaki Bight Earthquake. USGS.

New Zealand is located on the boundary beneath the Australian and Pacific Plates. Beneath the islands the Pacific Plate is being subducted beneath the Australian Plate. This causes a great deal of friction which causes Earthquakes where the boundary between the two plates is close to the surface; this is to the east of North Island, but onshore on South Island, where it can lead to strong Earthquakes. Technically such quakes also occur where the plate margin is deeper, but these are felt less strongly as the rocks between the boundary and the surface absorb much of the energy, making strong tremors much less frequent on North Island. As the Pacific Plate sinks deeper into the Earth it is partially melted by the friction and the heat of the planet's interior. Some of the melted material then rises through the overlying Australian Plate, fuelling the volcanoes of New Zealand.

 The subduction zone beneath New Zealand, and how if fuels Earthquakes and volcanoes. Te Ara.

Witness reports of Earthquakes can help scientists to understand these events, and the underlying geologic processes that cause them. If you felt either of these quakes then you can report it to the GeoNet here. 

See also...

https://sciencythoughts.blogspot.com/2019/12/one-person-dead-and-up-to-twenty-seven.htmlhttps://sciencythoughts.blogspot.com/2019/09/number-of-measles-cases-reported-in-new.html
https://sciencythoughts.blogspot.com/2019/04/tourists-warned-to-keep-away-from.htmlhttps://sciencythoughts.blogspot.com/2018/10/three-hunters-rescued-alive-after-their.html
https://sciencythoughts.blogspot.com/2018/09/homes-evacuated-after-chemical-spill-in.htmlhttps://sciencythoughts.blogspot.com/2017/01/evacuatios-after-ammonia-leak-at-plant.html
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Comet C/2010 U3 (Boattini) makes its closest approach to the Earth.

Comet C/2010 U3 (Boattini) will make its closest approach to the Earth on Tuesday 28 January 2020, reaching a distance of 8.22 AU from the Earth (822% of the distance between the Earth and the Sun, or 1 229 500 000 km, or slightly less than the distance at which the planet Saturn orbits the Sun). The comet will not be visible at this time as it is behind the Sun.

Comet C/2010 U3 (Boattini) (between the two vertical lines at the centre of the image), seen on 29 May 2019 from Japan. Toshihiko Ikemura/Hirohisa Sato/Seiichi Yoshida.

C/2010 U3 (Boattini) was discovered on 31 October 2010 by astronomer Andrea Boattini, working at the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation C/2019 U3 implies that it was the third comet discovered in the second half of October 2010 (period 2010 U). Surprisingly C/2010 U3 (Boattini) was found to be active (i.e. producing a coma due to material being ablated from its surface by solar heating in November 20015, while at a distance of 25.8 AU from the Sun (slightly inside the orbit of the planet Neptune), making it the most distant object upon which such activity has been seen.

Modelled morphology of comet C/2010 U3 (Boattini) with inclusion of the Lorentz force. Dates in GMT and scale bars are labelled in each panel, in arc seconds ( the whole sky - including the but behind the Earth - is divided into 360 degrees, each degree into 60 minutes and each minute into 60 seconds). The yellow arrows mark the position angles of the antisolar directions, while the white ones mark the position angles of negative heliocentric velocity. Equatorial north is up and east is left. Hui et al (2019).

C/2010 U3 (Boattini) has an unknown period and a highly eccentric trajectory tilted at an angle of 55.5° to the plane of the Solar System, that takes it from 8.45 AU from the Sun at perihelion (845% of the distance between the Earth and the Sun) to an unknown outer orbital point, somewhere beyond the Kuiper Belt, in the Oort Cloud. It is considered to be a hyperbolic comet, an object from the Oort Cloud (or possibly even the interstellar space beyond), that has been nudged onto a trajectory that takes it through the Inner Solar System by an encounter with another Oort Cloud body or possibly the gravity of another star or other extra-Solar System object. Such comets are not expected to make return visits to the Inner Solar System, but rather are thrown out of the Solar System altogether by a gravitational slingshot caused by their close encounter with the Sun. However in the case of C/2010 U3 (Boattini) the current passage through the Inner Solar System is not thought to be the first; it is calculated to have made a similar visit around 1.86 million years ago.
 
 The calculated trajectory and current position of C/2010 U3 (Boattini). Minor Planet Center.
 
See also...
 
https://sciencythoughts.blogspot.com/2020/01/comet-114pwiseman-skiff-reaches.htmlhttps://sciencythoughts.blogspot.com/2020/01/cyanide-gas-detected-in-coma-of.html
 
https://sciencythoughts.blogspot.com/2019/12/interstellar-comet-2iborisov-makes-its.htmlhttps://sciencythoughts.blogspot.com/2019/12/comet-114pwiseman-skiff-approaches-earth.html
https://sciencythoughts.blogspot.com/2019/10/comet-c2018-n2-asassn-makes-its-closest.htmlhttps://sciencythoughts.blogspot.com/2019/09/comet-c2018-w2-africano-approaches.html
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Sunday, 26 January 2020

Trying to understand the cause of the 22 December 2018 Anak Krakatau volcanic tsunami.

During the 1883 Krakatau eruption and tsunami, which is estimated to have killed over 36 000 people, 12 km³ of dense rock equivalent was erupted; a caldera collapse occurred as a result, leaving only small and steep subaerial remnants of the former volcano edifice along the rim of a 7 km wide deep-water caldera basin. Volcanism continued after the 1883 events, eventually producing Anak Krakatau ('the son of Krakatau'), where several additional smaller tsunamis were triggered at this site by processes such as underwater explosions. Anak Krakatau may be preconditioned for landslide triggered tsunamis, as it is situated on a steep morphological cliff. This edifice probably first breached the sea surface in 1928 and gradually formed a 150 m high tuff ring by 1959. A gradual shift in activity then occurred toward the southwest, resulting in further growth of the edifice over the cliff and toward the deep submarine caldera basin. This lead to recent concerns about a possible landslide from the southwestern island flank and the corresponding generation of a tsunami. These concerns and scientific assessments turned into reality following an intense (but unidentified) increase in precursor activity. Although flank motion was identified, the hazard was not systematically monitored. On 22 December 2018, this volcanic centre once again became the source of a tsunami that struck the highly vulnerable Sumatran and Java coasts. According to the Indonesian National Disaster Management Authority, the 22 December 2018 tsunami caused over 430 fatalities, injured 14 000 people, and displaced 33 000 more along the Sunda Strait. The tsunami risk of this area is particularly high as the coast is very popular with both locals and tourists and is home to over 20 million people within a 100 km distance from the volcano.

In a paper published in the journal Nature Comunications on 1 October 2019, Thomas Walter of the German Research Centre for Geosciences, Mahmud Haghshenas Haghighi, also of the German Research Centre for Geosciences, and of the Institute of Photogrammetry and GeoInformation at Leibniz University Hannover, Felix Schneider, again of the German Research Centre for Geosciences, Diego Coppola of the Dipartimento di Scienze della Terra at the Università di Torino, Mahdi Motagh, again of the German Research Centre for Geosciences, and the Institute of Photogrammetry and GeoInformation at Leibniz University Hannover, Joachim Saul, Andrey Babeyko, and Torsten Dahm, again of the German Research Centre for Geosciences, Valentin Troll of the Department of Earth Sciences at Uppsala University, and the Faculty of Geological Engineering at Universitas Padjajaran, Frederik Tilmann, again of the German Research Centre for Geosciences, and of the Institute for Geological Sciences at the Freie Universität Berlin, Sebastian Heimann, again of the German Research Centre for Geosciences,  Sébastien Valade, once again of the German Research Centre for Geosciences, and of the Department of Computer Vision & Remote Sensing at the Technische Universität Berlin, Rahmat Triyono of the Earthquake and Tsunami Center at the Indonesian Agency for Meteorology, Climatology and Geophysics, Rokhis Khomarudin of the LAPAN Remote Sensing Application Center, Nugraha Kartadinata of the Volcano Research and Monitoring Division of the Geological Agency of Indonesia, Marco Laiolo, again of the Dipartimento di Scienze della Terra at the Università di Torino, Francesco Massimetti, once again of the Dipartimento di Scienze della Terra at the Università di Torino, and of the Dipartimento di Scienze della Terra at the Università di Firenze, and Peter Gaebler of the Federal Institute for Geosciences and Natural Resources, discuss the causes of the 22 December 2018 Anak Krakatau volcanic tsunami, and the predictability of future similar events.

Volcanic islands are typically fast-growing edifices that rest on a complex morphology and weak substrata, and they are frequently made up of highly fragmented, mechanically unstable material. Therefore, many volcanic islands rise rapidly but also erode swiftly via volcano flank instability, leading to irregular shapes and embayments owing to sector collapses. Geomorphic amphitheatres are common subaerial remnants of fast lateral landslides; these events often recur at the same location. They may result in distal run-out submarine deposits, which demonstrate the intense dynamics of tsunamigenic mass movements in the oceans. In fact, data collected from around the world reveal that historical volcano-induced tsunamis have caused significant damage and loss; about 130 events have been recorded from 80 different source volcanoes since 1600 AD. These events have been caused by the entry of pyroclastic flows into the ocean and their submarine continuation, by caldera collapse, and by landslides entering the ocean, or combinations thereof. Among the progenitors of these events, seventeen historically identified source volcanoes are located in Southeast Asia. Volcano-induced tsunamis have probably led to the demise of ancient civilisations and are responsible for approximately one quarter of all fatalities attributed to volcanic activity.

It has been over 135 years since the infamous volcano-induced Krakatau tsunami occurred on 27 August 1883. A common problem of such events is that they are rare, and thus, although volcanic islands introduce numerous recognisable threats such as instability, sector collapse and tsunamis, little is known about their precursor activity and possible strategies to mitigate the associated risks. Moreover, the preparation and initiation of sector collapses are complex, insomuch that they could possibly be associated with faulting, slumping, and pyroclastic flows or combinations thereof. Consequently, at present, no consensus exists regarding what constitutes a reliable precursor signal for sector collapse on a volcanic island.

Location of Anak Krakatau. Walter et al. (2019).

By combining different ground and satellite data Walter et al. outline the details of the complex hazard cascade leading up to the events on the 22nd December 2018. Our study reveals that Anak Krakatau showed clear signs of flank motion and elevated volcanic activity prior to sector collapse which triggered the destructive tsunami.

Satellite monitoring and ground observations reveal that Anak Krakatau was in an elevated stage of activity throughout 2018. An analysis of infrared data recorded by the thermal sensors of the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite indicates that a new intense eruptive phase initiated at Anak Krakatau on 30 June 2018. This eruptive phase was the most intense recorded since systematic data acquisition began in 2000 and was characterised by a mean volcanic radiative power of 146 MW, which is roughly 100 times the long-term thermal emission average (1.6 MW) recorded between 2000 and June 2018. This thermal activity was associated with persistent Strombolian to Vulcanian activity (ejection of incandescent cinders driven by the bursting of gas bubbles and dense clouds of ash-laden gas exploding from the crater) and the emplacement of eruptive deposits along the centre and the western and southern flanks of the volcano. This eruptive phase continued for 175 days until 22 December 2018, when the activity suddenly evolved into a sector collapse.

Thermal emissions recorded by satellite data. Visual summary of the total extent of thermal anomalies, as measured from the Sentinel 2 images showing the emplacement of several hot and new material depositions on the southwestern flank of Anak Krakatau during July-December 2018. Whether these hot materials are due to lava flows or due to clastic deposition cannot deduced from this data. The approximate extension of land inundated by new lava flows and/or covered by hot ejecta (about 0.85 km²), is shown on the lower right panel. Walter et al. (2019).

An estimate of the erupted volume derived from thermal data indicates that the eruption phase produced approximately 25 500 km³ of deposits, implying a mean output rate of 1.7 m³ per second from June 2018 to just prior to the collapse event. Thus, the load acting on the summit and especially the southern flanks of the island progressively increased over this time by about 54 million tons (assuming a mean rock density of 2110 kg per cubic metre).The 2018 eruptive period was punctuated by 11 pulses with time averaged discharge rates higher than 3 m³ per second. The occurrence of these effusive pulses peaked between September and October 2018, with the three highest time averaged discharge rates of 10.5, 33.4, and 50.9  m³ per second, on 9, 15, and 22 September 2018, respectively. Starting in October 2018, the rate of these pulses declined, both in intensity and in frequency, except for a period in mid-November with a peak time averaged discharge rate of 23.2 3 m³ per second. The general decrease in activity after mid-October is also suggested by the trend in the development of the cumulative volume of erupted materials.

According to satellite images from the European Sentinel 2 mission, at least 0.85 km² of the island (28% of the total area) was covered with abundant hot ejecta and new deposits. Many of these entered the sea, adding 0.1 km² of land surface to the southern shore of the island (the island area increased from 2.93 to 3.03 km²) by early December 2018, as assessed by shoreline edge detection analysis. A compositional analysis of ash sampled during the intense eruption phase on 22 July 2018 indicates a basaltic andesite composition, which overlaps with the typical compositional spectrum displayed by Anak Krakatau in recent decades.

Eruptions and island perimeter growth map. (a) Several selected Sentinel 2 images (band combination of 12, 11, 4) showing the emplacement of hot and new material (red-yellow) on the southern flank of Anak Krakatau during the increased eruptive activity in 2018. (b) Island perimeter maps derived from satellite radar amplitude images show little variation from January 2018 to June 2018, followed by southward growth from June 2018 to December 2018 prior to the sector collapse (light grey). The black lines indicate the new scarps formed by the sector collapse. The outer outline indicates the post collapse island perimeter in January 2019 (dark grey). (c) Land area change based on monthly island perimeter analysis. The area changed gradually prior to the flank collapse, but more rapidly after the collapse event. Walter et al. (2019).

Interferometric synthetic aperture radar analysis and time-series analysis show that the southwestern and southern flanks of Krakatau were already slowly subsiding and moving westward at the beginning of the analysis window in January 2018, despite the absence of significant thermal anomalies. The deformation that occurred in the subsequently collapsed sector was advancing at an approximately constant rate with a peak of almost 20 mm (or about 4mm per month) in the satellites’ line-of-sight direction until the volcanic activity increased in late June 2018, at which point the deformation markedly accelerated (to about 10mm per month). In addition, short-term eruptive pulses in Sepember–October 2018 resulted in a minor step change. Therefore, the data show that increased eruption rates coincide with increases in flank movement. An analysis of the deformation field pattern reveals that it affected over one-third of the island, exhibiting a moderate gradient on the west side and a well-identified gradient on the southeast side. The cumulative deformation pattern indicates a progressively sliding flank that can be explained by a deep décollement plane, simulated as a rectangular dislocation, with a dip (steepest angle of descent of a tilted bed or feature) of 35°, a strike of 163° (quadrant compass bearing in terms of east or west of true north), and a slip of 3.36m. Notably, deformation also affected the island summit, and therefore potentially shearing its main magmatic and hydrothermal-plumbing systems.

Island perimeter monitoring. (a) Selected Sentinel 1 GRD images before the collapse, on the day of the collapse (the collapse was at 13.55 GMT, the GRD image at 22.33 GMT) and afterwards. Arising from higher radar reflectivity, steep cliffs of landslide amphitheatre can be partly identified in the 22 December 2018 image (dashed black line). On 25 December 2018, the southwest sector blurs due to eruption plume affecting the radar path1,2. The image on 27 December 2018 reveals the complete amphitheatre geometry (partially infilled in the northwest) and the new coastline (shifted by up to 260 m) in the northeast. Dashed black lines in second and fourth image represent approximate outlines of satellite headwalls, which might indicate two outlines, enclosing approximately 0.63km² and 0.84km², which is 44% and 58% of the deformation area identified in interferometric synthetic aperture radar time series, respectively. (b) An island perimeter detection algorithm was used to determine the growth of Anak Krakatau from 1 January 2018 to 31 January 2019. Walter et al. utilised the Google Earth Engine cloud computing environment³ by first computing monthly stacks from the Sentinel 1 ground range detected (GRD) scenes to reduce the speckle and then segmenting the stacked images using an adaptive threshold to separate land from water bodies. The figure shows the examples of December 2018 and January 2019, for which the stacking has been done to overcome the speckle effect at the cost of averaging short-term variations in the coastline. Each stack considers all Sentinel 1A/B GRD images, ascending and descending, acquired from the first to the last days of a month, except for December 2018, for which GRD images only until December 21 (before the sector collapse) were considered. The enlarged views of the backscatter channel of the GRD images in December 2018 and January 2019 and the average images used to delineate the outlines indicated by red polygons. (c) Monthly evolution of the coastline of Anak Krakatau. Note that largest changes on the southern flank occurred during June-August 2018. Walter et al. (2019).

The dynamics of the moving flank were relatively slow; as a consequence, seismic stations installed on the mainland for tsunami early warning were hardly able to record this type of movement. Then, conditions started to change shortly before the sector collapse event. Satellite thermal data show a pulse (5.6 m³ per second) on 22 December 2018 at 06.50 GMT, just a few hours before the onset of the collapse. Compared with the thermal pulses recorded earlier in 2018, this eruption was relatively small. Infrasound records show the release of continuous high-frequency energy (0.5–5 Hz) from Krakatau, indicating high levels of volcanic activity in the hours prior to the collapse followed by a brief period of quiescence. Both the intense activity earlier in the day and the quiet period were further confirmed by eyewitness accounts. Seismic stations then suddenly recorded a high-frequency event (2–8 Hz4b), just ~115 s before the flank collapsed on 22 December 2018, representing the last and most immediate precursor, or even trigger, of the main sector collapse. The seismic signal originated at Anak Krakatau and was associated with either an earthquake or an explosion with seismic amplitudes that exceeded even those of the sector collapse in the 4-8 Hz frequency band and was even recorded by infrasound stations at large distances. The coda (1–8 Hz) of this event is unusually long compared with those of tectonic earthquakes of comparable magnitude; in fact, the coda is still discernible when the onset signal of the catastrophic sector collapse becomes identifiable.

Deformation maps and island perimeter changes. (a) Interferometric synthetic aperture radar time series showing the movement of the ground in the satellites’ line-of-sight (LOS) direction, generated for the period between 1 January and 22 December 2018 for one ascending and two descending tracks. The different viewing geometries reveal the deformation of the southwestern flank of Anak Krakatau. (b) Average vertical (colours, red is down) and E-W (black vector symbols) movement computed from the different interferometric synthetic aperture radar viewing geometries. (c) Deformation trends for the different tracks (average pixel values in the inner sector collapse scarp outline) showing pronounced trend changes in June–September 2018, labeled I and II, and identifying the 22 December 2018 collapse event. Walter et al. (2019).

Local, regional, and even some teleseismic seismic stations show clear signatures of the tsunami-triggering event. The abrupt onset of a short-period seismic signal is followed by about 5 minutes of strong emissions at 0.1–4 Hz, approximately coinciding with a long-period signal (0.01–0.03 Hz) occurring over a shorter duration (~90 s) that Walter et al. interpret as the seismic signature of the main mass movement of the landslide. The onset times of the short-period signals at stations in Sumatra and Java are consistent with the location of the volcano and an origin time of 13:55:49 GMT. The inversion of low-pass filtered (0.01–0.03 Hz) surface waves reveals an event with a moment magnitude of 5.3. A significant non-double-couple component is retrieved from the inversion of low-pass filtered seismograms, indicating a linear vector dipole oriented to the southwest at 222° and a dip angle of 12° (or alternatively representing tensile opening mixed with a shear rupture dipping about 61° to the southwest). As these parameters are close to those of the pre-eruptive décollement plane derived from interferometric synthetic aperture radar data (northwest–southeast strike and southwest dip), we conjecture that it was this plane that constituted the failure plane during the sector collapse. Following the main event, a nearly continuous tremor-like signal exhibiting a slowly decreasing intensity with frequencies 0.7–4Hz was recorded at all nearby stations and was attributed to strong volcanic explosions.

Interferometric synthetic aperture radar deformation modelling. (a. Unwrapped mean line of sight (LOS) velocity data for the investigated tracks and viewing geometries, used for modelling. Landslides may include a plastic flow rheology, but may be modelled using elastic dislocations especially for slow-moving landslides, in order to assess the geometric and dynamic parameters of the landslide detachment plane. (b) Walter et al. use the rectangular dislocation (RD13) to search for the source of deformation. The RD is based on the angular dislocation in a half-space and provides geometrical parameters allowing to assess shear and tensile faults in an elastic medium. Parameters that determine the orientation of the RD plane are described by the strike angle (α) and the dip angle (δ), while the ‘plunge angle’ (θ) is the angle between the upper edge of the RD and the intersection of the free surface with the extended RD plane. In this work, the deformation modelling considers the 2018 mean displacement velocity prior to the sector collapse. As the tracks provide different viewing geometries (ascending and descending), the method is sensitive to both vertical and horizontal displacements. No further weighting was applied. Walter et al. implement the RD in a non-linear inversion scheme based on the genetic algorithm (GA), where the objective function (OF) that is minimised is the L2-norm of the model residuals. The solution space chosen was initially wide (dimension was free, plunge 0 degrees, dip from 0 to 90 degrees, strike from 0 to 360 degrees, rake from -100 to -80 degrees). Walter et al. define a GA population size of 60, a mutation rate of 0.15, and 1000 iterations, with a Poisson ratio of 0.25. (c) Residuals created by calculating the difference of the data and the model. Largest absolute residual values exceed 10 cm in those regions of young material depositions in the southern sector of the island. Walters et al. examined this feature and conjecture that there some signal may arise from compaction or cooling of young volcanoclastic material. Independent tests have been made where we were masking out the southern sector, but found generally comparable results for the RD source, implying that the new material addition in the southern sector of the island had minor effect only on the inversion. Walter et al. (2019).

The effects of this event were recorded extensively. The Australian infrasound array located over 1150 km to the southwest of Anak Krakatau recorded a high-energy impulse at 15.01.09 GMT, which translates to a modelled origin time of 13:55:49 GMT at the Krakatau site. This timing is consistent with the origin time of the short-period seismic signal at Anak Krakatau (identified as the landslide signal). The duration of the impulse is broadly comparable to the long-period seismic signal and indicates that subaerial sliding lasted for about one minute only. Both the seismic records at local stations and the infrasound records from the Australian infrasound array continued to be dominated by coherent emissions from Anak Krakatau (presumably related to strong volcanic eruptive activity there) for at least several hours. The dominant frequency of the eruption signature in the infrasound signal shifted by nearly an order of magnitude (from about 0.8–4 Hz prior to the landslide to 0.1–0.7 Hz afterwards). Even the closest local stations did not pick up a clear signature of any pre-landslide eruption, but post-landslide eruptions dominated the seismograms of stations even a few hundred kilometers away. Together, these observations suggest a profound change in eruptive style following the landslide. Furthermore, on 23 December 2018 at 06.31 GMT, a large sulphur dioxide cloud was detected, likely resulting from the decapitated and degassing hydrothermal system. In contrast, no similarly strong degassing was detected in the weeks prior to the flank collapse event. 

 Bounce-point distribution from rays starting at Krakatau. The infrasound wave was clearly identified at the Australian infrasound array (southwestern corner of map), but an infrasound phase of the main event was also identified at seismic stations LWLI and MDSI. Networks GE and IA are indicated with red and green symbols, respectively. Walters et al. (2019).

Tsunami arrivals were recorded by four tide gauge stations on the Sumatra and Java coasts. Backtracing from these four stations, using the classic tsunami travel time approach, suggests that the source location corresponds to the southwestern part of Anak Krakatau and that the source origin time corresponds to that revealed by the broadband seismic analysis. Therefore, the backtracing simulation shows that the tsunami was triggered by the long-period landslide during the first minutes of the event and not by the following volcanic eruptions.

Tsunami travel time backtracing. The isochrons show all possible tsunami source points for each station at approximately 13.56 (the seismically determined onset time of the slope instability). The isochrons intersect at Anak Krakatau. It was necessary to shift the picked arrival times by 1-3 minutes to make all four isochrons overlap and agree with the seismic origin time (marked as DT+x in the figure). Given that a coarse global bathymetry was used, this small discrepancy is expected AT: arrival time, OT: (seismic) origin time. Walters et al. (2019).

The full extent of the sector collapse event initially remained hidden owing to intense post-collapse eruptive activity but became visible when the eruption intensity decreased again by 27 December 2018. As a result, a new and steep amphitheatre enclosing a deep valley became distinguishable on the southwestern sector of the island. The deposition of new material shifted the coastlines. The collapsed area is readily identified in satellite radar imagery and is located in the area that was subsiding and moving laterally outward prior to the collapse event. The area affected by landslides, however, is smaller than the area affected by precursory deformation; accordingly, we estimate that only 45–60% of the deforming subaerial flank actually failed. High-resolution camera drone records in January 2019 allow the partial derivation of a digital elevation model. By comparing the digital elevation models from before and after the event, we ascertain that the sector collapse reduced the height of the island from 320 to 120 m and removed the former edifice peak, thereby decapitating the main eruption conduit. Detailed volumetric estimates obtained upon differencing the two digital elevation models suggest an estimated volume loss of 102 000 000 m³, which is a minimum estimate, as it does not consider the volume gained by new eruptive deposits (which may exceed another 100 000 000 m³); furthermore, the submarine collapse volume is not included and necessitates forthcoming bathymetric surveys. Tephra deposition occurred immediately after the sector collapse (between 22 and 25 December 2018, as determined by satellite radar images), causing a shift in the perimeter of the island and overprinting the collapse scar geometry.

Morphological changes by sector collapse, material deposition, and subsequent erosion. (a)–(c) TerraSAR-X satellite radar images in high-resolution spotlight mode showing the extents of changes associated with sector collapse and with the formation and erosion of the new coastline, tuff ring, and crater lake before and after the 22 December 2018 collapse. (d), (e) Before and after comparison of the morphology deduced from TanDEM-X data (before, light-gray shading) and camera drone data (after, dark gray shading) revealing profound topographic changes, material loss and deposition of new materials. Erosion was carving gullies on the volcano flanks by early January 2018. Walters et al. (2019).

Profound changes continued to occur in the weeks following the catastrophic event. Numerous small slumps deposited material into the landslide amphitheatre and an explosion tuff ring formed inside the decapitated volcano conduit area. The eruption site now appears slightly shifted to the southwest, hosting a new 400 m wide water-filled crater. Thermal activity was detected after the collapse, possibly linked to ongoing eruptions. Although the collapse of the southwestern sector into the ocean was associated with a considerable volume loss, area calculations of the island reveal rapid regrowth (over 10%) from December 2018 to January 2019, which was mainly associated with the (re-)deposition of pyroclastic material.

Cascade of precursors leading up to the 22 December 2018 sector collapse event. (a) Precursors include flank motion (white arrow), eruptions (represented as eruption cloud), and increasing eruptive deposits (red shaded areas) as assessed by satellite data (thermal, interferometric synthetic aperture radar). A décollement (black line beneath the island) dips southwest, but faulting had not yet breached the surface. Approximately 2 minutes before the collapse, a seismic event was recorded (shown by seismic trace symbol). (b) The landslide collapse along a failure plane (black curved line beneath island) showed a 1–2-minute long low-frequency signal (seismic waveform). Infrasound instruments (speaker symbol) measured the collapse before the tsunami waves arrived. The collapse decapitated the island (grey shaded area). The tsunami (blue wave) caused damage and loss along the coast. (c) Post-collapse volcanic explosions occurred coincident with increased degassing (grey plume) caused by unloading (arrow symbol); the old topography is indicated (black dashed line). New eruptive deposits increased the island area (red shaded areas). Finally, rapid erosion deeply carved incisions into the fresh eruption deposits. Walters et al. (2019).

On the basis of the remotely sensed displacement data and thermal analysis, we conclude that the Krakatau volcano showed clear signs of flank motion and elevated volcanic activity prior to the 22 December 2018 sector collapse. The long-term hazard owing to Anak Krakatau’s steep volcanic edifice had already been described, and thus, the collapse event and subsequent tsunami were anticipated hazards. The month-scale precursors included the strongest thermal activity recorded at Anak Krakatau in about 20 years and an accelerated flank motion; these characteristics made Anak Krakatau one of the most rapidly deforming volcano flanks known on Earth prior to its collapse. In fact, deformation was already identified along the southwestern flank of Anak Krakatau in interferometric synthetic aperture radar time series over 10 years before December 2018. but this deformation was not interpreted to be a potential precursor of a larger sector collapse. Compared with other volcanoes that exhibited flank deformation prior to sector collapse, the movement at Anak Krakatau also corresponded with eruption pulses, possibly associated with pressure changes in the volcano interior, and therefore Anak Krakatau shares a similar behaviour with volcanoes elsewhere.

Destruction over the island as identified in Sentinel 2 data and as seen from drone photos. (a) Sentinel-2 bands 8, 4, and 3 in combination with the RGB channels show vegetation as red and loss of vegetation after eruption in dark gray. (b) The drone images reveal the profound destruction on the adjacent island of Kecil, most likely associated with devastation due to surge and gas plumes. The upper image shows coastline erosion on western Kecil. Lower image shows view over Kecil, looking to the north. Walters et al. (2019).

Walters et al. investigated whether changes in composition could explain the increase in magma production at Anak Krakatau prior to its collapse, but our analysis of syn-deformation tephra samples suggest that the material was not significantly different from the material erupted in past decades, implying that deep magmatic changes were likely not directly responsible for the observed dynamic changes at the surface. The orientation of the main sliding plane of the collapse event could be identified from the seismic records of the collapse, suggesting a steeply southwesterly dipping failure nodal plane. The strike and dip of this plane are geometrically in agreement with the amphitheatre morphology and also notably with the inferred dislocation plane of precursory creep motion. Therefore, we conclude that the landslide décollement had already developed before the collapse.

Furthermore, as a significant proportion of the island and its shallow plumbing system was removed by the flank collapse, unloading may affect (also future) post-collapse compositions. the structural evolution, magma pathways, and eruption locations. 

A remaining question is whether the landslide of Anak Krakatau was triggered by volcanic or seismic activity. Our observations indicate that the climax of the eruptive phase was recorded in late September 2018, about 3 months prior to the flank collapse. Indeed, from September to December 2018, the volume of newly deposits followed a generally decreasing trend. In addition, sulphur dioxide gas emissions were low in the weeks prior to the collapse. Moreover, because the deformation rate remained almost constant throughout this period, Walters et al. suggest that only minor change, if any, was attributable to the accumulation of magma into the shallow portions of the edifice. However, the intense activity witnessed throughout the year likely increased the overall instability of the edifice owing to the rapid accumulation of new material. In fact, studies elsewhere show that slope instability at volcanoes is not always associated with eruptive phases. This relationship is observed because slope instability changes over time; in addition, fault planes and other zones of weakness are strongly affected by pore pressure changes, hydrothermal activity, and mechanical weakening by alteration, as well as by sea erosion and oversteepening. A similar but much smaller sequence recently occurred also at Mount Etna, where a short-term increase in the magma supply and eruption rate was accompanied by magmatic intrusion, leading to the collapse of an unstable cone. This example demonstrates that under such critical conditions, minor internal and external perturbations can potentially trigger a collapse and eruption, leading to a disaster. From this perspective, Walters et al.'s hypothesis that the seismic event identified two minutes before the Anak Krakatau landslide acted as an external trigger is plausible but remains to be tested further.

Volcano-induced tsunamis are thought to be rare and are therefore not commonly considered in tsunami early warning centres. Historic documents reveal, however, that Southeast Asia experiences volcano-induced tsunami hazards relatively frequently, with 17 events during the 20th century and at least 14 events during the 19th century, defining a recurrence rate of one event every 5–8 years. A volcano-induced tsunami from Anak Krakatau was anticipated, but accurate predictions were impossible owing to a lack of understanding of the processes involved. Hence, the study of the 22 December 2018 sector collapse at Anak Krakatau provides important new information about the precursors and processes that culminated in the disaster.

The tsunami reached the coastal towns of Jambu, Ciwandan, Agung, and Panjang within 31, 38, 39, and 57 minutes, respectively. The tsunami waves were overtaken by the faster seismic waves and the infrasound signals of the strong explosive eruption, that were associated with the landslide and decapitation of the hot interior of the volcano followed by steam-driven phreatomagmatic explosions.

In fact, the seismic records following the sector collapse event indicate that tremor activity continued for hours, resembling the volcanic tremors associated with steam-driven explosions elsewhere, although the large distance between the volcano and the seismic network may have blurred this interpretation. The eruptive style of the post-decapitation eruptions was different from that of the pre-decapitation eruptions. This is indicated by the different frequency contents in the seismic and infrasound data and the onset of significant sulphur dioxide emissions on 22–23 December 2018, in agreement with the period characterised by a reduction in radar amplitudes, the deposition of erupted material and a shift in the coastline, as seen in the ground-range detected images on 22–25 December 2018. The productivity of the eruptions also appeared to increase, as evidenced by the marked growth of the island area after the collapse. This suggests a major effect of unloading on the magmatic and hydrothermal interior of the volcano.

It appears that a perfect storm of magma-tectonic processes at Anak Krakatau culminated in the 22 December 2018 tsunami disaster. Leading up to the event, different sensors, and methods measured distinct anomalous behaviours, which in hindsight can be deemed precursory. However, at the time and when considered individually, none of the parameters, including the thermal anomalies, flank motion, anomalous degassing, seismicity, and infrasound data, were sufficiently conclusive to shed light on the events that were about to unfold.

Walters et al.'s study demonstrates that volcano sector collapses and the resulting tsunamis might be effectively anticipated by continuously monitoring the various preparation stages. Long-term flank motion, changes in thermal emission, and short-term seismic events precede the collapse, which itself was well monitored by low-frequency seismic waveforms and infrasound stations. Assessments of these parameters could be implemented in available early warning systems. Therefore, the next-generation tsunami early warning system must consider multiparametric observations, since our study reveals that a multitude of changes signified an unprecedented level of activity at Anak Krakatau prior to 22 December 2018. Walters et al. hence recommend a dedicated search for island volcanoes that are susceptible to flank collapses and those with a history of tsunamis, and we advise the development of appropriate monitoring programs to identify critical systems at these sites.

Because the Anak Krakatau sector collapse and tsunami are rare events, insights such as those reported by Walters et al. yield vital information on precursor processes and aid in refining existing monitoring and early warning technologies.

See also...

https://sciencythoughts.blogspot.com/2019/10/eruption-on-mount-merapi-java.htmlhttps://sciencythoughts.blogspot.com/2019/07/eruption-on-tangkuban-perahu-volcano.html
https://sciencythoughts.blogspot.com/2019/06/large-eruption-on-mount-sinabung-sumatra.htmlhttps://sciencythoughts.blogspot.com/2019/05/flights-cancelled-after-eruption-on.html
https://sciencythoughts.blogspot.com/2019/04/eruption-on-mount-agung-bali.htmlhttps://sciencythoughts.blogspot.com/2019/03/magnitude-55-earthquake-on-lombok.html
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Saturday, 25 January 2020

Magnitude 6.7 Earthquake in Malatya Province, Turkey.

The United States Geological Survey recorded a Magnitude 6.7 Earthquake at a depth of 10 km, about 4 km to the northeast of the  city of Doğanyol in Malatya Province, southeast Turkey, at about 8.55 pm local time (at about 5.55 pm GMT) on Friday 24 January 2020. The Earthquake is reported to have caused a number of building collapses across the area, as well as a large number of deaths and injuries. At the time of writing 22 people are known to have died due to the Earthquake, four in Malatya Province and eighteen in Elazığ Province, with more than 1200 people injured, though these figures are expected to rise further as the rubble from collapsed buildings is cleared. The Earthquake was felt across eastern and central Turkey, as well as in Georgia, Iran, Iraq, Syria, Lebanon, Jordan and Israel.

Collapsed buildings in eastern Turkey following a Magnitude 6.7 Earthquake on Friday 24 January 2020. Hürriyet Daily News.

Malatya Province lies at the boundary between the Eurasian Plate to the north and east, the Anatolian Plate to the west and the Arabian Plate to the south. The Arabian Plate  is being pushed north and west by the movement of the African Plate, further to the south. This leads to a zone of tectonic activity within the province, as the Arabian and Anatolian plates are pushed together, along the East Anatolian Fault, and past one-another, along the Dead Sea Transform.

The approximate location of the 24 January 2020 Malatya Province Earthquake. USGS.

This movement also leads to a zone of faulting along the northern part of Turkey, the North Anatolian Fault Zone, as the Anatolian Plate is pushed past the Eurasian Plate, which underlies the Black Sea and Crimean Peninsula  (transform faulting). This is not a simple process, as the two plates constantly stick together, then break apart as the pressure builds up, leading to Earthquakes, which can be some distance from the actual fault zone.

The northward movement of the African and Arabian Plates also causes folding and uplift in the Caucasus Mountains, which separate Georgia from Russia. Again this is not a smooth process, with the rocks sticking together, then moving sharply as the pressure builds up enough to break them apart, which can also lead to Earthquakes in the region.

 Plate movements and fault zones around the Anatolian Plate. Mike Norton/Wikimedia Commons.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
 
See also...
 
https://sciencythoughts.blogspot.com/2019/09/magnitude-58-earthquake-beneath-sea-of.htmlhttps://sciencythoughts.blogspot.com/2019/01/three-confirmed-deaths-and-two-people.html
https://sciencythoughts.blogspot.com/2018/07/collapse-at-unlicensed-coal-mine-kills.htmlhttps://sciencythoughts.blogspot.com/2018/04/magnitude-48-earthquake-in-antalya.html
https://sciencythoughts.blogspot.com/2017/10/magnitude-46-earthquake-off-bodrum.htmlhttps://sciencythoughts.blogspot.com/2017/09/magnitude-49-earthquake-of-southeast.html
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Undestanding the relationship between carbon dioxide and the formation and environmental impact of Large Igneous Provinces.

Large Igneous Provinces are defined as the geologically rapid emplacement (over 100 000-1 000 000 years) of hundreds of thousands to millions of cubic kilometres of lava at the surface and the associated intrusive bodies. They are dominated by thick successions of lavas known as flood basalts. These vast igneous provinces have formed several times throughout Earth’s history, on almost all of the major continents and also in the oceans. Large igneous provinces are often found far from plate boundaries. Detailed studies of individual Large Igneous Provinces have shown that they are formed of igneous rocks with diverse compositions, ranging from tholeiitic basalts, to occasional rhyolites, to strongly alkaline magmas such as lamproites and carbonatites. The generation and emplacement of Large Igneous Province magmas is linked to rapid, large-scale outgassing of volatile molecules and elements, including sulphur, water, halogens, and carbon dioxide. This surface outgassing is facilitated by extensive subterranean magmatic plumbing systems that form important pathways for the transfer of mantle and crustal carbon to the atmosphere. Among magmatic gases, carbon dioxide (CO₂) is particularly vital to the life cycle of Large Igneous Province magmatism and its climatic consequences. The centrality of CO₂ in the environmental perturbations that coincide with some Large Igneous Provinces, such as the Deccan Traps (India), Siberian Traps (Russia), Karoo–Ferrar (southern Africa and Antarctica, respectively), Ontong Java Plateau (Pacific Ocean), Columbia River Basalt Group (northwestern USA), and the Central Atlantic Magmatic Province (northwest Africa, southwest Europe, northeast and southeast North America), renders Large Igneous Province-driven climate stress an important palaeoclimate analog for the present-day climate. However, the origins, budget, isotopic composition, and fate of Large Igneous Province carbon remain pressing and challenging questions due to the evanescence of CO₂ in carbon-saturated mafic magmatic systems.

The hardened lava flows of the Deccan Traps, in western India, may have played a role in the demise of the Dinosaurs. Gerta Keller/Science.

In a paper published in the journal Elements on 2 October 2019, Benjamin Black of the Department of Earth and Atmospheric Sciences at the City College of New York, and Sally Gibson of the Department of Earth Sciences at the University of Cambridge, discuss the relationship between the outgassing of carbon dioxide and the emplacement of Large Igneous Provinces.

Water, CO₂, sulphur, and halogens are among the most abundant constituents in volcanic gases. While some fraction of these gases is released during volcanic eruptions, the remainder may be discharged diffusely through crust, unaccompanied by volcanic eruptions. This cryptic degassing can form an important part of the overall gas release budget, especially for CO₂. Furthermore, if the establishment of lithospheric plumbing systems during the initial development of a Large Igneous Province precedes the onset of flood basalt volcanism at the surface, or if intrusive magmas solidify after the last eruptions, cryptic degassing could either precede or postdate active volcanism.

Because no Large Igneous Provinces are forming at the present day, studies of recent, analogous, volcanic activity provide an important source of information about the release of Large Igneous Province carbon. Eruptions of tholeiitic flood basalt lavas (subalkaline basalts, basalts that contain less sodium than some other basalts) may bear similarities to present-day fissure eruptions in Hawai‘i (Kï'lauea Volcano) and Iceland (Laki and Holuhraun Volcanoes). The smaller volume alkaline magmas associated with some Large Igneous Province may have had similar origins to those in the East African Rift.

Lava from a fissure on the  Kï'lauea Volcano, Hawai'i,  flows through a well-established channel to the ocean south of Kapoho. USGS.

At sites of present-day volcanism, both eruptive and cryptic CO₂ release can be constrained through ground-based and airborne measurements. More direct constraints on carbon in Large Igneous Province magmas can potentially be provided by petrologic and geochemical methods, but such data are scarce and require cautious interpretation. Blebs of melt trapped inside growing crystals (commonly known as 'melt inclusions') can record magmatic water, CO₂, sulphur, and halogen concentrations at the time of entrapment and prior to eruption. However, any CO₂ that has already exsolved (come out of solution and escaped) at the time of melt entrapment cannot be reliably reconstructed. Alternatively, trace element ratios such as CO₂/Niobium and CO₂/Barium from un-degassed samples can be used to infer original CO₂ content for partly degassed, primitive magmas. This is because Niobium and Barium exhibit a similar incompatible behaviour to CO₂ during mantle melting but do not degas.

Although all Large Igneous Province consist of a wide variety of magma types, estimates of volatile contents have mainly been derived for tholeiitic flood basalts. Most previous estimates of the CO₂ content of flood basalts have relied on analogous, well-studied basaltic systems, such as Kï'lauea and Laki, to arrive at concentrations of 0.5–0.9 % CO₂ (by weight) in flood basalt magmas. Given that in continental settings, flood basalt magmas may require several percent weight of dissolved water and/or exsolved CO₂ to be sufficiently buoyant to erupt through low-density felsic continental crust (crust enhanced in silicon, oxygen, aluminium, sodium, and potassium), these CO₂ concentrations may be underestimates.

Lave fields deposited during the 1783-84 Laki Eruption in Iceland. Ulrich Latzenhofer/Fotopedia.

Alternative estimates of the CO₂ concentrations in flood basalt magmas have been derived from olivine-hosted melt inclusions. These are rare and typically found in primitive flood basalts, so they may not necessarily be representative of the main phase of more fractionated tholeiitic magmatism. Moreover, analyses of CO₂ in olivine-hosted melt inclusions in tholeiitic magmas reflect only the dissolved amount, which forms an indeterminate fraction of the total CO2 released per km3 of magma. Thus far, measurements of CO₂ in olivine-hosted melt inclusions from flood basalts have overlooked the CO₂ in shrinkage vapour bubbles (which may dominate the total CO₂ content) and so are minimum estimates. If Large Igneous Province magmas reach CO₂ saturation at high pressures, CO₂ concentrations in melt inclusions represent lower limits on initial concentrations. Consequently, it is not surprising that CO₂  measurements for melt inclusions in the Siberian Traps are lower than the estimates for flood basalts based on measurements from Hawai‘i and Iceland.

The extent of the End Permian Siberian Traps Volcanism. Jo Weber/Wikimedia Commons.

The use of volatile/nonvolatile trace element ratios (such as CO₂/niobium and CO₂/barium) to estimate original CO₂ concentrations must be applied with care to flood basalt magmas. This is because processes such as recharge, assimilation, and fractional crystallisation in crustal magma chambers can significantly modify concentrations of strongly incompatible trace elements. Primitive high-Magnesium oxide lavas, known as picrites, may sidestep this issue and thereby provide a window into initial CO₂ concentrations. Using this approach, barium and niobium concentrations in picrites from the Siberian Traps and the North Atlantic Igneous Province suggest original melt CO₂ concentrations of between 0.1 and 2 CO₂ by weight. This large range for flood basalts, along with the current lack of data for more alkaline magmas, emphasises the need for further direct constraints on carbon in Large Igneous Provinces.

Large igneous provinces occupy broad areal extents, up to 1 000 000 km² and are widely believed to have resulted from the impingement and lateral spreading of upwelling high-temperature mantle plume heads with diameters of up to 2000 km at the base of the lithosphere. According to their site of emplacement, large igneous provinces may be categorised as oceanic or continental. Large Igneous Provinces emplaced on the continents may draw carbon from three main reservoirs: the convecting mantle, the subcontinental lithospheric mantle, and sedimentary rocks and fluids in the crust. The main source of carbon for oceanic large igneous provinces is the convecting mantle.

 Summary diagram of Large Igneous Province (LIP) carbon fluxes ( expressed as mega tonnes of carbon per year: Mt C y ̄¹), isotope ratios (rxpressed as δ13C, in units of per mil, ‰), and various types of geological reservoirs, all placed in context. The carbonatite ledge represents a major inflection in the CO²-bearing peridotite solidus. Black & Gibson (2019).

Carbon dioxide behaves highly incompatibly during mantle melting, meaning that it partitions almost entirely into the melt phase. Consequently, the initial CO² concentrations of magmas are determined by the carbon concentration of their mantle source and by the degree of partial melting. The high helium³/helium⁴ ratios in some large igneous province magmas suggest their parental melts are formed from deep-sourced, primordial material brought up in mantle plumes. In addition to primordial carbon, mantle plumes are also likely to contain carbon that has been recycled, due to subduction and subsequent entrainment of oceanic crust by the plume. However, the fate of carbon during plate tectonic recycling is not well known. Furthermore, while some Large Igneous Province melts have incompatible trace element and strontium, neodymium, lead and hafnium isotopic ratios similar to oceanic basalts, and they appear to be derived solely from a mantle plume source, the geochemistry of many large igneous province melts testifies to additional contributions from the overlying lithosphere.

Variations in the depth and degree of melting in upwelling mantle plumes associated with flood basalts are well-established, for example, from incompatible trace element ratios that reflect the presence or absence of garnet in the residue during melting. Numerical models indicate that the extent of partial melting that occurs during upwelling of mantle plumes is primarily controlled by the temperature of the convecting mantle and the thickness of the overlying lithosphere. If the lithosphere becomes thinner through the course of Large Igneous Province magmatism, due to synemplacement (fracturing) extension or erosion, the amount of melting will be lowest at the earliest stages of plume impact and the carbon concentration in these melts will be high. Because the lithosphere is of nonuniform thickness, the amount of melting in the plume will also vary spatially at any given time, and regions of pre-existing thinning or weakness will focus plume upwelling and melting.

Thinning or removal of the subcontinental lithospheric mantle may also potentially mobilize carbon. While this large and ancient Earth reservoir has been proposed as a major repository for volatiles, in part due to the infiltration of small-fraction, volatile rich, convecting, mantle-sourced melts over long periods of geological time, its carbon budget and isotopic composition are poorly known. The most concentrated accumulation of carbon is likely to be either at a depth of approximately 75 km, where experimental studies have shown that there is a depression in the CO₂-bearing peridotite solidus, or in regions where redox freezing traps carbon as graphite or diamonds, which can later be oxidized by percolating carbonatitic melts.

Mantle plume–lithosphere interactions through the life cycle of continental Large Igneous Provinces (LIPs) can influence the melting regime of the convecting mantle and the subcontinental mantle lithosphere. (1) During the initial impingement of a mantle plume (red) beneath thick continental lithosphere (yellow and brown), low-degree partial melts from metasomatised lithospheric mantle predominate. (2) Over the course of Large Igneous Province magmatism, the lithosphere undergoes significant thinning through extension and/or foundering. Formation of tholeiitic basalts through adiabatic decompression melting in the mantle plume imposes a maximum lithospheric thickness during main-phase flood basalt magmatism of about 70 km. Black & Gibson (2019).

A final, but potentially important, source of carbon to Large Igneous Province magmas comes from crustal fluids and sedimentary rocks (such as evaporites, coals, carbonates, and hydrocarbons). The flux of carbon from these crustal sources depends on the country rock lithologies, the input and distribution of heat, and the fraction of gases that reach the atmosphere.

Large igneous provinces are commonly emplaced over several million years, but the main pulse of flood basalt volcanism occurs on timescales of a million years or less. The flux of CO₂ is, therefore, likely to vary during Large Igneous Province emplacement. and through the course of individual Large Igneous Province eruptions. Both the evolving CO₂ flux and the ratio of carbon isotopes depend on magma emplacement rates, melting conditions, carbon sources, and flushing of CO₂ through the magmatic system as a fluid phase that is not bound to magma transfer. All of these factors are likely to shift through the life cycle of a Large Igneous Province.

The tempo of magma emplacement can be constrained through studies of geochronology, physical vulcanology, palaeomagnetism, radiogenic isotope systems, or proxies such as mercury deposition. These lines of evidence retain significant uncertainties, but they do generally support the existence of short-timescale variations in volcanic activity superposed on gradually shifting long-term mean volcanic fluxes.

Geochronologic studies show that low-degree, incompatible trace element–rich, lithospheric melts often pre- and postdate the main phase outpourings of flood basalts. If lithospheric removal occurs during Large Igneous Province emplacement, this could also trigger a pulse of devolatilisation from both the foundering lithospheric material and the residual subcontinental lithosphere under a steeper geothermalmal gradient.

Thermomechanical transitions may modulate the depths of magma storage and, therefore, the country rock lithologies that are to be subjected to heating and devolatilisation; crustal metamorphism also requires heating of large volumes of rock, and outgassing from the cold upper crust may, therefore, lag behind the onset of volcanism.

On the timescales of individual eruptions, carbon outgassing can be decoupled from volcanic flux, for example when CO₂ partitions into a fluid phase. Ground-based measurements of the Holuhraun (Iceland) fissure eruption of 2014–2015 revealed that CO₂/SO₂ ratios in the volcanic plume were higher by a factor of 10 during the earliest days of the eruption.

Flipping the causal relationship, evolving CO₂ concentrations during fractionation of magmas in the deep crust have also been hypothesised to exert control over the eruptibility of flood basalt magmas. Carbon dioxide may, therefore, play a role in shaping the tempo of volcanic activity and outgassing.

The depths at which Large Igneous Province magmas become saturated in an exsolved CO₂-rich phase, and the mobility and fate of the exsolved fluid, are critical to understanding their overall carbon outgassing history. The proportion of a magmatic volatile substance, one that is initially dissolved in the melt, that reaches the atmosphere can be thought of as the outgassing efficiency. For CO₂, it is commonly assumed that the outgassing efficiency is close to 100% for extrusive flood basalt magmas, due to the very low solubility of CO₂ in basaltic melt at one atmosphere pressure. In conjunction with an assumed CO₂ content of 0.5% weight in a primitive basaltic melt, this efficiency implies an approximate CO₂ yield of 14 megatonnes per  cubic kilometre of erupted magma. Importantly, if the CO₂ outgassing efficiency deviates significantly from 100%, carbon isotope fractionation due to partial degassing may shift the net isotope ratio of the carbon that is released.

Estimates of CO₂ outgassing that are based solely on emplacement rates of flood basalts do not account for the potential flux of CO₂ from associated intrusive magmas. Outgassing from CO₂ saturated magma bodies in the permeable upper crust may take place through gradual, passive degassing in conjunction with emissions during eruptions. In the less permeable lower crust and lithospheric mantle, dike formation and magma ascent may provide one of the only avenues for CO₂-rich exsolved fluids to reach the surface. In this case, CO₂ initially exsolved at depth could 'flush' shallower magmas, increasing CO₂ release beyond what would be expected from the volume of erupted flood basalts. The hypothesis of large-scale CO₂ flushing in the complex magmatic plumbing systems associated with flood basalts receives some support from studies of Icelandic fissure eruptions. Comparison between trace element concentrations and melt inclusion CO₂ content from the 1783–1784 Laki (Iceland) fissure eruption suggests that about 60% of the initial CO₂ cargo was degassed in the lower-to-middle crust.

 A fissure eruption in Hawai'i. Wikimedia Commons.

The importance of deep intrusive degassing depends on the relative volumes of intrusive and extrusive magmas. Based on petrology and seismic imaging of high-velocity layers near the Moho (the Mohorovičić Discontinuity,  the boundary between the Earth's crust and the mantle), previous researchers have inferred that the ultramafic cumulates that may underlie Large Igneous Provinces are comparable in volume to the erupted lavas. A range in intrusive/extrusive ratio of 0.5 to 4 implies that 30%–80% of Large Igneous Province magmas do not erupt. The efficiency with which these deep intrusive magmas degas and transfer CO₂ to the atmosphere is uncertain. It has been estimated than a 40–60% degassing rate of intrusive magmas occurred  in the Laki plumbing system, but this rate could be higher if crystallisation in deep magma bodies drove further CO₂ exsolution. Assuming homogeneous CO₂ content, 50% intrusive degassing, and efficient transfer of CO₂ to the atmosphere via flushing through the magmatic system and eruption, the magmas that do erupt could carry 1.25 to 3 times their native CO₂ content assuming an intrusive/extrusive ration of 0.5 to 4. This 'excess carbon' load is, in some ways, analogous to the 'excess sulfur' released from some arc volcanic eruptions that tap a sulphur-rich exsolved phase.

The Mohorovičić Discontinuity, or Moho (red). Geology.com

The fluxes of CO₂ during Large Igneous Province magmatism, and, consequently, the viability of CO₂ as a significant driver of climate change, have been the subject of debate. Perhaps the best evidence for major perturbation of the global carbon cycle by Large Igneous Provinces comes from carbon isotope records and independent palaeoclimate and  atmospheric CO₂ content proxies. In brief, these lines of evidence indicate four major aspects of carbon cycle disruption: (1) strong warming coinciding with some Large Igneous Provinces (e.g., the Siberian Traps and the North Atlantic Igneous Province), with more ambiguous evidence in other cases; (2) spikes in atmospheric CO₂ content; (3) negative carbon isotope excursions (drop in the proportion of carbon¹² in sediments); (4) ocean acidification. Taken together with geochronology aligning these environmental changes to Large Igneous Province magmatism, the most straightforward interpretation is that they were primarily caused by carbon release related to Large Igneous Province emplacement, possibly supplemented with light carbon from clathrates (chemical substances consisting of a lattice of water molecules that traps or contains other molecules) or from the metamorphism of sedimentary organic material. Accurately determining the CO₂ budget of diverse Large Igneous Province magmas is critical to testing hypotheses regarding the causal links between magmatism, warming, and carbon cycle perturbation.

On the timescales of silicate weathering and water–rock reactions, Large Igneous Provinces may play an important role as carbon sinks, because CO₂ reacts with calcium and magnesium in basalts to form carbonate minerals. By analogy to mid-ocean ridge settings, where the balance between net outgassing and net sequestration of carbon is uncertain, the capacity for  Large Igneous Provincecarbon sequestration on longer timescales may rival the magnitude of potential outgassing. This balance, and the timescales of atmospheric CO₂ draw down, may depend on the extent of subaerial versus submarine volcanism in a given Large Igneous Province and the potential for water–rock reactions in hydrothermal systems.

Large igneous provinces are dominated volumetrically by vast outpourings of flood basalt lavas and their intrusive equivalents. The frequent association of alkaline and carbonatite magmatism with flood basalts in Large Igneous Provinces attests to the petrologic importance of carbon during the generation of Large Igneous Province magmas, though when and how plumbing systems associated with diverse magma types interact with each other is an unresolved question. Previous estimates of the CO₂ content of flood basalts range from 0.5% to 0.9% by weight. Revised estimates of the CO₂ content of Hawaiian (Kï'lauea) and Icelandic (Laki) basalts, along with incompatible trace elements from flood basalt picrites, suggest that primitive flood basalt magmas may commonly comprise about 1% CO₂ by weight, or possibly more. However, such primitive magmas comprise a small fraction of the erupted lavas. More detailed numerical models and geochemical measurements are required to understand the carbon concentrations in flood basalts and other magma types in Large Igneous Provincess. For example, the carbon delivery potential of voluminous tholeiite lavas in flood basalt successions is important but poorly constrained. While their parental magmas result from high degrees of partial melting and are, therefore, less likely to be intrinsically CO₂-rich they may receive a boost from CO₂-rich fluids released from deep intrusive magmas.




A fissure formed during the 1783-4 Laki eruption in Iceland. Alan Robock/Eos. 

Based on the shifting contributions of deep convecting mantle, lithospheric mantle, and crustal sources, together with the variable flux from deep intrusive magmas, the CO₂ flux from Large Igneous Province magmas probably evolves through the entire magmatic cycle. As a consequence, Large Igneous Province carbon emissions are unlikely to scale directly with volumetric eruption rates, which offer, at best, a partial picture of the tempo of carbon outgassing. Nevertheless, gross estimates based on erupted volume provide a starting place for situating Large Igneous Provinces in the context of the global carbon cycle. For a total Large Igneous Province magma volume of 1 000 000–10 000 000 km³ emplaced over 100 000–1 000 000 years, and carrying  about 1% weight in CO₂, the mean annual flux would be 30–3000 mega tonnes of CO₂ per/year, with the potential for orders of magnitude deviation from this mean rate over the course of the magmatic cycle. The Laki fissure eruption released an estimated 300–900 mega tonnes of CO₂ over the course of 8 months. For comparison, the present-day global subaerially released flux of CO₂ from all volcanoes has been estimated at 300–600 mega tonnes of CO₂ per/year. During geologically brief intervals of intense outgassing, Large Igneous Provinces are likely to dominate the global flux of deep carbon to the atmosphere.

See also...

https://sciencythoughts.blogspot.com/2020/01/fluctuations-in-mercury-and-organic.htmlhttps://sciencythoughts.blogspot.com/2020/01/understanding-climate-change-before-and.html
https://sciencythoughts.blogspot.com/2018/10/looking-for-connection-between-columbia.htmlhttps://sciencythoughts.blogspot.com/2017/08/understanding-conection-between.html
https://sciencythoughts.blogspot.com/2016/04/using-mercury-to-assess-role-of-central.htmlhttps://sciencythoughts.blogspot.com/2014/04/the-cause-of-end-permian-extinction.html
 
 
 
 
 
 
 
 
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