Predicting eruptions in monogenetic volcanic fields.

Seismic activity and fumerol (gas) emissions are well established as predictors of eruptions on stratovolcanoes (volcanic mountains which undergo repeated eruptions), but predicting eruptions in other kinds of volcanoes is more problematic. Monogenetic fields are areas of volcanic activity where sporadic eruptions occur at different locations, rather than at a single site. Most such sites only suffer very irregular eruptions, but the fields are often home to hot spring systems or other features which make them attractive to humans, leading to settlement in potentially hazardous areas, making finding a method for predicting eruptions in these areas a priority.

In a paper publishd in the journal Geology on 5 February 2016, Helena Albert of the Central Geophysical Observatory at the Spanish Geographic Institute, Fidel Costa of the Earth Observatory of Singapore and Asian School of the Environment at the Nanyang Technological University and Joan Martí of the Institute of Earth Sciences Jaume Almera, discuss a number of historic eruptions at a number of different monogentic fields, with a view to understanding the processes driving volcanic activity at these sites and the possibility of predicting future eruptions in such areas.

Albert et al. examined ten historic eruptions at five mongenetic fields; the 1704-05 Tenerife eruption (part of the Canary Islands Volcanic Field), the 1909 Tenerife eruption, the 1949 La Palma eruption (also in the Canaries), the 1971 La Palma eruption, the 2011 El Hierro eruption (again in the Canaries), the 1759 Michoacan eruption (in the Michoacan-Guanajuato region of Mexico), the 1943 Michoacan eruption, the 1943 Goropu Mountains eruption (part of the Owen Stanley Range of Papua New Guinea), the 1973 Heimaey Island eruption (Iceland) and the 1989 Higashi-Izu eruption (on the Izu Peninsula, Japan).

 Emissions from the 2011 El Hierro eruption, which happened offshore to the south of the island. Guardia Civil.

Of these, only the 2011 El Hierro eruption was monitored with high quality modern seismic equipment, with three others having lower quality records available. However witness accounts of earthquakes provide insight into seismic activity in all of these fields prior to the onset of eruptive activity. All of the eruptions were proceded by periods of seismic activity, with the longest such recorded period preceding the 2011 El Hierro eruption, where tremmors were recorded 4-5 years ahead of the eruption, and the shortest being the 1973 Heimay Island eruption, where seismic activity was only noticed 2 days before the main eruption.

Lava flow from the 1973 Heimay Island eruption which entered the town of Vestmannaeyjar, destroying about half of the homes there and causing the population to evacuate to the Icelandic mainland. Will Perry/The Reykjavík Museum of Photography.

Albert et al. next looked at samples of erupted lava from all of the eruptions except the 1943 Goropu Mountains eruption and the 1989 Higashi-Izu eruption. In each case they found that the lavas were mineralogically mixed, that is to say included minerals thought to come from different magmatic intrusions. Magma extruded from deep within the Earth is thought to be more-or-less entirely liquid, however if it becomes trapped in chambers close to the surface (5-15 km underground) it cools foming a mush with crystals suspended in a liquid matrix. Different minerals are formed at different temperatures, and larger crystals form more slowly (implying the mush has remained cool enough for the crystals to form but hot enough for the matrix to remain liquid for a longer period of time), giving each magmatic intrusion its own distinctive makeup. Lavas with mixed compostitions contain crystals which cannot have formed in a single intrusion.

Ruins of the San Juan Parangaricutiro Church which was destroyed by the 1943 Michoacan eruption. Sparks Mexico/Wikipedia.

From this Albert et al. conclude that each of these volcanoc fields is fed by a an extensive shallow plumbing system with an extensive system of dykes at mid-crustal levels recieving intermittent intrusions of deep magma. While a full understanding of the processes behind this is beyond the scope of this study, the majority of these eruptions were preceded by a rise in seismic activity which began months to years before the main eruptive episodes, giving the potential for the prediction of such eruptions using dedicated seismic monitoring networks.

 Possible plumbing system configuration and evolution of events that may occur below monogenetic volcanoes (schematic and not to scale). The depth of the subvolcanic system may vary from 5 to 15 km. The depth of the magma source is also variable but is at least 20 km. (A) Intrusion of magma approximately one or two years prior to the eruption, stalling of magma at 5–15 km due to the loss of buoyancy or freezing of dikes, and mixing processes registered by the crystals. Crustal assimilation occurs in some cases. Seismic activity is felt by the population in some cases. (B) Renewal of magma intrusion, progressive opening of the path between deep and shallow reservoirs, and mixing processes registered by the crystals. Crustal assimilation occurs in some cases. The seismic activity is commonly felt by the population. (C) Continued intrusion of mafic magma leads to easier transfer from deep to shallow reservoirs, and this allows the magma to finally erupt. Magma mixing (and crustal assimilation in some cases) are recorded by the crystals. Seismicity is felt by the population. Also shown the possibility that magma is directly transfer from the mantle to the surface. Albert et al. (2015).

See also...

Magnitude 5.4 Earthquake off the coast of El Hierro.                                                          The United States Geological Survey recorded a Magnitude 5.4 Earthquake at a depth of 22.8 km roughly 31 km to the northwest of the island of El Hierro in the Canaries, slightly after 5.45 pm local time (which is GMT) on Friday 27 December 2013. There are...

The floating stones of El Hierro.               When volcanic activity began off the south coast of El Hierro, in the Canary Islands, in October 2010, it led to the production of large plumes of pumice-like rocks that floated on the surface of the ocean, producing a distinctive volcanic 'stain'. Pumice is a volcanic rock which contains large vesicles (sealed, gas filled cavities) which cause it to float on the water, which is formed by superheated magma coming...

Earthquake of the coast of El Hierro.           On Friday 29 March 2013, slightly after 5.00 pm GMT, the United States Geological Survey recorded a Magnitude 4.6 Earthquake at a depth of 17.3 km, roughly 30 km southwest of El Hierro in the Canary Islands...

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Magnitude 5.4 Earthquake off the coast of El Hierro.

The United States Geological Survey recorded a Magnitude 5.4 Earthquake at a depth of 22.8 km roughly 31 km to the northwest of the island of El Hierro in the Canaries, slightly after 5.45 pm local time (which is GMT) on Friday 27 December 2013. There are no reports of any damage or injuries relating to this quake, however it was felt quite strongly on El Hierro, and triggered a number of small landslides, resulting in the temporary closure of several roads as a precautionary measure. The quake was also felt on the islands of Las Palmas and La Gomera. 

Dust from a rockfall on El Hierro triggered by the 27 December 2013 Earthquake. Julio del Castillo Vivero/Actualidad Volcánica de Canarias/Earthquake Report.

The Canary Islands are a group of volcanic islands fueled by a mantle plume rising through the African Plate, on which they are situated. The plume is rising from deep within the Earth, and is independent of the movement of the tectonic plates at the Earth's surface. As the plate moves relative to the hotspot new volcanic islands form on its surface, each over the hotspot when it forms, with the oldest islands of the chain in the east (the African Plate is being pushed east by the expansion of the Atlantic Ocean, but the hotspot is relatively motionless). Earthquake activity to the south of El Hierro began in July 2011, since when there has been considerably activity including a number of volcanic eruptions. In December 2011 it was confirmed that a new volcanic fissure had opened up beneath the sea to the south of El Hierro, and a new volcanic island is apparently in the process of being born.

The approximate location of the 27 December 2013 El Hierro Earthquake. Google Maps.

See also The floating stones of El Hierro, Magnitude 5.9 Earthquake in the Azores, Earthquake of the coast of El Hierro, Fresh volcanic activity on El Hierro and Ongoing volcanic activity on El Hierro in the Canary Islands.

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The floating stones of El Hierro.

When volcanic activity began off the south coast of El Hierro, in the Canary Islands, in October 2010, it led to the production of large plumes of pumice-like rocks that floated on the surface of the ocean, producing a distinctive volcanic 'stain'. Pumice is a volcanic rock which contains large vesicles (sealed, gas filled cavities) which cause it to float on the water, which is formed by superheated magma coming into contact with seawater and is a common product of submarine eruptions. However closer inspection of the floating stones of El Hierro resembled pumice only superficially, being comprised largely of wollastonite (a metamorphicly altered form of limestone) and having quartz sand inclusions. These strange rocks became known as restingolites, after the village of La Restinga, on the southern tip of El Hierro.

In a paper published in the journal Solid Earth on 13 March 2012, a team of scientists led by Valentin Troll of the Department of Earth Sciences at Uppsala University and the Istituto Nazionale di Geofisica e Vulcanologia in Rome develop a theory on the origin of these restingolites.

Restingolites from El Hierro. Troll et al. (2012).

Troll et al. conclude that the floating stones of El Hierro derive not from hot magma, but rather from the metamorphic alteration of sediments on the seafloor as magma welled up around them, heating the sediments and causing a mixture of melting and degassing that produced a pumice-like sedimentary rock, often with a thin crust of true volcanic material, that they refer to as xenopumice, as they are pumice-like in appearance, but xenolithic in origin (i.e. they derive from material alien to the actual magma-flow). They note that similar clasts have been found within volcanically derived sediments at other sites in the Canaries, which suggests that the formation of the El Hierro xenopumice stones was not a unique event, and may be common to the early stages of many submarine eruptions.

Sketch showing the internal structure of El Hierro Island. Ascending magma is interacting with the pre-volcanic sedimentary rocks, and Troll et al. suggest that the early floating stones found at El Hierro are the products of magma-sediment interaction beneath the volcano. Pre-island sedimentary material was carried to the ocean floor during magma ascent and eruption and melted and vesiculated while immersed in magma to develop a pumice-like texture (xenopumice). Once erupted onto the ocean floor, these xenopumices separated from the host lava and floated on the sea surface due to their high vesicularity (i.e. their low density). Troll et al. (2012).

See also Earthquake off the coast of El Hierro, The biology of pumice rafts, Pumice raft linked to Havre Seamount, not Monowai, Pumice raft suggests eruption from Mount Monowai and Fresh volcanic activity on El Hierro.

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Earthquake of the coast of El Hierro.

On Friday 29 March 2013, slightly after 5.00 pm GMT, the United States Geological Survey recorded a Magnitude 4.6 Earthquake at a depth of 17.3 km, roughly 30 km southwest of El Hierro in the Canary Islands. This is a moderately large Earthquake, but is unlikely to have been felt this far offshore.

The location of the 29 March 2013 Earthquake. Google Maps.

The Canary Islands are a group of volcanic islands fueled by a mantle plume rising through the African Plate, on which they are situated. The plume is rising from deep within the Earth, and is independent of the movement of the tectonic plates at the Earth's surface. As the plate moves relative to the hotspot new volcanic islands form on its surface, each over the hotspot when it forms, with the oldest islands of the chain in the east (the African Plate is being pushed east by the expansion of the Atlantic Ocean, but the hotspot is relatively motionless). Earthquake activity to the south of El Hierro began in July 2011, since when there has been considerably activity including a number of volcanic eruptions. In December 2011 it was confirmed that a new volcanic fissure had opened up beneath the sea to the south of El Hierro, and a new volcanic island is apparently in the process of being born.

See also Fresh volcanic activity on El Hierro, Ongoing volcanic activity on El Hierro in the Canary Islands, Tourists evacuated due to volcanic activity on El Hierro in the Canary Islands and Volcanoes and Earthquakes on Sciency Thoughts YouTube.

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Fresh volcanic activity on El Hierro.

El Hierro is the southwestern-most of the Canary Islands, a Spanish territory off the coast of southern Morocco. It is a shield volcano (shallow-dome shaped volcano formed from layers of lava) rising 1500 m above sea-level. The current island forms part of the rim of the crater of a larger, more ancient volcano, El Golfo, which is thought to have collapsed 130 000 years ago. The center of this ancient crater lies to the southwest of the modern island. Until 2011 the volcano has been essentially inert throughout recorded history, with the last rumored (but unconfirmed) activity in the 1790s.

In July 2011 the island began to suffer frequent small tremors, which persisted throughout the summer, increasing in intensity sharply in September. In the second week of October these became more severe still, prompting a temporary evacuation of the southernmost tip of the island. This was followed by a series of submarine eruptions to the south of the island, marked by the production of turbulent gas rings and patches of floating lava (pumice). These persisted till March 2012, when they abruptly stopped. At the same time Earthquake activity on the island dropped abruptly, though it never actually stopped.

Annotated satellite photograph of El Hierro island on 26 October 2011 highlighting the emission area (discolored brown water, circled) and volcanic material transported by ocean currents, appearing light blue-green; settlement names are underlined. (Inset) Geographic location of El Hierro island, in the Canary Islands, off the West coast of Africa. Smithsonian Global Volcanism Program.

On 24 June 2011 the island suffered a magnitude 3.1 quake located of the north shore, according to the  Instituto Geográfico Nacional. This marked the onset of a renewed increase in seismic activity, with a series of Earthqakes following that moved first south, then westward across the island, finally migrating off the southwest coast, where tremors continued into July. Such migrating quakes under or around volcanoes are often caused by the movement of magma bodies beneath the visible cone, and may signal the onset of new volcanic activity.

See also Ongoing volcanic activity on El Hierro in the Canary Islands, Tourists evacuated due to volcanic activity on El Hierro in the Canary Islands and Volcanoes on Sciency Thoughts YouTube.

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Ongoing volcanic activity on El Hierro in the Canary Islands.

The island of El Hierro in the Canaries is the peak of a shield volcano (a volcanic mountain made up of layers of lava, with a wider, less cone-shaped, profile than the more notable stratovolcanoes) rising 1500 m above the sea. It is about 1.2 million years old, but has been relatively inactive during recorded history, with one observed eruption in 1793, and some lava flows dated to the 1600s. With no recorded history of large scale volcanic eruptions the island has developed as a popular tourist spot, and has a permanent population of over 10 000. Then in July this year (2011) El Hierro began to change.

The island of El Hierro.

From the start of July onwards seismologists began to record small earthquakes beneath El Hierro. These were minor, none larger than a 3 on the Richter Scale, but shallow, with an average depth of about 10 km, and as time went on they were becoming more frequent. This is something that volcanologists pay careful attention to, as it can be a sign of magma movements beneath a volcano, heralding a forthcoming eruption. This lead scientists from the Instituto Volcanologico de Canarias to place a network of GPS monitors around the island, to measure any movements. On 24 August they reported that the volcano had inflated by about 1 cm during the previous month.

By 3 September El Hierro was experiencing up to 250 minor tremors a day, with some of the quakes as shallow as 2 km. There was then a fall off in the number and intensity of quakes, lasting until the 27th, when the quakes abruptly resumed. The local government issued calls for calm, but closed schools on the island as well as the only significant road tunnel, Los Roquillos. On the 28th there were reports of rocks emerging from the Pico de Malpaso summit, though local authorities have denied this. People living close to the mountain were evacuated, but as a precaution against landslides. At about this point newspapers started to point out that there had been previous predictions that a major eruption in the Canaries could cause an Atlantic-wide tsunami. Authorities in the Canaries denied that there was any risk of this unless there was a submarine eruption on El Hierro, something thought highly unlikely.

On the 29th evacuees were allowed to return to their homes, as the area of seismic activity had moved offshore into the Las Calmas Sea, to the south of the island. This was thought to be caused by magma moving into a new chamber beneath the island, but was not seen as a cause for alarm, as only about 10% of magma movements result in an eruption at the surface. These quakes persisted for the next couple of weeks, waxing and waning in strength.

On 10 October 2011 a small submarine eruption in the Las Calmas Sea was reported, 5 km from La Restinga, the most southerly point on the island, at a depth of about 600 m. A significant number of dead fish were seen close to the sight. The next day the alert level for La Restinga was raised from yellow to red, and an emergency meeting was called at the village's football field, where the villagers were informed they were to be evacuated to the north of the island. A 4 nautical mile (7.4 km) shipping exclusion zone was also placed around the eruption.

On 12 October two new volcanic fissures were discovered, one at a depth of 750 m, 3.7 km from the shore and the other at a depth of 500 m, 2.8 km from the shore. These apparently turned the water around them green, and gave off a strong sulphurous smell. This combination of dead fish, water discolouration and odour was interpreted as indicating the vents were emitting gas rather than lava, and it was hoped that this might cause the pressure in the magma chamber to fall, decreasing earthquake activity and reducing the risk of a full-scale eruption.

Helicopter footage of one of the green patches.

A satellite image showing the extent of water discolouration to the south of El Hierro; to give a sense of scale El Hierro is about 25 km from east to west. Satellite Image courtesy of RapidEye.

On 15 October volcanic pumice (a volcanic rock with many gas filled pores) was observed floating on the surface of the sea, confirming that the eruptions were not confined to just gas. At this point some scientists began to talk about the possibility of a new island emerging to the south of El Hierro. A day later giant gas bubbles were observed reaching the surface of the sea close to the shore.

A giant gas bubble off the coast of El Hierro.

Throughout this period of eruptions there have been discussions of the possibility of a major tsunami originating from El Hierro. Scientists at the scene have repeatedly suggested that they feel this is a highly unlikely outcome, however this is largely based upon informed guesswork.

In 2001 Stephen Ward of the Institute of Geophysics and Planetary Physics at the University of California Santa Cruz and Simon Day of the Benfield Grieg Hazard Research Centre at the Department of Geological Sciences, University College London published a paper in the journal Geophysical Research Letters in which they speculated that a volcanic eruption on La Palma in the Canaries could provoke an Atlantic-wide tsunami with devastating consequences.

In October 2011 a team lead by Pablo Dávila Harris of the Department of Geology at the University of Leicester published a paper in the journal Geology discussing the likelyhood of a similar tsunami being caused by a volcanic eruption on Tenerife.

Ultimately we simply do not know the probability of an eruption on El Hierro, or many other Atlantic islands, causing a major tsunami. In the Pacific, where the dangers of a major tsunami are much more obvious, there is a well established tsunami warning centre, and a similar system is under construction for the Indian Ocean. The danger of a major tsunami in the Atlantic is clearly lower than in either of these other oceans, but should such an event occur it is likely to have devastating consequences, both economically and in human terms. While the costs of such a warning system are likely to be unattractive to governments in the current economic climate, a considerable amount of money has already been spent looking for extra-terrestrial objects with the potential to impact the Earth; a less likely scenario, and one which we would be less clear how to react to.

See also Tourists evacuated due to volcanic activity on El Hierro, Eruptions on Mount Nabro and Volcanoes on Sciency thoughts YouTube.

Tourists evacuated due to volcanic activity on El Hierro in the Canary Islands.

El Hierro is the southwesternmost of the Canary Islands, the tip of an ancient shield volcano that rises 1500 m above sea level. The Canary Islands are all volcanic in origin, sitting on a volcanic hotspot that has been moving east to west throwing up volcanoes for at least 60 million years (strictly speaking the hotspot has been staying still while the overlying Atlantic crust moves east). Technically El Hierro is part of the rim of the crater of an older volcano, El Golfo, which collapsed around 130 000 years ago. The island appeared above the sea about 1.2 million years ago, in the east the oldest of the islands, Feurteventura, is thought to be about 20.6 million years old.

There are seven major islands in the Canaries; El Hierro, La Palma, La Gomera, Tenerife, Gran Canaria, Fuerteventura and Lanzarote. All of these except La Gomera have their own volcano; La Gomera probably did but is highly eroded, to the extent that it is not possible to be certain where the volcanic eruptions that produced the island were centered. There are also a number of seamounts (submarine volcanoes) associated with the islands; notable South and Henry Seamounts, to the south and southeast of El Hierro, and Casablanca Seamount to the east of Fuerteventura.

Despite being the youngest of the islands, El Hierro has not been active for a long time. The last reported eruption on the island was in 1793, when lava erupted from a vent on the northwest of the island for about a month. Prior to this there may have been eruptions in 1692 and 1677. Significant lava flows on the island have been dated to 550 BC, 950 BC and 4790 BC.

In July this year (2011) the Instituto Geográfico Nacional began to detect small earthquakes occurring deep beneath the island. These quakes were happening at a depth of about 10 km, though all were small, with magnitudes bellow 3 on the Richter Scale. This sort of activity near a volcano is often indicative of magma movements beneath the surface which may lead to a future volcanic eruption. Since July there have been over 7200 such minor earthquakes on Hierro, but in September they suddenly became much more frequent, with over 900 occuring between the 8th and 19th of the month, three large enough to have been felt by people living at the surface.

The epicenters of earthquakes on El Hierro since July 2011.

On Wednesday this week (28 September) rocks were seen being thrown from the Pico de Malpaso summit, the highest peak on the island. At this point the local police made the decision to begin evacuating people from the immediate area. Initially 53 people have been evacuated from properties close to the mountain, but preparations are being made to evacuate up to 2000 of the islands' 11 000 inhabitants. The authorities do not expect a major eruption on the island, but are concerned about the hazards presented by volcanic bombs (hot rocks thrown from volcanoes).

The threat of a major volcanic eruption in the Canary Islands is taken seriously by geologists. In 1999 a team led by Simon Day of the Benefield Grieg Hazard Research Centre at University College London published a paper in the Journal of Volcanology and Geothermal Research in which they concluded that a major eruption in the Canary Islands could lead to a flank collapse, triggering a tsunami that could devastate the coasts of countries around the Atlantic Ocean, including the US, Brazil and Western Europe.

The evolution of a hypothetical tsunami centered on La Palma in the Canaries.

See also The dangers of a modern Laki-style eruption in Iceland, Scientists predict eruption of Axial Seamount and Volcanoes on Sciency Thoughts YouTube.