What happens when a volcano meets a subduction zone?

This week a reader in New Zealand asked me what happens when a volcano meets a subduction zone? This is a slightly complex question, as there are different answers for different types of volcano, so I've written a full article to answer the question in full.

The question was asked with reference to the Kermadec Islands, which are an island arc created as one section of ocean plate is subducted beneath another. As the underlying Pacific Plate disappears beneath the overlying Australian Plate it is partially melted. The lighter minerals from the underlying plate then rise up through the overlying plate, forming a row of volcanoes, the island arc at the surface.

How volcanoes are formed in the Kermadec Islands.

Eventually all of the underlying plate will be consumed, and the subduction zone will encounter the far side of the plate, which may have another subduction zone with an island arc, or possibly a continent. Neither of these can be easily consumed, so the subduction zone closes, forming a suture. This starves the volcanoes of new material and they cease activity. Where two ocean subduction zones meet then one of the subduction zones may survive, with the two sutured island arcs sitting on top of the remaining subduction zone. Some scientists think that the Earth's earliest continents formed in this way, by the accretion of large numbers of suture zones. Suture zones tend to be mineral rich, and are often exploited by mining companies for their natural wealth.

The formation of a suture zone, in what is now part of the Himalayas.

The next common source of volcanic activity are mid-ocean ridges, where new crust is being created by convection currents within the Earth's mantle. Mid Ocean ridges underly the volcanoes of Iceland and St. Helena; the East African Rift volcanoes are thought to occur where a new ocean ridge is forming, breaking the continent apart.

New ocean crust forming at a mid-ocean ridge.

These structures can be subducted if they are overrun by a subduction zone. This disrupts the convection currents that from the ridge, stopping the volcanic activity, but does not form a suture, so that there is a break in the subducting material, between what are effectively two plates. The San Andreas Fault in California is formed this way. The overlying surface that people build cities upon is all part of the North American Plate, but beneath the surface the ancient Farallon Spreading Ridge is being subducted, forming a deep fault that occasionally causes spectacular and devastating earthquakes.

At the San Andreas Fault the subduction of a spreading ridge has lead to spectacular faulting at the surface, but volcanic activity has stopped.

The third type of volcanoes found on Earth are hotspot volcanoes. These occur away from plate margins, and drift across the surface of the Earth apparently independently of the movement of the continents. The volcanoes of Hawaii, the Canaries and the Seychelles are hotspot volcanoes. Many scientists believe that these volcanoes sit on top of mantle plumes that originate deep within the Earth; deeper than the convection currents that cause the movement of the continents, and therefore independent of them.

The Hawaiian Islands depicted sitting on top of a mantle plume.

Under this model a mantle plume is largely independent of the movement of the plates. It's origin is deep bellow the forces that cause ocean spreading and subduction zones, and it ought to be able to pass under these structures without major effect. It is thought that the Hawaiian Hotspot may have crossed Siberia during the Mesozoic, and that the Seychelles Hotspot once lay beneath India.

The movement of volcanic hotspots across the Pacific Ocean.

However not all scientists agree with this interpretation of volcanic hotspots. Some scientists believe in a shallow origin for these structures. Under this model a volcanic hotspot is a spreading ridge that has failed to expand into a new area of ocean creation. Such hotspots would have origins with shallow convection currents in the Earth's mantle, and would be disrupted by an encounter with a subduction zone, with the volcanoes dying and being sutured to the overlying plate. The hotspot might then persist for a while as a localized centre of earthquake activity.

See also Eruptions on Mount Nyamuragira in the Virunga National Park, Eruptions on Mount Nabro, The End of the Cretaceous and Volcanoes on Sciency Thoughts YouTube.

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NASA may have discovered an exoplanet smaller than the Earth.

This week at the First Kepler Science Conference at the Ames Research Park NASA scientists announced that the Kepler Space Telescope had potentially discovered an exoplanet smaller than the Earth. At the moment the team are reluctant to say that they are able to confirm this discovery, but a paper is being prepared for the journal Nature, which only publishes major discoveries that have not been documented elsewhere, which suggests that they have serious data to support their claim.

The new planet is in the Kepler-20 system, formerly known as KOI-70 (Kepler Object of Interest-70), which now has three confirmed planets, Kepler-20b, Kepler-20c and Kepler-20d and two unconfirmed planets, KOI-70.04 and KOI-70.05; the star is now referred to as Kepler-20A (letters are capitalized to indicate stars), but the unconfirmed objects retain the old designaton for the system until they are confirmed. All five orbit closer to their star than Mercury does to the sun. The three confirmed planets have been observed to cause a regular dimming of the star as they transit (pass in front of) it, and to cause perturbations in its rotation (wobbles) due to their mass. The two unconfirmed planets have been observed to transit the star, but are two small to cause any visible movement in the star, and will so need to be confirmed by another method. The smaller planets can be detected when they transit the star because they are so close to it; a small close planet causes as much dimming as a larger more distant one.

An artist's impression of the Kepler-20 system.

It is KOI-70.04 that is potentially smaller than the Earth, about 80-90% of the size of our planet. It is the second closest object to the star, at a orbiting every 6.1 days at a distance of about 9.6 million kilometers or 6.4% of the distance from Earth to the Sun. It is estimated that this would give the planet a surface temperature of 600°C.

The three larger planets have masses of 8.7 times that of the Earth (Kepler-20b), 16.1 times the mass of the Earth (Kepler-20c) and somewhere under 20.1 times the mass of the Earth (Kepler-20d). All three have radiuses between 1.9 and 3 times that of the Earth.

No further information is available at this time; no data on KOI-70.05 has been released, although it is presumably the closest object to the star.

See also NASA announces the discovery of Kepler-22b, The Atmosphere of GJ1214b and Exolanets on Sciency Thoughts YouTube.

New dinosaurs from old draws.

In 1916 the father and son dinosaur hunting team Charles and Levi Sternberg mounted an expedition to the Steveville Badlands of Alberta, Canada to collect dinosaur bones for the Natural History Museum in London. The area had proven a bountiful source of dinosaur bones for previous expeditions, having first been worked by Lawrence Lamb in 1898, and has remained a centre for dinosaur hunting to this day, forming part of Dinosaur Provincial Park, an area so rich in dinosaur bones that palaeontologists have attempted to erect stratigraphic biozones based upon them. Erecting biozones is a method of using fossils to date rocks; this uses fossils with known chronological ranges to establish the maximum and minimum age of rocks containing them; usually biozones are erected using shellfish, micro-organisms, pollen, rodents' teeth or other such small widespread fossils.

Alberta proved to be a productive area for the Sternbergs, but failed to impress the Museum; the then Keeper of Geology, Arthur Smith Woodward, described the fossils returned as 'rubbish'. Woodward was a powerful and influential figure at the time, though not a wise one. He gained a reputation for using his influence to marginalize promising younger palaeontologists who he saw as a threat to his status, and had an unscientific tendency towards self promotion, attaching himself to newsworthy discoveries and burying others. He was eventually discredited by the Piltdown forgery.

The items sent by the Sternbergs to Woodward did not contain large, complete fossil dinosaurs that would make good displays in the Museum, so they were stored away and forgotten, and never really assessed for their scientific interest. The Sternbergs were apparently unconcerned by this; they were veteran fossil hunters, and if the British Museum was not interested in their fossils, then there were plenty of other museums that were.

Ninety years later the specimens were re-examined by modern scientists, who realized the significance of the fossils. Amongst other treasures were two partial skulls from a previously unknown Ceratopsid Dinosaur, which has now been formerly described by a team lead by Andrew Farke of the Raymond M. Alf Museum of Paleontology in California in a paper in the journal Acta Palaeontologica Polonica.

The new dinosaur has been given the name Sinops sternbergorum (Spinops meaning 'spineface' and sternbergorum in honor of its' discoverers. Sinops lived about 76 million years ago, in the Late Cretaceous. It is thought to have been about six meters long in life, and to have weighed about two tonnes. Like other Ceratopsid Dinosaurs it had a prominent facial shield, with several horns.

Artist's rendering of the skull bones of Sinops sterbergorum. Copyright Lukas Panzrin, courtesy of the Raymond M. Alf Museum of Paleontology.

This is where things got interesting. When Farke et al. examined the nasal horn of Sinops they realized that it erupted from a different part of the skull to that of better described Ceratopsid Dinosaurs. This was not entirely unexpected; it had been suggested of some other species, but in these cases the fossils had been imperfect and somewhat ambivalent. In the case of Sinops there was no doubt; the horn was sufficiently different from that of other ceratopsids that it must have arisen separately.

Sinops is a member of a group of ceratopsids called the Centrosaurinae. This group are considered basal or (misleadingly) primitive within the ceratopsid group. What this actually means is that the group divided from the other Ceratopsid Dinosaurs early in the family's history; it is not closely related to the most familiar ceratopsid, Triceratops.

The earliest known centrosaur derives from Late Cretaceous Oldman Formation of Alberta the is Albertaceratops nesmoi; it is about a million and a half years older than Sinops at roughly 77.5 million years old. It is known from a single skull and a few fragmentary bones from the rest of the body. Albertaceratops is bore long brow horns and a bony ridge on its nose where most ceratopsids have a horn, which suggests that it came from a lineage in which the horn had not yet arisen, this supports the idea that the nasal horn subsequently evolved in separate lineages. Albertaceratops also had two small, hook-like horns pointing outward at the top of it's crest.

An impression of Albertaceratops nesmoi by paleoartist Nobu Tamura.

The next centrosaur to appear in the fossil record is Centrosuarus, which first appears in the Oldman Formation of Alberta, about 65.5 million years ago, and continues into the overlying Dinosaur Park Formation. The genus after which the group is named. Centrosaurus had a large nasal horn, small brow horns and a row of small horns around the frill. Centrosaurus could be up to six meters long, but was not especially heavily built. There are two described species, C. apertus, in which the small brow horns curve upwards, and C. brinkmani, in which they curve to the side. It has a much better record than most other species, with several vast bone beads known. These are though to have formed where large numbers of individuals gathered around drying water holes and eventually perished.

A reconstruction of the head of Centrosaurus brinkmani by Julius Csotonyi of the Royal Tyrell Museum in Alberta.

The beds immediately overlying those containing Centrosaurus contain another species of centrosaur, Styracosaurus albertensis. This was a slightly shorter animal from tip to tail, but was much heavier built, reaching 1.8 m at the shoulder, and being loner in the body and shorter in the tail; it could probably weigh nearly 3 tonnes. Like Centrosaurus, Styracosaurus has been found in large bone beds, thought to have formed in mass death events around drying water holes in times of draught. Styracosaurus had a large, straight, nasal horn and almost non-existent brow horns. The edge of the frill bore a number of large horns rivaling the nasal horn in length. The nasal horn has not been found intact, so the exact size and development pattern of this horn is unclear.

A reconstruction of Styracosaurus albertensis by Nobu Tamura.

Diabloceratops eatoni lived 75 million years ago in what is now Utah. It is known only from a partial skull, with a pair of large spikes on the top of its neck frill, small brow horns and one or two small nasal horns (again this part of the skull is not well preserved). Diabloceratops is considered to be from a lineage that derived from the earliest centrosaurs separately; all other known centrosaurs are thought to be more clesely related to one-another than to Diabloceratops.

A reconstruction of Diabloceratops eatoni, by natural history illustrator Andrey Atuchin.

Einiosuarus procurvicornis is known from a number of bone beds from Montana, between 74.5 and 74 million years old. It reached about six meters in length, had a forward curving nasal horn, brow horns reduced to a pair of knobs, and a pair of horns on the top of its small neck frill.

Reconstruction of Einiosaurus procurvicornis by illustrator Sergey Krasovskiy.

From the same time in Montana came Rubeosaurus ovatus, known only from two partial skulls. Rubeosaurus appears to have had a large nasal horn, and at least two pairs of horns on the neck frill. Some juvenile fossils attributed to the genus Brachyceratops may also belong to this species.

Above these species in the same deposits in Montana, the species Achelousaurus horneri dates from about 74 million years ago, and is known from three skulls and a partial skeleton. It appears to have been about six meters in length, and to have had boney ridges in the place of the nasal and brow horns, though it did have a pair of horns on the top of the neck frill.

An illustration of Achelousaurus horneri by Sergey Krasovskiy.

The latest surviving genus of centrosaur appears to have been Pachyrhinosaurus. Two species of this are known from Alberta, P. lakustai dating from between 73.5 and 72.5 million year ago and P. canadensis from about 71.5 to 71 million years ago. A third species, P. perotorum has been found in Alaska, and is even younger, at between 70 and 69 million years old, making it the last known surviving centrosaur. Pachyrhinosaurus was about eight meters long when fully grown, and weighed about eight tonnes. The nose and brow horns were replaced by bony ridges, and the neck frill bore a single pair of small horns in P. canadensis and some specimens of P. lakustai. P. perotorum and some specimens of P. lakstai lacked these horns. In P. Canadensis and P. perotorum the brow ridges were close together, but in P. lakustai they were further apart.

Pachyrhinosaurus canadensis by Nobu Tamura.

See also Nesting behavior and parental care in an Ornithischian Dinosaur? Head-butting in a Pachycephalosuar, and Dinosaurs on Sciency Thoughts YouTube.

NASA announces the discovery of Kepler-22b, a planet it its star's habitable zone.

On 5 December 2011 the NASA released a press statement to the effect that it had found a planet in the centre of the 'Goldilocks Zone' of a star 587 light years from Earth in the constellation of Cygnus. The Goldilocks Zone is the area in a star system where there is likely to be liquid water on an Earth-like planet; it is neither too hot, nor too cold, like the Baby Bear's porridge in the folk-tale. The system has been dubbed Kepler-22 (i.e. the 22nd planetary system discovered by the Kepler Space Telescope), with the star named Kepler-22A and the planet Kepler-22b (the first object in a system is 'a', the second 'b' etc.; stars are capitalized, planets are not).

A comparison of the habitable zones of our solar system and the Kepler-22 system.

Kepler-22A is a very sun-like star, 97% of the mass of our sun and 98% its volume. Kepler-22b obits this star every 290 days, at 85% of the distance at which the Earth orbits the sun. Keppler-22b planet has 2.4 times the radius of the Earth, though its mass is unknown; it was detected as it transits it's star; it does not exert enough gravitational pull to cause the star to wobble detectably (the other means by which we commonly discover new planets), which is how the mass of exoplanets can be determined. This is not greatly surprising, most planets that cause measurable wobbles in their stars are large planets orbiting small stars. It is estimated that if Kepler-22b was Earth-like then that its average surface temperature would be 22°C (the average surface temperature on Earth is 14°C).

This discovery has provoked a great deal of interest in the popular press, and has been widely hailed as a potential home for extra-terrestrial life, something NASA has done little to downplay. However the estimations of the habitability of Kepler-22b are based upon the assumption that it might be an earthlike planet. At 2.4 times the volume of the Earth Keppler-22b is likely to have a much thicker atmosphere, leading to a greater greenhouse effect, thereby raising the planet's surface temperature. This can make a considerable difference to a planet's temperature; without the greenhouse effect Venus would probably have a mild, pleasant, climate and Earth would be a frozen snowball. Even on Earth changes in the atmosphere are thought to have caused spectacular changes in climate over geological timescales, from an average surface temperature of -57°C during the Cryogenian, 700-750 million years ago to 21°C in the Cambrian 542-488 million years ago. Worst case scenarios for anthropogenic global warming suggest that the Earth's average surface temperature could rise by 6°C within the next hundred years, which would be devastating for human life (and much non-human life), but which would not make the planet uninhabitable. On Kepler-22b a much thicker atmosphere could potentially make the planet to hot for liquid water, and therefore unsuitable for life as we understand it.

NASA intend to follow up the initial observation with studies of the system using the Spitzer Space Telescope and ground-based observatories.

See also The Atmosphere of GJ1214b, Reconstructing cloud cover for ancient Earth, Could there be life on Gliese 581d? and Exoplanets on Sciency Thoughts YouTube.

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The atmosphere of GJ1214b.

The planet GJ1214b was discovered in December 2009 by the MEarth Project at Fred Lawrence Whipple Observatory in Arizona and described in a paper in the journal Nature. It orbits the star GJ1214A (A implies the first body in a system, B the second; stars are capitalized, planets are not), approximately 40 light years from Earth in the constellation of Ophiuchus. GJ1214A is a cool Red Dwarf star, only one fifth the size of our sun and 0.3% as bright. GJ1214b orbits this star every 38 hours, at 1.4% the distance at which our planet orbits the sun.

An artists impression of GJ1214b and it's star.

The planet was discovered by the tidal effect it exerts upon its star; a large planet orbiting close to a small star exerts a considerable gravitational pull, causing the star to wobble back and forth as the planet orbits it. This can be detected by sensitive telescopes using the Doppler effect, as the star moves towards us the light waves it emits are slightly compressed, making it appear slightly blueish (to very sensitive spectrometers, not human astronomers), as it moves away the light waves are expanded, making it appear slightly reddish.

Although GJ1214b was discovered by the Doppler method, it also transits (passes in front of) its star, enabling scientists to estimate both the the mass of the planet (by the Doppler method) and its radius (by the dimming it causes when it passes in front of its star) and therefore its density (by multiplying the two). This reveals a planet with some rather unusual properties. GJ1214b is 6.55 times as massive as the Earth, and has a radius 2.678 that of the Earth. This implies a planet with a much lower density than the Earth, which is surprising for a planet this size.

It was initially suggested that the planet could be made up largely of ice, which is far less dense than rock. But the surface temperature of GJ1214b is estimated to be between 399 and 555 K, or 126-282 °C. Another proposal is that GJ1214b might be an ocean planet entirely covered by an ocean hundreds of kilometers deep, with an icy core kept solid by the pressure of the water above it. Water can remain a liquid at temperatures above 100 °C if the pressure is high enough, but the although GJ1214b is big, its low density gives it an estimated gravity of 0.91 that of the Earth, so it is highly unlikely that water could remain a liquid at its surface. It has also been suggested that GJ1214b might be a relatively small (still bigger than the Earth) rocky planet with a thick, dense, atmosphere. But no mechanism has been suggested by which a small, hot, planet could retain such an atmosphere so close to a star, so if this is the case then the planet must be very young, unlikely orbiting a Red Dwarf star estimated to be 6 billion years old, or have moved recently into its current position, which is even harder to explain.

On 28 November 2011 a team led by Zachory Berta of the Harvard-Smithsonian Centre for Astrophysics published a paper on the arXiv online database at Cornell University Library, detailing the results of a spectroscopic study of the atmosphere of GJ1214b using the Wild Field Camera 3 on the Hubble Space Telescope. This found that the upper atmosphere of GJ1214b probably contains a thick layer of cloud, made up of some form of largish molecules, rather than a simple gas such as hydrogen; Berta suggest that this may be water (H₂O).

Any gas made up of molecules containing more than one type of atoms tends to have a greenhouse effect; it absorbs light (energy) at a variety of wavelengths, but emits it in the infra-red part of the spectrum. The obvious model for this in our solar system is Venus, which has an atmosphere containing large amounts of Carbon Dioxide (CO₂) and Sulphuric Acid (H₂SO₄), and a dramatic runaway greenhouse effect. This would suggest that GJ1214b is likely to be hotter, and less Earthlike, than has previously been suggested. The term 'super-Earth' has been used to describe planets of sizes intermediate between Earth and Neptune, but it would appear that in the case of GJ1214b it would appear that 'super-Venus' might be more appropriate.

See also A new study of the HAT-P-13 planetary system, The Keper-18 planetary system, Just how big is Iota Draconis b? and Exoplanets on Sciency Thoughts Youtube.

New eruptions on Mount Tungurahua, Ecuador. November 2011.

Mount Tungurahua is a stratovolcano (a 'conventional' cone-shaped volcano, the sort you see in Hollywood movies) located in the Sangay National Park in Ecuador, overlooking the town of Baños de Agua Santa. The town's major industry is tourism, attracting visitors to visit the volcano, the hot springs associated with the volcano, and the Amazon Rainforest.

Tungurahua has been intermittently active since 1999 (prior to which it had been inactive for about 75 years), with major eruptions in August 2006, February 2008, May 2010, December 2010 and April 2011. The 2006 eruption killed seven people; two volcanologists and a local family.

The August 2006 eruption on Mount Tungurahua.

On 27 November 2011 a number of earthquakes were felt in the area, followed by pyroclastic flows (flows of heavy superheated gas containing large amounts of hot ash, rocks and debris) were observed on the northern and northwestern flanks of the volcano. Later in the afternoon a series of explosions were heard near the summit of the volcano, and pyroclastic flows were observed on the southern and southwestern flanks. The following day there were further explosions, and pyroclastic flows on the south flank. A plume of ash and hot gas rose 3 km above the volcano, several local towns reported ash-falls, and nearby villages were evacuated with the help of the National Secretariat for Risk Management.

By the beginning of December the Geophysical Institute of Ecuador reported that the amount of gas being produced by the volcano had dropped dramatically, with no further pyroclastic flows. The volcano is still undergoing frequent earthquakes, but these do not necessarily imply an eruption is immanent. The situation is being monitored by the Tungurahua Volcanological Observatory in Guadelupe, 14 km to the north.

The current Tungurahua volcano is the third on the site, referred to by volcanologists as Tungurahua III. The first volcano on the site, Tungurahua I, built up and then collapsed some time in the Mid-Pleistocene. This was followed by Tungurahua II, which started to grow about 14 000 years ago, then collapsed about 3000 years ago. The current volcano has been growning since this time, and lies within the caldera of Tungurahua II.

Like all South American volcanoes Tungurahua owes its existence to the subduction of the Nazca Plate (which underlies the southeast Pacific) beneath South America. The Nazca Plate is being pushed from the east and forced down into the Earth's interior beneath South America. As it sinks rocks in the crust melt, and the lighter portions of it rise up through the overlying South American Plate to form volcanoes at the surface. These are dotted throughout the Andes Mountains; a range of mountains that is formed by a mixture of volcanism and crumpling of the South American Plate where is is forced against the Nazca Plate.

The subduction of the Nazca Plate beneath South America creates the Andes Mountains.

See also Earthquake in Jujuy Province, The Puyehue Eruption, The Puyehue-Cordón Caulle Volcanic Complex and Volcanoes on Sciency Thoughts YouTube.

The Open University run a course in Volcanoes, earthquakes and tsunamis by distance learning. This can be counted towards a degree, but does not need to be taken as part of a degree course. The course costs £170 for UK residents and £405 for overseas residents, though financial support may be available for UK and European residents.

Nine killed at illegal rare-earth mine in Guangxi Zhuang Autonomous Region, southern China.

On Saturday 26 November 2011, at about 3.30 in the afternoon, a major landslide occurred at a rare earth element mine in Cangwu County in the Guangxi Zhuang Autonomous Region in southern China. The mine has been described as illegal by local authorities, who have made a number of arrests connected with the incident. It was initially thought that seven workers had been killed, but nine bodies have now been recovered and rescuers are still searching the (extensive) site. Rescue attempts are being coordinated by the Mayor of Wuzhou City, Wang Kai.

Rescuers uncover a vehicle, inside which one of the victims was found.

Rare earth elements, or lanthanides, are elemental metals with atomic numbers ranging from 57 to 71, also referred to as 'f-block elements'. In addition the elements scandium and yttrium are commonly known as rare earth elements, because they are generally found in the same deposits as the lanthanides. These elements have proved to be extremely useful in modern electrical items such as mobile phones, causing their value to rise rapidly in recent years, which has lead to problems with illegal extraction in China and some other countries.

The Periodic Table showing the rare earth elements; the lanthanides plus scandium and yttrium.

Despite their name, the rare earth elements are not especially rare, they are found at trace levels in many rocks, and are far more abundant than traditional precious metals such as gold. However deposits containing significant amounts of rare earth elements are unusual, and the elements are difficult to extract. The commonest method of rare earth element extraction is to pump large quantities of strong acid into mineral bearing rocks, in the hope of dissolving the desired elements. This appears to have been the method used in Cangwu, on this occasion dissolving more than the miners had bargained for, and causing a significant collapse in rocks beneath the site.

China currently produces about 90% of the world's rare earth elements, with Chinese companies controlling much of the production outside of China. This has lead to complaints by some countries that China is establishing a monopoly in the elements. Despite this legitimate Chinese producers have struggled to keep up with the demands of the country's rapidly growing industry, leading to a ready market for rare earth elements produced by unlicensed, unregulated operations such as the one at Cangwu, which the Chinese authorities have struggled to control.

An additional problem with rare earth element production is that the ores that contain these elements often also contain the radioactive elements thorium and uranium, which will also come out of the rocks with acid leaching.. In one notable case a mine at Bukit Merah in Malaysia became so radioactive that the Malaysian authorities became convinced that the Japanese owners of the plant, Mitsubishi, were secretly using the site as a dump for radioactive waste produced elsewhere.

See also At least 21 miners killed in disaster in Yunnan Province and Disaster at Gleision Colliery.