Magnitude 4.8 Earthquake off the south coast of the Reykjanes Peninsula, Iceland.

The United States Geological Survey recorded a Magnitude 4.8 Earthquake at a depth of 10.3 km roughly 8 km south of the Reykjanes Peninsula in southwest Iceland, slightly before 7.35 am local time (which is GMT) on Sunday 13 October 2013. There are no reports of any damage or casualties arising from this quake, but strong shaking was felt as far away as Hella, over 100 km to the east and the quake was felt as far away as Hólmavík, 250 km to the north.

The approximate location of the 13 October 2013 Reykjanes Peninsula Earthquake. Google Maps.

Iceland lies directly upon the Mid-Atlantic Ridge, a chain of (mostly) submerged volcanoes running the length of the Atlantic Ocean along which the ocean is splitting apart, with new material forming at the fringes of the North American and European Plates beneath the sea (or, in Iceland, above it). This leads most obviously to the famed volcanicity of Iceland; but Earthquakes here are very common too.

See also Steam explosions on Mount Kverkfjöll, Earthquake off the north coast of Iceland, Large Earthquake near Jan Mayen Island, The dangers of a modern Laki style eruption in Iceland and The Hekla Volcano - is the Gateway to Hell about to open?

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Submarine eruption to the northwest of Jebel Zubair.

The Zubair Archipelago are a group of volcanic islands off the southwest coast of Yemen; they are essentially a shield volcano on the Red Sea Rift with a number of vents. The had been quiet from about 1848 until December 2011, when a series of eruptions from a vent to the northeast of Jebel Zubair island (the largest island of the group) began, persisting through January 2012. A new eruption began on 28 September 2013 a new series of eruptions began, this time to the northwest of Jebel Zubair and the southeast of the 2011/12 eruptions. This eruption has the form of a steam plume and increase in atmospheric sulphur dioxide (SO₂), detected by NASA's Terra Satellite, and further observed over the following days. To date no ash or pumice produced by this eruption has been detected, suggesting that it is not (yet) a major event.

The approximate location of the Jebel Zubair eruption. Google Maps.

The Red Sea Rift is a spreading boundary between two tectonic plates, the African Plate and the Arabian, where new oceanic crust is being formed. Arabia was formerly part of the African Plate, but split away about 30 million years ago. The Great Rift Valley of Africa is a continuation of this rift, that is slowly splitting Africa in two from the north to the south.

See also The movement of deep magma beneath the Afar Depression, Magnitude 5.0 Earthquake beneath coastal Eritrea, Earthquakes beneath the Gulf of Aden, Earthquake beneath the Red Sea and The magma chamber beneath the Erta Ale Volcano.

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The death of a hydrothermal vent community.

The East Pacific Rise runs from the Southern Ocean to the Gulf of California, where it passes into the subduction zone beneath the North American Plate, becoming the San Andreas Fault. A number of deepsea hydrothermal vent communities have been discovered along the East Pacific Rise, some of which have been studied extensively. One of these was the site known as Bio9, located at 9°50' North, which was discovered in 1991, and studied extensively over the next few years. The site thrived until 1995, but appeared to sicken and die over the next two years.

In a paper published in the journal Geochemical Transactions on 27 January 2012, Michael Hentscher and Wolfgang Bach of the Department of Geosciences at the University of Bremen discuss the history of the Bio9 hydrothermal vent community over the period 1991-1997, and theorize about how changes to the chemistry of the water emerging from the vent may have brought about the demise of the vent community.

Diagram showing the chemical cycle of a healthy hydrothermal vent community. From Hentscher & Bach (2012).

The Bio9 community was dominated by tubeworms of the genus Riftia when it was first discovered. These worms obtain nutrition from symbiotic bacteria that live within their bodies, bacteria that oxidize hydrogen sulphide from the mineral rich waters. These worms were thriving in 1991, but in 1994 the colonies appeared to have developed rusty red spots. In 1995 the colonies were largely covered by the rusty spots, and by 1997 the colonies had largely died off.

Healthy Riftia tubeworms. NOAA.

Hentscher & Bach theorize that the rusty material was colonies of red, iron-oxidizing bacteria, and that their growth reflected an increase in the amount of iron ions in the water emerging from the vents; they do not believe the bacteria directly harmed the tubeworms.

The death of the tubeworms between 1995 and 1997 would therefore not be directly related to the spread of the red rust in 1994-5. Hentscher & Bach suspect that this was due to a fall in the level of hydrogen sulphide within the vent water, resulting in the death of first the symbiotic bacteria within the worms, and then the worms themselves.

The chemicals emitted by terrestrial volcanic springs and fumaroles change over time, and there is no reason to suspect that deep-sea hydrothermal vents would behave any differently. If this is the case then the death of this vent community would be a part of the natural life-cycle of the tubeworms, and hydrothermal vent communities in other parts of the world, with different biological communities, probably suffer similar die-offs.

See also A deep-sea hydrothermal vent community discovered in the Mariana Trench, Deepest hydrothermal vent communities yet found discovered in the Caribbean and New deep-sea hydrothermal vent ecosystem discovered in the Southern Ocean.

Deepest hydrothermal vent communities yet found discovered in the Caribbean.

Hydrothermal vents occur on the boundaries between plates in the deep sea, as water percolates through super-heated volcanic rocks soaking up minerals, and is released back into the ocean. They are often called 'black-smokers' because of the dark colouration of the mineral-rich water. The water coming from hydrothermal vents often has temperatures of hundreds of degrees centigrade, well above the boiling point of water at the surface, but remains a liquid in the high pressure environment of the deep sea. The water is also often highly acidic, and laden with potentially toxic metal ions. Despite this deep-sea hydrothermal vents have been shown to support whole communities of animals, witch depend not on energy from green, photosynthesizing plants for survival, but rather of bacteria able to generate energy by consuming the toxic chemicals from the hydrothermal vents.

On 10 January 2012 a team lead by Douglas Connelly of The National Oceanography Centre at the University of Southampton and Jonathan Copley of the Department of Ocean and Earth Science, also at the University of Southampton, published a paper on Nature Communications in which they describe the discovery of two systems of hydrothermal vents on the Mid-Cayman Ridge, 4960 m and 2300 m beneath the Caribbean Sea.

The location of the new hydrothermal vent sites. From Connelly et al. (2012).

The Von Damm Vent field is located on the upper slopes of an undersea mountain, Mount Dent, 13 km to the west of the active spreading centre, at a depth of 2300 m. The water at the vent is rich in hydrogen sulphide (H₂S), and while enriched in metals relative to the surrounding water, is metal poor compared to other deep-sea vents; it appears colourless at the vent rather than black.

The Beebe Vent Field is located 4960 m beneath the surface, 20% deeper than any previousy discovered vent. The water issuing from the vent is not as rich in hydrogen sulphide as the water at the Von Damm site, but it is extremely rich in copper and iron. This creates a visible plume of black water that reaches 1100 m above the vents. The Southampton researchers estimate that for the column to remain discrete from the surrounding water for this far the water must be emerging at about 500 °C (they did not have the means to take a direct temperature reading at this depth).

(a & b) A colony of shrimps on a vent at the Von Damm Vent Field; the water issuing from the vent is colourless. (c & d) Sulphide chimneys issuing black 'smoke'; coloured water at the Beebe Vent Field, which gets its colour from dissolved copper and iron ions. From Connelly et al. (2012)

Both sites had thriving biological communities, dominated by dense colonies of shrimps, containing at least two species of shrimp. The Von Damm site also had extensive mussel beds. Fish, snails, sea anemones and squat lobsters were also present at both sites. This is similar to the fauna found on deep-sea vents on the Mid-Atlantic Ridge.

(a) A a dense colony of shrimps at the Beebe Vent Field. (b) Sea anemones and microbial mats at the Beebe Vent Field. (c) Dead mussel shells on Mount Dent. (d) Possible tube-worm casts on Mount Dent.

See also New deep-sea hydrothermal vent ecosystem discovered in the Southern Ocean.

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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