̽��ֱ�� of Cambridge - tectonics

Scientists 'see' puzzling features deep in Earth’s interior

cmm201 — Thu, 19 May 2022 09:00:00 +0000

̽��ֱ��enigmatic area of rock, which is located almost directly beneath the Hawaiian Islands, is one of several ultra-low velocity zones – so-called because earthquake waves slow to a crawl as they pass through them.

̽��ֱ��research, published in Nature Communications, is the first to reveal the complex internal variability of one of these pockets in detail, shedding light on the landscape of Earth’s deep interior and the processes operating within it.

“Of all Earth’s deep interior features, these are the most fascinating and complex. We’ve now got the first solid evidence to show their internal structure - it’s a real milestone in deep earth seismology,” said lead author Zhi Li, PhD student at Cambridge’s Department of Earth Sciences.

Earth’s interior is layered like an onion: at the centre sits the iron-nickel core, surrounded by a thick layer known as the mantle, and on top of that a thin outer shell — the crust we live on. Although the mantle is solid rock, it is hot enough to flow extremely slowly. These internal convection currents feed heat to the surface, driving the movement of tectonic plates and fuelling volcanic eruptions.

Scientists use seismic waves from earthquakes to 'see' beneath Earth’s surface — the echoes and shadows of these waves reveal radar-like images of deep interior topography. But, until recently, 'images' of the structures at the core-mantle boundary, an area of key interest for studying our planet’s internal heat flow, have been grainy and difficult to interpret.

̽��ֱ��researchers used the latest numerical modelling methods to reveal kilometre-scale structures at the core-mantle boundary. According to co-author Dr Kuangdai Leng, who developed the methods while at the ̽��ֱ�� of Oxford, “We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before.” Leng, who is currently based at the Science and Technology Facilities Council, says that this means they can improve the resolution of the images by an order of magnitude compared to previous work.

̽��ֱ��researchers observed a 40% reduction in the speed of seismic waves travelling at the base of the ultra-low velocity zone beneath Hawaii. This supports existing proposals that the zone contains much more iron than the surrounding rocks – meaning it is denser and more sluggish. “It’s possible that this iron-rich material is a remnant of ancient rocks from Earth’s early history or even that iron might be leaking from the core by an unknown means,” said project lead Dr Sanne Cottaar from Cambridge Earth Sciences.

̽��ֱ��research could also help scientists understand what sits beneath and gives rise to volcanic chains like the Hawaiian Islands. Scientists have started to notice a correlation between the location of the descriptively-named hotspot volcanoes, which include Hawaii and Iceland, and the ultra-low velocity zones at the base of the mantle. ̽��ֱ��origin of hotspot volcanoes has been debated, but the most popular theory suggests that plume-like structures bring hot mantle material all the way from the core-mantle boundary to the surface.

With images of the ultra-low velocity zone beneath Hawaii now in hand, the team can also gather rare physical evidence from what is likely the root of the plume feeding Hawaii. Their observation of dense, iron-rich rock beneath Hawaii would support surface observations. “Basalts erupting from Hawaii have anomalous isotope signatures which could either point to either an early-Earth origin or core leaking, it means some of this dense material piled up at the base must be dragged to the surface,” said Cottaar.

More of the core-mantle boundary now needs to be imaged to understand if all surface hotspots have a pocket of dense material at the base. Where and how the core-mantle boundary can be targeted does depend on where earthquakes occur, and where seismometers are installed to record the waves.

̽��ֱ��team’s observations add to a growing body of evidence that Earth’s deep interior is just as variable as its surface. “These low-velocity zones are one of the most intricate features we see at extreme depths – if we expand our search, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core-mantle boundary,” said Li.

They now plan to apply their techniques to enhance the resolution of imaging of other pockets at the core-mantle boundary, as well as mapping new zones. Eventually they hope to map the geological landscape across the core-mantle boundary and understand its relationship with the dynamics and evolutionary history of our planet.

Reference:
Zhi Li, Kuangdai Leng, Jennifer Jenkins, Sanne Cottaar. 'Kilometer-scale structure on the core–mantle boundary near Hawaii.' Nature Communications (2022), DOI: 10.1038/s41467-022-30502-5

New research led by the ̽��ֱ�� of Cambridge is the first to obtain a detailed 'image' of an unusual pocket of rock at the boundary layer with Earth’s core, some three thousand kilometres beneath the surface.

Of all Earth’s deep interior features, these are the most fascinating and complex

Zhi Li

gnuckx

Etna volcano eruption, 12 January 2011

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. Images, including our videos, are Copyright © ̽��ֱ�� of Cambridge and licensors/contributors as identified. All rights reserved. We make our image and video content available in a number of ways – as here, on our main website under its Terms and conditions, and on a range of channels including social media that permit your use and sharing of our content under their respective Terms.

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Earth's interior is swallowing up more carbon than thought

cmm201 — Mon, 26 Jul 2021 09:59:43 +0000

They found that the carbon drawn into Earth’s interior at subduction zones - where tectonic plates collide and dive into Earth’s interior - tends to stay locked away at depth, rather than resurfacing in the form of volcanic emissions.

Their findings, published in Nature Communications, suggest that only about a third of the carbon recycled beneath volcanic chains returns to the surface via recycling, in contrast to previous theories that what goes down mostly comes back up.

One of the solutions to tackle climate change is to find ways to reduce the amount of CO₂ in Earth’s atmosphere. By studying how carbon behaves in the deep Earth, which houses the majority of our planet’s carbon, scientists can better understand the entire lifecycle of carbon on Earth, and how it flows between the atmosphere, oceans and life at the surface.

̽��ֱ��best-understood parts of the carbon cycle are at or near Earth’s surface, but deep carbon stores play a key role in maintaining the habitability of our planet by regulating atmospheric CO₂ levels. “We currently have a relatively good understanding of the surface reservoirs of carbon and the fluxes between them, but know much less about Earth’s interior carbon stores, which cycle carbon over millions of years,” said lead author Stefan Farsang, who conducted the research while a PhD student at Cambridge's Department of Earth Sciences.

There are a number of ways for carbon to be released back to the atmosphere (as CO₂) but there is only one path in which it can return to the Earth’s interior: via plate subduction. Here, surface carbon, for instance in the form of seashells and micro-organisms which have locked atmospheric CO₂ into their shells, is channelled into Earth’s interior. Scientists had thought that much of this carbon was then returned to the atmosphere as CO₂ via emissions from volcanoes. But the new study reveals that chemical reactions taking place in rocks swallowed up at subduction zones trap carbon and send it deeper into Earth’s interior - stopping some of it coming back to Earth’s surface.

̽��ֱ��team conducted a series of experiments at the European Synchrotron Radiation Facility, “ ̽��ֱ��ESRF has world-leading facilities and the expertise that we needed to get our results,” said co-author Simon Redfern, Dean of the College of Science at NTU Singapore, “ ̽��ֱ��facility can measure very low concentrations of these metals at the high pressure and temperature conditions of interest to us.” To replicate the high pressures and temperatures of subductions zones, they used a heated ‘diamond anvil’, in which extreme pressures are achieved by pressing two tiny diamond anvils against the sample.

̽��ֱ��work supports growing evidence that carbonate rocks, which have the same chemical makeup as chalk, become less calcium-rich and more magnesium-rich when channelled deeper into the mantle. This chemical transformation makes carbonate less soluble – meaning it doesn’t get drawn into the fluids that supply volcanoes. Instead, the majority of the carbonate sinks deeper into the mantle where it may eventually become diamond.

“There is still a lot of research to be done in this field,” said Farsang. “In the future, we aim to refine our estimates by studying carbonate solubility in a wider temperature, pressure range and in several fluid compositions.”

̽��ֱ��findings are also important for understanding the role of carbonate formation in our climate system more generally. “Our results show that these minerals are very stable and can certainly lock up CO₂ from the atmosphere into solid mineral forms that could result in negative emissions,” said Redfern. ̽��ֱ��team have been looking into the use of similar methods for carbon capture, which moves atmospheric CO₂ into storage in rocks and the oceans.

“These results will also help us understand better ways to lock carbon into the solid Earth, out of the atmosphere. If we can accelerate this process faster than nature handles it, it could prove a route to help solve the climate crisis,” said Redfern.

Reference:
Farsang, S, Louvel, M, Zhao, C et al. Deep carbon cycle constrained by carbonate solubility. Nature Communications (2021). DOI: 10.1038/s41467-021-24533-7

Adapted from a news release by the ESRF

Scientists from Cambridge ̽��ֱ�� and NTU Singapore have found that slow-motion collisions of tectonic plates drag more carbon into Earth’s interior than previously thought.

We currently have a relatively good understanding of the surface reservoirs of carbon and the fluxes between them, but know much less about Earth’s interior carbon stores, which cycle carbon over millions of years

Stefan Farsang

NASA Goddard Space Flight Center

Alaska’s Pavlof Volcano: NASA’s View from Space

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Earthquake rocks Afghanistan and Pakistan – an area prone to magnitude 7 quakes

Anonymous — Tue, 27 Oct 2015 12:35:31 +0000

A devastating earthquake struck the Hindu Kush region of north-east Afghanistan just after lunchtime on October 26, rocking communities as far away as Tajikistan, Pakistan and even India. A devastating earthquake struck the Hindu Kush region of north-east Afghanistan just after lunchtime on October 26, rocking communities as far away as Tajikistan, Pakistan and even India.

̽��ֱ��strong quake, estimated at magnitude 7.5 by the US Geological Survey (USGS), had its origins more than 200km deep beneath Earth’s surface, and was felt as strong shaking across a very wide area. Casualties have been reported from across the region, with widespread landslips causing potential further damage to infrastructure.

So far it has been reported that 150 people have died, but this number is likely to rise.

̽��ֱ��quake is the second large shake to hit the Alpine-Himalayan earthquake belt this year, following the one that devastated Nepal in April. A region stretching from the Mediterranean through Anatolia, Iran and Central Asia into the mountains of South-East Asia, the Alpine-Himalayan belt is the home of around a fifth of the world’s largest earthquakes.

Tectonic plates collide. LennyWikipedia~commonswiki, CC BY-SA

̽��ֱ��earthquake was driven by collision between the Eurasian tectonic plate to the north and the Indian plate to the south. ̽��ֱ��area marks the scar of the closure of an ancient ocean, the Thethys, which once separated the continents of Gondwana, including most of the landmasses in today’s southern hemisphere, and Laurasia, made up of most of the countries that are today in the northern hemisphere.

̽��ֱ��Hindu Kush has experienced many such earthquakes before today, and this latest appears to follow closely the pattern of those of the past. Preliminary analysis by the USGS indicates that it was caused by a deep fault in which rocks thrust past each other instantaneously. They point out that seven earthquakes of magnitude 7 or more have hit within 250km of the current earthquake over the past century. Most recently the magnitude 7.4 earthquake, some 20km west of the latest event, killed over 150 people in March 2002.

This type of deep fault, a near-vertical a thrust fault, is a process that has previously been associated with the tearing off of sections of ancient ocean floor sinking into the Earth’s mantle beneath today’s continent. Researchers have previously suggested that earthquakes in the Hindu Kush can be caused by the break off of strips of such slabs, stretching and tearing free, on geological time scales, as they fall deep into the mantle.

Whatever the geological triggers for the quake, grieving communities will now be gathering themselves together and guarding against the inevitable aftershocks. With increased understanding of the risks that Earth poses along this seismic belt, it is important to be aware and prepare for future large earthquakes. If buildings are not to be destroyed time and again, it is important to adopt and adhere to construction and planning codes. A key step in promoting legal enforcement is educating the community about the risks, as well as how to respond as safely as possible during an earthquake.

Efforts such as the “Earthquakes without Frontiers” continue to highlight the risks of earthquakes, and have drawn attention to the tectonic forces that stand poised to strike along Tethys’ former shores.

Simon Redfern, Professor in Earth Sciences, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Professor Simon Redfern (Department of Earth Sciences) discusses the devastating earthquake that struck Afghanistan on October 26 and the geological triggers that caused it.

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Topography of Hindu Kush.

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For image use please see separate credits above.

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Earthquakes without frontiers

sc604 — Mon, 26 Oct 2015 13:31:11 +0000

̽��ֱ��Ganges is India’s most iconic river, flowing from the Himalaya to the Bay of Bengal, and its massive river basin is one of the most fertile and densely populated regions in the world. ̽��ֱ��Ganges flows through 29 cities with a population over 100,000, 23 cities with a population between 50,000 and 100,000, and close to 50 towns.

But someday – perhaps tomorrow or perhaps in 100 years – a massive earthquake will hit the region, and the consequences could be catastrophic: as many as a million lives in the Ganges river basin could be at risk, primarily because buildings have not been constructed to be earthquake resilient, despite the fact that the relevant building codes are in place.

Of course, earthquakes don’t respect borders, and India is not alone in being at risk due to poorly constructed buildings. Northern India lies in the Alpine–Himalayan earthquake belt, which stretches from the Mediterranean to the Pacific. It is the second-most seismically active region in the world, and responsible for around 20% of the world’s largest earthquakes. ̽��ֱ��belt is being created by ongoing plate tectonics: as the African, Arabian and Indian plates continue to move northwards, they collide with the Eurasian plate.

̽��ֱ��earthquake belt includes the most famous of the great trade routes, the Silk Road, which follows the edges of deserts and mountains, and high plateaus like Tibet. ̽��ֱ��landscape of the Silk Road has been shaped by earthquakes over millions of years: forcing mountains upwards and making life in the desert possible by controlling where water comes to the surface.

As the earthquake faults grind rocks together they make an impermeable clay, which often forces water to the surface along spring lines, determining where people live. To the casual observer, it seems as if the major earthquakes in this part of the world often seem to ‘target’ towns and cities but, in reality, people are often simply living where the water is, which is also where earthquakes happen.

Between 2 and 2.5 million people have died in earthquakes since 1900. Approximately two thirds of those deaths occurred in earthquakes in the continental interiors – places like northern India. Over that time, advances in the scientific understanding of earthquakes have been translated into impressive resilience in places where the hazard is well understood, which are mainly on the edges of the oceans. Comparable advances have not, however, taken place in most parts of the continental interiors, where the hazard is still much less well identified and poorly understood.

“Earthquake science has progressed so that we’re now much better at recognising the signals in the landscape that tell us whether a particular place is dangerous,” says Professor James Jackson, Head of Cambridge’s Department of Earth Sciences. “We can’t tell you exactly when an earthquake is going to happen, but we can say it will happen, not least because it’s happened before. If it’s happened before, it will happen again. What we can do, however, is to understand earthquakes better and use that knowledge to help make buildings safer.”

Four years ago, with funding from the Natural Environment Research Council, Jackson and colleagues from other universities in the UK established Earthquakes Without Frontiers (EWF), an international partnership bringing together earthquake scientists from across the great earthquake belt, from China to Italy, in order to share expertise. “But it soon became clear that the project was about much more than earthquake science, and the real issue was how to translate science into effective policy, which requires an understanding of the social context in which people live,” says Jackson.

With additional funding from the Economic and Social Research Council, EWF expanded to include social science and policy dimensions. ̽��ֱ��project, which runs until 2017, has three overarching objectives: to increase knowledge of earthquake hazards across the region; to establish greater resiliency against these hazards; and to establish a well-networked interdisciplinary partnership to support local earthquake scientists. Within Asia, there are more than 50 national level stakeholders who are working with EWF on earthquake risk reduction.

Across much of the earthquake belt, people live in large cities, mostly in poorly built apartment blocks and buildings that have not been designed to withstand earthquakes. Large cities such as Tehran, Almaty and Bishkek have all been destroyed multiple times by earthquakes, and it’s only a matter of time before the next one hits. ̽��ֱ��problem that EWF faces is convincing the public and policy makers of the importance of making towns and cities more earthquake resilient.

“In these big cities, everyday life is difficult enough: they’re very congested, they have huge problems with traffic, air quality, water quality, food supply and poverty,” explains Jackson. “And quite understandably, the risk of an earthquake seems quite remote compared to daily worries. But that doesn’t make the threat go away.”

“We face two main problems: the first is that there is a lack of awareness of the fact that seismologists cannot predict earthquakes – it’s just not something we are able to do or will be able to do,” says Dr Supriyo Mitra of the Indian Institute of Science Education and Research Kolkata. Mitra obtained his PhD at Cambridge, and is now one of the key Indian academic collaborators on the project, primarily working in Indian-administered Kashmir. “ ̽��ֱ��other problem is that there is a lot of resistance to making buildings safe. It is an additional cost, but it’s a necessity and we need to get that across to people.”

Perhaps the most important change that can be made to increase earthquake resilience in these areas is the enforcement of building codes. ̽��ֱ��building codes in Los Angeles and Tehran are similar, but the difference is that in Los Angeles, most buildings are constructed according to those codes, while in Tehran most are not, so as a result, Los Angeles is highly resilient to earthquakes, while Tehran remains very vulnerable.

“Enforcement comes not just from legal enforcement, but education,” adds Jackson. “People are really starting to realise that this is important. And once you educate the public, it rises up the agenda because the public insists that it does.

“There are going to be around a billion new homes built across Asia over the next 10 years – let’s build them so they are safe.”

̽��ֱ��Alpine–Himalayan belt, which stretches from the Mediterranean to the Pacific, is one of the world’s most seismically active regions. Now, a combination of earth science, social science and education is being used to help the region become more resilient to earthquakes, protecting lives and property.

We can’t tell you exactly when an earthquake is going to happen, but we can say it will happen, not least because it’s happened before. If it’s happened before, it will happen again.

James Jackson

Timothy Smith, U.S. Navy

̽��ֱ��city of Muzafarabad, Pakistan lays in ruins after the 2005 Kashmir earthquake that hit the region.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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