Satellites reveal melting of rocks under volcanic zone, deep in Earth’s mantle

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Mount Ngauruhoe, in the foreground, and Mount Ruapehu are two of the active volcanoes in the Taupo volcanic zone.
Guillaume Piolle/Wikimedia Commons, CC BY-ND

Simon Lamb, Victoria University of Wellington and Timothy Stern, Victoria University of Wellington

Volcanoes erupt when magma rises through cracks in the Earth’s crust, but the exact processes that lead to the melting of rocks in the Earth’s mantle below are difficult to study.

In our paper, published today in the journal Nature, we show how it is possible to use satellite measurements of movements of the Earth’s surface to observe the melting process deep below New Zealand’s central North Island, one of the world’s most active volcanic regions.

Rifting in the Taupo volcanic zone

The solid outer layer of the Earth is known as the crust, and this overlies the Earth’s mantle. But these layers are not fixed. They are broken up into tectonic plates that slowly move relative to each other.

It is along the boundaries of the tectonic plates that most of the geological action at the Earth’s surface occurs, such as earthquakes, volcanic activity and mountain building. This makes New Zealand a particularly dynamic place, geologically speaking, because it straddles the boundary between the Australian and Pacific plates.

The central region of the North Island is known as the Taupo volcanic zone, or TVZ. It is named after Lake Taupo, the flooded crater of the region’s largest volcano, and it has been active for two million years. Several volcanoes continue to erupt regularly.

The TVZ is the southern tip of a zone of expansion, or rifting, in the Earth’s crust that extends offshore for thousands of kilometres, all the way north in the Pacific Ocean to Tonga. Offshore, this takes place through sea floor spreading in the Havre Trough, creating both new oceanic crust and a narrow sliver of a plate right along the edge of the Australian tectonic plate. Surprisingly, this spreading is going on at the same time as the adjacent Pacific tectonic plate is sliding beneath the Australian plate in a subduction zone, triggering some of the major earthquakes in the region.

Sea floor spreading results in melting of the Earth’s mantle, but it is very difficult to observe this process directly in the deep ocean. However, sea floor spreading in the Havre Trough transitions abruptly onshore into the volcanic activity in the TVZ. This provides an opportunity to observe the melting in the Earth’s mantle on land.

Lake Taupo is the caldera of the region’s largest volcano.
NASA/Wikimedia Commons, CC BY-ND

In general, volcanic activity happens whenever there is molten rock at depth, and therefore the volcanism in the North Island indicates vast volumes of molten rock beneath the surface. However, it has been a tricky problem to understand exactly what is causing the melting in the first place, because the underlying rocks are buried by thick layers of volcanic material.

We have tackled this problem using data from Global Positioning System (GPS) sensors, some of which form part of New Zealand’s GeoNet network and some that have been used in measurement campaigns since 1995. The sensors measure horizontal and vertical shifts in the Earth’s surface to millimetre precision, and our research is based on data collected over the past two decades.

Bending of the earth’s surface

The GPS measurements in the Taupo volcanic zone reveal that it is widening east-west at a rate of 6-15 millimetres per year – in other words, the region, overall, is expanding, as we anticipated from our previous geological understanding. But it was surprising to discover that, at least for the past 15 years, a roughly 70-kilometre stretch is undergoing strong horizontal contraction and is also rapidly subsiding, quite the opposite of what one might anticipate.

Also unexpectedly, the contracting zone is surrounded by regions that are expanding, but also uplifting. Trying to make sense of these observations turned out to be the key to our new insight into the process of melting beneath the TVZ.

We found that the pattern of contraction and subsidence, together with expansion and uplift, in the context of the overall rifting of the TVZ, could be explained by a simple model that involves the bending and curving of an elastic upper crust, pulled downwards or pushed upwards by an underlying vertical driving force. The size of the region that is behaving like this, extending for about 100 kilometres in width and 200 kilometres in length, requires this force to originate nearly 20 kilometres underground, in the Earth’s mantle.

This diagram illustrates a patch of suction stress along the axis of the underlying upwelling mantle flow beneath the Taupo volcanic zone.
Simon Lamb, CC BY-ND

Melting the mantle

When tectonic plates drift apart on the sea floor, the underlying mantle rises up to fill the gap. This upwelling triggers melting, and the reason for this is that hot, but solid, mantle rocks undergo a reduction in pressure as they move upwards and closer to the Earth’s surface. This drop in pressure, rather than a change in temperature, begins the melting of the mantle.

But there is another property of this upwelling mantle flow, because it also creates a suction force that pulls down the overlying crust. This force comes about because as part of the flow, the rocks have to effectively “turn a corner” near the surface from a predominantly vertical flow to a predominantly horizontal one.

It turns out that the strength of this force depends on how stiff or sticky the mantle rocks are, measured in terms of viscosity (it is difficult to drive the flow of highly viscous or sticky fluids, but easy in runny ones).

Experimental studies have shown that the viscosity of rocks deep in the Earth is very sensitive to how much molten material they contain, and we propose that changes in the amount of melt provide a powerful mechanism to change the viscosity of the upwelling mantle. If mantle rocks don’t contain much melt, they will be much stickier, causing the overlying crust to be pulled down rapidly. If the rocks have just melted, then this makes the flow of the rocks runnier, allowing the overlying crust to spring back up again.

We also know that the movements that we observe at the surface with GPS must be relatively short lived, geologically speaking, lasting for no more than a few hundred or few thousand years. Otherwise they would result in profound changes to the landscape and we have no evidence for that.

Using GPS, we can not only measure the strength of the suction force, but we can “see” where, for how long, and by how much the underlying mantle is melting. This melt will eventually rise up through the crust to feed the overlying volcanoes.

This research helps us to understand how volcanic systems work on a variety of time scales, from human to geological. In fact, it may be that the GPS measurements made over just the last two decades have captured a change in the amount of mantle melt at depth, which could herald the onset of increased volcanic activity and associated risk in the future. But we don’t have measurements over a long enough time period yet to make any confident predictions.

The ConversationThe key point here is, nevertheless, that we have entered a new era whereby satellite measurements can be used to probe activity 20 kilometres beneath the Earth’s surface.

Simon Lamb, Associate Professor in Geophysics, Victoria University of Wellington and Timothy Stern, Professor of Geophysics, Victoria University of Wellington

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

Volcanoes under the ice: melting Antarctic ice could fight climate change

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Furious winds keep the McMurdo Dry Valleys in Anarctica free of snow and ice. Calcites found in the valleys have revealed the secrets of ancient subglacial volcanoes.
Stuart Rankin/Flickr, CC BY-NC

Silvia Frisia, University of Newcastle

Iron is not commonly famous for its role as a micronutrient for tiny organisms dwelling in the cold waters of polar oceans. But iron feeds plankton, which in turn hold carbon dioxide in their bodies. When they die, the creatures sink to the bottom of the sea, safely storing that carbon.

How exactly the iron gets to the Southern Ocean is hotly debated, but we do know that during the last ice age huge amounts of carbon were stored at the bottom of the Southern Ocean. Understanding how carbon comes to be stored in the depth of the oceans could help abate CO2 in the atmosphere, and Antarctica has a powerful role.

Icebergs and atmospheric dust are believed to have been the major sources of this micronutrient in the past. However, in research published in Nature Communications, my colleagues and I examined calcite crusts from Antarctica, and found that volcanoes under its glaciers were vital in delivering iron to the ocean during the last ice age.

Today, glacial meltwaters from Greenland and the Antarctic peninsula supply iron both in solution and as tiny particles (less than 0.0001mm in diameter), which are readily consumed by plankton. Where glaciers meet bedrock, minute organisms can live in pockets of relatively warm water. They are able to extract “food” from the rock, and in doing so release iron, which then can be carried by underwater rivers to the sea.

Volcanic eruptions under the ice can create underwater subglacial lakes, which, at times, discharge downstream large masses of water that travel to the ice margin and beyond, carrying with them iron in particle and in solution.

The role of melting ice in climate change is as yet poorly understood. It’s particularly pertinent as scientists predict the imminent collapse of part of the Larsen C ice shelf.

Researchers are also investigating how to reproduce natural iron fertilisation in the Southern Ocean and induce algal blooms. By interrogating the volcanic archive, we learn more about the effect that iron fertilisation from meltwater has on global temperatures.

A polished wafer of the subglacial calcites. The translucent, crystalline layers formed while in pockets of water, providing nourishment to microbes. The opaque calcite with rock fragments documents a period when waters discharged from a subglacial lake formed by a volcanic eruption, carrying away both iron in solution and particles of iron.

The Last Glacial Maximum

During the Last Glacial Maximum, a period 27,000 to 17,000 years ago when glaciers were at their greatest extent worldwide, the amount of CO2 in the atmosphere was lowered to 180 parts per million (ppm) relative to pre-industrial levels (280 ppm).

Today we are at 400 ppm and, if current warming trends continue, a point of no return will be reached. The global temperature system will return to the age of the dinosaurs, when there was little difference in temperature from the equator to the poles.

If we are interested in providing a habitable planet for our descendants, we need to mitigate the quantity of carbon in the atmosphere. Blooms of plankton in the Southern Ocean boosted by iron fertilisation were one important ingredient in lowering CO2 in the Last Glacial Maximum, and they could help us today.

The Last Glacial Maximum had winds that spread dust from deserts and icebergs carrying small particles into the Southern Ocean, providing the necessary iron for algal blooms. These extreme conditions don’t exist today.

Hidden volcanoes

Neither dust nor icebergs alone, however, explain bursts of productivity recorded in ocean sediments in the Last Glacial Maximum. There was another ingredient, only discovered in rare archives of subglacial processes that could be precisely dated to the Last Glacial Maximum.

Loss of ice in Antartica’s Dry Valleys uncovered rusty-red crusts of calcite plastered on glacially polished rocks. The calcites have tiny layers that can be precisely dated by radiometric techniques.

A piece of subglacial calcite coating pebbles. This suggests that the current transporting the pebbles was quite fast, like a mountain stream. The pebbles were deposited at the same time as the opaque layer in the calcite formed.

Each layer preserves in its chemistry and DNA a record of processes that contributed to delivering iron to the Southern Ocean. For example, fluorine-rich spherules indicate that underwater vents created by volcanic activity injected a rich mixture of minerals into the subglacial environment. This was confirmed by DNA data, revealing a thriving community of thermophiles – microorganisms that live in very hot water only.

Then, it became plausible to hypothesise that volcanic eruptions occurred subglacially and formed a subglacial lake, whose waters ran into an interconnected system of channels, ultimately reaching the ice margin. Meltwater drained iron from pockets created where ice met bedrock, which then reached the ocean – thus inducing algal blooms.

We dated this drainage activity to a period when dust flux does not match ocean productivity. Thus, our study indicates that volcanoes in Antarctica had a role in delivering iron to the Southern Ocean, and potentially contributed to lowering CO2 levels in the atmosphere.

The ConversationOur research helps explain how volcanoes act on climate change. But it also uncovers more about iron fertilisation as a possible way to mitigate global warming.

Silvia Frisia, Associate Professor, School of Environmental and Life Sciences , University of Newcastle

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

Frozen cones on Pluto – the first discovery of ice volcanoes?

Helen Maynard-Casely, Australian Nuclear Science and Technology Organisation

Ice volcanoes have shaped my life, and until today I didn’t even know if they actually existed. Now, thanks to NASA’s New Horizons spacecraft, there’s a good chance we’ve found a frozen volcanic cone on the surface of Pluto.

The first type of scientist I ever wanted to be was a volcanologist. Aged 12 the prospect of running up and down volcanoes and finding out what make them tick really enthused me.

Then, a pivotal moment for me, I must have been about 16, I watched ‘The Planets’ on TV and heard scientists talk of the possibility of ice volcanoes on the moons of Jupiter and Saturn (or cryo-volcanoes given that they would erupt a temperatures below −150 °C). For me, the fact that the solar system could possibly build volcanoes out of materials other than rock was captivating.

I steered away from an undergrad in geophysics to planetary science and my future of investigating icy stuff was set.

We’ve been searching for ice volcanoes in the solar system for a while and so far no ‘smoking caldera’ has turned up. For instance, we know that the surfaces of the icy moons Europa (orbiting Jupiter) and Titan (orbiting Saturn) are geologically young. However, the puzzle as to how they resurface is continuing as no ‘cryo-volcanic’ features have yet been spotted on these moons.

But now, 16 years after I watched that program, we’ve actually now got the first hint of a volcano of ice sitting on another body. In the pictures that New Horizon’s took of the Southern edge of Sputnik Platina, two volcano features have been spotted. They’ve been informally named Wright and Piccard Mons (I’ve been reliably informed that ‘Piccard’ is in reference to August Piccard the physicist and explorer).

Topographic maps of Piccard Mons (left) and Wright Mons (right).

It is early days in the discovery, but as Dr Oliver White, one of team of scientists looking through New Horizon’s data, said ‘These are big mountains with a large hole in their summit, and on Earth that generally means one thing – a volcano”.

The surface of Pluto hovers about 44 K (about -230°C) so, I’m sure you’re wondering how anything can be fluid enough at those chilly temperatures to erupt. This is because the ice that makes up these mountains is not pure, it will contain a significant amount of substances like methane, nitrogen and ammonia.

Freezing curve of ammonia-water system.

When mixed with water these materials, especially ammonia, cause an effect known as ‘freezing point depression’ lowering the temperature that the water becomes solid. In fact, anything that dissolves in water will have this effect, but ammonia is particularly effective at it – lowering the freezing temperature to -100 °C. Ok, so that’s not quite the -230°C of the surface so then this raises the possibility that internal heating may have play a role on Pluto too.

New Horizons is only a fifth of the way through downloading all of the data it collected as it shot past Pluto, there’s hopefully a lot more of these features yet to be identified. More importantly for knowing more about Wright and Piccard Mons is the spectroscopy data that’s on it way. Analysing the sunlight reflected off them will hopefully give us a hint of their chemistry. Once we have that, then we can start to build models of how these things have built and speculate if they are still active or not.

As well as sending all the data it has already collected, New Horizons is now on its way to the next encounter. Little nudges last week to the frighteningly fast trajectory is propelling the spacecraft towards 2014 MU69, a Kuiper Belt object that it will hopefully fly past in 2019. Given all that New Horizon’s has discovered (from only a fifth of the data) it is rather exciting to think what we are going to see further out.

The Conversation

Helen Maynard-Casely, Instrument Scientist, Australian Nuclear Science and Technology Organisation

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

Undersea Volcanoes May Be Impacting Climate Change


A new study claims that volcanic eruptions along the ocean floor may impact earth’s climate cycle and that predictive models, including those that analyze humanity’s impact on climate change, may need to be modified.

“People have ignored seafloor volcanoes on the idea that their influence is small—but that’s because they are assumed to be in a steady state, which they’re not,” said Maya Tolstoy, a geophysicist and author of the study that appeared in Geophysical Research Letters and was also reported on in Science Daily.

Until now, scientists presumed that seafloor volcanoes exuded lava at a slow and steady pace, but Tolstoy thinks that not only do the volcanoes erupt in bursts, they follow remarkably consistent patterns that range anywhere from two weeks to 100,000 years.

The reason why the study is important is because it offers up the idea that undersea volcanoes may contribute to the beginning of…

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