Victoria’s volcano count rises

Geologists have discovered three previously unrecorded volcanoes in volcanically active southeast Australia.

The new Monash University research, published in the Australian Journal of Earth Sciences, gives a detailed picture of an area of volcanic centres already known to geologists in the region.

Covering an area of 19,000 square kilometres in Victoria and South Australia, with over 400 volcanoes, the Newer Volcanics Province (NVP) features the youngest volcanoes in Australia including Mount Schank and Mount Gambier.

Focusing on the Hamilton region, lead researcher Miss Julie Boyce from the School of Geosciences said the surprising discovery means additional volcanic centres may yet be discovered in the NVP.

“Victoria’s latest episode of volcanism began about eight million years ago, and has helped to shape the landscape. The volcanic deposits, including basalt, are among the youngest rocks in Victoria but most people know little about them,”Miss Boyce said.

“Though it’s been more than 5000 years since the last volcanic eruption in Australia, it’s important that we understand where, when and how these volcanoes erupted. The province is still active, so there may be future eruptions.”

The largest unrecorded volcano is a substantial maar-cone volcanic complex – a broad, low relief volcanic crater caused by an explosion when groundwater comes into contact with hot magma – identified 37 kilometres east of Hamilton.

Miss Boyce said the discoveries were made possible only by analysing a combination of satellite photographs, detailed NASA models of the topography of the area and the distribution of magnetic minerals in the rocks, alongside site visits to build a detailed picture of the Hamilton region of the NVP.

“Traditionally, volcanic sites are analysed by one or two of these techniques. This is the first time that this multifaceted approach has been applied to the NVP and potentially it could be used to study other volcanic provinces worldwide.”

The NVP is considered active, as carbon dioxide is released from the Earth’s mantle in several areas, where there is a large heat anomaly at depth. With an eruption frequency of one volcano every 10,800 years or less, future eruptions may yet occur.

It’s hoped that this multifaceted approach will lead to a better understanding of the distribution and nature of volcanism, allowing for more accurate hazard analysis and risk estimates for future eruptions.

New evidence for oceans of water deep in the Earth

Researchers from Northwestern University and the University of New Mexico report evidence for potentially oceans worth of water deep beneath the United States. Though not in the familiar liquid form — the ingredients for water are bound up in rock deep in the Earth’s mantle — the discovery may represent the planet’s largest water reservoir.

The presence of liquid water on the surface is what makes our “blue planet” habitable, and scientists have long been trying to figure out just how much water may be cycling between Earth’s surface and interior reservoirs through plate tectonics.

Northwestern geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt have found deep pockets of magma located about 400 miles beneath North America, a likely signature of the presence of water at these depths. The discovery suggests water from the Earth’s surface can be driven to such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle.

The findings, to be published June 13 in the journal Science, will aid scientists in understanding how the Earth formed, what its current composition and inner workings are and how much water is trapped in mantle rock.

“Geological processes on the Earth’s surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight,” said Jacobsen, a co-author of the paper. “I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades.”

Scientists have long speculated that water is trapped in a rocky layer of the Earth’s mantle located between the lower mantle and upper mantle, at depths between 250 miles and 410 miles. Jacobsen and Schmandt are the first to provide direct evidence that there may be water in this area of the mantle, known as the “transition zone,” on a regional scale. The region extends across most of the interior of the United States.

Schmandt, an assistant professor of geophysics at the University of New Mexico, uses seismic waves from earthquakes to investigate the structure of the deep crust and mantle. Jacobsen, an associate professor of Earth and planetary sciences at Northwestern’s Weinberg College of Arts and Sciences, uses observations in the laboratory to make predictions about geophysical processes occurring far beyond our direct observation.

The study combined Jacobsen’s lab experiments in which he studies mantle rock under the simulated high pressures of 400 miles below the Earth’s surface with Schmandt’s observations using vast amounts of seismic data from the USArray, a dense network of more than 2,000 seismometers across the United States.

Jacobsen’s and Schmandt’s findings converged to produce evidence that melting may occur about 400 miles deep in the Earth. H2O stored in mantle rocks, such as those containing the mineral ringwoodite, likely is the key to the process, the researchers said.

“Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles,” said Schmandt, a co-author of the paper. “If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found.”

If just one percent of the weight of mantle rock located in the transition zone is H2O, that would be equivalent to nearly three times the amount of water in our oceans, the researchers said.

This water is not in a form familiar to us — it is not liquid, ice or vapor. This fourth form is water trapped inside the molecular structure of the minerals in the mantle rock. The weight of 250 miles of solid rock creates such high pressure, along with temperatures above 2,000 degrees Fahrenheit, that a water molecule splits to form a hydroxyl radical (OH), which can be bound into a mineral’s crystal structure.

Schmandt and Jacobsen’s findings build on a discovery reported in March in the journal Nature in which scientists discovered a piece of the mineral ringwoodite inside a diamond brought up from a depth of 400 miles by a volcano in Brazil. That tiny piece of ringwoodite — the only sample in existence from within the Earth — contained a surprising amount of water bound in solid form in the mineral.

“Whether or not this unique sample is representative of the Earth’s interior composition is not known, however,” Jacobsen said. “Now we have found evidence for extensive melting beneath North America at the same depths corresponding to the dehydration of ringwoodite, which is exactly what has been happening in my experiments.”

For years, Jacobsen has been synthesizing ringwoodite, colored sapphire-like blue, in his Northwestern lab by reacting the green mineral olivine with water at high-pressure conditions. (The Earth’s upper mantle is rich in olivine.) He found that more than one percent of the weight of the ringwoodite’s crystal structure can consist of water — roughly the same amount of water as was found in the sample reported in the Nature paper.

“The ringwoodite is like a sponge, soaking up water,” Jacobsen said. “There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water. This mineral can contain a lot of water under conditions of the deep mantle.”

For the study reported in Science, Jacobsen subjected his synthesized ringwoodite to conditions around 400 miles below the Earth’s surface and found it forms small amounts of partial melt when pushed to these conditions. He detected the melt in experiments conducted at the Advanced Photon Source of Argonne National Laboratory and at the National Synchrotron Light Source of Brookhaven National Laboratory.

Jacobsen uses small gem diamonds as hard anvils to compress minerals to deep-Earth conditions. “Because the diamond windows are transparent, we can look into the high-pressure device and watch reactions occurring at conditions of the deep mantle,” he said. “We used intense beams of X-rays, electrons and infrared light to study the chemical reactions taking place in the diamond cell.”

Jacobsen’s findings produced the same evidence of partial melt, or magma, that Schmandt detected beneath North America using seismic waves. Because the deep mantle is beyond the direct observation of scientists, they use seismic waves — sound waves at different speeds — to image the interior of the Earth.

“Seismic data from the USArray are giving us a clearer picture than ever before of the Earth’s internal structure beneath North America,” Schmandt said. “The melting we see appears to be driven by subduction — the downwelling of mantle material from the surface.”

The melting the researchers have detected is called dehydration melting. Rocks in the transition zone can hold a lot of H2O, but rocks in the top of the lower mantle can hold almost none. The water contained within ringwoodite in the transition zone is forced out when it goes deeper (into the lower mantle) and forms a higher-pressure mineral called silicate perovskite, which cannot absorb the water. This causes the rock at the boundary between the transition zone and lower mantle to partially melt.

“When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit,” Schmandt said. “This is called dehydration melting.”

“Once the water is released, much of it may become trapped there in the transition zone,” Jacobsen added.

Just a little bit of melt, about one percent, is detectible with the new array of seismometers aimed at this region of the mantle because the melt slows the speed of seismic waves, Schmandt said.

Scientists may have identified echoes of ancient Earth

A group of scientists believe that a previously unexplained isotopic ratio from deep within the Earth may be a signal from material from the time before the Earth collided with another planet-sized body, leading to the creation of the Moon. This may represent the echoes of the ancient Earth, which existed prior to the proposed collision 4.5 billion years ago. This work is being presented at the Goldschmidt conference in Sacramento, California.

The currently favoured theory says that the Moon was formed 4.5 billion years ago, when the Earth collided with a Mars-sized mass, which has been given the name “Theia”. According to this theory, the heat generated by the collision would have caused the whole planet to melt, before some of the debris cooled and spun off to create the Moon.

Now however, a group of scientists from Harvard University believe that they have identified a sign that only part of the Earth melted, and that an ancient part still exists within the Earth’s mantle.

According to lead researcher Associate Professor Sujoy Mukhopadhyay (Harvard):

“The energy released by the impact between the Earth and Theia would have been huge, certainly enough to melt the whole planet. But we believe that the impact energy was not evenly distributed throughout the ancient Earth. This means that a major part of the impacted hemisphere would probably have been completely vaporised, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting”.

The team has analysed the ratios of noble gas isotopes from deep within the Earth’s mantle, and has compared these results to isotope ratios closer to the surface. The found that 3He to 22Ne ratio from the shallow mantle is significantly higher than the equivalent ratio in the deep mantle.

Professor Mukhopadhyay commented, “This implies that the last giant impact did not completely mix the mantle and there was not a whole mantle magma ocean”.

Additional evidence comes from analysis of the 129-Xenon to 130-Xenon ratio. It is known that material brought to the surface from the deep mantle (via mantle plumes) has a lower ratio than that normally found nearer the surface, for example in the basalts from mid-ocean ridges. Since 129-Xenon is produced by radioactive decay of 129-Iodine, these xenon isotopes put a time stamp on the formation age of the ancient parcel of mantle to within the first 100 million years of Earth’s history.

Professor Mukhopadhyay continued “The geochemistry indicates that there are differences between the noble gas isotope ratios in different parts of the Earth, and these need to be explained. The idea that a very disruptive collision of the Earth with another planet-sized body, the biggest event in Earth’s geological history, did not completely melt and homogenize the Earth challenges some of our notions on planet formation and the energetics of giant impacts. If the theory is proven correct, then we may be seeing echoes of the ancient Earth, from a time before the collision”.

Commenting, Professor Richard Carlson (Carnegie Institute of Washington), Past President of the Geochemical Society said:

“This exciting result is adding to the observational evidence that important aspects of Earth’s composition were established during the violent birth of the planet and is providing a new look at the physical processes by which this can occur”.

Four-billion-year-old rocks yield clues about Earth’s earliest crust

University of Alberta Ph.D. student Jesse Reimink studied some of the oldest rocks on Earth to find out how the earliest continents formed. -  Bryan Alary/University of Alberta
University of Alberta Ph.D. student Jesse Reimink studied some of the oldest rocks on Earth to find out how the earliest continents formed. – Bryan Alary/University of Alberta

It looks like just another rock, but what Jesse Reimink holds in his hands is a four-billion-year-old chunk of an ancient protocontinent that holds clues about how the Earth’s first continents formed.

The University of Alberta geochemistry student spent the better part of three years collecting and studying ancient rock samples from the Acasta Gneiss Complex in the Northwest Territories, part of his PhD research to understand the environment in which they formed.

“The timing and mode of continental crust formation throughout Earth’s history is a controversial topic in early Earth sciences,” says Reimink, lead author of a new study in Nature Geoscience that points to Iceland as a solid comparison for how the earliest continents formed.

Continents today form when one tectonic plate shifts beneath another into the Earth’s mantle and cause magma to rise to the surface, a process called subduction. It’s unclear whether plate tectonics existed 2.5 billion to four billion years ago or if another process was at play, says Reimink.

One theory is the first continents formed in the ocean as liquid magma rose from the Earth’s mantle before cooling and solidifying into a crust.

Iceland’s crust formed when magma from the mantle rises to shallow levels, incorporating previously formed volcanic rocks. For this reason, Reimink says Iceland is considered a theoretical analogue on early Earth continental crust formation.

Working under the supervision of co-author Tom Chacko, Reimink spent his summers in the field collecting rock samples from the Acasta Gneiss Complex, which was discovered in the 1980s and found to contain some of the Earth’s oldest rocks, between 3.6 and four billion years old. Due to their extreme age, the rocks have undergone multiple metamorphic events, making it difficult to understand their geochemistry, Reimink says.

Fortunately, a few rocks-which the research team dubbed “Idiwhaa” meaning “ancient” in the local Tlicho dialect-were better preserved. This provided a “window” to see the samples’ geochemical characteristics, which Reimink says showed crust-forming processes that are very similar to those occurring in present-day Iceland.

“This provides the first physical evidence that a setting similar to modern Iceland was present on the early Earth.”

These ancient rocks are among the oldest samples of protocontinental crust that we have, he adds, and may have helped jump-start the formation of the rest of the continental crust.

Reimink, who came to the U of A to work with Chacko, says the university’s lab resources are “second to none,” particularly the Ion Microprobe facility within the Canadian Centre for Isotopic Microanalysis run by co-author Richard Stern, which was instrumental to the discovery.

“That lab is producing some of the best data of its kind in the world. That was very key to this project.”

Lower mantle chemistry breakthrough

Breaking research news from a team of scientists led by Carnegie’s Ho-kwang “Dave” Mao reveals that the composition of the Earth’s lower mantle may be significantly different than previously thought. These results are to be published by Science.

The lower mantle comprises 55 percent of the planet by volume and extends from 670 and 2900 kilometers in depth, as defined by the so-called transition zone (top) and the core-mantle boundary (below). Pressures in the lower mantle start at 237,000 times atmospheric pressure (24 gigapascals) and reach 1.3 million times atmospheric pressure (136 gigapascals) at the core-mantle boundary.

The prevailing theory has been that the majority of the lower mantle is made up of a single ferromagnesian silicate mineral, commonly called perovskite (Mg,Fe)SiO3) defined through its chemistry and structure. It was thought that perovskite didn’t change structure over the enormous range of pressures and temperatures spanning the lower mantle.

Recent experiments that simulate the conditions of the lower mantle using laser-heated diamond anvil cells, at pressures between 938,000 and 997,000 times atmospheric pressure (95 and 101 gigapascals) and temperatures between 3,500 and 3,860 degrees Fahrenheit (2,200 and 2,400 Kelvin), now reveal that iron bearing perovskite is, in fact, unstable in the lower mantle.

The team finds that the mineral disassociates into two phases one a magnesium silicate perovskite missing iron, which is represented by the Fe portion of the chemical formula, and a new mineral, that is iron-rich and hexagonal in structure, called the H-phase. Experiments confirm that this iron-rich H-phase is more stable than iron bearing perovskite, much to everyone’s surprise. This means it is likely a prevalent and previously unknown species in the lower mantle. This may change our understanding of the deep Earth.

“We still don’t fully understand the chemistry of the H-phase,” said lead author Li Zhang, also of Carnegie. “But this finding indicates that all geodynamic models need to be reconsidered to take the H-phase into account. And there could be even more unidentified phases down there in the lower mantle as well, waiting to be identified.”

Ice-loss moves the Earth 250 miles down

At the surface, Antarctica is a motionless and frozen landscape. Yet hundreds of miles down the Earth is moving at a rapid rate, new research has shown.

The study, led by Newcastle University, UK, and published this week in Earth and Planetary Science Letters, explains for the first time why the upward motion of the Earth’s crust in the Northern Antarctic Peninsula is currently taking place so quickly.

Previous studies have shown the earth is ‘rebounding’ due to the overlying ice sheet shrinking in response to climate change. This movement of the land was understood to be due to an instantaneous, elastic response followed by a very slow uplift over thousands of years.

But GPS data collected by the international research team, involving experts from Newcastle University, UK; Durham University; DTU, Denmark; University of Tasmania, Australia; Hamilton College, New York; the University of Colorado and the University of Toulouse, France, has revealed that the land in this region is actually rising at a phenomenal rate of 15mm a year – much greater than can be accounted for by the present-day elastic response alone.

And they have shown for the first time how the mantle below the Earth’s crust in the Antarctic Peninsula is flowing much faster than expected, probably due to subtle changes in temperature or chemical composition. This means it can flow more easily and so responds much more quickly to the lightening load hundreds of miles above it, changing the shape of the land.

Lead researcher, PhD student Grace Nield, based in the School of Civil Engineering and Geosciences at Newcastle University, explains: “You would expect this rebound to happen over thousands of years and instead we have been able to measure it in just over a decade. You can almost see it happening which is just incredible.

“Because the mantle is ‘runnier’ below the Northern Antarctic Peninsula it responds much more quickly to what’s happening on the surface. So as the glaciers thin and the load in that localised area reduces, the mantle pushes up the crust.

“At the moment we have only studied the vertical deformation so the next step is to look at horizontal motion caused by the ice unloading to get more of a 3-D picture of how the Earth is deforming, and to use other geophysical data to understand the mechanism of the flow.”

Since 1995 several ice shelves in the Northern Antarctic Peninsula have collapsed and triggered ice-mass unloading, causing the solid Earth to ‘bounce back’.

“Think of it a bit like a stretched piece of elastic,” says Nield, whose project is funded by the Natural Environment Research Council (NERC). “The ice is pressing down on the Earth and as this weight reduces the crust bounces back. But what we found when we compared the ice loss to the uplift was that they didn’t tally – something else had to be happening to be pushing the solid Earth up at such a phenomenal rate.”

Collating data from seven GPS stations situated across the Northern Peninsula, the team found the rebound was so fast that the upper mantle viscosity – or resistance to flow – had to be at least ten times lower than previously thought for the region and much lower than the rest of Antarctica.

Professor Peter Clarke, Professor of Geophysical Geodesy at Newcastle University and one of the authors of the paper, adds: “Seeing this sort of deformation of the earth at such a rate is unprecedented in Antarctica. What is particularly interesting here is that we can actually see the impact that glacier thinning is having on the rocks 250 miles down.”

Deep origins to the behavior of Hawaiian volcanoes

A 300-m-high fountain during episode 8 of the 1959 Kilauea Iki eruption from close to the Byron Ledge overlook.  7 am (HST) on 11 December 1959. -  Hawaiian Volcano Observatory, U.S. Geological Survey
A 300-m-high fountain during episode 8 of the 1959 Kilauea Iki eruption from close to the Byron Ledge overlook. 7 am (HST) on 11 December 1959. – Hawaiian Volcano Observatory, U.S. Geological Survey

Kīlauea volcano, on the Big Island of Hawai’i, typically has effusive eruptions, wherein magma flows to create ropy pāhoehoe lava, for example. However, Kīlauea less frequently erupts more violently, showering scoria and blocks over much of the surface of the island. To explain the variability in Kīlauea’s eruption styles, a team including Bruce Houghton, the Gordon Macdonald Professor of Volcanology in Geology and Geophysics at the University of Hawai’i at Mānoa (UHM) School of Ocean and Earth Science and Technology (SOEST) and colleagues from the University of Cambridge (UC) and Don Swanson from the Hawaiian Volcano Observatory (HVO) of the U.S. Geological Survey analyzed 25 eruptions that have taken place over the past 600 years.

The team’s research shows that the ultimate fate of a magma at Kīlauea, that is if the eruption will be effusive or explosive, is strongly influenced by the variability in composition of the deep magma – with more gas-rich magmas producing more explosive eruptions. “Gas-rich magmas are ‘predisposed’ to rise quickly through the Earth’s mantle and crust and erupt powerfully,” Houghton explained.

One of the biggest challenges in volcanic forecasting is to predict at an early stage the full path that an eruption will follow. Monitoring gives scientists an indication where an eruption will occur but not always the probable form it will take.

“Other statistics like a volcano’s volume, eruption rate, and duration are keys to real-time hazard and risk mapping,” said Houghton. “They are the target of approaches like ours.”

This investigation, published this week in Nature Geoscience, required careful analysis of the physical and chemical properties of eruption products over the last 600 years. Swanson and Houghton supplied a framework of very well-characterized eruptions using a detailed classified scheme for the size and power of the eruptions. UC performed nano-scale measurements of the original gas content of the magmas as ‘frozen’ in tiny packets of chilled melt inside large crystals in the magma.

This new look at the eruption history at Kīlauea has led to new understanding of what causes eruption style there. “Pre-existing wisdom had it that the form of an eruption was principally decided during the last kilometer of rise towards the surface. But now we know the content of dissolved gas at the deep source is a key,” said Houghton.

In the future, Houghton and colleagues hope to offer even more accurate models by estimating just how fast magma does rise at Kīlauea prior to eruption by using the rates at which the trapped original gasses can ‘leak’ out of the trapped magma.

Taking the pulse of mountain formation in the Andes

Sedimentary deposits near Cerdas in the Altiplano plateau of Bolivia are shown. These rocks contain ancient soils used to decipher the surface temperature and surface uplift history of the southern Altiplano. -  Photo by Carmala Garzione/University of Rochester.
Sedimentary deposits near Cerdas in the Altiplano plateau of Bolivia are shown. These rocks contain ancient soils used to decipher the surface temperature and surface uplift history of the southern Altiplano. – Photo by Carmala Garzione/University of Rochester.

Scientists have long been trying to understand how the Andes and other broad, high-elevation mountain ranges were formed. New research by Carmala Garzione, a professor of earth and environmental sciences at the University of Rochester, and colleagues sheds light on the mystery.

In a paper published in the latest Earth and Planetary Science Letters, Garzione explains that the Altiplano plateau in the central Andes-and most likely the entire mountain range-was formed through a series of rapid growth spurts.

“This study provides increasing evidence that the plateau formed through periodic rapid pulses, not through a continuous, gradual uplift of the surface, as was traditionally thought,” said Garzione. “In geologic terms, rapid means rising one kilometer or more over several millions of years, which is very impressive.”

It’s been understood that the Andes mountain range has been growing as the Nazca oceanic plate slips underneath the South American continental plate, causing the Earth’s crust to shorten (by folding and faulting) and thicken. But that left two questions: How quickly have the Andes risen to their current height, and what was the actual process that enabled their rise?

Several years ago (2006-2008), Garzione and several colleagues provided the first estimates of the timing and rates of the surface uplift of the central Andes (“Mountain Ranges Rise Much More Rapidly than Geologists Expected”) by measuring the ancient surface temperatures and rainfall compositions preserved in the soils of the central Altiplano, a plateau in Bolivia and Peru that sits about 12,000 feet above sea level. Garzione concluded that portions of the dense lower crust and upper mantle that act like an anchor on the base of the crust are periodically detached and sink through the mantle as the thickened continental plate heats up. Detachment of this dense anchor allows the Earth’s low density upper crust to rebound and rise rapidly.

More recently, Garzione and Andrew Leier, an assistant professor of Earth and Ocean Sciences at the University of South Carolina, used a relatively new temperature-recording technique in two separate studies in different regions of the Andes to determine whether pulses of rapid surface uplift are the norm, or the exception, for the formation of mountain ranges like the Andes.

Garzione and Leier (“Stable isotope evidence for multiple pulses of rapid surface uplift in the Central Andes, Bolivia”) both focused on the bonding behavior of carbon and oxygen isotopes in the mineral calcite that precipitated from rainwater; their results were similar.

Garzione worked in the southern Altiplano, collecting climate records preserved in ancient soils at both low elevations (close to sea level), where temperatures remained warm over the history of the Andes, and at high elevations where temperatures should have cooled as the mountains rose. The calcite found in the soil contains both the lighter isotopes of carbon and oxygen-12C and 16O-as well as the rare heavier isotopes-13C and 18O. Paleo-temperature estimates from calcite rely on the fact that heavy isotopes form stronger bonds. At lower temperatures, where atoms vibrate more slowly, the heavy isotope 13C-18O bonds would be more difficult to break, resulting in a higher concentration of 13C-18O bonds in calcite, compared to what is found at warmer temperatures. By measuring the abundance of heavy isotope bonds in both low elevation (warm) sites and high elevation (cooler) sites over time, Garzione used the temperature difference between the sites to estimate the elevation of various layers of ancient soils at specific points in time.

She found that the southern Altiplano region rose by about 2.5 kilometers between 16 million and 9 million years ago, which is considered a rapid rate in geologic terms. Garzione speculates that the pulsing action relates to a dense root that grows at the boundary of the lower crust and upper mantle. As the oceanic plate slips under the continental plate, the continental plate shortens and thickens, increasing the pressure on the lower crust. The basaltic composition of the lower crust converts to a very high-density rock called eclogite, which serves as an anchor to the low-density upper crust. As this root is forced deeper into the hotter part of the mantle, it heats to a temperature where it can be rapidly removed (over several million years), resulting in the rapid rise of the mountain range.

“What we are learning is that the Altiplano plateau formed by pulses of rapid surface uplift over several million years, separated by long periods (several tens of million years) of stable elevations,” said Garzione. “We suspect this process is typical of other high-elevation ranges, but more research is needed before we know for certain.”

Hot mantle drives elevation, volcanism along mid-ocean ridges

Scientists have found that temperature deep in Earth's mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth's crust. -  Dalton Lab / Brown University
Scientists have found that temperature deep in Earth’s mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth’s crust. – Dalton Lab / Brown University

Scientists have shown that temperature differences deep within Earth’s mantle control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor. The findings, published April 4 in the journal Science, shed new light on how temperature in the depths of the mantle influences the contours of the Earth’s crust.

Mid-ocean ridges form at the boundaries between tectonic plates, circling the globe like seams on a baseball. As the plates move apart, magma from deep within the Earth rises up to fill the void, creating fresh crust as it cools. The crust formed at these seams is thicker in some places than others, resulting in ridges with widely varying elevations. In some places, the peaks are submerged miles below the ocean surface. In other places – Iceland, for example – the ridge tops are exposed above the water’s surface.

“These variations in ridge depth require an explanation,” said Colleen Dalton, assistant professor of geological sciences at Brown and lead author of the new research. “Something is keeping them either sitting high or sitting low.”

That something, the study found, is the temperature of rocks deep below Earth’s surface.

By analyzing the speeds of seismic waves generated by earthquakes, the researchers show that mantle temperature along the ridges at depths extending below 400 kilometers varies by as much as 250 degrees Celsius. High points on the ridges tend to be associated with higher mantle temperatures, while low points are associated with a cooler mantle. The study also showed that volcanic hot spots along the ridge – volcanoes near Iceland as well as the islands of Ascension, Tristan da Cunha, and elsewhere – all sit above warm spots in Earth’s mantle.

“It is clear from our results that what’s being erupted at the ridges is controlled by temperature deep in the mantle,” Dalton said. “It resolves a long-standing controversy and has not been shown definitively before.”

A CAT scan of the Earth

The mid-ocean ridges provide geologists with a window to the interior of the Earth. The ridges form when mantle material melts, rises into the cracks between tectonic plates, and solidifies again. The characteristics of the ridges provide clues about the properties of the mantle below.

For example, a higher ridge elevation suggests a thicker crust, which in turn suggests that a larger volume of magma was erupted at the surface. This excess molten rock can be caused by very hot temperatures in the mantle. The problem is that hot mantle is not the only way to produce excess magma. The chemical composition of the rocks in Earth’s mantle also controls how much melt is produced. For certain rock compositions, it is possible to generate large volumes of molten rock under cooler conditions. For many decades it has not been clear whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.

To distinguish between these two possibilities, Dalton and her colleagues introduced two additional data sets. One was the chemistry of basalts, the rock that forms from solidification of magma at the mid-ocean ridge. The chemical composition of basalts differs depending upon the temperature and composition of the mantle material from which they’re derived. The authors analyzed the chemistry of nearly 17,000 basalts formed along mid-ocean ridges around the globe.

The other data set was seismic wave tomography. During earthquakes, seismic waves are sent pulsing through the rocks in the crust and mantle. By measuring the velocity of those waves, scientists can gather data about the characteristics of the rocks through which they traveled. “It’s like performing a CAT scan of the inside of the Earth,” Dalton said.

Seismic wave speeds are especially sensitive to the temperature of rocks. In general, waves propagate more quickly in cooler rocks and more slowly in hotter rocks.

Dalton and her colleagues combined the seismic data from hundreds of earthquakes with data on elevation and rock chemistry from the ridges. Correlations among the three data sets revealed that temperature deep in the mantle varied between around 1,300 and 1,550 degrees Celsius underneath about 61,000 kilometers of ridge terrain. “It turned out,” said Dalton, “that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range.”

The study showed that as ridge elevation falls, so does mantle temperature. The coolest point beneath the ridges was found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean. The hottest spot was near Iceland, which is also the ridges’ highest elevation point.

Iceland is also where scientists have long debated whether a mantle plume – a vertical jet of hot rock originating from deep in the Earth – intersects the mid-ocean ridge. This study provides strong support for a mantle plume located beneath Iceland. In fact, this study showed that all regions with above-average temperature are located near volcanic hot spots, which points to mantle plumes as the culprit for the excess volume of magma in these areas.

Understanding a churning planet

Despite being made of solid rock, Earth’s mantle doesn’t sit still. It undergoes convection, a slow churning of material from the depths of the Earth toward the surface and back again.

“Convection is why we have plate tectonics and earthquakes,” Dalton said. “It’s also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth.”

Two factors influence how that convection works: variations in the composition of the mantle and variations in its temperature. This work, says Dalton, points to temperature as a primary factor in how convection is expressed on the surface.

“We get consistent and coherent temperature measurements from the mantle from three independent datasets,” Dalton said. “All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth.”

The findings from this study will also be useful in future research using seismic waves, Dalton says. Because the temperature readings as indicated by seismology were backed up by the other datasets, they can be used to calibrate seismic readings for places where geochemical samples aren’t available. This makes it possible to estimate temperature deep in Earth’s mantle all over the globe.

That will help geologists gain a new insights into how processes deep within the Earth mold the ground beneath our feet.

Off-rift volcanoes explained

Potsdam: Rift valleys are large depressions formed by tectonic stretching forces. Volcanoes often occur in rift valleys, within the rift itself or on the rift flanks as e.g. in East Africa. The magma responsible for this volcanism is formed in the upper mantle and ponds at the boundary between crust and mantle. For many years, the question of why volcanoes develop outside the rift zone in an apparently unexpected location offset by tens of kilometers from the source of molten magma directly beneath the rift has remained unanswered. A team of scientists from the GFZ German Research Centre for Geosciences, University of Southampton and University Roma Tre (Italy) have shown that the pattern of stresses in the crust changes when the crust thins due to stretching and becomes gravitationally unloaded. As a consequence of this stress pattern, the path of the magma pockets ascending from the ponding zone is deviated diagonally to the sides of the rift. Eventually, the magma pockets emerge at distances of tens, sometime hundreds of kilometers from the rift axis, creating the so-called off-rift volcanoes.

The scientists used a numerical model that simulates the propagation of the magma pockets, called dikes, to demonstrate a previously unknown control of rift topography on the trajectory of magma transport. The surface location of the volcanoes depend on the geometry of the rift valleys, explains GFZ researcher Francesco Maccaferri: “We find that in broad, shallow rift valleys, the magma will ascend vertically above the source of magma. In deep, narrow valleys the modification of the stress pattern is very intense and the magma path is strongly deviated.” Since in the latter case the initial path of the dikes is almost horizontal, in extreme cases the magma can arrest as a horizontal intrusion and form a pile of stacked sheet-like bodies without any surface volcanism. This is confirmed in rift valleys around the worl

The phenomenon is a dynamic one: “If the tectonic extension continues and the rift reaches a mature stage of evolution, the pile of the magma sheets can reach the shallow crust. Our model predicts correctly that additional magma-filled sheets will then orient vertically and propagate laterally along the middle of the rift.”adds Eleonora Rivalta from GFZ.

Rift valleys are one of the main tectonic features of our planet. They form both between diverging tectonic plates or within plates which undergo tectonic extension. The generation of magma at depth beneath rift valleys and the divergence of the plates through magma intrusions has been an object of research for tens of years, but the link between deep sources and surface volcanism have previously been missing. The new model may be invoked to explain both off-rift volcanism or the lack of volcanism in million years old rift valleys in Europe.