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.

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.

Source of Galapagos eruptions is not where models place it

Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. -  Douglas Toomey
Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. – Douglas Toomey

Images gathered by University of Oregon scientists using seismic waves penetrating to a depth of 300 kilometers (almost 200 miles) report the discovery of an anomaly that likely is the volcanic mantle plume of the Galapagos Islands. It’s not where geologists and computer modeling had assumed.

The team’s experiments put the suspected plume at a depth of 250 kilometers (155 miles), at a location about 150 kilometers (about 100 miles) southeast of Fernandina Island, the westernmost island of the chain, and where generations of geologists and computer-generated mantle convection models have placed the plume.

The plume anomaly is consistent with partial melting, melt extraction, and remixing of hot rocks and is spreading north toward the mid-ocean ridge instead of, as projected, eastward with the migrating Nazca plate on which the island chain sits, says co-author Douglas R. Toomey, a professor in the UO’s Department of Geological Sciences.

The findings — published online Jan. 19 ahead of print in the February issue of the journal Nature Geoscience — “help explain why so many of the volcanoes in the Galapagos are active,” Toomey said.

The Galapagos chain covers roughly 3,040 square miles of ocean and is centered about 575 miles west of Ecuador, which governs the islands. Galapagos volcanic activity has been difficult to understand, Toomey said, because conventional wisdom and modeling say newer eruptions should be moving ahead of the plate, not unlike the long-migrating Yellowstone hotspot. </p

The separating angles of the two plates in the Galapagos region cloud easy understanding. The leading edge of the Nazca plate is at Fernandina. The Cocos plate, on which the islands’ some 1,000-kilometer-long (620-miles) hotspot chain once sat, is moving to the northeast.

The suspected plume’s location is closer to Isabella and Floreana islands. While a dozen volcanoes remain active in the archipelago, the three most volatile are Fernandina’s and the Cerro Azul and Sierra Negra volcanoes on the southwest and southeast tips, respectively, of Isabella Island, the archipelago’s largest landmass.

The plume’s more southern location, Toomey said, adds fuel to his group’s findings, at three different sites along the globe encircling mid-ocean ridge (where 85 percent of Earth’s volcanic activity occurs), that Earth’s internal convection doesn’t always adhere to modeling efforts and raises new questions about how ocean plates at the Earth’s surface — the lithosphere — interact with the hotter, more fluid asthenosphere that sits atop the mantle.

“Ocean islands have always been enigmatic,” said co-author Dennis J. Geist of the Department of Geological Sciences at the University of Idaho. “Why out in the middle of the ocean basins do you get these big volcanoes? The Galapagos, Hawaii, Tahiti, Iceland — all the world’s great ocean islands – they’re mysterious.”

The Galapagos plume, according to the new paper, extends up into shallower depths and tracks northward and perpendicular to plate motion. Mantle plumes, such as the Galapagos, Yellowstone and Hawaii, generally are believed to bend in the direction of plate migration. In the Galapagos, however, the volcanic plume has decoupled from the plates involved.

“Here’s an archipelago of volcanic islands that are broadly active over a large region, and the plume is almost decoupled from the plate motion itself,” Toomey said. “It is going opposite than expected, and we don’t know why.”

The answer may be in the still unknown rheology of the gooey asthenosphere on which the Earth’s plates ride, Toomey said. In their conclusion, the paper’s five co-authors theorize that the plume material is carried to the mid-ocean ridge by a deep return flow centered in the asthenosphere rather than flowing along the base of the lithosphere as in modeling projections.

“Researchers at the University of Oregon are using tools and technologies to yield critical insights into complex scientific questions,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. “This research by Dr. Toomey and his team sheds new light on the volcanic activity of the Galapagos Islands and raises new questions about plate tectonics and the interaction between the zones of the Earth’s mantle.”

Co-authors with Toomey and Geist were: doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, Calif.; Emilie E.E. Hooft of the UO Department of Geological Sciences; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.

The National Science Foundation (grants OCE-9908695, OCE-0221549 and EAR-0651123 to the UO; OCE-0221634 to the Carnegie Institution of Washington and EAR-11452711 to the University of Idaho) supported the research.

Source of Galapagos eruptions is not where models place it

Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. -  Douglas Toomey
Birds flock not far from a volcano on Isabella Island, where two still active volcanoes are located. The location of the mantle plume, to the southeast of where computer modeling had put it, may explain the continued activity of volcanoes on the various Galapagos Islands. – Douglas Toomey

Images gathered by University of Oregon scientists using seismic waves penetrating to a depth of 300 kilometers (almost 200 miles) report the discovery of an anomaly that likely is the volcanic mantle plume of the Galapagos Islands. It’s not where geologists and computer modeling had assumed.

The team’s experiments put the suspected plume at a depth of 250 kilometers (155 miles), at a location about 150 kilometers (about 100 miles) southeast of Fernandina Island, the westernmost island of the chain, and where generations of geologists and computer-generated mantle convection models have placed the plume.

The plume anomaly is consistent with partial melting, melt extraction, and remixing of hot rocks and is spreading north toward the mid-ocean ridge instead of, as projected, eastward with the migrating Nazca plate on which the island chain sits, says co-author Douglas R. Toomey, a professor in the UO’s Department of Geological Sciences.

The findings — published online Jan. 19 ahead of print in the February issue of the journal Nature Geoscience — “help explain why so many of the volcanoes in the Galapagos are active,” Toomey said.

The Galapagos chain covers roughly 3,040 square miles of ocean and is centered about 575 miles west of Ecuador, which governs the islands. Galapagos volcanic activity has been difficult to understand, Toomey said, because conventional wisdom and modeling say newer eruptions should be moving ahead of the plate, not unlike the long-migrating Yellowstone hotspot. </p

The separating angles of the two plates in the Galapagos region cloud easy understanding. The leading edge of the Nazca plate is at Fernandina. The Cocos plate, on which the islands’ some 1,000-kilometer-long (620-miles) hotspot chain once sat, is moving to the northeast.

The suspected plume’s location is closer to Isabella and Floreana islands. While a dozen volcanoes remain active in the archipelago, the three most volatile are Fernandina’s and the Cerro Azul and Sierra Negra volcanoes on the southwest and southeast tips, respectively, of Isabella Island, the archipelago’s largest landmass.

The plume’s more southern location, Toomey said, adds fuel to his group’s findings, at three different sites along the globe encircling mid-ocean ridge (where 85 percent of Earth’s volcanic activity occurs), that Earth’s internal convection doesn’t always adhere to modeling efforts and raises new questions about how ocean plates at the Earth’s surface — the lithosphere — interact with the hotter, more fluid asthenosphere that sits atop the mantle.

“Ocean islands have always been enigmatic,” said co-author Dennis J. Geist of the Department of Geological Sciences at the University of Idaho. “Why out in the middle of the ocean basins do you get these big volcanoes? The Galapagos, Hawaii, Tahiti, Iceland — all the world’s great ocean islands – they’re mysterious.”

The Galapagos plume, according to the new paper, extends up into shallower depths and tracks northward and perpendicular to plate motion. Mantle plumes, such as the Galapagos, Yellowstone and Hawaii, generally are believed to bend in the direction of plate migration. In the Galapagos, however, the volcanic plume has decoupled from the plates involved.

“Here’s an archipelago of volcanic islands that are broadly active over a large region, and the plume is almost decoupled from the plate motion itself,” Toomey said. “It is going opposite than expected, and we don’t know why.”

The answer may be in the still unknown rheology of the gooey asthenosphere on which the Earth’s plates ride, Toomey said. In their conclusion, the paper’s five co-authors theorize that the plume material is carried to the mid-ocean ridge by a deep return flow centered in the asthenosphere rather than flowing along the base of the lithosphere as in modeling projections.

“Researchers at the University of Oregon are using tools and technologies to yield critical insights into complex scientific questions,” said Kimberly Andrews Espy, vice president for research and innovation and dean of the UO Graduate School. “This research by Dr. Toomey and his team sheds new light on the volcanic activity of the Galapagos Islands and raises new questions about plate tectonics and the interaction between the zones of the Earth’s mantle.”

Co-authors with Toomey and Geist were: doctoral student Darwin R. Villagomez, now with ID Analytics in San Diego, Calif.; Emilie E.E. Hooft of the UO Department of Geological Sciences; and Sean C. Solomon of the Lamont-Doherty Earth Observatory at Columbia University.

The National Science Foundation (grants OCE-9908695, OCE-0221549 and EAR-0651123 to the UO; OCE-0221634 to the Carnegie Institution of Washington and EAR-11452711 to the University of Idaho) supported the research.

Researchers describe unusual Mars rock

The first rock that scientists analyzed on Mars with a pair of chemical instruments aboard the Curiosity rover turned out to be a doozy – a pyramid-shaped volcanic rock called a “mugearite” that is unlike any other Martian igneous rock ever found.

Dubbed “Jake_M” – after Jet Propulsion Laboratory engineer Jake Matijevic – the rock is similar to mugearites found on Earth, typically on ocean islands and in continental rifts. The process through which these rocks form often suggests the presence of water deep below the surface, according to Martin Fisk, an Oregon State University marine geologist and member of the Mars Science Laboratory team.

Results of the analysis were published this week in the journal Science, along with two other papers on Mars’ soils.

“On Earth, we have a pretty good idea how mugearites and rocks like them are formed,” said Fisk, who is a co-author on all three Science articles. “It starts with magma deep within the Earth that crystallizes in the presence of 1-2 percent water. The crystals settle out of the magma and what doesn’t crystallize is the mugearite magma, which can eventually make its way to the surface as a volcanic eruption.”

Fisk, who is a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences, said the most common volcanic rocks typically crystallize in a specific order as they cool, beginning with olivine and feldspar. In the presence of water, however, feldspar crystallizes later and the magma will have a composition such as mugearite.

Although this potential evidence for water deep beneath the surface of Mars isn’t ironclad, the scientists say, it adds to the growing body of studies pointing to the presence of water on the Red Planet – an ingredient necessary for life.

“The rock is significant in another way,” Fisk pointed out. “It implies that the interior of Mars is composed of areas with different compositions; it is not well mixed. Perhaps Mars never got homogenized the way Earth has through its plate tectonics and convection processes.”

In another study, scientists examined the soil diversity and hydration of Gale Crater using a ChemCam laser instrument. They found hydrogen in all of the sites sampled, suggesting water, as well as the likely presence of sulphates. Mars was thought to have three stages – an early phase with lots of water, an evaporation phase when the water disappeared leaving behind sulphate salts, and a third phase when the surface soils dried out and oxidized – creating the planet’s red hue.

“ChemCam found hydrogen in almost every place we found iron,” Fisk said.

The third study compared grains of rock on the surface with a darker soil beneath at a site called the Rocknest Sand Shadow. Some of the sand grains are almost perfectly round and may have come from space, Fisk said.

Researchers describe unusual Mars rock

The first rock that scientists analyzed on Mars with a pair of chemical instruments aboard the Curiosity rover turned out to be a doozy – a pyramid-shaped volcanic rock called a “mugearite” that is unlike any other Martian igneous rock ever found.

Dubbed “Jake_M” – after Jet Propulsion Laboratory engineer Jake Matijevic – the rock is similar to mugearites found on Earth, typically on ocean islands and in continental rifts. The process through which these rocks form often suggests the presence of water deep below the surface, according to Martin Fisk, an Oregon State University marine geologist and member of the Mars Science Laboratory team.

Results of the analysis were published this week in the journal Science, along with two other papers on Mars’ soils.

“On Earth, we have a pretty good idea how mugearites and rocks like them are formed,” said Fisk, who is a co-author on all three Science articles. “It starts with magma deep within the Earth that crystallizes in the presence of 1-2 percent water. The crystals settle out of the magma and what doesn’t crystallize is the mugearite magma, which can eventually make its way to the surface as a volcanic eruption.”

Fisk, who is a professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences, said the most common volcanic rocks typically crystallize in a specific order as they cool, beginning with olivine and feldspar. In the presence of water, however, feldspar crystallizes later and the magma will have a composition such as mugearite.

Although this potential evidence for water deep beneath the surface of Mars isn’t ironclad, the scientists say, it adds to the growing body of studies pointing to the presence of water on the Red Planet – an ingredient necessary for life.

“The rock is significant in another way,” Fisk pointed out. “It implies that the interior of Mars is composed of areas with different compositions; it is not well mixed. Perhaps Mars never got homogenized the way Earth has through its plate tectonics and convection processes.”

In another study, scientists examined the soil diversity and hydration of Gale Crater using a ChemCam laser instrument. They found hydrogen in all of the sites sampled, suggesting water, as well as the likely presence of sulphates. Mars was thought to have three stages – an early phase with lots of water, an evaporation phase when the water disappeared leaving behind sulphate salts, and a third phase when the surface soils dried out and oxidized – creating the planet’s red hue.

“ChemCam found hydrogen in almost every place we found iron,” Fisk said.

The third study compared grains of rock on the surface with a darker soil beneath at a site called the Rocknest Sand Shadow. Some of the sand grains are almost perfectly round and may have come from space, Fisk said.

Beneath Earth’s surface, scientists find long ‘fingers’ of heat

Slow-moving seismic waves, hotter than surrounding material, interact with plumes rising from the mantle to affect the formation of hotspot volcanic islands. -  Illustration: Scott French
Slow-moving seismic waves, hotter than surrounding material, interact with plumes rising from the mantle to affect the formation of hotspot volcanic islands. – Illustration: Scott French

Scientists seeking to understand the forces at work beneath the surface of the Earth have used seismic waves to detect previously unknown “fingers” of heat, some of them thousands of miles long, in Earth’s upper mantle. Their discovery, published Sept. 5 in Science Express, helps explain the “hotspot volcanoes” that give birth to island chains such as Hawai’i and Tahiti.

Many volcanoes arise at collision zones between the tectonic plates, but hotspot volcanoes form in the middle of the plates. Geologists have hypothesized that upwellings of hot, buoyant rock rise as plumes from deep within Earth’s mantle – the layer between the crust and the core that makes up most of Earth’s volume – and supply the heat that feeds these mid-plate volcanoes.

But some hotspot volcano chains are not easily explained by this simple model, a fact which suggests there are more complex interactions between these hot plumes and the upper mantle. Now, a computer modeling approach, developed by University of Maryland seismologist Vedran Lekic and colleagues at the University of California Berkeley, has produced new seismic wave imagery which reveals that the rising plumes are, in fact, influenced by a pattern of finger-like structures carrying heat deep beneath Earth’s oceanic plates.

Seismic waves are waves of energy produced by earthquakes, explosions and volcanic eruptions, which can travel long distances below Earth’s surface. As they travel through layers of different density and elasticity, their shape changes. A global network of seismographs records these changing waveforms. By comparing the waveforms from hundreds of earthquakes recorded at locations around the world, scientists can make inferences about the structures through which the seismic waves have traveled.

The process, known as seismic tomography, works in much the same way that CT scans (computed tomography) reveal structures hidden beneath the surface of the human body. But since we know much less about the structures below Earth’s surface, seismic tomography isn’t easy to interpret. “The Earth’s crust varies a lot, and being able to represent that variation is difficult, much less the structure deeper below” said Lekic, an assistant professor of geology at the College Park campus.

Until recently, analyses like the one in the study would have taken up to 19 years of computer time. While studying for his doctorate with the study’s senior author, UC Berkeley Prof. Barbara Romanowicz, Lekic developed a method to more accurately model waveform data while still keeping computer time manageable, which resulted in higher-resolution images of the interaction between the layers of Earth’s mantle.

By refining this method, a research team led by UC Berkeley graduate student Scott French found finger-like channels of low-speed seismic waves flowing about 120 to 220 miles below the sea floor, and stretching out in bands about 700 miles wide and 1,400 miles apart. The researchers also discovered a subtle but important difference in speed: at this depth, seismic waves typically travel about 2.5 to 3 miles per second, but the average seismic velocity in the channels was 4 percent slower. Because higher temperatures slow down seismic waves, the researchers infer that the channels are hotter than the surrounding material.

“We estimate that the slowdown we’re seeing could represent a temperature increase of up to 200 degrees Celsius,” or about 390 degrees Fahrenheit, said French, the study’s study lead author. At these depths, absolute temperatures in the mantle are about 1,300 degrees Celsius, or 2,400 degrees Fahrenheit, the researchers said.

Geophysicists have long theorized that channels akin to those revealed in the computer model exist, and are interacting with the plumes in Earth’s mantle that feed hotspot volcanoes. But the new images reveal for the first time the extent, depth and shape of these channels. And they also show that the fingers align with the motion of the overlying tectonic plate. The researchers hypothesize that these channels may be interacting in complex ways with both the tectonic plates above them and the hot plumes rising from below.

“This global pattern of finger-like structures that we’re seeing, which has not been documented before, appears to reflect interactions between the upwelling plumes and the motion of the overlying plates,” Lekic said. “The deflection of the plumes into these finger-like channels represents an intermediate scale of convection in the mantle, between the large-scale circulation that drives plate motions and the smaller scale plumes, which we are now starting to image.”

“The exact nature of those interactions will need further study,” said French, “but we now have a clearer picture that can help us understand the ‘plumbing’ of Earth’s mantle responsible for hotspot volcano islands like Tahiti, Reunion and Samoa.”

Beneath Earth’s surface, scientists find long ‘fingers’ of heat

Slow-moving seismic waves, hotter than surrounding material, interact with plumes rising from the mantle to affect the formation of hotspot volcanic islands. -  Illustration: Scott French
Slow-moving seismic waves, hotter than surrounding material, interact with plumes rising from the mantle to affect the formation of hotspot volcanic islands. – Illustration: Scott French

Scientists seeking to understand the forces at work beneath the surface of the Earth have used seismic waves to detect previously unknown “fingers” of heat, some of them thousands of miles long, in Earth’s upper mantle. Their discovery, published Sept. 5 in Science Express, helps explain the “hotspot volcanoes” that give birth to island chains such as Hawai’i and Tahiti.

Many volcanoes arise at collision zones between the tectonic plates, but hotspot volcanoes form in the middle of the plates. Geologists have hypothesized that upwellings of hot, buoyant rock rise as plumes from deep within Earth’s mantle – the layer between the crust and the core that makes up most of Earth’s volume – and supply the heat that feeds these mid-plate volcanoes.

But some hotspot volcano chains are not easily explained by this simple model, a fact which suggests there are more complex interactions between these hot plumes and the upper mantle. Now, a computer modeling approach, developed by University of Maryland seismologist Vedran Lekic and colleagues at the University of California Berkeley, has produced new seismic wave imagery which reveals that the rising plumes are, in fact, influenced by a pattern of finger-like structures carrying heat deep beneath Earth’s oceanic plates.

Seismic waves are waves of energy produced by earthquakes, explosions and volcanic eruptions, which can travel long distances below Earth’s surface. As they travel through layers of different density and elasticity, their shape changes. A global network of seismographs records these changing waveforms. By comparing the waveforms from hundreds of earthquakes recorded at locations around the world, scientists can make inferences about the structures through which the seismic waves have traveled.

The process, known as seismic tomography, works in much the same way that CT scans (computed tomography) reveal structures hidden beneath the surface of the human body. But since we know much less about the structures below Earth’s surface, seismic tomography isn’t easy to interpret. “The Earth’s crust varies a lot, and being able to represent that variation is difficult, much less the structure deeper below” said Lekic, an assistant professor of geology at the College Park campus.

Until recently, analyses like the one in the study would have taken up to 19 years of computer time. While studying for his doctorate with the study’s senior author, UC Berkeley Prof. Barbara Romanowicz, Lekic developed a method to more accurately model waveform data while still keeping computer time manageable, which resulted in higher-resolution images of the interaction between the layers of Earth’s mantle.

By refining this method, a research team led by UC Berkeley graduate student Scott French found finger-like channels of low-speed seismic waves flowing about 120 to 220 miles below the sea floor, and stretching out in bands about 700 miles wide and 1,400 miles apart. The researchers also discovered a subtle but important difference in speed: at this depth, seismic waves typically travel about 2.5 to 3 miles per second, but the average seismic velocity in the channels was 4 percent slower. Because higher temperatures slow down seismic waves, the researchers infer that the channels are hotter than the surrounding material.

“We estimate that the slowdown we’re seeing could represent a temperature increase of up to 200 degrees Celsius,” or about 390 degrees Fahrenheit, said French, the study’s study lead author. At these depths, absolute temperatures in the mantle are about 1,300 degrees Celsius, or 2,400 degrees Fahrenheit, the researchers said.

Geophysicists have long theorized that channels akin to those revealed in the computer model exist, and are interacting with the plumes in Earth’s mantle that feed hotspot volcanoes. But the new images reveal for the first time the extent, depth and shape of these channels. And they also show that the fingers align with the motion of the overlying tectonic plate. The researchers hypothesize that these channels may be interacting in complex ways with both the tectonic plates above them and the hot plumes rising from below.

“This global pattern of finger-like structures that we’re seeing, which has not been documented before, appears to reflect interactions between the upwelling plumes and the motion of the overlying plates,” Lekic said. “The deflection of the plumes into these finger-like channels represents an intermediate scale of convection in the mantle, between the large-scale circulation that drives plate motions and the smaller scale plumes, which we are now starting to image.”

“The exact nature of those interactions will need further study,” said French, “but we now have a clearer picture that can help us understand the ‘plumbing’ of Earth’s mantle responsible for hotspot volcano islands like Tahiti, Reunion and Samoa.”

Sea level influenced tropical climate during the last ice age

The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. -  Pedro DiNezio
The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. – Pedro DiNezio

Scientists look at past climates to learn about climate change and the ability to simulate it with computer models. One region that has received a great deal of attention is the Indo-Pacific warm pool, the vast pool of warm water stretching along the equator from Africa to the western Pacific Ocean.

In a new study, Pedro DiNezio of the International Pacific Research Center, University of Hawaii at Manoa, and Jessica Tierney of Woods Hole Oceanographic Institution investigated preserved geological clues (called “proxies”) of rainfall patterns during the last ice age when the planet was dramatically colder than today. They compared these patterns with computer model simulations in order to find a physical explanation for the patterns inferred from the proxies.

Their study, which appears in the May 19, online edition of Nature Geoscience, not only reveals unique patterns of rainfall change over the Indo-Pacific warm pool, but also shows that they were caused by the effect of lowered sea level on the configuration of the Indonesian archipelago.

“For our research,” explains lead-author Pedro DiNezio at the International Pacific Research Center, “we compared the climate of the ice age with our recent warmer climate. We analyzed about 100 proxy records of rainfall and salinity stretching from the tropical western Pacific to the western Indian Ocean and eastern Africa. Rainfall and salinity signals recorded in geological sediments can tell us much about past changes in atmospheric circulation over land and the ocean respectively.”

“Our comparisons show that, as many scientists expected, much of the Indo-Pacific warm pool was drier during this glacial period compared with today. But, counter to some theories, several regions, such as the western Pacific and the western Indian Ocean, especially eastern Africa, were wetter,” adds co-author Jessica Tierney from Woods Hole Oceanographic Institute.

In the second step, the scientists matched these rainfall and salinity patterns with simulations from 12 state-of-the-art climate models that are used to also predict future climate change. For this matching they applied a method of categorical data comparison called the ‘Cohen’s kappa’ statistic. Though widely used in the medical field, this method has not yet been used to match geological climate signals with climate model simulations.

“We were taken aback that only one model out of the 12 showed statistical agreement with the proxy-inferred patterns of the rainfall changes. This model, though, agrees well with both the rainfall and salinity indicators – two entirely independent sets of proxy data covering distinct areas of the tropics,” says DiNezio.

The model reveals that the dry climate during the glacial period was driven by reduced convection over a region of the warm pool called the Sunda Shelf. Today the shelf is submerged beneath the Gulf of Thailand, but was above sea level during the glacial period, when sea level was about 120 m lower.

“The exposure of the Sunda Shelf greatly weakened convection over the warm pool, with far-reaching impacts on the large-scale circulation and on rainfall patterns from Africa to the western Pacific and northern Australia,” explains DiNezio.

The main weakness of the other models, according to the authors, is their limited ability to simulate convection, the vertical air motions that lift humid air into the atmosphere. Differences in the way each model simulates convection may explain why the results for the glacial period are so different.

“Our research resolves a decades-old question of what the response of tropical climate was to glaciation,” concludes DiNezio. “The study, moreover, presents a fine benchmark for assessing the ability of climate models to simulate the response of tropical convection to altered land masses and global temperatures.

Sea level influenced tropical climate during the last ice age

The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. -  Pedro DiNezio
The exposed Sunda Shelf during glacial times greatly affected the atmospheric circulation. The shelf is shown on the left for present-day as the light-blue submerged areas between Java, Sumatra, Borneo, and Thailand, and on the right for the last ice age as the green exposed area. – Pedro DiNezio

Scientists look at past climates to learn about climate change and the ability to simulate it with computer models. One region that has received a great deal of attention is the Indo-Pacific warm pool, the vast pool of warm water stretching along the equator from Africa to the western Pacific Ocean.

In a new study, Pedro DiNezio of the International Pacific Research Center, University of Hawaii at Manoa, and Jessica Tierney of Woods Hole Oceanographic Institution investigated preserved geological clues (called “proxies”) of rainfall patterns during the last ice age when the planet was dramatically colder than today. They compared these patterns with computer model simulations in order to find a physical explanation for the patterns inferred from the proxies.

Their study, which appears in the May 19, online edition of Nature Geoscience, not only reveals unique patterns of rainfall change over the Indo-Pacific warm pool, but also shows that they were caused by the effect of lowered sea level on the configuration of the Indonesian archipelago.

“For our research,” explains lead-author Pedro DiNezio at the International Pacific Research Center, “we compared the climate of the ice age with our recent warmer climate. We analyzed about 100 proxy records of rainfall and salinity stretching from the tropical western Pacific to the western Indian Ocean and eastern Africa. Rainfall and salinity signals recorded in geological sediments can tell us much about past changes in atmospheric circulation over land and the ocean respectively.”

“Our comparisons show that, as many scientists expected, much of the Indo-Pacific warm pool was drier during this glacial period compared with today. But, counter to some theories, several regions, such as the western Pacific and the western Indian Ocean, especially eastern Africa, were wetter,” adds co-author Jessica Tierney from Woods Hole Oceanographic Institute.

In the second step, the scientists matched these rainfall and salinity patterns with simulations from 12 state-of-the-art climate models that are used to also predict future climate change. For this matching they applied a method of categorical data comparison called the ‘Cohen’s kappa’ statistic. Though widely used in the medical field, this method has not yet been used to match geological climate signals with climate model simulations.

“We were taken aback that only one model out of the 12 showed statistical agreement with the proxy-inferred patterns of the rainfall changes. This model, though, agrees well with both the rainfall and salinity indicators – two entirely independent sets of proxy data covering distinct areas of the tropics,” says DiNezio.

The model reveals that the dry climate during the glacial period was driven by reduced convection over a region of the warm pool called the Sunda Shelf. Today the shelf is submerged beneath the Gulf of Thailand, but was above sea level during the glacial period, when sea level was about 120 m lower.

“The exposure of the Sunda Shelf greatly weakened convection over the warm pool, with far-reaching impacts on the large-scale circulation and on rainfall patterns from Africa to the western Pacific and northern Australia,” explains DiNezio.

The main weakness of the other models, according to the authors, is their limited ability to simulate convection, the vertical air motions that lift humid air into the atmosphere. Differences in the way each model simulates convection may explain why the results for the glacial period are so different.

“Our research resolves a decades-old question of what the response of tropical climate was to glaciation,” concludes DiNezio. “The study, moreover, presents a fine benchmark for assessing the ability of climate models to simulate the response of tropical convection to altered land masses and global temperatures.