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.

The biggest mass extinction and Pangea integration

Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. -  ©Science China Press
Relationships between geosphere disturbances and mass extinction during the Late Permian and Early Triassic are shown. – ©Science China Press

The mysterious relationship between Pangea integration and the biggest mass extinction happened 250 million years ago was tackled by Professor YIN Hongfu and Dr. SONG Haijun from State Key Laboratory of Geobiology and Environmental Geology, China University of Geosciences (Wuhan). Their study shows that Pangea integration resulted in environmental deterioration which further caused that extinction. Their work, entitled “Mass extinction and Pangea integration during the Paleozoic-Mesozoic transition”, was published in SCIENCE CHINA Earth Sciences.2013, Vol 56(7).

The Pangea was integrated at about the beginning of Permian, and reached its acme during Late Permian to Early Triassic. Formation of the Pangea means that the scattered continents of the world gathered into one integrated continent with an area of nearly 200 million km2. Average thickness of such a giant continental lithosphere should be remarkably greater than that of each scattered continent. Equilibrium principle implies that the thicker the lithosphere, the higher its portion over the equilibrium level, hence the average altitude of the Pangea should be much higher than the separated modern continents. Correspondingly, all oceans gathered to form the Panthalassa, which should be much deeper than modern oceans. The acme of Pangea and Panthalassa was thus a period of high continent and deep ocean, which should inevitably induce great regression and influence the earth’s surface system, especially climate.

The Tunguss Trap of Siberia, the Emeishan Basalt erupted during the Pangea integration. Such global-scale volcanism should be evoked by mantle plume and related with integration of the Pangea. Volcanic activities would result in a series of extinction effects, including emission of large volume of CO2, CH4, NO2 and cyanides which would have caused green house effects, pollution by poisonous gases, damage of the ozone layer in the stratosphere, and enhancement the ultra-violet radiation.

Increase of CO2 concentration and other green house gases would have led to global warming, oxygen depletion and carbon cycle anomaly; physical and chemical anomalies in ocean (acidification, euxinia, low sulfate concentration, isotopic anomaly of organic nitrogen) and great regression would have caused marine extinction due to unadaptable environments, selective death and hypercapnia; continental aridity, disappearance of monsoon system and wild fire would have devastated the land vegetation, esp. the tropical rain forest.

The great global changes and mass extinction were the results of interaction among earth’s spheres. Deteriorated relations among lithosphere, atmosphere, hydrosphere, and biosphere (including internal factors of organism evolution itself) accumulated until they exceeded the threshold, and exploded at the Permian-Triassic transition time. Interaction among bio- and geospheres is an important theme. However, the processes from inner geospheres to earth’s surface system and further to organism evolution necessitate retardation in time and yields many uncertainties in causation. Most of the processes are now at a hypothetic stage and need more scientific examinations.

The complicated birth of a volcano

Snow storms, ice and glaciers – these are the usual images we associate with the Antarctic. But at the same time it is also a region of fire: the Antarctic continent and surrounding waters are dotted with volcanoes – some of them still active and others extinct for quite some time. The Marie Byrd Seamounts in the Amundsen Sea are in the latter group. Their summit plateaus are today at depths of 2400-1600 meters. Because they are very difficult to reach with conventional research vessels, they have hardly been explored, even though the Marie Byrd Seamounts are fascinating formations. They do not fit any of the usual models for the formation of volcanoes. Now geologists from GEOMAR Helmholtz Centre for Ocean Research Kiel were able to find a possible explanation for the existence of these seamounts on the basis of rare specimens. The study is published in the international journal “Gondwana Research“.

Classic volcanologists differentiate between two types of fire mountains. One type is generated where tectonic plates meet, so the earth’s crust is already cracked to begin with. The other type is formed within the earth’s plates. “The latter are called intraplate volcanoes. They are often found above a so-called mantle plume. Hot material rises from the deep mantle, collects under the earth’s crust, makes its way to the surface and forms a volcano,” said Dr. Reinhard Werner, one of the authors of the current paper. One example are the Hawaiian Islands. But neither of the above models fits the Marie Byrd Seamounts. “There are no plate boundary in the vicinity and no plume underground,” says graduate geologist Andrea Kipf from GEOMAR, first author of the study.

To clarify the origin of the Marie Byrd Seamounts, in 2006 the Kiel scientists participated in an expedition of the research vessel POLARSTEN in the Amundsen Sea. They salvaged rock samples from the seamounts and subjected these to thorough geological, volcanological and geochemical investigations after returning to the home labs. “Interestingly enough, we found chemical signatures that are typical of plume volcanoes. And they are very similar to volcanoes in New Zealand and the Antarctic continent,” says geochemist Dr. Folkmar Hauff, second author of the paper.

Based on this finding, the researchers sought an explanation. They found it in the history of tectonic plates in the southern hemisphere. Around 100 million years ago, remains of the former supercontinent Gondwana were located in the area of present Antarctica. A mantle plume melted through this continental plate and cracked it open. Two new continents were born: the Antarctic and “Zealandia”, with the islands of New Zealand still in evidence today. When the young continents drifted in different directions away from the mantle plume, large quantities of hot plume material were attached to their undersides. These formed reservoirs for future volcanic eruptions on the two continents. “This process explains why we find signatures of plume material at volcanoes that are not on top of plumes,” says Dr. Hauff.

But that still does not explain the Marie Byrd Seamounts because they are not located on the Antarctic continent, but on the adjacent oceanic crust instead. “Continental tectonic plates are thicker than the oceanic ones. This ensures, among other things, differences in temperature in the underground,” says volcanologist Dr. Werner. And just as air masses of different temperatures create winds, the temperature differences under the earth’s crust generate flows and movements as well. Thus the plume material, that once lay beneath the continent, was able to shift under the oceanic plate. With disruptions due to other tectonic processes, there were cracks and crevices which allowed the hot material to rise, turn into magma and then- about 60 million years ago – allowed the Marie Byrd Seamounts to grow. “This created islands are comparable to the Canary Islands today,” explains Andrea Kipf. “Some day the volcanoes became extinct again, wind and weather eroded the cone down to sea level, and other geological processes further eroded the seamounts. Finally, the summit plateaus arrived at the level that we know today,” the PhD student describes the last step of the development.

Based on the previously little investigated Marie Byrd Seamounts, the researchers were able to show another example of how diverse and complex the processes are, that can cause volcanism. “We are still far from understanding all of these processes. But with the current study, we can contribute a small piece to the overall picture,” says Dr. Werner.

Corn syrup model splits Yellowstone’s mantle plume in 2

A corn syrup mantle plume rises near the subducting plate, which induces fluid flow that distorts and deforms the plume as it rises toward the surface. -  Kelsey Druken
A corn syrup mantle plume rises near the subducting plate, which induces fluid flow that distorts and deforms the plume as it rises toward the surface. – Kelsey Druken

One of the greatest controversies in science is what’s underneath the Yellowstone supervolcano. The controversy surrounds a unique relationship between a mantle plume (like the one that powers Hawaiian volcanoes) and the subduction zone off the Washington-Oregon coast. Cutting-edge research using a common kitchen ingredient is explored in the latest issue of EARTH Magazine.

Recently published research explores this problem in 3-D, using a model created with corn syrup, fiberglass and a series of hydraulic pistons. What the scientists saw was a plume sliced in half by the subducting plate. Before this research, different scientific teams had only investigated the subducting tectonic plate or the mantle plume, but not both at the same time.

The resulting model of a bifurcated mantle plume potentially answers key questions about the Yellowstone supervolcano. Read about how these results impact volcano research in Washington, Oregon, Montana, Wyoming and the South Pacific in the July issue of EARTH Magazine: http://bit.ly/153lVat. For complete access – including to see the corn syrup apparatus subscribe to Earth Magazine at: http://www.earthmagazine.org/digital.

Don’t miss the other great articles in the July issue of EARTH Magazine. Uncover ancient earthquake damage in a Roman Mausoleum, Arctic ozone depletion, and plankton growth caused by Icelandic volcanoes, all in this month’s issue of EARTH, now available on the digital newsstand at http://www.earthmagazine.org/digital.

Carbon cycle reaches Earth’s lower mantle, Science study reports

<IMG SRC="/Images/347637653.jpg" WIDTH="350" HEIGHT="175" BORDER="0" ALT="Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth's deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS”>
Like an insect in amber, mineral inclusions trapped in diamonds can reveal much about the Earth’s deep interior. The study by Walter et al. in Science reveals mineral inclusions that originated in oceanic crust subducted into the lower mantle. – Image © Science/AAAS

The carbon cycle, upon which most living things depend, reaches much deeper into the Earth than generally supposed-all the way to the lower mantle, researchers report.

The findings, which are based on the chemistry of an unusual set of Brazilian diamonds, will be published online by the journal Science, at the Science Express Web site, on 15 September. Science is published by AAAS, the non-profit, international science society.

“This study shows the extent of Earth’s carbon cycle on the scale of the entire planet, connecting the chemical and biological processes that occur on the surface and in the oceans to the far depths of Earth’s interior,” said Nick Wigginton, associate editor at Science.

“Results of this kind offer a broader perspective of planet Earth as an integrated, dynamic system,” he said.

The carbon cycle generally refers to the movement of carbon through the atmosphere, oceans, and the crust. Previous observations suggested that the carbon cycle may even extend to the upper mantle, which extends roughly 400 kilometers into the Earth. In this region, plates of ocean crust-bearing a carbon-rich sediment layer-sink beneath other tectonic plates and mix with the molten rock of the mantle.

Seismological and geochemical studies have suggested that oceanic crust can sink all the way to the lower mantle, more than 660 kilometers down. But actual rock samples with this history have been hard to come by.

Michael Walter of the University of Bristol and colleagues in Brazil and the United States analyzed a set of “superdeep” diamonds from the Juina kimberlite field in Brazil. Most diamonds excavated at Earth’s surface originated at depths of less than 200 kilometers. Some parts of the world, however, have produced rare, superdeep diamonds, containing tiny inclusions of other material whose chemistry indicates that the diamonds formed at far greater depths.

The Juina-5 diamonds studied by Walter and colleagues contain inclusions whose bulk compositions span the range of minerals expected to form when basalt melts and crystallizes under the extreme high pressures and temperatures of the lower mantle.

Thus, these inclusions probably originated when diamond-forming fluids incorporated basaltic components from oceanic lithosphere that had descended into the lower mantle, the researchers have concluded.

If this hypothesis is correct, then the carbon from which the diamonds formed may have been deposited originally within ocean crust at the seafloor. A relative abundance of light carbon isotopes in the Juina-5 diamonds supports this idea, since this lighter form of carbon is found at the surface but not generally in the mantle, the authors say.

The diamond inclusions also include separate phases that appear to have “unmixed” from the homogenous pool of material. This unmixing likely happened as the diamonds traveled upward hundreds of kilometers into the upper mantle, the researchers say.

After the diamonds formed in the lower mantle, they may have been launched back near the surface by a rising mantle plume, Walter and colleagues propose.