How productive are the ore factories in the deep sea?

About ten years after the first moon landing, scientists on earth made a discovery that proved that our home planet still holds a lot of surprises in store for us. Looking through the portholes of the submersible ALVIN near the bottom of the Pacific Ocean in 1979, American scientists saw for the first time chimneys, several meters tall, from which black water at about 300 degrees and saturated with minerals shot out. What we have found out since then: These “black smokers”, also called hydrothermal vents, exist in all oceans. They occur along the boundaries of tectonic plates along the submarine volcanic chains. However, to date many details of these systems remain unexplained.

One question that has long and intensively been discussed in research is: Where and how deep does seawater penetrate into the seafloor to take up heat and minerals before it leaves the ocean floor at hydrothermal vents? This is of enormous importance for both, the cooling of the underwater volcanoes as well as for the amount of materials dissolved. Using a complex 3-D computer model, scientists at GEOMAR Helmholtz Centre for Ocean Research Kiel were now able to understand the paths of the water toward the black smokers. The study appears in the current issue of the world-renowned scientific journal “Nature“.

In general, it is well known that seawater penetrates into the Earth’s interior through cracks and crevices along the plate boundaries. The seawater is heated by the magma; the hot water rises again, leaches metals and other elements from the ground and is released as a black colored solution. “However, in detail it is somewhat unclear whether the water enters the ocean floor in the immediate vicinity of the vents and flows upward immediately, or whether it travels long distances underground before venting,” explains Dr. Jörg Hasenclever from GEOMAR.

This question is not only important for the fundamental understanding of processes on our planet. It also has very practical implications. Some of the materials leached from the underground are deposited on the seabed and form ore deposits that may be of economically interest. There is a major debate, however, how large the resource potential of these deposits might be. “When we know which paths the water travels underground, we can better estimate the quantities of materials released by black smokers over thousands of years,” says Hasenclever.

Hasenclever and his colleagues have used for the first time a high-resolution computer model of the seafloor to simulate a six kilometer long and deep, and 16 kilometer wide section of a mid-ocean ridge in the Pacific. Among the data used by the model was the heat distribution in the oceanic crust, which is known from seismic studies. In addition, the model also considered the permeability of the rock and the special physical properties of water.

The simulation required several weeks of computing time. The result: “There are actually two different flow paths – about half the water seeps in near the vents, where the ground is very warm. The other half seeps in at greater distances and migrates for kilometers through the seafloor before exiting years later.” Thus, the current study partially confirmed results from a computer model, which were published in 2008 in the scientific journal “Science”. “However, the colleagues back then were able to simulate only a much smaller region of the ocean floor and therefore identified only the short paths near the black smokers,” says Hasenclever.

The current study is based on fundamental work on the modeling of the seafloor, which was conducted in the group of Professor Lars Rüpke within the framework of the Kiel Cluster of Excellence “The Future Ocean”. It provides scientists worldwide with the basis for further investigations to see how much ore is actually on and in the seabed, and whether or not deep-sea mining on a large scale could ever become worthwhile. “So far, we only know the surface of the ore deposits at hydrothermal vents. Nobody knows exactly how much metal is really deposited there. All the discussions about the pros and cons of deep-sea ore mining are based on a very thin database,” says co-author Prof. Dr. Colin Devey from GEOMAR. “We need to collect a lot more data on hydrothermal systems before we can make reliable statements”.

Computer models solve geologic riddle millions of years in the making

An international team of scientists that included USC’s Meghan Miller used computer modeling to reveal, for the first time, how giant swirls form during the collision of tectonic plates – with subduction zones stuttering and recovering after continental fragments slam into them.

The team’s 3D models suggest a likely answer to a question that has long plagued geologists: why do long, curving mountain chains form along some subduction zones – where two tectonic plates collide, pushing one down into the mantle?

Based on the models, the researchers found that parts of the slab that is being subducted sweep around behind the collision, pushing continental material into the mountain belt.

With predictions confirmed by field observations, the 3D models show a characteristic pattern of intense localized heating, volcanic activity and fresh sediments that remained enigmatic until now.

“The new model explains why we see curved mountains near colliding plates, where material that has been scraped off of one plate and accreted on another is dragged into a curved path on the continent,” Miller said.

Miller collaborated with lead author Louis Moresi from Monash University and his colleagues Peter Betts (also from Monash) and R. A. Cayley from the Geological Survey of Victoria in Australia. Their research was published online by Nature on March 23.

Their research specifically looked at the ancient geologic record of Eastern Australia, but is also applicable to the Pacific Northwest of the United States, the Mediterranean, and southeast Asia. Coastal mountain ranges from Northern California up to Alaska were formed by the scraping off of fragment of the ancient Farallon plate as it subducted beneath the North American continent. The geology of the Western Cordillera (wide mountain belts that extend along all of North America) fits the predictions of the computer model.

“The amazing thing about this research is that we can now interpret arcuate-shaped geological structures on the continents in a whole new way,” Miller said. “We no longer need to envision complex motions and geometries to explain the origins of ancient or modern curved mountain belts.”

The new results from this research will help geologists interpret the formation of ancient mountain belts and may prove most useful as a template to interpret regions where preservation of evidence for past collisions is incomplete – a common, and often frustrating, challenge for geologists working in fragmented ancient terrains.

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.

Earth’s crust was unstable in the Archean eon and dripped down into the mantle

Earth’s mantle temperatures during the Archean eon, which commenced some 4 billion years ago, were significantly higher than they are today. According to recent model calculations, the Archean crust that formed under these conditions was so dense that large portions of it were recycled back into the mantle. This is the conclusion reached by Dr. Tim Johnson who is currently studying the evolution of the Earth’s crust as a member of the research team led by Professor Richard White of the Institute of Geosciences at Johannes Gutenberg University Mainz (JGU). According to the calculations, this dense primary crust would have descended vertically in drip form. In contrast, the movements of today’s tectonic plates involve largely lateral movements with oceanic lithosphere recycled in subduction zones. The findings add to our understanding of how cratons and plate tectonics, and thus also the Earth’s current continents, came into being.

Because mantle temperatures were higher during the Archean eon, the Earth’s primary crust that formed at the time must have been very thick and also very rich in magnesium. However, as Johnson and his co-authors explain in their article recently published in Nature Geoscience, very little of this original crust is preserved, indicating that most must have been recycled into the Earth’s mantle. Moreover, the Archean crust that has survived in some areas such as, for example, Northwest Scotland and Greenland, is largely made of tonalite-trondhjemite-granodiorite complexes and these are likely to have originated from a hydrated, low-magnesium basalt source. The conclusion is that these pieces of crust cannot be the direct products of an originally magnesium-rich primary crust. These TTG complexes are among the oldest features of our Earth’s crust. They are most commonly present in cratons, the oldest and most stable cores of the current continents.

With the help of thermodynamic calculations, Dr. Tim Johnson and his collaborators at the US-American universities of Maryland, Southern California, and Yale have established that the mineral assemblages that formed at the base of a 45-kilometer-thick magnesium-rich crust were denser than the underlying mantle layer. In order to better explore the physics of this process, Professor Boris Kaus of the Geophysics work group at Mainz University developed new computer models that simulate the conditions when the Earth was still relatively young and take into account Johnson’s calculations.

These geodynamic computer models show that the base of a magmatically over-thickened and magnesium-rich crust would have been gravitationally unstable at mantle temperatures greater than 1,500 to 1,550 degrees Celsius and this would have caused it to sink in a process called ‘delamination’. The dense crust would have dripped down into the mantle, triggering a return flow of mantle material from the asthenosphere that would have melted to form new primary crust. Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to provide a source of the tonalite-trondhjemite-granodiorite complexes. The dense residues of these processes, which would have a high content of mafic minerals, must now reside in the mantle.

Scientists anticipated size and location of 2012 Costa Rica earthquake

Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica's Nicoya Peninsula in 2010. -  Lujia Feng
Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, performs a GPS survey in Costa Rica’s Nicoya Peninsula in 2010. – Lujia Feng

Scientists using GPS to study changes in the Earth’s shape accurately forecasted the size and location of the magnitude 7.6 Nicoya earthquake that occurred in 2012 in Costa Rica.

The Nicoya Peninsula in Costa Rica is one of the few places where land sits atop the portion of a subduction zone where the Earth’s greatest earthquakes take place. Costa Rica’s location therefore makes it the perfect spot for learning how large earthquakes rupture. Because earthquakes greater than about magnitude 7.5 have occurred in this region roughly every 50 years, with the previous event striking in 1950, scientists have been preparing for this earthquake through a number of geophysical studies. The most recent study used GPS to map out the area along the fault storing energy for release in a large earthquake.

“This is the first place where we’ve been able to map out the likely extent of an earthquake rupture along the subduction megathrust beforehand,” said Andrew Newman, an associate professor in the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

The study was published online Dec. 22, 2013, in the journal Nature Geoscience. The research was supported by the National Science Foundation and was a collaboration of researchers from Georgia Tech, the Costa Rica Volcanological and Seismological Observatory (OVSICORI) at Universidad Nacional, University California, Santa Cruz, and the University of South Florida.

Subduction zones are locations where one tectonic plate is forced under another one. The collision of tectonic plates during this process can unleash devastating earthquakes, and sometimes devastating tsunamis. The magnitude 9.0 earthquake off the coast of Japan in 2011 was due to just such a subduction zone eaerthquake. The Cascadia subduction zone in the Pacific Northwest is capable of unleashing a similarly sized quake. Damage from the Nicoya earthquake was not as bad as might be expected from a magnitude 7.6 quake.

“Fortunately there was very little damage considering the earthquake’s size,” said Marino Protti of OVSICORI and the study’s lead author. “The historical pattern of earthquakes not only allowed us to get our instruments ready, it also allowed Costa Ricans to upgrade their buildings to be earthquake safe.”

Plate tectonics are the driving force for subduction zones. As tectonic plates converge, strain temporarily accumulates across the plate boundary when portions of the interface between these tectonic plates, called a megathrust, become locked together. The strain can accumulate to dangerous levels before eventually being released as a massive earthquake.

“The Nicoya Peninsula is an ideal natural lab for studying these events, because the coastline geometry uniquely allows us to get our equipment close to the zone of active strain accumulation,” said Susan Schwartz, professor of earth sciences at the University of California, Santa Cruz, and a co-author of the study.

Through a series of studies starting in the early 1990s using land-based tools, the researchers mapped regions where tectonic plates were completely locked along the subduction interface. Detailed geophysical observations of the region allowed the researchers to create an image of where the faults had locked.

The researchers published a study a few months before the earthquake, describing the particular locked patch with the clearest potential for the next large earthquake in the region. The team projected the total amount of energy that could have developed across that region and forecasted that if the locking remained similar since the last major earthquake in 1950, then there is presently enough energy for an earthquake on the order of magnitude 7.8 there.

Because of limits in technology and scientific understanding about processes controlling fault locking and release, scientists cannot say much about precisely where or when earthquakes will occur. However, earthquakes in Nicoya have occurred about every 50 years, so seismologists had been anticipating another one around 2000, give or take 20 years, Newman said. The earthquake occurred in September of 2012 as a magnitude 7.6 quake.

“It occurred right in the area we determined to be locked and it had almost the size we expected,” Newman said.

The researchers hope to apply what they’ve learned in Costa Rica to other environments. Virtually every damaging subduction zone earthquake occurs far offshore.

“Nicoya is the only place on Earth where we’ve actually been able to get a very accurate image of the locked patch because it occurs directly under land,” Newman said. “If we really want to understand the seismic potential for most of the world, we have to go offshore.”

Scientists have been able to reasonably map portions of these locked areas offshore using data on land, but the resolution is poor, particularly in the regions that are most responsible for generating tsunamis, Newman said. He hopes that his group’s work in Nicoya will be a driver for geodetic studies on the seafloor to observe such Earth deformation. These seafloor geodetic studies are rare and expensive today.

“If we want to understand the potential for large earthquakes, then we really need to start doing more seafloor observations,” Newman said. “It’s a growing push in our community and this study highlights the type of results that one might be able to obtain for most other dangerous environments, including offshore the Pacific Northwest.”

Global map to predict giant earthquakes

A team of international researchers, led by Monash University’s Associate Professor Wouter Schellart, have developed a new global map of subduction zones, illustrating which ones are predicted to be capable of generating giant earthquakes and which ones are not.

The new research, published in the journal Physics of the Earth and Planetary Interiors, comes nine years after the giant earthquake and tsunami in Sumatra in December 2004, which devastated the region and many other areas surrounding the Indian Ocean, and killed more than 200,000 people.

Since then two other giant earthquakes have occurred at subduction zones, one in Chile in February 2010 and one in Japan in March 2011, which both caused massive destruction, killed many thousands of people and resulted in billions of dollars of damage.

Most earthquakes occur at the boundaries between tectonic plates that cover the Earth’s surface. The largest earthquakes on Earth only occur at subduction zones, plate boundaries where one plate sinks (subducts) below the other into the Earth’s interior. So far, seismologists have recorded giant earthquakes for only a limited number of subduction zone segments. But accurate seismological records go back to only ~1900, and the recurrence time of giant earthquakes can be many hundreds of years.

“The main question is, are all subduction segments capable of generating giant earthquakes, or only some of them? And if only a limited number of them, then how can we identify these,” Dr Schellart said.

Dr Schellart, of the School of Geosciences, and Professor Nick Rawlinson from the University of Aberdeen in Scotland used earthquake data going back to 1900 and data from subduction zones to map the main characteristics of all active subduction zones on Earth. They investigated if those subduction segments that have experienced a giant earthquake share commonalities in their physical, geometrical and geological properties.

They found that the main indicators include the style of deformation in the plate overlying the subduction zone, the level of stress at the subduction zone, the dip angle of the subduction zone, as well as the curvature of the subduction zone plate boundary and the rate at which it moves.

Through these findings Dr Schellart has identified several subduction zone regions capable of generating giant earthquakes, including the Lesser Antilles, Mexico-Central America, Greece, the Makran, Sunda, North Sulawesi and Hikurangi.

“For the Australian region subduction zones of particular significance are the Sunda subduction zone, running from the Andaman Islands along Sumatra and Java to Sumba, and the Hikurangi subduction segment offshore the east coast of the North Island of New Zealand. Our research predicts that these zones are capable of producing giant earthquakes,” Dr Schellart said.

“Our work also predicts that several other subduction segments that surround eastern Australia (New Britain, San Cristobal, New Hebrides, Tonga, Puysegur), are not capable of producing giant earthquakes.”

Slippery fault unleashed destructive Tohoku-Oki earthquake and tsunami

UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. -  IODP
UCSC researcher Patrick Fulton is shown with core samples from the drilling operation. – IODP

For the first time, scientists have measured the frictional heat produced by the fault slip during an earthquake. Their results, published December 5 in Science, show that friction on the fault was remarkably low during the magnitude 9.0 Tohoku-Oki earthquake that struck off the coast of Japan in March 2011 and triggered a devastating tsunami.

“The Tohoku fault is more slippery than anyone expected,” said Emily Brodsky, a geophysicist at the University of California, Santa Cruz, and coauthor of three papers on the Tohoku-Oki earthquake published together in Science. All three papers are based on results from the international Japan Trench Fast Drilling Project (JFAST), which Brodsky helped organize.

Because friction generates heat (like rubbing your hands together), taking the temperature of a fault after an earthquake can provide a measure of the fault’s frictional resistance to slip. But that hasn’t been easy to do. “It’s been difficult to get this measurement because the signal is weak and it dissipates over time, so we needed a big earthquake and a rapid response,” said Brodsky, a professor of Earth and planetary sciences at UCSC.

The JFAST expedition drilled across the Tohoku fault in 2012 and installed a temperature observatory in one of three boreholes nearly 7 kilometers below the ocean surface. The logistically and technically challenging operation successfully recovered temperature measurements and other data as well as core samples from across the fault.

The low resistance to slip on the fault may help explain the large amount of slip–an unprecedented 50 meters of displacement–that occurred during the earthquake, according to UC Santa Cruz researcher Patrick Fulton, who is first author of the paper focusing on the temperature measurements. An abundance of weak, slippery clay material in the fault zone–described in the two companion papers–may account for the low friction during the earthquake, he said.

The Tohoku-Oki earthquake occurred in a “subduction zone,” a boundary between two tectonic plates where one plate is diving beneath another–in this case, the Pacific plate dives beneath the Eurasian plate just east of Japan. Fulton explained that the epicenter, where the earthquake started, was much deeper than the shallow portion of the fault examined by JFAST. One of the surprising things about the earthquake, in addition to the 50 meters of slip, was that the fault ruptured all the way to the surface of the seafloor.

“The large slip at shallow depths contributed to the tsumani that caused so much damage in Japan. Usually, these earthquakes don’t rupture all the way to the surface,” Fulton said.

The strain that is released in a subduction zone earthquake is thought to build up in the deep portion of the fault where the two plates are “locked.” The shallow portion of the fault was not expected to accumulate a large amount of stress and was considered unlikely to produce a large amount of slip. The JFAST results show that the frictional stress on the shallow portion of the fault was very low during the earthquake, which means that either the stress was low to begin with or all of the stress was released during the earthquake.

“It’s probably a combination of both–the fault was pretty slippery to begin with, and whatever stress was on the fault at that shallow depth was all released during the earthquake,” Fulton said.

An earlier paper by JFAST researchers, published in Science in February 2013 (Lin et al.), also suggested a nearly total stress drop during the earthquake based on an analysis of geophysical data collected during drilling.

“We now have four lines of evidence that frictional stress was low during the earthquake,” Brodsky said. “The key measure is temperature, but those results are totally consistent with the other papers.”

One of the new papers (Ujiie et al.) presents the results of laboratory experiments on the material recovered from the fault zone. Tests showed very low shear stress (resistance to slip) attributable to the abundance of weak, slippery clay material. The other paper (Chester et al.) focuses on the geology and structure of the fault zone. In addition to the high clay content, the researchers found that the fault zone was surprisingly thin (less than 5 meters thick).

J. Casey Moore, a research professor of Earth sciences at UCSC and coauthor of the Chester et al. paper, said he suspects the clay layer observed in the Tohoku fault zone may play an important role in other fault zones. “Looking for something like that clay may give us a tool to understand the locations of earthquakes that cause tsunamis. It’s potentially a predictive tool,” Moore said.

According to Brodsky, measuring the frictional forces on the fault is the key to a fundamental understanding of earthquake mechanics. “We’ve been hamstrung without in situ measurements of frictional stress, and we now have that from the temperature data,” she said. “It’s hard to say how generalizable these results are until we look at other faults, but this lays the foundation for a better understanding of earthquakes and, ultimately, a better ability to identify earthquake hazards.”

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.

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.”

Rising mountains, cooling oceans prompted spread of invasive species 450 million years ago

This slab of rock contains fossils of invasive species that populated the continent of Laurentia 450 million years ago after a major ecological shift occurred. Ohio University geologists found that rising mountains and cooling oceans prompted the spread of these invasive species. -  Alycia Stigall
This slab of rock contains fossils of invasive species that populated the continent of Laurentia 450 million years ago after a major ecological shift occurred. Ohio University geologists found that rising mountains and cooling oceans prompted the spread of these invasive species. – Alycia Stigall

New Ohio University research suggests that the rise of an early phase of the Appalachian Mountains and cooling oceans allowed invasive species to upset the North American ecosystem 450 million years ago.

The study, published recently in the journal PLOS ONE, took a closer look at a dramatic ecological shift captured in the fossil record during the Ordovician period. Ohio University scientists argue that major geological developments triggered evolutionary changes in the ancient seas, which were dominated by organisms such as brachiopods, corals, trilobites and crinoids.

During this period, North America was part of an ancient continent called Laurentia that sat near the equator and had a tropical climate. Shifting of the Earth’s tectonic plates gave rise to the Taconic Mountains, which were forerunners of the Appalachian Mountains. The geological shift left a depression behind the mountain range, flooding the area with cool water from the surrounding deep ocean.

Scientists knew that there was a massive influx of invasive species into this ocean basin during this time period, but didn’t know where the invaders came from or how they got a foothold in the ecosystem, said Alycia Stigall, an Ohio University associate professor of geological sciences who co-authored the paper with former Ohio University graduate student David Wright, now a doctoral student at Ohio State University.

“The rocks of this time record a major oceanographic shift, pulse of mountain building and a change in evolutionary dynamics coincident with each other,” Stigall said. “We are interested in examining the interactions between these factors.”

Using the fossils of 53 species of brachiopods that dominated the Laurentian ecosystem, Stigall and Wright created several phylogenies, or trees of reconstructed evolutionary relationships, to examine how individual speciation events occurred.

The invaders that proliferated during this time period were species within the groups of animals that inhabited Laurentia, Stigall explained. Within the brachiopods, corals and cephalopods, for example, some species are invasive and some are not.

As the geological changes slowly played out over the course of a million years, two patterns of survival emerged, the scientists report.

During the early stage of mountain building and ocean cooling, the native organisms became geographically divided, slowly evolving into different species suited for these niche habitats. This process, called vicariance, is the typical method by which new species originate on Earth, Stigall said.

As the geological changes progressed, however, species from other regions of the continent began to directly invade habitats, a process called dispersal. Although biodiversity may initially increase, this process decreases biodiversity in the long term, Stigall explained, because it allows a few aggressive species to populate many sites quickly, dominating those ecosystems.

This is the second time that Stigall and her team have found this pattern of speciation in the geological record. A study published in 2010 on the invasive species that prompted a mass extinction during the Devonian period about 375 million years ago also discovered a shift from vicariance to dispersal that contributed to a decline in biodiversity, Stigall noted.

It’s a pattern that’s happening during our modern biodiversity crisis as well, she said.

“Only one out of 10 invaders truly become invasive species. Understanding the process can help determine where to put conservation resources,” she said.