Great earthquakes, water under pressure, high risk

The largest earthquakes occur where oceanic plates move beneath continents. Obviously, water trapped in the boundary between both plates has a dominant influence on the earthquake rupture process. Analyzing the great Chile earthquake of February, 27th, 2010, a group of scientists from the GFZ German Research Centre for Geosciences and from Liverpool University found that the water pressure in the pores of the rocks making up the plate boundary zone takes the key role (Nature Geoscience, 28.03.2014).

The stress build-up before an earthquake and the magnitude of subsequent seismic energy release are substantially controlled by the mechanical coupling between both plates. Studies of recent great earthquakes have revealed that the lateral extent of the rupture and magnitude of these events are fundamentally controlled by the stress build-up along the subduction plate interface. Stress build-up and its lateral distribution in turn are dependent on the distribution and pressure of fluids along the plate interface.

“We combined observations of several geoscience disciplines – geodesy, seismology, petrology. In addition, we have a unique opportunity in Chile that our natural observatory there provides us with long time series of data,” says Onno Oncken, director of the GFZ-Department “Geodynamics and Geomaterials”. Earth observation (Geodesy) using GPS technology and radar interferometry today allows a detailed mapping of mechanical coupling at the plate boundary from the Earth’s surface. A complementary image of the rock properties at depth is provided by seismology. Earthquake data yield a high resolution three-dimensional image of seismic wave speeds and their variations in the plate interface region. Data on fluid pressure and rock properties, on the other hand, are available from laboratory measurements. All these data had been acquired shortly before the great Chile earthquake of February 2010 struck with a magnitude of 8.8.

“For the first time, our results allow us to map the spatial distribution of the fluid pressure with unprecedented resolution showing how they control mechanical locking and subsequent seismic energy release”, explains Professor Oncken. “Zones of changed seismic wave speeds reflect zones of reduced mechanical coupling between plates”. This state supports creep along the plate interface. In turn, high mechanical locking is promoted in lower pore fluid pressure domains. It is these locked domains that subsequently ruptured during the Chile earthquake releasing most seismic energy causing destruction at the Earth’s surface and tsunami waves. The authors suggest the spatial pore fluid pressure variations to be related to oceanic water accumulated in an altered oceanic fracture zone within the Pacific oceanic plate. Upon subduction of the latter beneath South America the fluid volumes are released and trapped along the overlying plate interface, leading to increasing pore fluid pressures. This study provides a powerful tool to monitor the physical state of a plate interface and to forecast its seismic potential.

Great earthquakes, water under pressure, high risk

The largest earthquakes occur where oceanic plates move beneath continents. Obviously, water trapped in the boundary between both plates has a dominant influence on the earthquake rupture process. Analyzing the great Chile earthquake of February, 27th, 2010, a group of scientists from the GFZ German Research Centre for Geosciences and from Liverpool University found that the water pressure in the pores of the rocks making up the plate boundary zone takes the key role (Nature Geoscience, 28.03.2014).

The stress build-up before an earthquake and the magnitude of subsequent seismic energy release are substantially controlled by the mechanical coupling between both plates. Studies of recent great earthquakes have revealed that the lateral extent of the rupture and magnitude of these events are fundamentally controlled by the stress build-up along the subduction plate interface. Stress build-up and its lateral distribution in turn are dependent on the distribution and pressure of fluids along the plate interface.

“We combined observations of several geoscience disciplines – geodesy, seismology, petrology. In addition, we have a unique opportunity in Chile that our natural observatory there provides us with long time series of data,” says Onno Oncken, director of the GFZ-Department “Geodynamics and Geomaterials”. Earth observation (Geodesy) using GPS technology and radar interferometry today allows a detailed mapping of mechanical coupling at the plate boundary from the Earth’s surface. A complementary image of the rock properties at depth is provided by seismology. Earthquake data yield a high resolution three-dimensional image of seismic wave speeds and their variations in the plate interface region. Data on fluid pressure and rock properties, on the other hand, are available from laboratory measurements. All these data had been acquired shortly before the great Chile earthquake of February 2010 struck with a magnitude of 8.8.

“For the first time, our results allow us to map the spatial distribution of the fluid pressure with unprecedented resolution showing how they control mechanical locking and subsequent seismic energy release”, explains Professor Oncken. “Zones of changed seismic wave speeds reflect zones of reduced mechanical coupling between plates”. This state supports creep along the plate interface. In turn, high mechanical locking is promoted in lower pore fluid pressure domains. It is these locked domains that subsequently ruptured during the Chile earthquake releasing most seismic energy causing destruction at the Earth’s surface and tsunami waves. The authors suggest the spatial pore fluid pressure variations to be related to oceanic water accumulated in an altered oceanic fracture zone within the Pacific oceanic plate. Upon subduction of the latter beneath South America the fluid volumes are released and trapped along the overlying plate interface, leading to increasing pore fluid pressures. This study provides a powerful tool to monitor the physical state of a plate interface and to forecast its seismic potential.

Ancient Indonesian climate shift linked to glacial cycle

Using sediments from a remote lake, researchers from Brown University have assembled a 60,000-year record of rainfall in central Indonesia. The analysis reveals important new details about the climate history of a region that wields a substantial influence on the global climate as a whole.

The Indonesian archipelago sits in the Indo-Pacific Warm Pool, an expanse of ocean that supplies a sizable fraction of the water vapor in Earth’s atmosphere and plays a role in propagating El Niño cycles. Despite the region’s importance in the global climate system, not much is known about its own climate history, says James Russell, associate professor of geological sciences at Brown.

“We wanted to assess long-term climate variation in the region,” Russell said, “not just to assess how global climate influences Indonesia, but to see how that feeds back into the global climate system.”

The data are published this week in the Proceedings of the National Academy of Sciences.

The study found that the region’s normally wet, tropical climate was interrupted by a severe dry period from around 33,000 years ago until about 16,000 years ago. That period coincides with peak of the last ice age, when glaciers covered vast swaths of the northern hemisphere. Climate models had suggested that glacial ice could shift the track of tropical monsoons, causing an Indonesian dry period. But this is the first hard data to show that was indeed the case.

It’s also likely, Russell and his colleagues say, that the drying in Indonesia created a feedback loop that amplified ice age cooling.

“A very large fraction of the Earth’s water vapor comes from evaporation of the ocean around Indonesia, and water vapor is the Earth’s most important greenhouse gas,” Russell said. “As you start varying the hydrological cycle of Indonesia, you almost have to vary the Earth’s water vapor concentration. If you reduce the water vapor content it should cool the climate globally. So the fact that we have this very strong drying in the tropics during glaciation would argue for a strong feedback of water vapor concentration to the global climate during glacial-interglacial cycles.”

Surprisingly absent from the data, Russell says, is the influence of other processes known to drive climate elsewhere in the tropics. In particular, there was no sign of climate change in Indonesia associated with Earth’s orbital precession, a wobble caused by Earth’s axis tilt that generates differences in sunlight in a 21,000-year cycle.

“There’s very little indication of the 21,000-year cycle that dominates much of the tropics,” he said. “Instead we see this very big set of changes that appear linked to the amount of ice on earth.”

To arrive at those conclusions, the researchers used sediment cores from Lake Towuti, an ancient lake on the island of Sulawesi in central Indonesia. By looking at how concentrations of chemical elements in the sediment change with depth, the researchers can develop a continuous record of how much surface runoff poured into the lake. The rate of runoff is directly related to the rate of rainfall.

In this case, Russell and his colleagues looked at titanium, an element commonly used to gauge surface runoff. They found a marked dip in titanium levels in sediments dated to between 33,000 and 16,000 years ago – a strong indicator that surface runoff slowed during that period.

That finding was buttressed by another proxy of rainfall: carbon isotopes from plant leaf wax. Leaves are covered with a carbon-based wax that protects them from losing too much water to evaporation. Different plants have different carbon isotopes in their leaf wax. Tropical grasses, which are adapted for dryer climates, tend to have the C-13 isotope. Trees, which thrive in wetter environs, use the C-12 isotope. The ratio of those two isotopes in the sediment cores is an indicator of the relative abundance of grass versus trees.

The cores showed an increase in abundance of grass in the same sediments that showed a decrease in surface runoff. Taken together, the results suggest a dry period strong enough to alter the region’s vegetation that was closely correlated with the peak glaciation in the northern hemisphere.

The next step for Russell and his colleagues is to see if this pattern is repeated in multiple glacial cycles. Glacial periods run on cycles of about 100,000 years. Core samples from deeper in the Lake Towuti sediment will show whether this drying evident during the last ice age also happened in previous ice ages. It’s estimated that Lake Tuwuti sediments record up to 800,000 years of climate data, and Russell recently received funding to take deeper cores.

Ultimately, Russell hopes his work will help to predict how the region might be influenced by human-forced global warming.

“This provides the kind of fundamental data we need to understand how the climate of this region operates on long timescales,” he said. “That can then anchor our understanding of how it might respond to global warming.”

Ancient Indonesian climate shift linked to glacial cycle

Using sediments from a remote lake, researchers from Brown University have assembled a 60,000-year record of rainfall in central Indonesia. The analysis reveals important new details about the climate history of a region that wields a substantial influence on the global climate as a whole.

The Indonesian archipelago sits in the Indo-Pacific Warm Pool, an expanse of ocean that supplies a sizable fraction of the water vapor in Earth’s atmosphere and plays a role in propagating El Niño cycles. Despite the region’s importance in the global climate system, not much is known about its own climate history, says James Russell, associate professor of geological sciences at Brown.

“We wanted to assess long-term climate variation in the region,” Russell said, “not just to assess how global climate influences Indonesia, but to see how that feeds back into the global climate system.”

The data are published this week in the Proceedings of the National Academy of Sciences.

The study found that the region’s normally wet, tropical climate was interrupted by a severe dry period from around 33,000 years ago until about 16,000 years ago. That period coincides with peak of the last ice age, when glaciers covered vast swaths of the northern hemisphere. Climate models had suggested that glacial ice could shift the track of tropical monsoons, causing an Indonesian dry period. But this is the first hard data to show that was indeed the case.

It’s also likely, Russell and his colleagues say, that the drying in Indonesia created a feedback loop that amplified ice age cooling.

“A very large fraction of the Earth’s water vapor comes from evaporation of the ocean around Indonesia, and water vapor is the Earth’s most important greenhouse gas,” Russell said. “As you start varying the hydrological cycle of Indonesia, you almost have to vary the Earth’s water vapor concentration. If you reduce the water vapor content it should cool the climate globally. So the fact that we have this very strong drying in the tropics during glaciation would argue for a strong feedback of water vapor concentration to the global climate during glacial-interglacial cycles.”

Surprisingly absent from the data, Russell says, is the influence of other processes known to drive climate elsewhere in the tropics. In particular, there was no sign of climate change in Indonesia associated with Earth’s orbital precession, a wobble caused by Earth’s axis tilt that generates differences in sunlight in a 21,000-year cycle.

“There’s very little indication of the 21,000-year cycle that dominates much of the tropics,” he said. “Instead we see this very big set of changes that appear linked to the amount of ice on earth.”

To arrive at those conclusions, the researchers used sediment cores from Lake Towuti, an ancient lake on the island of Sulawesi in central Indonesia. By looking at how concentrations of chemical elements in the sediment change with depth, the researchers can develop a continuous record of how much surface runoff poured into the lake. The rate of runoff is directly related to the rate of rainfall.

In this case, Russell and his colleagues looked at titanium, an element commonly used to gauge surface runoff. They found a marked dip in titanium levels in sediments dated to between 33,000 and 16,000 years ago – a strong indicator that surface runoff slowed during that period.

That finding was buttressed by another proxy of rainfall: carbon isotopes from plant leaf wax. Leaves are covered with a carbon-based wax that protects them from losing too much water to evaporation. Different plants have different carbon isotopes in their leaf wax. Tropical grasses, which are adapted for dryer climates, tend to have the C-13 isotope. Trees, which thrive in wetter environs, use the C-12 isotope. The ratio of those two isotopes in the sediment cores is an indicator of the relative abundance of grass versus trees.

The cores showed an increase in abundance of grass in the same sediments that showed a decrease in surface runoff. Taken together, the results suggest a dry period strong enough to alter the region’s vegetation that was closely correlated with the peak glaciation in the northern hemisphere.

The next step for Russell and his colleagues is to see if this pattern is repeated in multiple glacial cycles. Glacial periods run on cycles of about 100,000 years. Core samples from deeper in the Lake Towuti sediment will show whether this drying evident during the last ice age also happened in previous ice ages. It’s estimated that Lake Tuwuti sediments record up to 800,000 years of climate data, and Russell recently received funding to take deeper cores.

Ultimately, Russell hopes his work will help to predict how the region might be influenced by human-forced global warming.

“This provides the kind of fundamental data we need to understand how the climate of this region operates on long timescales,” he said. “That can then anchor our understanding of how it might respond to global warming.”

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.

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.

The causes and consequences of global climate warming that took place 56 million years ago studied

This image shows continental sediments in the Esplugafreda ravine, a small tributary of the Noguera Ribagorzana river, in the extreme west of the province of Lleida and close to the village of Aren (Huesca). -  UPV/EHU-University of the Basque Country
This image shows continental sediments in the Esplugafreda ravine, a small tributary of the Noguera Ribagorzana river, in the extreme west of the province of Lleida and close to the village of Aren (Huesca). – UPV/EHU-University of the Basque Country

The growing and justified concern about the current global warming process has kindled the interest of the scientific community in geological records as an archive of crucial information to understand the physical and ecological effects of ancient climate changes. A study by the UPV/EHU’s Palaeogene Study Group deals with the behaviour of the sea level during the Palaeocene-Eocene Thermal Maximum (PETM) 56 million years ago and has ruled out any connection. The study has been published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

“The fall in sea level did not unleash the emission of greenhouse gases during the Palaeocene-Eocene Thermal Maximum (PETM),” pointed out Victoriano Pujalte, lecturer in the UPV/EHU’s Department of Stratigraphy and Palaeontology, and lead researcher of the study.

The Palaeocene-Eocene Thermal Maximum (PETM) was a brief interval (in geological terms, it “only” lasted about 200,000 years) of extremely high temperatures that took place 56 million years ago as a result of a massive emission of greenhouse gases into the atmosphere. The global temperature increase is reckoned to have been between 5º C and 9º C. It was recorded in geological successions worldwide and was responsible for a great ecological impact: the most striking from an anthropological point of view was its impact on mammals, but it also affected other organisms, including foraminifera and nannofossils (marine microorganisms that are at the base of the trophic chain) and plants.

However, what actually caused this warming remains a controversial issue. The most widely accepted hypothesis suggests that it was due to the destabilising of methane hydrates that remained frozen on ocean floors. “Some authors, like Higgins and Schrag (2006), for example, proposed that a fall in sea level could have caused or co-contributed towards the unleashing of the emission of methane or CO2,” pointed out Victoriano Pujalte, lecturer in the UPV/EHU’s Department of Stratigraphy and Palaeontology, and lead researcher in the study. According to this hypothesis, “the marine sediments that were submerged in the sea were exposed when the sea level fell, and were responsible for the CO2 emissions,” he added. That is what, to a certain extent, prompted this study. Others not only reject that possibility but also the fall in sea level itself. “We set out to try and establish the behaviour of the sea level during that time interval, the PETM,” said Pujalte.

There is no cause-effect relationship

The studies were carried out mainly in the Pyrenees between Huesca and Lérida, specifically in the Tremp-Graus Basin, and also in Zumaia (Gipuzkoa, Basque Country). The Palaeocene-Eocene rocks have outcropped extensively in both areas, in other words, exposed on the surface, and they represent a whole range of ancient atmospheres, both continental and marine. “They provide a unique opportunity to explore the effects of changes in sea level and to analyse their effects,” added Pujalte.

The most useful indicators are the stable oxygen and carbon isotopes. The oxygen ones provide information on palaeotemperatures, but any sign of them can only be retrieved in deep-sea sample cores. The carbon isotopes provide data on variations in CO2 content in the atmosphere and in the oceans, and they can also be retrieved in ancient rocks that have outcropped in above-ground plots of land. In general, the variations of both isotopes run parallel, given that an increase in the proportion of CO2 is coupled with an increase in temperature.

The results obtained indicate that the PETM was in fact preceded by a fall in sea level, the size of which is estimated to have been about 20 metres and the maximum descent of which probably occurred about 75 million years before the start of the PETM. “However, it is doubtful that the descent was the cause of the PETM, although it could have contributed towards it,” pointed out Victoriano Pujalte. “They occurred at the same time, but there is no cause-effect relationship.”

Furthermore, the researchers observed that the rise in the sea level continued after the PETM, when the global temperature returned to normal levels. “Its origin was not only caused, therefore, by the thermal expansion of the oceans linked to the warming,” said Pujalte. “It is suggested that the most likely cause of it was the volcanic activity documented in the North Sea during the end of the Palaeocene and start of the Eocene; this activity was related to the expansion of the oceanic ridge in the North Atlantic,” he concluded.

The causes and consequences of global climate warming that took place 56 million years ago studied

This image shows continental sediments in the Esplugafreda ravine, a small tributary of the Noguera Ribagorzana river, in the extreme west of the province of Lleida and close to the village of Aren (Huesca). -  UPV/EHU-University of the Basque Country
This image shows continental sediments in the Esplugafreda ravine, a small tributary of the Noguera Ribagorzana river, in the extreme west of the province of Lleida and close to the village of Aren (Huesca). – UPV/EHU-University of the Basque Country

The growing and justified concern about the current global warming process has kindled the interest of the scientific community in geological records as an archive of crucial information to understand the physical and ecological effects of ancient climate changes. A study by the UPV/EHU’s Palaeogene Study Group deals with the behaviour of the sea level during the Palaeocene-Eocene Thermal Maximum (PETM) 56 million years ago and has ruled out any connection. The study has been published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

“The fall in sea level did not unleash the emission of greenhouse gases during the Palaeocene-Eocene Thermal Maximum (PETM),” pointed out Victoriano Pujalte, lecturer in the UPV/EHU’s Department of Stratigraphy and Palaeontology, and lead researcher of the study.

The Palaeocene-Eocene Thermal Maximum (PETM) was a brief interval (in geological terms, it “only” lasted about 200,000 years) of extremely high temperatures that took place 56 million years ago as a result of a massive emission of greenhouse gases into the atmosphere. The global temperature increase is reckoned to have been between 5º C and 9º C. It was recorded in geological successions worldwide and was responsible for a great ecological impact: the most striking from an anthropological point of view was its impact on mammals, but it also affected other organisms, including foraminifera and nannofossils (marine microorganisms that are at the base of the trophic chain) and plants.

However, what actually caused this warming remains a controversial issue. The most widely accepted hypothesis suggests that it was due to the destabilising of methane hydrates that remained frozen on ocean floors. “Some authors, like Higgins and Schrag (2006), for example, proposed that a fall in sea level could have caused or co-contributed towards the unleashing of the emission of methane or CO2,” pointed out Victoriano Pujalte, lecturer in the UPV/EHU’s Department of Stratigraphy and Palaeontology, and lead researcher in the study. According to this hypothesis, “the marine sediments that were submerged in the sea were exposed when the sea level fell, and were responsible for the CO2 emissions,” he added. That is what, to a certain extent, prompted this study. Others not only reject that possibility but also the fall in sea level itself. “We set out to try and establish the behaviour of the sea level during that time interval, the PETM,” said Pujalte.

There is no cause-effect relationship

The studies were carried out mainly in the Pyrenees between Huesca and Lérida, specifically in the Tremp-Graus Basin, and also in Zumaia (Gipuzkoa, Basque Country). The Palaeocene-Eocene rocks have outcropped extensively in both areas, in other words, exposed on the surface, and they represent a whole range of ancient atmospheres, both continental and marine. “They provide a unique opportunity to explore the effects of changes in sea level and to analyse their effects,” added Pujalte.

The most useful indicators are the stable oxygen and carbon isotopes. The oxygen ones provide information on palaeotemperatures, but any sign of them can only be retrieved in deep-sea sample cores. The carbon isotopes provide data on variations in CO2 content in the atmosphere and in the oceans, and they can also be retrieved in ancient rocks that have outcropped in above-ground plots of land. In general, the variations of both isotopes run parallel, given that an increase in the proportion of CO2 is coupled with an increase in temperature.

The results obtained indicate that the PETM was in fact preceded by a fall in sea level, the size of which is estimated to have been about 20 metres and the maximum descent of which probably occurred about 75 million years before the start of the PETM. “However, it is doubtful that the descent was the cause of the PETM, although it could have contributed towards it,” pointed out Victoriano Pujalte. “They occurred at the same time, but there is no cause-effect relationship.”

Furthermore, the researchers observed that the rise in the sea level continued after the PETM, when the global temperature returned to normal levels. “Its origin was not only caused, therefore, by the thermal expansion of the oceans linked to the warming,” said Pujalte. “It is suggested that the most likely cause of it was the volcanic activity documented in the North Sea during the end of the Palaeocene and start of the Eocene; this activity was related to the expansion of the oceanic ridge in the North Atlantic,” he concluded.

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.

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.