Catastrophic Flooding Changes The Course Of British History





Three dimensional perspective view showing details of the Channel valley (pink is shallow; blue is deep).  The erosional scours carved into floor of the valley are clearly evicent. - Image Credit: Imperial College London
Three dimensional perspective view showing details of the Channel valley (pink is shallow; blue is deep). The erosional scours carved into floor of the valley are clearly evicent. – Image Credit: Imperial College London

A catastrophic megaflood separated Britain from France hundreds of thousands of years ago, changing the course of British history, according to research published in the journal Nature today.



The study, led by Dr Sanjeev Gupta and Dr Jenny Collier from Imperial College London, has revealed spectacular images of a huge valley tens of kilometres wide and up to 50 metres deep carved into chalk bedrock on the floor of the English Channel.



Using high-resolution sonar waves the team captured images of a perfectly preserved submerged world in the channel basin. The maps highlight deep scour marks and landforms which were created by torrents of water rushing over the exposed channel basin.



To the north of the channel basin was a lake which formed in the area now known as the southern North Sea. It was fed by the Rhine and Thames, impounded to the north by glaciers and dammed to the south by the Weald-Artois chalk ridge which spanned the Dover Straits. It is believed that a rise in the lake level eventually led to a breach in the Weald-Artois ridge, carving a massive valley along the English Channel, which was exposed during a glacial period.



At its peak, it is believed that the megaflood could have lasted several months, discharging an estimated one million cubic metres of water per second. This flow was one of the largest recorded megafloods in history and could have occurred 450,000 to 200,000 years ago.



The researchers believe the breach of the ridge, and subsequent flooding, reorganised the river drainages in north-west Europe by re-routing the combined Rhine-Thames River through the English Channel to form the Channel River.


The breach and permanent separation of the UK also affected patterns of early human occupation in Britain. Researchers speculate that the flooding induced changes in topography creating barriers to migration which led to a complete absence of humans in Britain 100,000 years ago.



Dr Sanjeev Gupta, from the Department of Earth Science & Engineering at Imperial said: “This prehistoric event rewrites the history of how the UK became an island and may explain why early human occupation of Britain came to an abrupt halt for almost 120 thousand years.”



Project collaborator, Dr Jenny Collier, also from the Department of Earth Science & Engineering, speculates on the potential for future discoveries on the continental shelves.



“The preservation of the landscape on the floor of the English Channel, which is now 30-50 m below sea-level, is far better than anyone would have expected. It opens the way to discover a host of processes that shaped the development of north-west Europe during the past million years or so,” said Dr Collier.



The Imperial research team collaborated with the UK Hydrographic Office and the Maritime Coastguard Agency (MCA) on the project. Data collected by the MCA and archived by the Hydrographic Office was originally sourced for civil safety at sea.

Glaciers and Ice Caps to Dominate Sea Level Rise Through 21st Century





When a glacier with its “toe in the water” thins, a larger fraction of its weight is supported by water and it slides faster and calves more ice into the ocean at the glacier terminus. – Photo Credit: Nicolle Rager Fuller, National Science Foundation

Ice loss from glaciers and ice caps is expected to cause more global sea rise during this century than the massive Greenland and Antarctic ice sheets, according to a new University of Colorado at Boulder study.



The researcher, primarily funded by the National Science Foundation (NSF) and NASA, concluded that glaciers and ice caps are currently contributing about 60 percent of the world’s ice to the oceans and the rate has been markedly accelerating in the past decade, said Emeritus Professor Mark Meier of CU-Boulder’s Institute of Arctic and Alpine Research, lead study author. The contribution is presently about 100 cubic miles of ice annually — a volume nearly equal to the water in Lake Erie — and is rising by about three cubic miles per year.



In contrast, the CU-Boulder team estimated Greenland is now contributing about 28 percent of the total global sea rise from ice loss and Antarctica is contributing about 12 percent. Greenland is not expected to catch up to glaciers and ice caps in terms of sea level rise contributions until the end of the century, according to the study.



A paper on the subject appears in the July 19 issue of Science Express, the online edition of Science magazine. Co-authors include CU-Boulder INSTAAR researchers Mark Dyurgerov, Ursula Rick, Shad O’Neel, Tad Pfeffer, Robert Anderson and Suzanne Anderson, as well as Russian Academy of Sciences scientist Andrey Glazovsky.



“One reason for this study is the widely held view that the Greenland and Antarctic ice sheets will be the principal causes of sea-level rise,” said Meier, former INSTAAR director and professor in geological sciences. “But we show that it is the glaciers and ice caps, not the two large ice sheets, that will be the big players in sea rise for at least the next few generations.”



The accelerating contribution of glaciers and ice caps is due in part to rapid changes in the flow of tidewater glaciers that discharge icebergs directly into the ocean, said the study. Many tidewater glaciers are undergoing rapid thinning, stretching and retreat, which causes them to speed up and deliver increased amounts of ice into the world’s oceans, said CU-Boulder geology Professor Robert Anderson, study co-author.



Water controls how rapidly glaciers slide along their beds, said Anderson. When a glacier with its “toe in the water” thins, a larger fraction of its weight is supported by water and it slides faster and calves more ice into the ocean at the glacier terminus.



“While this is a dynamic, complex process and does not seem to be a direct result of climate warming, it is likely that climate acts as a trigger to set off this dramatic response,” said Anderson, also an INSTAAR researcher.



The human impact of this accelerated sea level rise could be dramatic. The team estimated accelerating melt of glaciers and ice caps could add from 4 inches to 9.5 inches of additional sea level rise globally by 2100. This does not include the expansion of warming ocean water, which could potentially double those numbers. A one-foot sea-level rise typically causes a shoreline retreat of 100 feet or more. The World Bank estimates that about 100 million people now live within about three feet of sea level.


“At the very least, our projections indicate that future sea-level rise may be larger than anticipated, and that the component due to glaciers and ice caps will continue to be substantial,” wrote the researchers in Science Express.



The team summarized satellite, aircraft and ground-based data from glaciers, ice caps, the Greenland ice sheet, the West Antarctic ice sheet and the East Antarctic ice sheet to calculate present and future rates of ice loss for the study.



Meier estimated there are several hundred thousand small glaciers and small, pancake-shaped ice caps in polar and temperate regions. They range from modest, high mountain glaciers to huge glaciers like the Bering Glacier in Alaska, which measures about 5,000 square miles in area and is nearly one-half mile thick in places.



The researchers used a mathematical “scaling” process to estimate more remote glacier volumes, thicknesses and trends by factoring in data like altitude, climate and geography. They used data gathered from around the world, including cold regions in Russia, Europe, China, Central Asia, Canada and South America.



While warming temperatures will likely cause many small high mountain glaciers in North America Europe to disappear by the end of the century, large ice fields and ice caps will continue to produce large amounts of melt water, Meier said. The scientists also believe many “cold” polar glaciers and ice caps will soon warm up enough to begin melting and contributing to sea rise.



The retreat of the Greenland and Antarctic ice sheets also is giving birth to new, smaller glaciers that are prime candidates for study by scientists. “It is incorrect to assume that the small glaciers will simply go away next century — they will continue to play a key role in the sea level story,” said Anderson.



Anderson also said that although the volume of ice locked up in Greenland is equal to roughly 23 feet in sea rise, only a small fraction is likely to be “pulled out” during the next century, most of it through outlet glaciers.



Many smaller “benchmark” glaciers around the world that have been under study for decades are expected to disappear by the end of the century, said Anderson. “We need to start gathering benchmark information on some of the larger glaciers that are unlikely to disappear, so that we can have a long-term record of their behavior.”



Anderson said outlet glaciers in Greenland behave much like tidewater glaciers in Canada and Alaska, making them very relevant for long-term study. “Since the world is becoming increasingly aware that sea-level rise is a very real problem, we need to acknowledge the role of all of the ice masses and understand the physical mechanisms by which they deliver water to the sea.”

Geologists Witness Unique Volcanic Mudflow in Action in New Zealand





This image of the lahar channel shows the area right after the collapse of New Zealand's Crater Lake's walls. - Photo Credit: University of Hawaii
This image of the lahar channel shows the area right after the collapse of New Zealand’s Crater Lake’s walls. – Photo Credit: University of Hawaii

Volcanologist Sarah Fagents from the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaii at Manoa had an amazing opportunity to study volcanic hazards first hand, when a volcanic mudflow broke through the banks of a volcanic lake at Mount Ruapehu in New Zealand.



Fagents and colleagues were there on a National Science Foundation (NSF)-funded project to study the long-forecast Crater Lake break-out lahar at Mount Ruapehu. A lahar is a type of mudflow composed of water and other sediment that flows down from a volcano, typically along a river valley.



Lahars are caused by the rapid melting of snow and/or glaciers during a volcanic eruption, or as in the case of Mount Ruapehu, the breakout of a volcanic lake.



“Lahars can be extremely hazardous, especially in populated areas, because of their great speed and mass,” said William Leeman, NSF program director for petrology and geochemistry. “They can flow for many tens of miles, causing catastrophic destruction along their path. The 1980 eruptions at Mount St. Helens, for example, resulted in spectacular lahar flows that choked virtually all drainages on the volcano, and impacted major rivers as far away as Portland, Ore.”



Fagents visited stretches of the lahar pathway before the breakout to assess pre-event channel conditions. Although the event was predicted to occur in 2007, the recent decreased filling rate of Crater Lake suggested that the lake bank actually would not be overtopped until 2008.



However, several days of intense rainfall and increased seepage through the bank ultimately caused it to collapse much sooner, on March 18, 2007.


A lahar warning system had been installed at Mount Ruapehu, and was hailed a success after it successfully alerted officials to the onset of the lahar. In total, about 1.3 million cubic meters of water were released from Crater Lake.



“We found a broad area covered in a veneer of mud and boulders,” said Fagents. “It was an unprecedented opportunity to see the immediate aftermath of such an event. It’s particularly motivating for the students who were along to learn first-hand about lahar processes in such a dynamic environment.”



Fagents and colleagues returned to New Zealand a month later to conduct a more detailed analysis of the deposit. “Because the Crater Lake breakout had been long forecast, there was an unprecedented amount of instrumentation installed in the catchment by our New Zealand colleagues to capture the event,” says Fagents.



“The 2007 event is the best studied lahar in the world,” she said.



Prediction of the effects of the events is of critical importance in populated volcanic regions. Many other volcanoes around the world, including Mount Rainier in Washington State, and Galunggung in Indonesia, are also considered particularly dangerous due to the risk of lahars, according to Leeman.



Fagents is developing a computer model to simulate lahar emplacement and to predict the associated hazards. “The intent is to adapt this model to account for different lahar triggering mechanisms, and for different locations, to make it widely applicable,” said Fagents. “The ultimate goal is provide a useful hazards assessment tool for future lahars.”

Geoscientists Investigate Art Rock Movement






A St Andrews researcher is taking part in a major scientific investigation of the ancient Spanish rocks said to inspire the work of surrealist artist Salvador Dali.



Dr Ian Alsop has just returned from fieldwork analysing 500 million year old rocks along the rugged coastline forming the Costa Brava of North East Spain. In a case of art imitating science, the landscape displaying `spectacular and peculiar geometries’ provided the inspiration for some of Salvador Dali’s most famous art works. One of the great 20th century surrealists, Dali – who was born and lived in the area – was said to be inspired by the ‘unrivalled’ rocks at Cap de Creus in his surrealist masterpieces.



Dr Alsop, a senior lecturer at the University’s School of Geography & Geosciences, is collaborating with colleagues from the Universitat Autonoma de Barcelona in a detailed scientific investigation of the rocks and structure of the largely unspoiled area. They hope that the study will reveal new insights into the products and processes responsible for the evolution of the Earth’s crust.



He said, “The rocks were originally deposited about 500 million years ago, but were subsequently compressed and deformed during mountain building or ‘orogeny’ 300 million years ago. The rocks were squeezed into fantastically folded and sheared geometries, and were also injected with molten magma which subsequently cooled into spectacular outcrops.



“It is these strange and sometimes grotesque exposures that are considered to have provided the inspiration for some of the surreal shapes in Dali’s greatest masterpieces such as “The persistence of memory” (known as the ‘melting clocks’ painting), currently on display in the Tate Modern.”


The unrivalled landscape is due to a combination of rock types, waves and wind in the area that provides a unique quality of exposure – resulting in fantastic three-dimensional rock formations.



The rocks now exposed at Cap de Creus provide superb small-scale ‘analogues’ of the behaviour of the Earth’s crust when sedimentary basins and mountain belts are created. The rocks can also tell geoscientists much about the way the Earth behaves during mountain building, when continental masses move towards one another at about the rate fingernails grow.



Dr Alsop explained, “The rocks of Cap de Creus provide an opportunity to collect and analyse an unrivalled data set of folds and fractures. This allows us a perhaps unparalleled glimpse of the products and processes responsible for the evolution of the Earth’s crust.”



The unique geology and weathering patterns observed in Cap de Creus are recognised not just as an inspiration for artists, but also as a special landscape now protected in a national park. The ongoing research by Dr Alsop and colleagues in to the nature of the deformed rocks is funded by grants from the Carnegie Trust and the Spanish Ministry of Science.

Scientist Uncovers Earth’s Mysterious Layer


Laboratory measurements of a high-pressure mineral believed to exist deep within the Earth show that the mineral may not, as geophysicists hoped, have the right properties to explain a mysterious layer lying just above the planet’s core.



A team of scientists, led by Sébastien Merkel of the University of California-Berkeley, now at CNRS/the University of Science of Technology of Lille, France, made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite. The work appears in the June 22 issue of the scientific journal Science.



The team included Allen McNamara of ASU’s School of Earth and Space Exploration, part of the College of Liberal Arts and Sciences. McNamara, a geophysicist, modeled the stresses the mineral typically would undergo as convection currents deep in Earth’s mantle cause it to rise and sink. Also on the team were Atsushi Kubo and Thomas Duffy, Princeton University ; Sergio Speziale, Lowell Miyagi and Hans-Rudolf Wenk, University of California-Berkeley; and Yue Meng, HPCAT, Carnegie Institution of Washington, Argonne , Ill.



“This the first time the deformation properties of this mineral have been studied at lower mantle temperatures and pressures,” McNamara says. “The goal was to observe where the weak planes are in its crystal structure and how they are oriented.”



The results of the combined laboratory tests and computer models, he says, show that post-perovskite doesn’t fit what is known about conditions in the lowermost mantle.



Earth’s mantle is a layer that extends from the bottom of the crust, about 25 miles down, to the planet’s core, 1,800 miles deep. Scientists divide the mantle into two layers separated by a wide transition zone centered around a depth of about 300 miles. The lower mantle lies below that zone.



Most of Earth’s lower mantle is made of a magnesium silicate mineral called perovskite. In 2004, earth scientists discovered that under the conditions of the lower mantle, perovskite can change into a high-pressure form, which they dubbed post-perovskite. Since its discovery, post-perovskite has been geophysicists’ favorite candidate to explain the composition of a mysterious layer that forms the bottom of Earth’s lower mantle.



Known to earth scientists as D” (dee-double-prime), this layer averages 120 miles thick and lies directly above Earth’s core. D” was named in 1949 by seismologist Keith Bullen, who found the layer from the way earthquake waves travel through the planet’s interior. But the nature of D” has eluded scientists since Bullen’s discovery.



“Our team found that while post-perovskite has some properties that fit what’s known about D”, our laboratory measurements and computer models show that post-perovskite doesn’t fit one particular essential property,” McNamara says.


That property is seismic anisotropy, he says, referring to the fact that earthquake waves passing through D” become distorted in a characteristic way.



“Down in the D” layer, the horizontal part of earthquake waves travel faster than the vertical parts,” McNamara says. “But in our laboratory measurements and models, post-perovskite produces an opposite effect on the waves. This appears to be a basic contradiction.”



McNamara notes that the laboratory measurements, made by team members at Princeton University and at Berkeley, were extremely difficult. They involved crushing tiny samples of perovskite on a diamond anvil until they changed into post-perovskite. The scientists then shot X-rays through the samples to identify the mineral crystals’ internal structure.



This information was used by other team members at the University of California-Berkeley to model how these crystals would deform as the mantle flows. The deformation results let the scientists predict how the crystals would affect seismic waves passing through them.



McNamara’s work modeled the slow churn of the mantle, in which convection currents in the rock rise and fall about as fast as fingernails grow – roughly an inch a year. He calculated stresses, pressures and temperatures to draw a detailed picture of where post-perovskite would be found. This let him profile the structure of the D” layer.



“All these computations have been in two dimensions,” he says. “Our next step is to go to three-dimensional modeling.”



So does their work rule out post-perovskite to explain the D” layer?



“Not completely,” McNamara says. “We’ve begun to study this newly found mineral in the laboratory, but the work isn’t yet over. It’s possible that post-perovskite does exist in the lowermost mantle, and another mineral is causing the seismic anisotropy we see there.”

Geophysicists Detect Molten Rock Layer Deep Below American Southwest


A sheet of molten rock roughly 10 miles thick spreads underneath much of the American Southwest, some 250 miles below Tucson. From the surface, you can’t see it, smell it or feel it.



But Arizona geophysicists Daniel Toffelmier and James Tyburczy detected the molten layer with a comparatively new and overlooked technique for exploring deep within Earth that uses magnetic eruptions on the sun.



Toffelmier, a hydrogeologist with Hargis + Associates Inc. in Mesa, graduated from ASU’s School of Earth and Space Exploration in 2006 with a master’s degree in geological sciences. Tyburczy, a professor of geoscience in the school, was Toffelmier’s thesis adviser. Their findings, which grew out of Toffelmier’s thesis, are presented in the June 21 issue of the scientific journal Nature.



“We had two goals in this research,” Tyburczy says. “We wanted to test a hypothesis about what happens to rock in Earth’s mantle when it rises to a particular depth – and we also wanted to test a computer modeling technique for studying the deep Earth.



“Finding that sheet of melt-rock tells us we we’re on the right track.”


Deep squeeze



In 2003, two Yale University geoscientists published a hypothesis about the composition and physical state of rocks in the Earth’s mantle. They proposed that mantle rock rising through a depth of 410 kilometers (about 250 miles) would give up any water mixed into its crystal structure, and the rock then would melt.



“This idea is interesting and fairly controversial among geophysicists,” Tyburczy says. “So Dan and I thought we’d test it.”



Geophysicists often study the planet’s structure using earthquake waves, which are good at detecting changes in rock density. For example, seismic waves show that Earth’s density abruptly alters at particular depths. The biggest change, or discontinuity, comes at the core-mantle boundary, about 2,900 kilometers (1,800 miles) deep. Another lies at a depth of 660 kilometers (410 miles), while the third most-prominent discontinuity occurs 410 kilometers (250 miles) down.



But seismic waves don’t tell scientists much about rocks’ chemical makeup, or about minor elements they contain, or their various mineral phases. Scientists need a different method to study mantle rocks that change composition as they shed water at 410 kilometers’ depth and become partly molten in the process.



A geophysical survey technique sensitive to these factors is called magnetotellurics, or geomagnetic depth sounding.



“Basically, this method measures changes in rocks’ electrical conductivity at different depths,” Toffelmier says.



Calibrated by laboratory work, magnetotelluric methods permit scientists to estimate the composition of rocks they won’t ever be able to hold in their hands.



“Rocks are semiconductors,” Tyburczy says. “And rocks with more hydrogen embedded in their structure conduct better, as do rocks that are partially molten.”



A common source for hydrogen is water, which can lodge throughout a mineral’s crystal structure.



But how to measure the conductivity of rocks buried hundreds of miles underfoot? The answer lies 93 million miles away, on the surface of the sun.

Outsourcing



The sun emits a continuous flow of charged atomic particles called the solar wind. This varies in strength as activity on the sun rises and falls. When gusts of particles reach Earth, they induce changes in the planet’s magnetosphere, causing in turn weak, but measurable electrical currents to flow through terrestrial rocks deep inside Earth.



Toffelmier and Tyburczy used electromagnetic field data collected by others for five regions of Earth: the American Southwest, northern Canada, the French Alps, a regionally averaged Europe and the northern Pacific Ocean. Only these few data sets contained information gathered over a long-enough period to be useful in the computer modeling.



“The long-period waves tell you about deep events and features, while short-period ones resolve shallower features,” Tyburczy says.



He says to think of it like an inverted cone extending down into Earth. The deeper you go, the wider the area that’s sampled – and the coarser the resolution.



The modeling approach Toffelmier and Tyburczy used was to start with an initial guess as to rock composition at different depths, run the model, compare the results to the actual field data, and then alter the run’s starting point. As they worked, they found that only the data for the southwestern United States showed signs of a water-bearing melt layer at the 410-kilometer (250-mile) depth.



“Without a melt zone at that depth, we can’t match the field observations,” Toffelmier says.



But, adds Tyburczy, “when we added a highly conductive melt zone, five to 30 kilometers (three to 20 miles) thick, we got a much better fit.”



The extent of the melt sheet is unknown, however, because the data set is limited in area. There’s little chance that any molten rock from it would erupt at the surface, the researchers say.



Seismic surveys show the 410-kilometer discontinuity is global in scope. But Toffelmier and Tyburczy’s work shows that melting at the 410-kilometer depth is patchy at best, and far from global. So the Yale hypothesis remains only partly confirmed.



So what’s next?



“Our modeling has been only in one dimension,” Tyburczy says. “We need to start looking in two and three dimensions. We also need to understand better how rocks and minerals change at the incredible pressures deep inside the Earth.”



“We’ve seen only the tip of the iceberg,” Toffelmier says.