Measuring Snow With A Bucket, A Windmill, And The Sun? Government Goes Off The Power Grid In Maine

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ScienceDaily (May 9, 2009) — In Maine, government scientists have figured out how to measure snowfall in remote areas with a bucket, a small windmill, and the sun -- all the while saving money, energy, and, ultimately helping to save lives.

What led to this energy-efficient ingenuity was the need to help the National Weather Service forecast and predict the risk of floods from spring snowmelt.
The problem was this: While the USGS has about 15 snowmelt measurement sites in Maine, they also needed a way to measure snowfall in remote areas where power grids are scarce. Emergency managers need accurate information to prepare for forthcoming hazards and energy companies need to plan ahead for how much water to expect in reservoirs.
"We needed to find an alternative power source," said Bob Lent, chief of the USGS Maine Water Science Center in Augusta. "So we cobbled together a small-scale commercial windmill to replace commercial AC power, and supplemented the windmill with solar panels. What we ended up with is a windmill that powers our measurements on windy and cloudy days, and solar panels that power them on calm, sunny days," said Lent. "And," he added, "not only will we get more accurate information, but the systems will pay for themselves in about 3 to 4 years since using the electricity-dependent devices cost between $200 and $400 a year."
A prototype system has been housed in use at the USGS office in Augusta for the past winter. It has proved so accurate, said Lent, that the USGS plans to install four snowfall sites around the state this summer using the same system.
Basically, the system looks like this: a gage is attached to a 5-gallon bucket that sits atop a simple wooden platform on a metal pole. The gage has a heating element to melt the snow as it collects in the cone of the bucket. The gage only turns on when snow is detected. Nearby is a data-collection box that is linked to the windmill and solar panels. When the bucket fills up with melted snow it tips over and empties. Each tip of the bucket measures 0.01 inches of precipitation and is recorded to the data recorder, which transmits the data and is updated on the web every hour.
"We are very optimistic about the utility of this system in other remote areas in the country and not just for snowfall measurements. It would be good for any remote site that needs more power than solar alone can deliver. For example, this could be used to measure water quality in the swamps of Florida as well as snowfall in Maine," Lent noted.
"It's a very small step in a very long journey of helping this country become greener, but this embodies what we need to be doing and the direction in which we need to be going," said Lent.
Adapted from materials provided by U.S. Geological Survey.

Rise Of Oxygen Caused Earth's Earliest Ice Age

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ScienceDaily (May 7, 2009) — Geologists may have uncovered the answer to an age-old question - an ice-age-old question, that is. It appears that Earth's earliest ice ages may have been due to the rise of oxygen in Earth's atmosphere, which consumed atmospheric greenhouse gases and chilled the earth.

Alan J. Kaufman, professor of geology at the University of Maryland, Maryland geology colleague James Farquhar, and a team of scientists from Germany, South Africa, Canada, and the U.S.A., uncovered evidence that the oxygenation of Earth's atmosphere - generally known as the Great Oxygenation Event - coincided with the first widespread ice age on the planet.
"We can now put our hands on the rock library that preserves evidence of irreversible atmospheric change," said Kaufman. "This singular event had a profound effect on the climate, and also on life."
Using sulfur isotopes to determine the oxygen content of ~2.3 billion year-old rocks in the Transvaal Supergroup in South Africa, they found evidence of a sudden increase in atmospheric oxygen that broadly coincided with physical evidence of glacial debris, and geochemical evidence of a new world-order for the carbon cycle.
"The sulfur isotope change we recorded coincided with the first known anomaly in the carbon cycle. This may have resulted from the diversification of photosynthetic life that produced the oxygen that changed the atmosphere," Kaufman said.
Two and a half billion years ago, before the Earth's atmosphere contained appreciable oxygen, photosynthetic bacteria gave off oxygen that first likely oxygenated the surface of the ocean, and only later the atmosphere. The first formed oxygen reacted with iron in the oceans, creating iron oxides that settled to the ocean floor in sediments called banded iron-formations - layered deposits of red-brown rock that accumulated in ocean basins around the worldwide. Later, once the iron was used up, oxygen escaped from the oceans and started filling up the atmosphere.
Once oxygen made it into the atmosphere, Kaufman's team suggests that it reacted with methane, a powerful greenhouse gas, to form carbon dioxide, which is 62 times less effective at warming the surface of the planet. "With less warming potential, surface temperatures may have plummeted, resulting in globe-encompassing glaciers and sea ice" said Kaufman.
In addition to its affect on climate, the rise in oxygen stimulated the rise in stratospheric ozone, our global sunscreen. This gas layer, which lies between 12 and 30 miles above the surface, decreased the amount of damaging ultraviolet sunrays reaching the oceans, allowing photosynthetic organisms that previously lived deeper down, to move up to the surface, and hence increase their output of oxygen, further building up stratospheric ozone.
"New oxygen in the atmosphere would also have stimulated weathering processes, delivering more nutrients to the seas, and may have also pushed biological evolution towards eukaryotes, which require free oxygen for important biosynthetic pathways," said Kaufman.
The result of the Great Oxidation Event, according to Kaufman and his colleagues, was a complete transformation of Earth's atmosphere, of its climate, and of the life that populated its surface. The study is published in the May issue of Geology.
Adapted from materials provided by University of Maryland.

World's Most Unusual Volcano: Origin Of Carbon-based Lavas Revealed

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ScienceDaily (May 7, 2009) — Scientists studying the world's most unusual volcano have discovered the reason behind its unique carbon-based lavas. The new geochemical analyses reveals that an extremely small degree of partial melting of typical minerals in the earth's upper mantle is the source of the rare carbon-derived lava erupting from Tanzania's Oldoinyo Lengai volcano.

Although carbon-based lavas, known as carbonatites, are found throughout history, the Oldoinyo Lengai volcano, located in the East African Rift in northern Tanzania, is the only place on Earth where they are actively erupting. The lava expelled from the volcano is highly unusual in that it contains almost no silica and greater than 50 percent carbonate minerals. Typically lavas contain high levels of silica, which increases their melting point to above 900°C (1652°F). The lavas of Oldoinyo Lengai volcano erupt as a liquid at approximately 540°C (1004°F). This low silica content gives rise to the extremely fluid lavas, which resembles motor oil when they flow.
A team of scientists from University of New Mexico, Scripps Institution of Oceanography at UC San Diego and Centre de Recherches Petrographiques et Geochimiques in Nancy, France, report new findings of volcanic gas emissions in a paper published in the May 7 issue of the journal Nature.
"The chemistry and isotopic composition of the gases reveal that the CO2 is directly sourced from the upper mantle below the East African Rift," said David Hilton, professor of geochemistry at Scripps Institution of Oceanography at UC San Diego and coauthor of the paper. "These mantle gases allow us to infer the carbon content of the upper mantle that is producing the carbonatites to be around 300 parts per million, a concentration that is virtually identical to that measured below mid-ocean ridges."
Mid-ocean ridges are underwater mountain ranges where the seafloor is spreading due to tectonic plates moving away from one another. Rift valleys, such as the one where Oldoinyo Lengai volcano is located, and mid-ocean ridges are considered to be distinct tectonic regions. However, this study has shown that their chemistries are identical, which led the scientists to suggest that the carbon contents of their mantle sources were not different but due to partial melting of typical minerals located in the earth's mantle.
"Since the volcano was under magma pressure during the eruption, we were able to collect pristine samples of the volcanic gases, with minimal air contamination," said Tobias Fischer, volcanologist at the University of New Mexico. The pristine samples collected during a 2005 eruption offered the scientists a deeper look at the processes taking place in the earth's upper mantle.
The geochemical analyses, some of which were conducted at Hilton's geochemical lab at Scripps Oceanography, revealed that magma from the upper mantle below both the oceans and continents is a uniform and well-mixed reservoir of "typical" volcanic gases such as carbon dioxide, nitrogen, argon and helium.
The lava expelled from the volcano is highly unusual in that it contains almost no silica and greater than 50 percent carbonate minerals. Typically lavas contain high levels of silica, which increases their melting point to above 900°C (1652°F). The lavas of Oldoinyo Lengai volcano are comprised of carbonatites, which erupts as a liquid at approximately 540°C (1004°F). This low silica content gives rise to the extremely fluid lavas, which resembles motor oil when they flow.
"These finding are significant because it shows that these extremely bizarre lavas and their parent magmas, nephelinites, were produced by melting of a typical upper mantle mineral assemblage without an extreme carbon content in the magma source," said geochemist Bernard Marty at the Centre de Recherches Petrographiques et Geochimiques in Nancy, France. "Rather, in order to make carbonatite lavas, all you need is a very low melt fraction of 0.3 percent or less."
Oldoinyo Lengai, like all volcanoes, emits carbon dioxide into the atmosphere as a gas. However, Lengai's magma is unusual in that it also contains high sodium contents. About one percent of the mantle-derived carbon emitted from Lengai goes into the carbonatite melt with the remainder being emitted into the atmosphere as CO2 gas. The CO2 released into the atmosphere by volcanoes worldwide is a small fraction when compared to man-made emissions.
Journal reference:
Fischer et al. Upper-mantle volatile chemistry at Oldoinyo Lengai volcano and the origin of carbonatites. Nature, 2009; 459 (7243): 77 DOI: 10.1038/nature07977
Adapted from materials provided by University of California - San Diego, via EurekAlert!, a service of AAAS.

How Much Oil Have We Used?

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ScienceDaily (May 8, 2009) — Estimates of how much crude oil we have extracted from the planet vary wildly. Now, UK researchers have published a new estimate in the International Journal of Oil, Gas and Coal Technology that suggests we may have used more than we think.

The idea that we are running out of oil is not a new one, but do we even know how much oil we have extracted from since the first commercial oil wells were sunk in the middle of the nineteenth century? In 2008, chemists Istvan Lakatos and Julianna Lakatos-Szabo of the Hungarian Academy of Sciences theorised that less than 100 billion tonne of crude oil has been produced since 1850 and that the average annual production rate is less than 700 million barrels per year.
They compared proven reserves and estimates of yet-to-find (YTF) resources and echoed the sentiment that we will soon face oil shortages even though a substantial part of those reserves remain in the ground untapped.
Now, John Jones in the School of Engineering, at the University of Aberdeen, UK, suggests that the figures cited by Istvan Lakatos and Julianna Lakatos-Szabo for which they give no references grossly underestimates how much oil we have used already. Jones says that we have used at least 135 billion barrels of oil since 1870, the period during which J.D. Rockefeller established The Standard Oil Company and began drilling in earnest.
The oil industry now spans several generations, says Jones, and has historically been as uninterested in how much oil has been drawn as were economists, day-to-day and annual figures being of much greater concern. However, in 2005, The Oil Depletion Analysis Centre (ODAC) in London provided a total figure of almost 1 trillion barrels of crude oil (944 billion barrels) since commercial drilling began. Even that figure does not add up, Jones explains.
He has calculated a better estimate by using the volume of a barrel (42 US gallons, or 0.16 cubic metres) and a crude oil density of 0.9 tonnes per cubic metre. ODAC's 944 billion barrels is thus the equivalent of 135 billion tonnes.
Jones explains that this figure is of the same order of magnitude as the estimate offered by Lakatos and Lakatos-Szabo, but is nevertheless 35% higher than ODAC's figure. "Their assertion that less than 100 billion tonnes has been produced is significantly inconsistent with the ODAC," says Jones. The implication is that either ODAC or the Hungarian team are incorrect in their estimates, and suggests that clarification of this important figure is now needed.
Journal references:
Jones et al. Total amounts of oil produced over the history of the industry. International Journal of Oil Gas and Coal Technology, 2009; 2 (2): 199 DOI: 10.1504/IJOGCT.2009.024887
Lakatos et al. Global oil demand and role of chemical EOR methods in the 21st century. International Journal of Oil Gas and Coal Technology, 2008; 1 (1/2): 46 DOI: 10.1504/IJOGCT.2008.016731
Adapted from materials provided by Inderscience, via AlphaGalileo.

Sea Salt Holds Clues To Climate Change

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ScienceDaily (May 7, 2009) — We know that average sea levels have risen over the past century, and that global warming is to blame. But what is climate change doing to the saltiness, or salinity, of our oceans? This is an important question because big shifts in salinity could be a warning that more severe droughts and floods are on their way, or even that global warming is speeding up.

Now, new research coming out of the United Kingdom (U.K.) suggests that the amount of salt in seawater is varying in direct response to man-made climate change. Working with colleagues to sift through data collected over the past 50 years, Peter Stott, head of climate monitoring and attribution at the Met Office in Exeter, England, studied whether or not human-induced climate change could be responsible for rises in salinity that have been recorded in the subtropical regions of the Atlantic Ocean, areas at latitudes immediately north and south of Earth’s tropics.
By comparing the data to climate models that correct for naturally occurring salinity variations in the ocean, Stott has found that man-made global warming -- over and above any possible natural sources of global warming, such as carbon dioxide given off by volcanoes or increases in the heat output of the sun -- may be responsible for making parts of the North Atlantic Ocean more salty.
Salinity levels are important for two reasons. First, along with temperature, they directly affect seawater density (salty water is denser than freshwater) and therefore the circulation of ocean currents from the tropics to the poles. These currents control how heat is carried within the oceans and ultimately regulate the world’s climate. Second, sea surface salinity is intimately linked to Earth’s overall water cycle and to how much freshwater leaves and enters the oceans through evaporation and precipitation. Measuring salinity is one way to probe the water cycle in greater detail.
In the last half-century or so, the subtropical Atlantic has been getting gradually saltier -- a less than 1 percent increase in real terms, but an effect that is nevertheless significant. “It might sound like quite a small change,” says Stott, “but the overall salinity of our oceans is naturally relatively steady, so it’s actually a lot of freshwater being factored out of the ocean.”
Stott’s analysis suggests that global warming is changing precipitation patterns over our planet. Higher temperatures increase evaporation in subtropical zones; the moisture is then carried by the atmosphere towards higher latitudes (towards the poles), and by trade winds across Central America to the Pacific, where it provides more precipitation. This process concentrates the salt in the water left behind in the North Atlantic, causing salinity to increase.
Water bearer
These are just the sort of effects that Gary Lagerloef and Amit Sen hope to uncover over the next few years. Lagerloef and Sen are, respectively, principal investigator and project manager of Aquarius, part of a brand new satellite mission due to be launched into orbit in May 2010. Aquarius is the first NASA instrument designed to track sea salinity from space and will be the primary payload on the SAC-D spacecraft, which has been built by the Argentinian Space Agency or Comision Nacional de Actividades Espaciales (CONAE). The three-year mission is named after the “cup-bearer to the gods” in Greek mythology.
Sea saltiness has been measured for centuries. Most of the data we have today consist of direct measurements taken at sea (traditionally by ships and, nowadays, more often by automated buoys and profiling floats). But there are vast areas of the ocean surface -- a quarter in total -- where salinity has never been measured. By covering the entire globe once every seven days, Aquarius will fill in the blanks and provide an unprecedented global picture of salinity.
Scientists measure salt levels using a practical salinity scale. One practical salinity unit or psu almost exactly represents the number of grams of salt in a kilogram of seawater. Salinities in the open ocean, free of ice or land mass, generally lie between 32 and 37 psu (the Pacific and Atlantic Oceans have maximum surface salinities around 35 and 37 respectively). “With our instruments we will be able to measure salinity to an accuracy of 0.2 psu,” explains Sen, who works at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “If you take half a gallon of water and put a pinch of salt in it, that’s about 0.2 psu. We will be able to detect that from space, while flying about 650 kilometers [about 404 miles] above Earth.”
This is no mean feat and is possible because of some impressive radiometer technology that will fly on board the spacecraft. A radiometer is essentially a sensitive radio receiver, which, in this instance, detects microwave radiation given off by the sea surface. The radiated power of the microwaves that are emitted enables scientists to calculate the saltiness of the water at the surface.
What’s special about the three radiometers designed for Aquarius is their calibration stability -- over a seven-day period, their temperature cannot stray more than 0.1 kelvin (0.18 degrees Fahrenheit). This calls for very precise thermal control and is the reason Aquarius will be able to measure salinity with unprecedented precision.
Boom boom
“We measure salinity in the top one to three centimeters of water because that is the crucial layer that connects the atmosphere and the oceans,” explains Simon Collins, instrument manager for Aquarius who is also based at JPL. “As such, one of the largest errors in our measurement comes from ripples in the surface of the sea.” To correct for this, Aquarius also carries with it a scatterometer -- a state-of-the-art radar instrument that senses roughness in the sea surface by booming microwave pulses down to the ocean and detecting the scattered pulses bounced back to the satellite.
While Aquarius has not yet set off, it has been a long journey for the project’s scientists and engineers, who are now ready to ship their instrument from JPL to Argentina. There it will be installed on the SAC-D spacecraft, before being transported to Brazil for functional and environmental testing and returned to the United States in April 2010, ready for its trip to space.
“People don’t realize that there is so much water and so little land,” Sen remarks. Aquarius, flying high above us, will shed light on El Niño and La Niña, phases of the world’s most powerful climate phenomena, reveal insights into how monsoons develop and, most importantly of all, how a pinch of salt can change our lives.
Journal references:
R. Curry, B. Dickson & I. Yashayaev. A change in the freshwater balance of the Atlantic Ocean over the past four decades. Nature, 2003; 426 (6968): 826 DOI: 10.1038/nature02206
P. A. Stott, R. T. Sutton & D. M. Smith. Detection and attribution of Atlantic salinity changes. Geophysical Research Letters, 2008; 35 (21): L21702 DOI: 10.1029/2008GL035874
Adapted from materials provided by NASA/Jet Propulsion Laboratory. Original article written by Amber Jenkins.

Erupting Undersea Volcano Near Island Of Guam Supports Unique Ecosystem

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ScienceDaily (May 5, 2009) — Scientists who have just returned from an expedition to an erupting undersea volcano near the Island of Guam report that the volcano appears to be continuously active, has grown considerably in size during the past three years, and its activity supports a unique biological community thriving despite the eruptions.

An international science team on the expedition captured dramatic new information about the eruptive activity of NW Rota-1.
"This research allows us, for the first time, to study undersea volcanoes in detail and close up," said Barbara Ransom, program director in NSF's Division of Ocean Sciences, which funded the research. "NW Rota-1 remains the only place on Earth where a deep submarine volcano has ever been directly observed while erupting."
Scientists first observed eruptions at NW Rota-1 in 2004 and again in 2006, said Bill Chadwick, an Oregon State University (OSU) volcanologist and chief investigator on the expedition. This time, however, they discovered that the volcano had built a new cone 40 meters high and 300 meters wide.
"That's as tall as a 12-story building and as wide as a full city block," Chadwick said. "As the cone has grown, we've seen a significant increase in the population of animals that lives atop the volcano. We're trying to determine if there is a direct connection between the increase in the volcanic activity and that population increase."
Animals in this unusual ecosystem include shrimp, crab, limpets and barnacles, some of which are new species.
"They're specially adapted to their environment," said Chadwick, "and are thriving in harsh chemical conditions that would be toxic to normal marine life.
"Life here is actually nourished by the erupting volcano."
Verena Tunnicliffe, a biologist from the University of Victoria, said that most of the animals are dependent on diffuse hydrothermal venting that provides basic food in the form of bacterial filaments coating the rocks.
"It appears that since 2006 the diffuse venting has spread and, with it, the vent animals," Tunnicliffe said. "There is now a very large biomass of shrimp on the volcano, and two species are able to cope with the volcanic conditions."
The shrimp reveal intriguing adaptations to volcano living.
"The 'Loihi' shrimp has adapted to grazing the bacterial filaments with tiny claws like garden shears," said Tunnicliffe. "The second shrimp is a new species--they also graze as juveniles, but as they grow to adult stage, their front claws enlarge and they become predators."
The Loihi shrimp was previously known only from a small active volcano near Hawaii--a long distance away. It survives on the fast-growing bacteria and tries to avoid the hazards of the volcanic eruptions. Clouds of these shrimp were seen fleeing volcanic bursts.
The other species attacks the Loihi shrimp and preys on marine life that wanders too close to the volcanic plumes and dies. "We saw dying fish, squid, etc., raining down onto the seamount, where they were jumped on by the volcano shrimp--a lovely adaptation to exploiting the noxious effects of the volcano," Tunnicliffe said.
The new studies are important because NW Rota-1 provides a one-of-a-kind natural laboratory for the investigation of undersea volcanic activity and its relation to chemical-based ecosystems at hydrothermal vents, where life on Earth may have originated.
"It is unusual for a volcano to be continuously active, even on land," Chadwick pointed out.
"This presents us with a fantastic opportunity to learn about processes we've never been able to directly observe before," he said. "When volcanoes erupt in shallow water they can be extremely hazardous, creating huge explosions and even tsunamis. But here, we can safely observe an eruption in the deep ocean and learn valuable lessons about how lot lava and seawater interact."
Chadwick said that volcanic plumes behave completely differently underwater than on land, where the eruption cloud is filled with steam and ash, and other gases are invisible.
"In the ocean, any steam immediately condenses and disappears and what is visible are clear bubbles of carbon dioxide and a dense cloud made of tiny droplets of molten sulfur, formed when sulfur dioxide mixes with seawater," Chadwick said. "These volcanic gases make the eruption cloud extremely acidic--worse than stomach acid--which is another challenge for biological communities living nearby."
Ocean acidification is a serious concern because of human-induced carbon dioxide accumulating in the atmosphere. "Submarine volcanoes are places where we can study how animals have adapted to very acidic conditions," Chadwick said.
During the April 2009 expedition, aboard the University of Washington's R/V Thompson, the scientists made dives with Jason, a remotely operated vehicle (ROV) operated by the Woods Hole Oceanographic Institution.
Chadwick said that "it was amazing how close Jason can get to the eruptive vent because the pressure at a depth of 520 meters [about 1,700 feet] in the ocean keeps the energy released from the volcano from becoming too explosive." Some of the most intriguing observations came when the volcano slowly pushed lava up and out of the erupting vent.
"As this was happening, the ground in front of us shuddered and quaked, and huge blocks were bulldozed out of the way to make room for new lava emerging from the vent," Chadwick said.
Part of the evidence that the volcano is in a constant state of eruption comes from an underwater microphone--or hydrophone--that was deployed a year ago at NW Rota-1 by OSU geologist Bob Dziak.
The hydrophone "listened" for the sounds of volcanic activity. The data it recorded clearly show that the volcano was active the entire year before the latest expedition. Another hydrophone and other instruments will monitor the volcano in the coming year.
The international team included scientists from OSU, the University of Washington, University of Victoria, University of Oregon, NOAA's Pacific Marine Environmental Laboratory, New Zealand and Japan.
This research was funded by the National Science Foundation (NSF).
Adapted from materials provided by National Science Foundation.

New Antarctic Seabed Sonar Images Reveal Clues To Sea-level Rise

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ScienceDaily (May 6, 2009) — Motorway-sized troughs and channels carved into Antarctica's continental shelves by glaciers thousands of years ago could help scientists to predict future sea-level rise, according to a report in the May issue of the journal Geology.

Using sonar technology from onboard ships, scientists from British Antarctic Survey (BAS) and the German Alfred Wegener Institute (AWI) captured the most extensive, continuous set of images of the seafloor around the Amundsen Sea embayment ever taken. This region is a major drain point of the West Antarctic Ice Sheet (WAIS) and considered by some scientists to be the most likely site for the initiation of major ice sheet collapse.
The sonar images reveal an 'imprint' of the Antarctic ice sheet as it was at the end of the last ice age around 10 thousand years ago. The extent of ice covering the continent was much larger than it is today. The seabed troughs and channels that are now exposed provide new clues about the speed and flow of the ice sheet. They indicate that the controlling mechanisms that move ice towards the coast and into the sea are more complex than previously thought.
Lead author Rob Larter from British Antarctic Survey said, "One of the greatest uncertainties for predicting future sea-level rise is Antarctica's likely contribution. It is very important for scientists and our society to understand fully how polar ice flows into the sea. Indeed, this issue was highlighted in 2007 by the Intergovernmental Panel on Climate Change (IPCC). Our research tells us more about how the ice sheet responded to warming at the end of the last ice age, and how processes at the ice sheet bed controlled its flow. This is a big step toward understanding of how the ice sheets are likely to respond to future warming.'
Background
The area of the Amundsen Sea embayment surveyed was 9950 km2. In the western Amundsen Sea embayment three 17-39 km wide troughs extend seaward from the modern ice shelf front. This is roughly with width of the English Channel. Individual streamlined features carved into the seabed are about as wide as a motorway.
Ice sheet
The Antarctic ice sheet retreated to near its present limit around 10 thousand years ago. It is the layer of ice up to 5000 m thick covering the Antarctic continent. It is formed from snow falling in the interior of the Antarctic which compacts into ice. The ice sheet slowly moves towards the coast, eventually breaking away as icebergs which gradually melt into the sea.
The ice sheet covering East Antarctica is very stable, because it lies on rock that is above sea level and is thought unlikely to collapse. The West Antarctic is less stable, because it sits on rock below sea level.
Ice shelf
An ice shelf is a thick (100-1000 m), floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are found in Antarctica, Greenland and Canada only.
Glacier
Just as rivers collect water and allow it to flow downhill a glacier is actually a "river" of ice. A glacier flows much more slowly than river. Rivers of ice within ice sheets account for most of the drainage into the oceans.
Continental shelf
The relatively shallow (generally up to 200 meters) seabed surrounding a continent where the depth gradually increases before it plunges into the deep ocean. Around Antarctica the continental shelf is up to 1600 m deep as a result of millions of years of glacial erosion. The deepest parts of the Antarctic continental shelf are near the present ice margin and depths generally decrease offshore.
Journal reference:
Larter et al. Subglacial bedforms reveal complex basal regime in a zone of paleo-ice stream convergence, Amundsen Sea embayment, West Antarctica. Geology, 2009; 37 (5): 411 DOI: 10.1130/G25505A.1
Adapted from materials provided by British Antarctic Survey.