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Warm rock keeps North America from drowning

25.06.2007
Much of North America would be underwater if it were not for the heat that makes rock buoyant, new research indicates. Scientists show how various regions of the continent are kept afloat by heat within Earth's rocky crust, and how far those regions would sink beneath sea level if they lacked that heat- induced lift.

Of coastal cities, New York City would sit 1,427 feet (435 meters) under the Atlantic, New Orleans would be 2,416 feet (736 meters) underwater and Los Angeles would rest 3,756 feet (1,145 meters) beneath the Pacific. Rather than perched a mile high (1.6 kilometers), Denver would be 727 feet (222 meters) below sea level.

"If you subtracted the heat that keeps North American elevations high, most of the continent would be below sea level, except the high Rocky Mountains, the Sierra Nevada, and the Pacific Northwest west of the Cascade Range," says Derrick Hasterok of the University of Utah in Salt Lake City, a researcher on the study.

Typically, the movements of "tectonic plates" of Earth's crust, which result in volcanoes, mountain-building collisions, and sinking or "subduction" of old seafloor, get the credit for determining elevation. However, Hasterok and his University of Utah coauthor David S. Chapman say tectonic forces contribute to elevation by affecting the composition and temperature of rock that they move. For example, as crustal plates collide to form mountains like the Himalayas, the mountains rise because the collision makes less dense crustal rock get thicker and warmer, thus more buoyant.

"We have shown for the first time that temperature differences within the Earth's crust and upper mantle explain about half of the elevation of any given place in North America," with most of the rest due to differences in what the rocks are made of, Chapman says.

Continents and mountains like the Rockies are kept afloat partly by heat from Earth's deep interior and heat from radioactive decay of uranium, thorium, and potassium in Earth's crust.

Chapman says it will take billions of years for North American rock to cool to the point it becomes denser, sinks, and puts much of the continent underwater. Coastal cities face flooding much sooner as sea levels rise due to global warming, he adds.

The researchers published their new findings on Saturday, 23 June as two reports in the Journal of Geophysical Research-Solid Earth - a publication of the American Geophysical Union.

In the new work, the team first analyzed results of previous experiments in which researchers have measured seismic waves moving through Earth's crust due to intentional explosions. The waves travel faster through colder, denser rock, and slower through hotter, less dense rock. Then, the Utah scientists used published data in which various kinds of rocks were measured in the laboratory to determine the rocks' densities and how fast seismic waves travel through them.

The combined data allowed the researchers to calculate how rock density varies with depth in the crust. They could then assess how much of any area's elevation is due to the thickness and composition of its rock and how much is due to the rock's heating and expansion. Finally, the researchers "removed the effects of composition of crustal rocks and the thickness of the crust to isolate how much a given area's elevation is related to the temperature of the underlying rock," Chapman says.

To calculate how regional elevations would change if temperature effects were removed, the researchers did not turn off all the heat, but imagined that a region's rock was as cold as some of North America's coldest crustal rock, which is still at 750 degrees Fahrenheit (400 degrees Celsius) at the base of the crust in Canada.

Hasterok says it has been well known for years that "elevations of different regions of the continents sit higher or lower relative to each other as a result of their density and thickness." By accounting for composition, thickness and, now, temperature of crustal rock in North America, scientists can more easily determine how much elevation is explained by forces such as upwelling plumes of molten rock like the "hot spot" beneath Yellowstone. The new method also will make it easier to identify areas where crustal rocks are unusually hot due to higher-than-average concentrations of radioactive isotopes.

Chapman says temperatures in Earth's crust and upper mantle often are inferred from measurements in boreholes drilled near the surface, whereas elevation reflects average rock temperatures down to 125 miles (201 kilometers) beneath Earth's surface. Inconsistencies in both measurements can be used to reveal the extent to which borehole temperatures are affected by global warming or changes in groundwater flow.

Although most locations would sink if the temperature influence were removed, some areas that sit atop rock that is colder than average would actually rise. For instance, Seattle sits above a plate of Earth's crust that is diving, or subducting, eastward at an angle. That slab of cold, former seafloor rock insulates the area west of the Cascades from heat deeper beneath the slab. Removing that heat-blocking action would warm the Earth's crust under Seattle, so it would expand and become more buoyant. Instead of its current position on the shores of the saltwater Puget Sound, Seattle would soar to an elevation of 5,949 feet (1812 meters).

Peter Weiss | AGU
Further information:
http://www.agu.org

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