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Without Hot Rock, Much Of North America Would Be Underwater


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Kara Sundstrom
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« on: December 20, 2008, 12:10:11 am »

A Lesson from the Abyssal Ocean Depths

Chapman says it may seem paradoxical, but "the answer to questions about the elevation of Earth's continental areas starts in the oceans."

The Earth's crust averages 4 miles thick beneath the oceans and 24 miles thick under continents. The crust and underlying layer, the upper mantle, together are known as the lithosphere, which has a maximum thickness of 155 miles. The lithosphere is broken up into "tectonic plates" that slowly drift, changing the shapes, locations and configurations of continents over the eons.

Ice floats on water because when water freezes it expands and becomes less dense. Rock and most other materials expand and become less dense when heated. 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. Most elevation that we can observe at the surface is a result of the buoyancy of the crust and upper mantle."

He adds that elevation changes also can stem from heating and expansion of rocks that makes them more buoyant -- a phenomenon named "thermal isostasy" that explains "why the hot mid-ocean ridges are much higher relative to the cold abyssal plains."

New ocean floor crust is produced by volcanic eruptions at undersea mountain ranges known as mid-ocean ridges. Molten and hot rock emerges to form new seafloor, which spreads away from a ridge like two conveyor belts moving opposite directions. As new seafloor crust becomes older and cooler over millions of years, it becomes denser and loses elevation. Chapman and Hasterok say there is a 10,000-foot elevation difference between the peaks of the mid-ocean ridges and older seafloor.

Given that, Chapman says he has been puzzled that differences in rock temperature never have been used to explain elevations on continents.

"Our goal was to show that temperature variations add a significant contribution, not only to the ocean floor, but also to continental elevation," Hasterok says. "For example, the Colorado Plateau sits 6,000 feet above sea level, while the Great Plains -- made of the same rocks [at depth] -- are much lower at 1,000 feet. We propose this is because, at the base of the crust, the Colorado Plateau is significantly warmer [1,200 degrees Fahrenheit] than the Great Plains [930 degrees Fahrenheit]."

When You're Hot You're High in North America

Chapman says that in the study, he and Hasterok "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."

First, they analyzed results of previous experiments in which scientists 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 they used published data in which various kinds of rocks were measured in the laboratory to determine both their density and how fast seismic waves travel through them.

The data allowed researchers to calculate how rock density varies with depth in the crust, and thus how much of any area's elevation is due to the thickness and composition of its rock, and how much is due to heating and expansion of the rock.

Seafloor crust has the same composition and thickness most places away from the tall mid-ocean ridges, so it is easy for scientists to observe how elevations vary with ocean crust temperature. But to determine the temperature effect on continents, "we wave this wand and create a transformed continental crust that is everywhere the same thickness [24 miles] and composition [2.85 times the density of water]," Chapman says. "Once we've done that, we can see the thermal effect."

That, in turn, made it possible to calculate how much heat flow contributes to elevation in each of 36 tectonic provinces -- sort of "mini-plates" -- of North America.

For example, the New England (Central) Appalachians Province has an average elevation of 897 feet, but if its rocky crust were cooled to that of old, colder continental crust like the Canadian Shield, the province would sit 563 feet below sea level, a drop of 1,460 feet. New York City, within that province, has an elevation listed as 33 feet. Subtract 1,460 feet and the Big Apple gets dunked 1,427 feet below sea level.


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Adapted from materials provided by University of Utah.

http://www.sciencedaily.com/releases/2007/06/070625080927.htm
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