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Warm rock keeps North America from drowning, geologists say

June 28, 2007
Courtesy American Geophysical Union
and World Science staff

Much of North Amer­i­ca would be un­der­wa­ter were it not for heat that makes rock buoy­ant, new re­search in­di­cates. Sci­en­tists list­ed var­i­ous parts of the con­ti­nent that stay afloat thanks to heat with­in Earth’s rocky crust, and how far those re­gions would sink be­neath sea lev­el if they lacked that heat-in­duced lift.

On the coast, New York City would sit 1,427 feet (435 me­ters) be­low the At­lan­tic; New Or­leans 2,416 feet (736 me­ters) un­der­wa­ter and Los An­ge­les 3,756 feet (1,145 me­ters) be­neath the Pa­cif­ic, re­search­ers said. 

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,” said Der­rick Has­terok of the Un­ivers­ity of Utah in Salt Lake City, a re­searcher on the stu­dy.

Typ­ic­ally, ge­ol­o­gists at­trib­ute dif­fer­ences in eleva­t­ion to move­ments of sec­tions of the Earth’s crust called tec­ton­ic plates, mountain-building col­li­sions, and sink­ing or “sub­duc­tion” of old seafloor. But 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 lighter 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 said. The find­ings were pub­lished on June 23 as two re­ports in the Jour­nal of Geo­phys­i­cal Research-Solid Earth.

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Much of North America would be underwater if it were not for the heat that makes rock buoyant, new research indicates. Scientists listed various parts 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. On the coast, New York City would sit 1,427 feet (435 meters) below the Atlantic; New Orleans 2,416 feet (736 meters) underwater and Los Angeles 3,756 feet (1,145 meters) beneath the Pacific, researchers said. 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,” said Derrick Hasterok of the University of Utah in Salt Lake City, a researcher on the study. Typically, geologists attribute differences in elevation to movements of sections of the Earth’s crust called tectonic plates, mountain-building collisions, and sinking or “subduction” of old seafloor. But 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 said. The findings were published on June 23 as two reports in the Journal of Geophysical Research-Solid Earth.