Not So High Without the Heat

A University of Utah study shows how various regions of North America are kept afloat by heat within Earth’s rocky crust, and how much of the continent would sink beneath sea level if not for heat that makes rock buoyant.

Of coastal cities, New York City would sit 1,427 feet under the Atlantic, Boston would be 1,823 feet deep, Miami would reside 2,410 feet undersea, New Orleans would be 2,416 underwater and Los Angeles would rest 3,756 feet beneath the Pacific.

Mile-high Denver’s elevation would be 727 feet below sea level and Salt Lake City, now about 4,220 feet, would sit beneath 1,293 feet of water. But high-elevation areas of the Rocky Mountains between Salt Lake and Denver would remain dry land.

“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 study co-author Derrick Hasterok, a University of Utah doctoral student in geology and geophysics.

“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, says the other co-author, David Chapman, a professor of geology and geophysics, and dean of the University of Utah Graduate School.

People usually think of elevations being determined by movements of “tectonic plates” of Earth’s crust, resulting in volcanism, mountain-building collisions of crustal plates, stretching apart and sinking of inland basins, and sinking or “subduction” of old seafloor. But Hasterok and Chapman say those tectonic forces act through the composition and temperature of rock they move. So 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.

The study – published online in the June issue of Journal of Geophysical Research-Solid Earth – is more than just an entertaining illustration of how 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.

So were we not under that plume during the Creaceous or did it abate somehow that would cause all those transgressive seas?