February 1, 2008 Geophysicists determined that tectonic mountain-building processes are not the only factor that determines elevation in North America. The temperature of the crust affects its density, and lower density crust will rise higher than colder, higher density crust. The heat in question comes from the Earthýs interior and also radioactive decay of various elements in the crust. Broadly, the Rocky Mountain region of the United States has the hottest crust, as well as the highest general elevation.
You may know how mountains formed, but do you know why they stay high up in the sky? Our mountain ranges show off some of the most majestic views around. We know collisions of the earth's crustal plates formed mountains like these, but now geophysicists say there's another reason some regions in the US sit higher than others.
"We now have an explanation for elevation for the continents, knowing that it's not just what types of rocks they're made of ý it's also how hot each region is," Derrick Hasterok, Ph.D. student at University of Utah, said.
New findings reveal that most of North America is kept afloat by heat within Earth's rocky crust and deeper mantle -- and if not for the heat, much of our continent would sink below sea level. "What we found out is that when you heat the continents they also expand and part of the consequence of that is that they sit up higher," David Chapman, Ph.D., geophysicist at the University of Utah, said.
The heat and temperature inside earth are critical for places like the rocky mountains, because it's warmer and more buoyant underneath, pushing the rockies up. But underneath lower coastal cities like New York and LA, it's cooler, less buoyant, with lower elevations. "The heat is caused by largely by radioactivity, which has a very very long time scale, so no one is going to sink into the ocean because of our discovery," Dr. Chapman said.
WHY DOES THE EARTH NEED TO MOVE? The Earth's crust is quite thin compared to the diameter of our planet. The crust is made up of sections called plates, ranging from very small to the size of an entire continent. Both the Earth's surface and its interior are constantly moving.
Beneath the rigid plates of the earth's crust, lies the mantle -- mobile rock that slowly moves in circles, like a pot of thick soup that is heated to boiling. The heated soup rises to the surface, spreads and begins to cool, then sinks back to the bottom of the pot, where it reheats and rises again. This cycle is repeated over and over; this is called convective flow. This process occurs much more slowly inside the Earth.
Because of the heat and pressure that builds up beneath the surface, the crust is constantly being stressed, which breaks it up. This is known as plate tectonics. When two plates move away from each other, deep cracks are opened through the crust, allowing magma to rise to the surface and then cool, making new crust. Crust is destroyed when one plate dives under another. Plates can also slide horizontally past each other.
The American Geophysical Union contributed to the information contained in the video portion of this report.