| Over geologic time, rocks deeper than about 120 km (75 miles) flow. Because of this, mountains float on this 'fluid' part of the mantle (called the asthenosphere). How high the mountains float depends on the makeup of the mountains just as the height a boat sits out of the water depends on how heavily loaded the boat is. There are two main ways to make mountains less dense and rise up higher. One is to change the thickness of the crust, the outer part of the Earth. The other is to change the weight of the upper part of the mantle, above the fluid part. |
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Rocks in the Earth's crust are lighter (less dense) than those in the Earth's mantle. Thus, the crust floats on the mantle just as ice floats on water. When ice floats, thicker pieces of ice will rise higher above the water. So in the photo at left, the thick blue ice cube rises above the thinner red ice cube. The same is often true for the Earth's crust: where it is thicker, the surface of the crust should be higher, and we should have mountains. If this is how the Sierra were made, we should find thick crust under the High Sierra in the eastern part of Yosemite National Park and thinner crust in the lower foothills in western Yosemite. | |
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| Can there be mountains if the crust is the same thickness inside and outside the mountains? Yes, and this is how the great mountain ranges of the ocean floor are made. There is a layer below the crust, awkwardly called the mantle lithosphereMantle lithosphere is the upper part of the mantle that has gotten colder and denser than mantle below it. It is heavier than the 'fluid' mantle below, so where it is thicker, elevations are lower. The oceans' mountain ranges have a thin mantle lithosphere, while the rest of the ocean has a thick mantle lithosphere and so are much deeper. |
See how this information is used to understand how the Sierra Nevada might have formed