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Our transcription: The physical fragmentation of rock is known as "mechanical weathering". The rocks we see are usually cracked to start with because of tectonic activity. Systems of natural cracks in rocks are called "joints." One common kind of joint results from a process known as "pressure release." The rock forming this great granitic dome in Yosemite National Park was originally buried kilometers beneath the surface. While a rock is below the surface, the layers of earth's crust above pressed down on it. With tectonic uplift, surface erosion reduces the thickness of the overlying rock. The downward pressure is reduced, and the rock expands primarily in an upward direction. At shallow depth, the expansion causes the rock to crack into sheets separated by joints parallel to the overlying surface. Exposed by erosion, plates of jointed granite then detach themselves and slide down slowly unpealing the face of the rock, layer by layer, like the stripping of skins from an onion. This is known as "exfoliation" -- literally, the stripping away of leaves. This opens the way for other types of mechanical weathering.
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