Weathering is the process that changes solid rock into sediments. Geologists use the word sediment to describe all different sizes of rock particles. Sediment includes really large pieces of rock, like boulders or gravel, but it also includes sand and much smaller particles, called silt and clay. In the process of weathering, rock is disintegrated and decomposed. Disintegration of rock happens as rock is broken into pieces. Once the pieces are separated from the rocks, erosion is the process that moves those pieces.
Gravity is one way that pieces of rock move, as broken pieces of rock fall or tumble from high places to lower ones. Gravity causes large and small pieces to fall from cliffs, as well as moving water in rivers and streams from mountaintops to the ocean. Wind and glaciers also move pieces of rock from one place to another. Wind moves sand sized and smaller pieces of rock through the air. Glaciers can move all sizes of particles, from extremely large boulders to the tiniest fragments.
Weathering happens at the Earth’s surface. When most rocks form, they are forming at very high temperatures and pressures. This is a very different environment than the low temperatures and pressures at Earth’s surface. When rocks reach Earth’s surface, weathering causes them to change form. The new form will include minerals that are stable at the low temperatures and pressures of Earth’s surface.
So while powerful forces on Earth, such as those resulting from plate tectonics, work to build huge mountains like the Himalayas or majestic volcanoes like Mt Fuji, the forces of weathering gradually wear away rocks, changing once tall mountains into hills and even plains. The Appalachian Mountains along the east coast of North America were once as tall as the Himalayas! So what happened?
No human being can watch for millions of years as mountains are slowly built, nor can we watch as those same mountains gradually wear away. However you probably have been able to ride your bike or walk along a brand new sidewalk or road. What do you experience? The new road or sidewalk is smooth and even. If it was made well, there won’t be any cracks or bumps.
Does that smooth surface stay that way? Certainly over millions of years, it will completely disappear, but we don’t have to wait that long. If you live in a part of the world that has cold winters, you may only have to wait one year to start seeing changes. We will talk next about what types of weathering change that brand new, smooth and even sidewalk into areas that are rough or cracked, chipped or buckled.
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