What is the process that breaks down rock at the earth's surface?

Rocks are broken down by various forms of erosion. Physical erosion is caused by water action wearing down the rocks and carrying them toward the ocean. Freezing also breaks down rocks. Water expands when it freezes. Water seeping into cracks as a liquid will expand and force the rock apart when the water freezes. Huge boulders of granite and broken off of cliffs this way. Wind carrying sand wears away rocks turning the rocks into sand.

Biological erosion, is the process in which organisms contribute to erosion, this is usually done by trees, in which their roots can force cracks in rocks, causing them to break apart, though trees can help in securing soil which can be eroded. Granite is also eroded by the action of organic agents as well as other rocks. The acids produced by the organic agents dissolves the inorganic mineral material.

Chemical erosion also destroys rocks. Oxidation weakens the chemical structure of the rocks making physical erosion easier. Acids eat away limestone dissolving the stone into Carbon Dioxide and water.

Erosion in all of its forms destroy rocks turning the rocks into sediments that in turn, turn back into rocks.