Weathering

The process of weathering alters rocks at the earth's surface and breaks them down over time into fine‐grained particles of sediment and soil. Weathering is the result of the interactions of air, water, and temperature on exposed rock surfaces and prepares the rock for erosion. Erosion is the movement of the particles by ice, wind, or water. The particles are then transported by that agent until they are deposited to form sedimentary deposits, which can be later eroded again or transformed into sedimentary rocks. The weathering of the sediment grains continues during erosion and transportation. Weathering is generally a long, slow process that is continuously active at the earth's surface.

There are two kinds of weathering: mechanical and chemical. Mechanical weathering is the process by which rocks are broken down into smaller pieces by external conditions, such as the freezing of water in cracks in the rock. The rock is chemically weathered when it reacts with rain, water, and the atmosphere to destroy chemical and mineralogical bonds and form new minerals. For example, feldspar crystals in a volcanic tuff commonly weather to form new clay minerals. Other minerals, such as calcite, dissolve. Quartz, on the other hand, is very resistant to weathering.

Chemical weathering weakens the bonds in rocks and makes them more vulnerable to decomposition and erosion. Thus, the weathered surface of a rock, whether it is an outcrop in the field or the stone walls of a building on a downtown street, looks different from the “fresh” interior of the rock. The most noticeable effect is the discoloration of the surface that results from the breakdown of minerals.

A spherically weathered boulder forms when the corners of an angular rock are broken down more quickly than the flat surfaces, forming rounded shapes. Differential weathering results when some rocks resist weathering more than other rocks, creating uneven rates of weathering and erosion. This phenomenon often forms arched natural bridges or spectacular mushroom‐shaped rock formations, where a broad, more‐resistant sandstone ledge is perched on a narrower column of less‐resistant shale that has eroded more quickly.