Weathering and Erosion

About this topic

Weathering breaks down rocks in place. Physical (mechanical) weathering includes freeze–thaw, where water expands as ice and cracks rock, and temperature changes and plant roots. Chemical weathering includes acid rain dissolving limestone and oxidation (rusting). Biological weathering is by organisms.

Erosion moves the material by water, wind, ice or gravity; deposition drops it elsewhere. Together they shape landscapes: valleys, canyons, beaches and deltas. They produce the sediments for sedimentary rocks and soil.

A classic example is the Grand Canyon, carved by the Colorado River over roughly 5 to 6 million years, exposing nearly two billion years of rock layers. In cold places, water seeps into cracks, freezes and expands by about 9 percent, wedging blocks apart, which is why mountain slopes are covered in broken rock. In cities, acid rain slowly dissolves marble statues and limestone buildings, as carbonic acid reacts with calcium carbonate.

Different agents leave different fingerprints: rivers produce rounded pebbles and V-shaped valleys, glaciers leave U-shaped valleys and deposit unsorted till, wind carries fine dust and makes dunes, and waves wear down cliffs and build beaches. Human actions speed it up: farming, deforestation and construction can increase erosion greatly. Engineers use terraces, vegetation and sea walls to control it. A frequent mistake is to confuse the terms: weathering breaks rock down in place, while erosion involves movement. Another is to think they are always destructive; they also build new landforms and produce soils and minerals that support life.