Weathering, Erosion and How Soil Forms

Soil is the loose material that covers most of the land on Earth. It is a mixture of tiny mineral pieces, decayed once-living matter, living organisms, air, and water.

Soil starts as solid rock called parent material. Weathering is the slow breakdown of that rock, and it happens in three ways. Physical weathering cracks rock into smaller pieces without changing what it is made of, like when water freezes in a crack and wedges it open. Chemical weathering changes the rock's minerals, like when slightly acidic rainwater dissolves limestone. Biological weathering comes from living things, like plant roots prying rock apart.

Weathering only breaks rock down. Erosion is different: it is the movement of the broken pieces by water, wind, or ice to a new place. Fast-moving water can carry more and larger particles than slow water, which is why steep streams carve valleys while slow rivers drop sediment.

Where weathered material stays put and mixes with the remains of plants and animals, soil builds up in layers called horizons. The top layers are dark and rich in organic matter. Deeper layers collect clay and minerals washed down from above, and near the bottom you find partly broken parent material sitting on solid bedrock. Scientists read these layers like a story of how the soil formed.

Because climate controls how fast rock weathers and how much life grows, soils differ around the world. Warm, wet climates weather rock quickly and can make deep soils, while cold or very dry climates weather rock slowly, so soils there stay thin.

Key words for understanding how soil forms:

Bedrock is the solid rock that lies beneath the soil. It is the starting material that soil eventually comes from.

The C horizon is the deepest soil layer. It holds the parent material, which is rock that is partly broken down but not yet real soil. People sometimes call it the soil base.

The A horizon sits near the surface. It is a mixture of organic material from dead plants and animals with the inorganic products of weathering. This dark, rich layer is where most roots grow and where farmers want deep, fertile soil.

The B horizon lies between the A and C horizons. Fine material like clay washes downward from the top layer and collects here, so the B horizon is an accumulation layer.

Clay is made of soil particles smaller than 0.002 millimeters across. Because clay particles are so tiny, clay holds water well but packs together tightly.

Together these layers make a soil profile. Reading the profile from top to bottom shows you the path from living surface litter, through rich topsoil, to washed-down minerals, to crumbling rock, and finally to solid bedrock.