Mountains look permanent, but they are being taken apart grain by grain. Every landscape is the result of a contest between forces that raise the land, such as plate collisions and volcanoes, and forces that wear it down. This level is about the second set, which work slowly but never stop.

Weathering and erosion

Two words are often used as if they meant the same thing. Weathering breaks rock down where it stands; erosion carries the pieces away. A cracked boulder on a hillside has been weathered; the sand washing off it in the next storm is being eroded. The two usually work together, since erosion exposes fresh rock to weathering.

Breaking rock apart

Mechanical weathering breaks rock into smaller pieces without changing what it is made of. The most powerful example is frost wedging. Water seeps into cracks, freezes and expands by about 9 percent, prying the rock apart a little more each winter. Plant roots widen cracks too, and in deserts, salt crystals growing in pores can crumble stone.

Chemical weathering changes the minerals themselves. Iron-bearing minerals rust, giving many soils and rocks their red and orange colors. Feldspar slowly turns into clay. The most striking case involves limestone. Rainwater absorbs carbon dioxide from the air and soil and becomes a weak carbonic acid, which slowly dissolves limestone. Over thousands of years this creates karst landscapes, with sinkholes, vanishing streams and caves. Mammoth Cave in Kentucky, the longest known cave system in the world, was dissolved this way. Where dripping water loses its carbon dioxide inside a cave, the dissolved mineral comes out again, building stalactites and stalagmites. Chemical weathering is fastest where it is warm and wet, which is why stone monuments crumble faster in humid cities than in deserts.

Rivers: the great sculptors

Running water is the most important agent of erosion on land. Streams carry sand and boulders that grind their beds deeper, cutting V-shaped valleys in uplands. In gentler country, rivers swing from side to side in loops called meanders and spread silt across wide floodplains during floods, building some of the most fertile farmland on Earth. A delta forms where a river slows as it enters a lake or the sea and drops the sediment it was carrying. The Greek historian Herodotus named the delta of the Nile for its shape, like the Greek letter delta.

Ice, wind and gravity

Glaciers cover less of the land today than rivers do, but where they flow they are enormously powerful. Armed with rocks frozen into their bases, they scrape and pluck at the land. A glacier typically carves a broad, U-shaped valley with steep sides and a wide floor, quite unlike the narrow V of a river valley. When glaciers melt, they leave scattered boulders called erratics, sometimes hundreds of kilometers from the rock they came from. In 1840 the Swiss-born naturalist Louis Agassiz published his study of glaciers in the Alps. Agassiz argued that erratic boulders and scratched bedrock showed that glaciers had once covered far larger areas, including much of northern Europe. Many scholars had blamed a great flood; Agassiz's ice age won out.

Wind erodes mainly in dry places, piling sand into dunes and spreading fine dust called loess, which forms rich soils in China and the American Midwest. Gravity, finally, moves material downhill on its own. Landslides and rockfalls are sudden, while soil creep is so slow that it shows only in tilted fences and curved tree trunks.

Soil: the thin living skin

Soil is weathered rock mixed with organic matter, air, water and countless living things, and nearly all our food depends on it. It builds up very slowly. Soil often takes centuries to form just a few centimeters, so it can be lost far faster than it is replaced. Today human activities such as farming, building and mining move more soil and rock each year than all the world's rivers, a sign of how large a geological force people have become.

Worked example: the Dust Bowl

In the 1910s and 1920s, high wheat prices and a run of wet years drew farmers onto the southern Great Plains of the United States. Tractors plowed up millions of acres of native grassland, whose deep roots had held the soil in place through droughts for thousands of years. Then, in the 1930s, the rains failed. With nothing to hold it, the dry topsoil blew away in towering black clouds; on 14 April 1935, remembered as Black Sunday, a single storm turned day into night across the plains. Seen through this level, it was erosion by wind, made catastrophic by removing the plants that protected soil that had taken centuries to form. The federal Soil Conservation Service, created in 1935 under Hugh Hammond Bennett, encouraged contour plowing, cover crops and rows of trees as windbreaks. Those practices, together with better farming habits, help explain why later droughts on the plains did far less damage.