Weathering, Erosion, and Soil

• WeatheringThe breaking down or changing of rock where it sits. A cracked boulder or a crumbly, rusty rock face has been weathered, but it has not gone anywhere yet.
• ErosionThe moving of weathered pieces, called sediment, by water, wind, ice, or gravity.
• DepositionThe dropping of sediment in a new place, when the water, wind, or ice carrying it slows down or melts.

• Frost wedgingWater seeps into a crack. When it freezes, it takes up about 9% more space, so the ice pushes the crack wider. The ice melts, more water trickles in, and it freezes again. After many freeze-thaw cycles, the rock splits. Frost wedging is strongest in places where the temperature swings above and below freezing again and again.
• AbrasionRock is worn away by other rock rubbing against it, like sand blown by wind or pebbles tumbling in a river. Abrasion is why river pebbles are smooth and round. Glaciers grind rock by abrasion too, but because the moving ice carries the pieces away, that counts as erosion (see Glaciers below).
• ExfoliationRock that formed deep underground was squeezed by the weight of the rock above it. When erosion removes that weight, the rock expands a little and cracks into curved sheets that peel off like layers of an onion. The rounded granite domes of Yosemite National Park, such as Half Dome, show this kind of sheeting.
• Plant rootsA seed lands in a crack and sprouts. As the root grows thicker, it pries the crack apart. Look at a sidewalk with a tree beside it to see roots at work.
• SaltSalty water soaks into tiny spaces in rock and evaporates. The salt crystals that are left grow and push the grains apart, so the rock crumbles. This happens a lot on sea cliffs and in deserts.

• Carbonic acid and cavesAs rain falls, it picks up carbon dioxide from the air and soil and becomes a weak acid called carbonic acid, much weaker than the juice of a lemon. Over thousands of years it dissolves calcite, the mineral limestone is made of. Underground, this opens up caves. Mammoth Cave in Kentucky, the world’s longest known cave system, has more than 400 miles (about 640 kilometers) of mapped passages.
• OxidationIron in minerals reacts with oxygen and water, the same reaction that makes rust. The rock turns reddish brown and crumbly. Many red rocks and red soils get their color from oxidized iron.
• HydrolysisWater reacts with feldspar, the most common mineral in granite, and slowly turns it into clay. That is why old granite can crumble into a gritty mix of quartz grains and clay.

• ClimateWarm, wet places have the fastest chemical weathering, because water and heat speed up chemical reactions. Cold places with many freeze-thaw days have lots of frost wedging. In dry deserts, chemical weathering is slow.
• Rock typeSome minerals resist weathering better than others. Quartz is very tough, so quartz sand lasts a long time. Limestone dissolves in weak acid, while granite holds up better.
• Surface areaThe more surface that is exposed, the faster rock weathers. Breaking one rock into many pieces gives water and air many more faces to attack.

• Running waterRivers are the biggest movers of sediment on land. The Colorado River began carving the Grand Canyon about 5 to 6 million years ago. Today the canyon is about 277 miles (446 kilometers) long and, on average, about 1 mile (1.6 kilometers) deep. When a river reaches the sea, it slows down and drops its sand, silt, and clay. The Mississippi River has built a huge delta into the Gulf of Mexico this way.
• WindWind can only lift small grains, like sand and dust, but in dry places with few plants it moves a lot of them. Where the wind slows down, it drops the sand in dunes. Great Sand Dunes National Park in Colorado has the tallest dunes in North America.
• GlaciersA glacier is a huge, slow river of ice. Rocks frozen into its base scrape and grind the land like sandpaper, and the ice plucks loose blocks from the bedrock. Glaciers turn narrow, V-shaped river canyons into wide U-shaped valleys. Yosemite Valley in California is a famous example; its last big glaciers peaked about 18,000 years ago. When glaciers melt, they drop their rocks in heaps called moraines and leave lone boulders called erratics.
• GravityGravity pulls rock and soil downhill. This is called mass movement. It can be fast, like a rockfall or a landslide, or slow, like soil creeping downhill a few millimeters a year. Rockfalls are the most active geologic process in Yosemite today.

• O horizonLeaf litter and humus on top. It is thick on a forest floor and may be missing in a farm field.
• A horizon (topsoil)Dark, crumbly, rich in humus, roots, worms, and microbes. Most plants grow here.
• E horizonA pale layer found in some soils. Water soaking down has washed clay, iron, and other minerals out of it.
• B horizon (subsoil)Where the clay and minerals washed down from above collect. It is often reddish or brown and harder to dig.
• C horizonPartly weathered chunks of the parent rock or sediment, with little living material.
• R layer (bedrock)Solid rock underneath. It is not soil, but over time it can become the parent material for new soil.

• SandThe biggest grains, 2 to 0.05 millimeters across. Sand feels gritty. Water drains through sandy soil quickly.
• SiltMedium grains, 0.05 to 0.002 millimeters. Silt feels smooth, like flour.
• ClayThe smallest bits, less than 0.002 millimeters. Clay feels sticky when wet, holds water, and can turn hard when dry.

• WindbreaksRows of trees slow the wind. In the 1930s, the Shelterbelt Project planted long belts of trees across the Plains.
• Contour plowing and terracesPlowing across a slope instead of up and down it, or cutting the slope into steps, keeps rain from rushing downhill and carrying soil away.
• Cover cropsPlanting crops like clover or rye after harvest keeps roots in the ground and covers bare soil.
• No-till farmingPlanting seeds without plowing leaves old stalks and roots in place to hold the soil.

Game: Break It, Move It, Drop It

Ten scenes from around the world. For each one, decide what is happening: is rock being broken down (weathering), carried away (erosion), or dropped in a new place (deposition)?

This game needs JavaScript. With JavaScript turned off, the ten scenes and their answers are listed below.

Quick quiz

Nine questions. Choose one answer for each, then press Check my answers.