52 Landslides-Low

Landslides FOCUS Question How do landslides change Earth's surface?

Moving Earth It's a rainy day in the mountains. The forest is quiet, except for the peaceful sound of raindrops. Suddenly, there is a long, loud roar as the surface on one side of a hill breaks off and slides away. The soil tears apart, churning like a washing machine, while trees and rocks are swept away. The wall of debris finally comes to a stop on the flat ground below, burying a road. This event is called a landslide. Landslides are natural disasters in which large masses of rock, mud, or soil suddenly collapse and move downhill. Landslides can quickly change the landscape.

Slow Versus Quick The shape of land changes all the time. Through the processes of weathering and erosion, water and wind wear away Earth's surface. Sometimes flowing water and blowing wind move rocks and soil a bit at a time. Gravity can move rocks and soil downhill over centuries. Eventually, mountains wear down, rivers carve valleys, and plains grow smooth and flat. However, landslides happen quickly. When a slope is very steep or the land becomes too loose or fragile, large masses of rock and soil can give way and fall all at once. There are three main types of landslides flows, slides and falls.

Flows A flow happens when land acts more like a liquid than a solid. It oozes or rushes downhill, tumbling as it goes. It buries the land below like a flood. A flow can be strong enough to break off trees and pick up large rocks. Flows almost always happen in soil made of small particles and usually take place when the soil is very wet. When soil becomes mud, a mudflow may form. The land easily flows away. A mudflow often occurs during periods of heavy rainfall. But small, loose bits of soil like sand or gravel can also flow when they are dry. This is a rockflow.

Slides and Falls A slide happens when an entire slab of land breaks off and rushes downhill. Usually there is a layer of soft or loose rock underground. The weak layer can't support the ground above it, so the top layer gives way. Slides happen more often where water seeps into the rock and weakens it. A fall happens when a chunk of rock breaks off all at once. These sudden crashes happen on very steep land, like cliffs, or places where the ground is hollow beneath the rock. Science in Your World The Old Man of the Mountain was a popular cliff in New Hampshire that looked like a human face. In 2003, the rocks of the "face" broke off and fell into the valley below.

What Causes Landslides? All landslides ultimately occur for the same reason— gravity. Landslides happen on sloped land, such as mountains, hills, canyons, and cliffs. What makes some slopes suddenly give way while others remain intact? Water Water is the main cause of landslides. When it rains or snows more than normal, landslides are more likely to occur. Heavy rain or flooding can turn soil into slick mud, and rocks become slippery— both of which cause flows. Gradual water erosion can also lead to a landslide. Waves create steep sea cliffs, and rivers carve out canyons. If a slope gets too steep, the land can suddenly give way. Water can also trickle into cracks in rock over time. When the water freezes, it expands, breaking the rock apart and sending it tumbling.