14.14: Landforms from Erosion and Deposition

What can cause devastation four months after a volcanic eruption?

Four months after Mt. Merapi in Indonesia erupted, there was more devastation. Rain continually washed ash down-slope in massive ash flows. Residents were unable to live well under these conditions.

Landforms and Gravity

Gravity is responsible for erosion by flowing water and glaciers. That’s because gravity pulls water and ice downhill. These are ways gravity causes erosion indirectly. But gravity also causes erosion directly. Gravity can pull soil, mud, and rocks down cliffs and hillsides. This type of erosion and deposition is called mass wasting. It may happen suddenly. Or it may occur very slowly, over many years.

Landslides

Landslides are the most dramatic, sudden, and dangerous types of mass wasting. Landslides are sudden falls of rock; by contrast, avalanches are sudden falls of snow.

Weathered material may fall away from a cliff because there is nothing to keep it in place. Rocks that fall to the base of a cliff make a talus slope. Sometimes as one rock falls, it hits another rock, which hits another rock, and begins a landslide.

A landslide can be very destructive (Figure below). It may bury or carry away entire villages. Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides can move as fast as 200 to 300 km/hour.

This landslide in California in 2008 blocked Highway 140.

Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. Wet soil becomes slippery and heavy. Earthquakes often trigger landslides. The shaking ground causes soil and rocks to break loose and start sliding.

Mudflows and Lahars

A mudflow is the sudden flow of mud down a slope because of gravity. Mudflows occur where the soil is mostly clay. Like landslides, mudflows usually occur when the soil is wet. Wet clay forms very slippery mud that slides easily. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path.

A lahar is mudflow that flows down a composite volcano (Figure below). Ash mixes with snow and ice melted by the eruption to produce hot, fast-moving flows. The lahar caused by the eruption of Nevado del Ruiz in Colombia in 1985 killed more than 23,000 people.

A lahar is a mudflow that forms from volcanic ash and debris. This lahar comes off of Tunguragua volcano in Ecuador.

Slump and Creep

Less dramatic types of mass wasting move Earth materials slowly down a hillside. Slump is the sudden movement of large blocks of rock and soil down a slope. (Figure below). All the material moves together in big chunks. Slumps may happen when a layer of slippery, wet clay is underneath the rock and soil on a hillside. Or they may occur when a river (or road) undercuts a slope. Slump leaves behind crescent-shaped scars on the hillside.

Slump material moves as a whole unit, leaving behind a crescent shaped scar.

Creep is the very slow movement of rock and soil down a hillside. Creep occurs so slowly you can’t see it happening. You can see only the effects of creep after years of movement (Figure below). The slowly moving ground causes trees, fence posts, and other structures on the surface to tilt downhill.

As the hillside moves down slope a tree tries to stand up straight. The tree ends up with a bent trunk.

Prevention and Awareness

Landslides cause $1 billion to $2 billion of damage in the United States each year. Mass wasting is responsible for traumatic and sudden loss of life and homes in many areas of the world.

Some communities have developed landslide warning systems. Around San Francisco Bay, the National Weather Service and the U.S. Geological Survey use rain gauges to monitor soil moisture. If soil becomes saturated, the weather service issues a warning. Earthquakes, which may occur on California’s abundant faults, can also trigger landslides.

To be safe from landslides:

• Be aware of your surroundings. Notice changes in the natural world.
• Look for cracks or bulges in hillsides, tilting of decks or patios, or leaning poles or fences when rainfall is heavy. Sticking windows and doors can indicate ground movement. This is because soil pushes slowly against a house and knocks windows and doors out of alignment.
• Look for landslide scars. Landslides are most likely to happen where they have occurred before.
• Plant vegetation and trees on the hillside around your home. This helps hold the soil in place.
• Help to keep a slope stable by building retaining walls. Installing good drainage in a hillside may keep the soil from getting saturated.

Summary

• Landslides are sudden and massive falls of rock down a slope. Landslides may be very destructive or even deadly. Slump and creep are slower types of mass wasting.
• Mudflows or lahars, which are volcanic mudflows, are mass movements that contain a lot of water.
• Mass wasting is more likely to occur on slopes that are wet, have weak rock, or are undercut. An earthquake or other ground shaking can trigger a landslide.
• To avoid being in a landslide, be aware of signs in a hillside, such as cracks or bulges and old landslide scars. To keep a slope stable, install good drainage or build retaining walls.

Review

1. What factors make it more likely that a place will have a landslide?
2. Pretend that you are going to buy a house on a hill. How do you know if the house might slide or creep down the hill?
3. What can be done to prevent landslides?
4. How is a landslide different from a mudflow? How is a lahar different from a mudflow?

Explore More

Use the resources below to answer the questions that follow.

1. What is creep?
2. How do trees compensate for creep?