landforms from erosion and deposition by gravity

landforms and gravity

Gravity shapes the Earths surface by moving weathered material from a higher place to a lower one. This occurs in a variety of ways and at a variety of rates, including sudden, dramatic events as well as slow, steady movements that happen over long periods of time. The force of gravity is constant and it is changing the Earths surface right now.

ground shaking

An earthquake, volcanic eruption, or even just a truck going by can shake unstable ground loose and cause a slide. Skiers and hikers may disturb the snow they travel over and set off an avalanche.

prevention and awareness

Landslides cause $1 billion to $2 billion damage in the United States each year and are responsible for traumatic and sudden loss of life and homes in many areas of the world. Some at-risk 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 Californias abundant faults, can also trigger landslides. To be safe from landslides: Be aware of your surroundings and 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 as soil pushes slowly against a house and knocks windows and doors out of alignment. Look for landslide scars because landslides are most likely to happen where they have occurred before. Plant vegetation and trees on the hillside around your home to help hold 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. Hillside properties in the San Francisco Bay Area and elsewhere may be prone to damage from landslides. Geologists are studying the warning signs and progress of local landslides to help reduce risks and give people adequate warnings of these looming threats.

mudflows and lahars

Added water creates natural hazards produced by gravity (Figure 1.2). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. 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 1.3). 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 Columbia in 1985 killed more than 23,000 people.

slump and creep

Less dramatic types of downslope movement move Earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (Figure 1.4). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope. Mudflows are common in southern California. A lahar is a mudflow that forms from vol- canic ash and debris. Slump material moves as a whole unit, leaving behind a crescent shaped scar. The trunks of these trees near Mineral King, California, were bent by snow creeping downhill when the trees were saplings. Click image to the left or use the URL below. URL:

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downslope movement by gravity

Erosion by gravity is called mass wasting. Mass wasting can be slow and virtually imperceptible, or rapid, massive, and deadly. 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.

landslides

Landslides are the most dramatic, sudden, and dangerous examples of Earth materials moved by gravity. Landslides are sudden falls of rock; by contrast, avalanches are sudden falls of snow. When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure 1.1). 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 are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami. 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.

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rock type

Layers of weak rock, such as clay, also allow more landslides. Wet clay is very slippery, which provides an easy surface for materials to slide over.

undercutting

If people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area. When construction workers cut into slopes for homes or roads, they must stabilize the slope to help prevent a landslide (Figure 1.6). Tree roots or even grasses can bind soil together. It is also a good idea to provide drainage so that the slope does not become saturated with water.

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contributing factors

There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot.

water

A little bit of water helps to hold grains of sand or soil together. For example, you can build a larger sand castle with slightly wet sand than with dry sand. However, too much water causes the sand to flow quickly away. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, allowing flow.

instructional diagrams

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questions

which of the following can trigger a landslide?

a) an earthquake.

b) a big rain, especially after a drought.

c) undercutting a steep slope.

-->  d) all of the above.

landslides cause a tiny amount of damage compared to other natural disasters.

a) true

-->  b) false

the best way to be safe from landslides is

a) never get on a sloped surface.

-->  b) be aware of the natural world and changes that might occur.

c) cut the trees around a house on a hill to keep the slope in place.

d) build a wall on the uphill side of your house to catch the dirt as it comes down.

which of the following is not true about landslides?

a) landslides are most likely to happen in a wet year that was proceeded by dry years.

b) landslides may destroy a small community.

c) landslides may move so fast because they ride on a cushion of air.

-->  d) landslides are most common in forested regions.

lahars are extremely damaging because they mix

-->  a) volcanic ash with ice and snow melted by an eruption.

b) a creeping slope and a lot of rainfall.

c) an earthquake with a landslide.

d) a & b

gravity shapes earths surface by moving weathered material down a slope.

-->  a) true

b) false

under what conditions will a mudflow form?

a) a time of drought.

b) heavy vegetation.

-->  c) clay-rich soil.

d) a steep slope with little topography.

what are the signs of creep?

-->  a) tree trunks curve.

b) houses split with half going downhill faster than the other half.

c) tons of mud and rock fly down the slope.

d) a block of rock and soil slumps downward.

conditions that will lead to landslides include

a) slopes that are undercut.

b) layers of weak rock, such as clay.

c) a lot of water, or no water.

-->  d) all of these.

diagram questions

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