A landslide is the movement of rock, earth, or debris down a sloped section of land. Landslides are caused by rain, earthquakes, volcanoes, or other factors that make the slope unstable
 
Geologists, scientists who study the physical formations of the Earth, sometimes describe landslides as one type of mass wasting. A mass wasting is any downward movement in which the Earth's surface is worn away. Other types of mass wasting include rockfalls and the flow of shore deposits called alluvium
 
Near populated areas, landslides present major hazards to people and property. Landslides cause an estimated 25 to 50 deaths and $3.5 billion in damage each year in the United States.
 
What Causes Landslides?
 
Landslides have three major causes: geology, morphology, and human activity.
 
Geology refers to characteristics of the material itself. The earth or rock might be weak or fractured, or different layers may have different strengths and stiffness.
 
Morphology refers to the structure of the land. For example, slopes that lose their vegetation to fire or drought are more vulnerable to landslides. Vegetation holds soil in place, and without the root systems of trees, bushes, and other plants, the land is more likely to slide away.
 
A classic morphological cause of landslides is erosion, or weakening of earth due to water. In April 1983, the town of Thistle, Utah, experienced a devastating landslide brought on by heavy rains and rapidly melting snow. A mass of earth eventually totaling 305 meters wide, 61 meters thick, and 1.6 kilometers long (1,000 feet wide, 200 feet thick, and one mile long) slid across the nearby Spanish Fork River, damming it and severing railroad and highway lines. The landslide was the costliest in U.S. history, causing over $400 million in damage and destroying Thistle, which remains an evacuated ghost town today.
 
Human activity, such as agriculture and construction, can increase the risk of a landslide. Irrigation, deforestation, excavation, and water leakage are some of the common activities that can help destabilize, or weaken, a slope.
 
Types of Landslides
 
There are many ways to describe a landslide. The nature of a landslide's movement and the type of material involved are two of the most common.
 
Landslide Movement
There are several ways of describing how a landslide moves. These include falls, topples, translational slides, lateral spreads, and flows.
 
In falls and topples, heavy blocks of material fall after separating from a very steep slope or cliff. Boulders tumbling down a slope would be a fall or topple.
 
In translational slides, surface material is separated from the more stable underlying layer of a slope. An earthquake may shake the loosen top layer of soil from the harder earth beneath in this type of landslide.
 
A lateral spread or flow is the movement of material sideways, or laterally. This happens when a powerful force, such as an earthquake, makes the ground move quickly, like a liquid.
 
Landslide Material
A landslide can involve rock, soil, vegetation, water, or some combination of all these. A landslide caused by a volcano can also contain hot volcanic ash and lava from the eruption. A landslide high in the mountains may have snow and snowmelt.
 
Volcanic landslides, also called lahars, are among the most devastating type of landslides. The largest landslide in recorded history took place after the 1980 eruption of Mount St. Helens in the U.S. state of Washington. The resulting flow of ash, rock, soil, vegetation and water, with a volume of about 2.9 cubic kilometers (0.7 cubic miles), covered an area of 62 square kilometers (24 square miles).
 
Other Factors
Another factor that might be important for describing landslides is the speed of the movement. Some landslides move at many meters per second, while others creep along at an centimeter or two a year. The amount of water, ice, or air in the earth should also be considered. Some landslides include toxic gases from deep in the Earth expelled by volcanoes. Some landslides, called mudslides, contain a high amount of water and move very quickly. Complex landslides consist of a combination of different material or movement types.
landslide
A landslide can topple trees, rocks, even buildings.

Martian Landslide
In December 2008, scientists announced that they had found evidence of the largest landslide ever. Because of a giant asteroid impact billions of years ago, the smooth northern hemisphere of Mars is sharply separated from the irregular southern highlands. Arabia Terra, a previously unexplained plateau between the two regions, is thought to have been formed by an enormous landslide immediately after the impact. The land mass that slid north to form Arabia Terra was the size of the entire United States!

Noun

the art and science of cultivating land for growing crops (farming) or raising livestock (ranching).

alluvium
Noun

gravel, sand, and smaller materials deposited by flowing water.

boulder
Noun

large rock.

bush
Noun

low-lying plant with many branches.

characteristic
Noun

physical, cultural, or psychological feature of an organism, place, or object.

complex
Adjective

complicated.

construction
Noun

arrangement of different parts.

costly
Adjective

expensive or having a lot of value.

creep
Verb

to move slowly and close to the ground.

dam
Verb

to block a flow of water.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

Noun

destruction or removal of forests and their undergrowth.

destroy
Verb

to ruin or make useless.

devastate
Verb

to destroy.

Noun

period of greatly reduced precipitation.

earth
Noun

soil or dirt.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

Noun

act in which earth is worn away, often by water, wind, or ice.

eruption
Noun

release of material from an opening in the Earth's crust.

estimate
Verb

to guess based on knowledge of the situation or object.

evacuate
Verb

to leave or remove from a dangerous place.

excavation
Noun

area that has been dug up or exposed for study.

expel
Verb

to eject or force out.

fall
Noun

movement of pieces of rock or soil downward in a landslide.

fire
Noun

a chemical process that releases heat and light due to burning.

flow
Noun

quick movement of material in a landslide, as if it were liquid.

fracture
Verb

to break.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

geologist
Noun

person who studies the physical formations of the Earth.

geology
Noun

study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

ghost town
Noun

urban area that has been abandoned by all residents.

hazard
Noun

danger or risk.

highway
Noun

large public road.

Noun

water in its solid form.

Noun

watering land, usually for agriculture, by artificial means.

lahar
Noun

flow of mud and other wet material from a volcano.

Noun

the fall of rocks, soil, and other materials from a mountain, hill, or slope.

lateral spread
Noun

movement of material sideways during a landslide.

lava
Noun

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

mass wasting
Noun

downward movement of rock, soil, and other material.

morphology
Noun

study of the form and structure of organisms or materials.

mudslide
Noun

rapid, downhill flow of soil and water. Also called a mudflow.

plant
Noun

organism that produces its own food through photosynthesis and whose cells have walls.

railroad
Noun

road constructed with metal tracks on which trains travel.

Noun

liquid precipitation.

rock
Noun

natural substance composed of solid mineral matter.

rockfall
Noun

sudden descent of large rocks.

root system
Noun

all of a plant's roots.

scheme
Noun

structure or diagram of the way information is studied, documented, and understood.

sever
Verb

to separate or cut away.

slope
Noun

slant, either upward or downward, from a straight or flat path.

snow
Noun

precipitation made of ice crystals.

snowmelt
Noun

water supplied by snow.

topple
Noun

movement of smaller pieces of rock or soil downward in a landslide.

toxic
Adjective

poisonous.

translational slide
Noun

movement of all surface material (including rocks, soil, and vegetation) downward during a landslide.

tree
Noun

type of large plant with a thick trunk and branches.

unstable
Adjective

unsteady or likely to fall apart.

vegetation
Noun

all the plant life of a specific place.

Noun

fragments of lava less than 2 millimeters across.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

vulnerable
Adjective

capable of being hurt.