• landslide
    A landslide can topple trees, rocks, even buildings.

    Photograph by Danielle Stevens, My Shot

    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!

    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.
  • Term Part of Speech Definition Encyclopedic Entry
    agriculture Noun

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

    Encyclopedic Entry: agriculture
    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.

    deforestation Noun

    destruction or removal of forests and their undergrowth.

    destroy Verb

    to ruin or make useless.

    devastate Verb

    to destroy.

    drought Noun

    period of greatly reduced precipitation.

    Encyclopedic Entry: drought
    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.

    erosion Noun

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

    Encyclopedic Entry: erosion
    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.

    ice Noun

    water in its solid form.

    Encyclopedic Entry: ice
    irrigation Noun

    watering land, usually for agriculture, by artificial means.

    Encyclopedic Entry: irrigation
    lahar Noun

    flow of mud and other wet material from a volcano.

    landslide Noun

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

    Encyclopedic Entry: landslide
    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.

    rain Noun

    liquid precipitation.

    Encyclopedic Entry: rain
    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.

    volcanic ash Noun

    fragments of lava less than 2 millimeters across.

    Encyclopedic Entry: volcanic ash
    volcano Noun

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

    Encyclopedic Entry: volcano
    vulnerable Adjective

    capable of being hurt.

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