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Earth Science 5.3 Mass Movements. Mass Movements. Key Concepts. Today students will identify. . . What is a mass movement What factors trigger mass movements How do geologists classify mass movements. Mass Movements.
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Earth Science 5.3 Mass Movements Mass Movements
Key Concepts • Today students will identify. . . • What is a mass movement • What factors trigger mass movements • How do geologists classify mass movements
Mass Movements • Earth’s land surface is not always flat, it consists of many sloped areas that are always changing. The force of gravity causes materials to move downslope. • The transfer of rock and soil downslope due to gravity is called mass movement
Mass Movements • The combined actions of weathering and mass movement produce most landslides. • Once weathering weakens and breaks rocks apart, mass movement carries the debris downslope. • There a stream usually carries it way. Streams are the most common of Earth’s landforms.
Triggers of Mass Movements • Gravity is the force behind mass movements however several factors make slopes more susceptible to the pull of gravity. • Among the factors that commonly trigger mass movements are • Saturation of surface materials with water • Oversteepening of slopes • Removal of vegetation • earthquakes
Water • Heavy rains and rapid melting of snow can trigger mass movements by saturating surface materials with water. • When the pores of sediments get filled with water, the particles slide past one another more easily resulting in mudslides
Oversteepened Slopes • Loose soil particles can maintain a relatively stable slope up to a certain angle. That angle ranges from about 25 to 40 degrees, depending on the size and shape of the particles. • If the angle of the slope exceeds the stable mass, mass movements become more likely . Such slopes are said to be oversteepened.
Removal of Vegetation • Plants make slopes more stable because their root systems bind the soil. • When plants are removed by forest fire or human activity (such as logging) the likelihood of a mass movement increases.
Earthquakes • Earthquakes are one of the most dramatic triggers of mass movements. • An earthquake and it’s aftershocks can dislodge enormous amounts of rock. In many areas these mass movements create more damage than the vibrations of the earthquake.
Types of Mass Movements • Scientists clarify mass movements based on the kind of material that moves, and the speed of the movement. • There are five basic types of mass movement • Rockfalls • Slides • Slumps • flows • creeps
Rockfalls • A rockfall occurs when rocks or rock fragments fall freely through the air. • This type of mass movement is common on slopes that are too steep for loose material to remain on the surface. • Many rockfalls occur from the mechanical weathering of rock caused by the freeze-thaw cycles or plant roots.
Slides • In a slide, a block of material moves suddenly along a flat, inclined surface. • Slides that include segments of bedrock are called rockslides • They often occur in high mountain areas. Rockslides are among the fastest mass movements reaching speeds of over 200 kilometers per hour.
Slumps • A slump is the downward movement of a block of material along a curved surface. • The material in a slump does not generally travel very fast or very far. • Slumps leave a crescent shaped cliff just above the slump. They are common in oversteepened slopes with thick accumulations of clay.
Flows • Flows are mass movements of materials containing a large amount of water, which move downslope as a thick fluid. • Flows that move quickly, called mudflows, are common in semiarid mountainous regions. In those regions protective vegetation is sparse. A heavy downpour or rapid snowmelt can flood canyons with a mixture of soil, rock, and water.
Earthflows • Earthflows are flows that move relatively slowly; from a millimeter to several meters per day. • Their movement may continue for years. • Earthflows occur in hillsides in wet regions. When water saturates the soil on a hillside, the material breaks away forming a tongue-shaped mass. • Earthflows range in size from a few meters to over 1 kilometer long.
Creep • The slowest type of mass movement is creep, which usually travels only a few millimeters or centimeters per year. • One factor that contributes to creep is alternating between freezing and thawing. Freezing expands the water in soil , lifting particles at right angles to the slope. Thawing causes contractions , which allows the particles to fall back toward the slope. • Each freeze-thaw cycle moves the particles a short distance downhill.
Concept Review. . . . • The transfer of rock and soil downslope due to gravity is called mass movement • Among the factors that commonly trigger mass movements are saturation of surface materials with water, oversteepening of slopes, removal of vegetation, and earthquakes • Geologists classify mass movements based on the kinds of materials that move, how it moves, and the speed of movement
Computer Lab • Go to the computer lab and write a short essay answering the following question: Your answer should be one page double-spaced with full clear sentences. • Loggers want to clear cut all the trees on a steep mountain slope that rolls down to the town below. This is northern Oregon and the rainy season is coming. The previous winter had extreme weather changes with many freeze-thaw cycles in rapid succession. • Explain carefully why clearcutting all the trees would be a bad thing at this time and what may be at risk if the trees are all eliminated. • List three types of preventative actions one could take to minimize these risks and explain why they are good preventative actions.
