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Mass Movement. Section 5.3. Mass Movements. The transfer of rock and soil downslope due to gravity is called mass movement . Ex: Landslides The combination of weathering and mass movement produce most landforms.
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Mass Movement Section 5.3
Mass Movements • The transfer of rock and soil downslope due to gravity is called mass movement. • Ex: Landslides • The combination of weathering and mass movement produce most landforms. • Once weathering weakens and breaks rock apart, mass movement moves the debris downslope. • There a stream usually carries is away. • Stream valleys are the most common of Earth’s landforms.
Mass Movements • Several factors make slopes more susceptible to the pull of gravity. • Saturation of surface materials with water. • Oversteepening of slopes. • Removal of vegetation. • Earthquakes. • Water: • Heavy rains and rapid melting of snow can trigger mass movement by saturating surface materials with water. • When the pores in sediment become filled with water, the particles slides past one another easily.
Mass Movements 2. Oversteepened Slopes: • Loose soil particles can maintain a relatively stable slope up to a certain angle (25 to 40°), depending on the size and shape of the particles. • If the steepness of the slope exceeds the stable angle, mass movement is likely. • Such slopes are said to be oversteepened. • This can result when: • Streams undercut a valley wall. • Waves pound against the base of a cliff. • People, through excavation during construction of roads/buildings.
Mass Movements 3. Removal of Vegetation: • Plants make slopes more stable because of their root systems. • When plants are removed, mass movements are likely. 4. Earthquakes: • Earthquakes are one of the most dramatic triggers of mass movements. • They can dislodge rock and unconsolidated material, which can cause more damage than the earthquake itself.
The process responsible for moving material downslope under the influence of gravity is called • Erosion • Weathering • Mass movement • Soil formation
What is the force behind mass movements? • The Sun’s energy • Flowing water • Gravity • Moving ice
Which of the following is not true about mass movements? • Some mass movements are too slow to be seen. • Mass movements always lead to landslides. • Gravity is the driving force behind all mass movements. • Mass movements are always downslope.
What factor commonly triggers mass movements? • Saturation of surface materials with water. • Earthquakes • Removal of vegetation • All of the above
Why can the removal of vegetation trigger mass movements? • The soil loses nutrients and begins the crumble. • The plant roots bind the soil and regolith together. • The shaking triggers mass movements. • The plant roots lubricate the loose sediment.
Oversteepened slopes often lead to mass movements because • Plants cannot grow on them. • The angle of their slope is between 10 and 20 degrees. • The angle of their slope is less than 20 degrees. • The angle of their slope is greater than 40 degrees.
During what season would you expect mass movements to be a greater threat? • A dry summer. • A wet spring before vegetation is growing. • A wet spring with lots of growing vegetation. • A dry autumn after the leaves have turned.
Mass Movements • Geologists classify mass movements based on the kind of material that moves, how it moves, and the speed of the movement. • Rockfalls: • Occurs when rocks or rock fragments fall freely through the air. • Common of slopes that are too steep for loose material to remain on the surface. • Result from the mechanical weathering of rock caused by freeze-thaw cycles or plant roots. • Sometimes trigger other mass movements.
Mass Movements 2. Slides: • A block of material moves suddenly along a flat, inclined surface., • Slides that include segments of bedrock are called rockslides. • Often occur in high mountain areas. • i.e. Andes, Alps, Rockies. • Rockslides are among the fastest mass movements (speeds over 200 kmph ≈ 125 mph). • Triggered by rain or melting snow.
Mass Movements 3. Slumps: • Is the downward movement of a block of material along a curved surface. • Usually does not travel very fast or very far. • Slumps leave a crescent-shaped cliff just above the slump. • Common on oversteepened slopes where the soil contains thick accumulations of clay.
Mass Movements 4. Flows: • Mass movements of material containing a large amount of water, which move downslope as a thick liquid. • Flows that move quickly, called mudflows, are common in semiarid mountainous regions. • i.e. Southern California • Follows the contours of the canyon, taking trees and boulders with it.
Mass Movements • Earthflows are flows that move relatively slow – from about a millimeter per day to several meters per day, and may continue for years. • Occurs most often on hillsides in wet regions. • When water saturates the soil and regolith on a hillside, the material breaks away, forming a tongue-shaped mass. • They range in size from a few meters long and less than 1 m deep to over 1 km long and more than 10 m deep.
Mass Movements 5. Creep: • The slowest type of mass movement. • Usually only travels a few millimeters or centimeters per year. • Because it is slow, you cannot directly observe it. • Alternating between freezing and thawing contributes to creep. • Effects are easy to recognize: • Structures once vertical tilt downhill. • Displacement of fences. • Cracks in walls and underground pipes.
A mass movement that involves the sudden movement of a block of material along a flat, inclined surface is called a • Slide • Rockfall • Slump • Flow
When a block of material moves downslope along a curved surface, the type of mass movement is called • A rockfall • A rockslide • A slump • Creep
What is the slowest type of mass movement? • A slump • A rockfall • An earthflow • Creep
A relatively rapid form of mass movement that is most common in dry mountainous regions is • Creep • A mudflow • A slump • An earthflow
Which of the following statements best describes a slump? • Slippage of a block of material moving along a curved surface. • Blocks of rock sliding down a slope. • Rapid flow of water-saturated debris, most common in mountainous regions. • Slow downhill movement of soil and regolith.
Alternate freezing and thawing often leads to • Creep • Slumps • Mudflows • Earthflows
