2 Key Concepts Today students will identify. . . What is a mass movement What factors trigger mass movementsHow do geologists classify mass movements
3 Mass MovementsEarth’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
4 Mass MovementsThe 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.
5 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 areSaturation of surface materials with waterOversteepening of slopesRemoval of vegetationearthquakes
6 WaterHeavy 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
7 Oversteepened SlopesLoose 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.
8 Removal of VegetationPlants 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.
9 EarthquakesEarthquakes 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.
10 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 movementRockfallsSlidesSlumpsflowscreeps
11 RockfallsA 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.
12 SlidesIn a slide, a block of material moves suddenly along a flat, inclined surface.Slides that include segments of bedrock are called rockslidesThey often occur in high mountain areas. Rockslides are among the fastest mass movements reaching speeds of over 200 kilometers per hour.
13 SlumpsA 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.
14 FlowsFlows 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.
15 EarthflowsEarthflows 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.
16 CreepThe 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.
17 Concept ReviewThe transfer of rock and soil downslope due to gravity is called mass movementAmong the factors that commonly trigger mass movements are saturation of surface materials with water, oversteepening of slopes, removal of vegetation, and earthquakesGeologists classify mass movements based on the kinds of materials that move, how it moves, and the speed of movement
18 Computer LabGo 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.