Mass Wasting in Geology
by Owen Borville
January 30, 2021
Learning, Geology, Science
Mass wasting is the movement of rock and sediment downslope under the influence of gravity and includes debris flows, mudflows, and landslides. Several factors influence the occurrence of mass wasting. A steep slope angle can trigger a slide or flow of sediment or rock. Weathering causes soil material to become weaker and more likely to slide or flow. Climate affects weathering, which in turn affects the chance of mass wasting. A large quantity of water in soil or rock can cause a slide or flow. Vegetation aids slope stability by absorbing water inside the slope and holding the slope material together. The overloading of a slope with material can decrease the strength of the slope and increase the chance of failure. The type of rock or material comprising the slope has an influence in the strength of the slope and therefore the chance of failure. Events such as earthquakes or volcanic eruptions can trigger a slope failure.
Types of mass wasting include falls, slides, and flows. A mass wasting event can involve a combination of mass wasting types and these are called complex movements. The angle of repose is the maximum slope at which sediment grains are stable.
Rock falls are a type of mass movement that occurs when a piece of rock of any size breaks off a steep slope or cliff and falls under the influence of gravity. The breaks occur along fractures or bedding planes in the rock and can be triggered by earthquakes or undercutting of slopes. Undercutting occurs as rock is eroded at the base of a cliff and the rock above becomes unstable and eventually breaks. Slide is a mass movement in which rock or soil material moves downslope under the influence of gravity.
Slump is a slow mass movement occurring on hillsides with a thick cover of loose rocky debris and commonly occurs after heavy rains saturate the soil. Slump can occur for a period of hours or up to several months. Slump can occur because of stream erosion at the base of a slope or by man-made road cuts at the base of a slope.
Mudflow or mudslide is a type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. Mudflows move rapidly downslope under the influence of gravity.
Debris flow is a flowing mixture of water-saturated debris that flows downslope under the force of gravity along channels or stream valleys and resembles masses of wet concrete. Debris flows consist of material varying in size from clay to blocks several tens of meters in dimension. Debris flows are formed when loose masses of unconsolidated wet debris become unstable. Water may be supplied by rainfall, by melting of snow or ice, or by overflow of a crater lake. Debris flows may be formed directly if lava or pyroclastic flows are erupted onto snow and ice. Earthflow is a type of landslide made up of a mixture of water-saturated loose soil, rock, organic matter and air, with a consistency similar to wet cement. Earthflows move downslope under the influence of gravity.
Quick Clay is found in former glaciated areas and is composed of clay and silt-sized particles of bedrock that were finely ground by glaciers and washed into the sea. Following the retreat of the sea, the clay was exposed and became part of the soil. Quick clay may liquefy if sufficiently disturbed by river erosion or by increases in precipitation, earthquakes, and human activities such as excavation and construction.
Solifluction is a slow downward flow of water-saturated soil caused by the meltwater from permafrost. Permafrost is defined on the basis of temperature as soil or rock that remains below zero degrees Celsius (0° C) throughout the year and forms when the ground cools sufficiently in the winter season to produce a frozen layer that persists throughout the following summer.
Creep is a very slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. Creep is most common in places with humid climate. Creep is so slow that it is often not recognized until the effects have taken place. Common effects of creep are slanted trees and utility poles in addition to cracked walls and streets.
by Owen Borville
January 30, 2021
Learning, Geology, Science
Mass wasting is the movement of rock and sediment downslope under the influence of gravity and includes debris flows, mudflows, and landslides. Several factors influence the occurrence of mass wasting. A steep slope angle can trigger a slide or flow of sediment or rock. Weathering causes soil material to become weaker and more likely to slide or flow. Climate affects weathering, which in turn affects the chance of mass wasting. A large quantity of water in soil or rock can cause a slide or flow. Vegetation aids slope stability by absorbing water inside the slope and holding the slope material together. The overloading of a slope with material can decrease the strength of the slope and increase the chance of failure. The type of rock or material comprising the slope has an influence in the strength of the slope and therefore the chance of failure. Events such as earthquakes or volcanic eruptions can trigger a slope failure.
Types of mass wasting include falls, slides, and flows. A mass wasting event can involve a combination of mass wasting types and these are called complex movements. The angle of repose is the maximum slope at which sediment grains are stable.
Rock falls are a type of mass movement that occurs when a piece of rock of any size breaks off a steep slope or cliff and falls under the influence of gravity. The breaks occur along fractures or bedding planes in the rock and can be triggered by earthquakes or undercutting of slopes. Undercutting occurs as rock is eroded at the base of a cliff and the rock above becomes unstable and eventually breaks. Slide is a mass movement in which rock or soil material moves downslope under the influence of gravity.
Slump is a slow mass movement occurring on hillsides with a thick cover of loose rocky debris and commonly occurs after heavy rains saturate the soil. Slump can occur for a period of hours or up to several months. Slump can occur because of stream erosion at the base of a slope or by man-made road cuts at the base of a slope.
Mudflow or mudslide is a type of landslide made up of a mixture of water-saturated rock debris and soil with a consistency similar to wet cement. Mudflows move rapidly downslope under the influence of gravity.
Debris flow is a flowing mixture of water-saturated debris that flows downslope under the force of gravity along channels or stream valleys and resembles masses of wet concrete. Debris flows consist of material varying in size from clay to blocks several tens of meters in dimension. Debris flows are formed when loose masses of unconsolidated wet debris become unstable. Water may be supplied by rainfall, by melting of snow or ice, or by overflow of a crater lake. Debris flows may be formed directly if lava or pyroclastic flows are erupted onto snow and ice. Earthflow is a type of landslide made up of a mixture of water-saturated loose soil, rock, organic matter and air, with a consistency similar to wet cement. Earthflows move downslope under the influence of gravity.
Quick Clay is found in former glaciated areas and is composed of clay and silt-sized particles of bedrock that were finely ground by glaciers and washed into the sea. Following the retreat of the sea, the clay was exposed and became part of the soil. Quick clay may liquefy if sufficiently disturbed by river erosion or by increases in precipitation, earthquakes, and human activities such as excavation and construction.
Solifluction is a slow downward flow of water-saturated soil caused by the meltwater from permafrost. Permafrost is defined on the basis of temperature as soil or rock that remains below zero degrees Celsius (0° C) throughout the year and forms when the ground cools sufficiently in the winter season to produce a frozen layer that persists throughout the following summer.
Creep is a very slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure. Creep is most common in places with humid climate. Creep is so slow that it is often not recognized until the effects have taken place. Common effects of creep are slanted trees and utility poles in addition to cracked walls and streets.