Describe the relationship between mass movement and gravity
Gravity is a common source of interaction between everything in nature, even in space that is formulated as spacetime. If we want describe the gravity interact. 4) Describe the relationship between mass movement and gravity. Mass movement is the down slope movement of earth materials under the. Mass movement is the movement of surface material caused by gravity. Water aids in the downslope movement of surface material in several ways. than 5, feet of elevation may exist between the ridge tops and the canyon bottoms.
Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.
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Slurry Flows high amounts of water Solifluction - flowage at rates measured on the order of centimeters per year of regolith containing water. Solifluction produces distinctive lobes on hill slopes see figure These occur in areas where the soil remains frozen and is then is thawed for a short time to become saturated with water.
Debris Flows- these occur at higher velocities than solifluction, and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down hill forming lobes with an irregular surface consisting of ridges and furrows. Mudflows- a highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams.
Thus, after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous. Mudflows on volcanoes are called lahars. Granular Flows low amounts of water Creep- the very slow, usually continuous movement of regolith down slope.
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Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles see figure They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff Grain Flows - usually form in relatively dry material, such as a sand dune, on a steep slope.
A small disturbance sends the dry unconsolidated grains moving rapidly down slope. Debris Avalanches - These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope.
They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions. Snow avalanches are similar, but usually involve only snow. Mass-Movements in Cold Climates Mass movements in cold climates is governed by the fact that water is frozen as ice during long periods of the year.
Ice, although it is solid, does have the ability to flow, and freezing and thawing cycles can also contribute to movement. Rock Glaciers - a lobe of ice-cemented rock debris mostly rocks with ice between the blocks that slowly moves downhill see figure Subaqueous Mass movement Mass movements also occur on slopes in the ocean basins.
Most slope failure can occur due to over-accumulation of sediment on slope or in a submarine canyon, or could occur as a result of a shock like an earthquake. Gigantic submarine slope failures are widespread on the ocean floor, particularly around islands like Hawaii and off the east and gulf coasts of North America. They are much larger than land-based slope failures and are an important process sculpting adjacent land.
When they occur, they create catastrophic tsunamis.
Triggering Events A mass movement can occur any time a slope becomes unstable. Sometimes, as in the case of creep or solifluction, the slope is unstable all of the time and the process is continuous.
But other times, triggering events can occur that cause a sudden instability to occur. Here we discuss major triggering events, but it should be noted that it if a slope is very close to instability, only a minor event may be necessary to cause a failure and disaster. This may be something as simple as an ant removing the single grain of sand that holds the slope in place. Shocks and vibrations - A sudden shock, such as an earthquake may trigger slope instability.
Minor shocks like heavy trucks rambling down the road, trees blowing in the wind, or human made explosions can also trigger mass movement events.
Slope Modification - Modification of a slope either by humans or by natural causes can result in changing the slope angle so that it is no longer at the angle of repose. A mass movement can then restore the slope to its angle of repose. Undercutting - streams eroding their banks or surf action along a coast can undercut a slope making it unstable.
Changes in Hydrologic Characteristics - heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass movement. Heavy rains can also saturate rock and increase its weight. Changes in the groundwater system can increase or decrease fluid pressure in rock and also trigger mass movements. Changes in slope strength - Weathering creates weaker material, and thus leads to slope failure.
Vegetation holds soil in place and slows the influx of water. Trees put down roots that hold the ground together and strengthen the slope. Removal of tress and vegetation either by humans or by a forest fire, often results in slope failures in the next rainy season. Volcanic Eruptions - produce shocks like explosions and earthquakes. They can also cause snow to melt or discharges from crater lakes, rapidly releasing large amounts of water that can be mixed with regolith to reduce grain to grain contact and result in debris flows, mudflows, and landslides.
Assessing Mass Movement Hazards As we have seen mass movements can be extremely hazardous and result in extensive loss of life and property. But, in most cases, areas that are prone to such hazards can be recognized with some geologic knowledge, slopes can be stabilized or avoided, and warning systems can be put in place that can reduce vulnerability.
Because there is usually evidence in the form of distinctive deposits and geologic structures left by recent mass movements, it is possible to construct maps of all areas prone to possible landslide hazards see the National Landslide hazards map at http: Detailed local maps can usually be obtained from individual state agencies.
Planners can use such hazards maps to make decisions about land use policies in such areas or, as will be discussed below, steps can be taken to stabilize slopes to attempt to prevent a disaster or minimize its effects.
Short-term prediction of mass-wasting events is somewhat more problematical. For earthquake triggered events, the same problems that are inherent in earthquake prediction are present. Slope destabilization and undercutting triggered events require constant monitoring. Mass movement hazards from volcanic eruptions can be predicted with the same degree of certainty that volcanic eruptions can be predicted, but again, the threat has to be realized and warnings need to be heeded.
Hydrologic conditions such as heavy precipitation can be forecast with some certainty, and warnings can be issued to areas that might be susceptible to mass movement processes caused by such conditions. Still, it is difficult of know exactly which hill slope of the millions that exist will be vulnerable to an event triggered by heavy rainfall. Some warning signs can be recognized by observations of things around you: Springs, seeps, or saturated ground in areas that have not typically been wet before.
New cracks or unusual bulges in the ground, street pavements or sidewalks. Soil moving away from foundations. Tilting or cracking of concrete floors and foundations. Broken water lines and other underground utilities. Leaning telephone poles, trees, retaining walls or fences Offset fence lines.
Sunken or down-dropped road beds. Rapid increase in creek water levels, possibly accompanied by increased turbidity soil content.
Sudden decrease in creek water levels though rain is still falling or just recently stopped. Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb. A faint rumbling sound that increases in volume is noticeable as the landslide nears.
Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris. Thus, all slopes should be assessed for potential landslide hazards. Mass movements can sometimes be avoided by employing engineering techniques to make the slope more stable. Steep slopes can be covered or sprayed with concrete covered or with a wire mesh to prevent rock falls.
Retaining walls could be built to stabilize a slope. If the slope is made of highly fractured rock, rock bolts may be emplaced to hold the slope together and prevent failure. Drainage pipes could be inserted into the slope to more easily allow water to get out and avoid increases in fluid pressure, the possibility of liquefaction, or increased weight due to the addition of water. Oversteepened slopes could be graded to reduce the slope to the natural angle of repose.
In mountain valleys subject to mudflows, plans could be made to rapidly lower levels of water in human-made reservoirs to catch and trap the mudflows. Trees or other vegetation could be planted on bare slopes to help hold soil. Some slopes, however, cannot be stabilized, or only stabilized at great expense. In these cases, humans should avoid these areas or use them for purposes that will not increase susceptibility of lives or property to mass movement hazards.
Conclusion Hopefully the information provided here will help you avoid death or loss of your property by mass movement Examples of questions on this material that could be asked on an exam What is the main force responsible for mass movements? How is this force affected by slope angle? In what ways does water added to a slope affect its stability? Define the following a angle of repose, b sensitive soils, c hydrocompacting clays, d groundwater, e quick clays What features in rock reduce the stability of slopes?
What are the major triggering events for mass movements? Is it possible to determine whether or not a slope has stability problems? The variety of downslope mass movements reflects the diversity of factors that are responsible for their origin. These factors affecting slope conditions will often combine with climatic factors such as precipitation and frost activity to produce downslope mass movement.
The types of mass movements caused by the above factors include: Sinking mass movements occur in relatively rapid fashion, known as subsidenceand in a gradual manner, called settlement. Subsidence involves a roof collapse or breakdown of a subsurface cavity such as a cave.
Extensive subsidence is evident in areas where coal, salt, and metalliferous ores are mined. Marine erosion sometimes causes the roof collapse of sea caves. Regions of karst topography will exhibit widespread subsidence in the form of sinkholes caused by underground drainage.
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Other types of subsidence caused by underground solutions have been found in chalk, gypsum, anhydrite, halite saltand loess terrains. The melting of ground ice also contributes to subsidence such as the formation of glacial kettles and depressions following the seasonal surface thaw of perennially frozen land.
The chemical decomposition of subsurface rocks and ores is also a cause of subsidence.