Download An overview of mass movement

Concept
The term mass wasting (sometimes called mass movement) encompasses a broad array of processes whereby earth material is transported down a slope by the force of
gravity. It is related closely to weathering, which is the breakdown of minerals or rocks at or near Earth's surface through physical, chemical, or biological processes,
and to erosion, the transport of material through a variety of agents, most of them flowing media, such as air or water. Varieties of mass wasting are classified
according to the speed and force of the process, from extremely slow creep to very rapid, dramatic slide or fall. Examples of rapid mass wasting include landslides and
avalanches, which can be the cause of widespread death and destruction when they occur in populated areas.
How It Works
Moving Earth and Rocks
In discussing mass wasting, the area of principal concern is Earth's surface rather than its interior. Thus, mass wasting is related most closely to the realm
of geomorphology, a branch of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description
and classification of various physical features on Earth. Though plate tectonics (which involves the movement of giant plates beneath the earth's surface) can
influence mass wasting, plate tectonics entails interior processes that humans usually witness only indirectly, by seeing their effects. Mass wasting, on the other hand,
often can be observed directly, particularly in its more rapid forms, such as rock fall.
There are three general processes whereby a piece of earth material can be moved from a high outcropping to the sea: weathering, mass wasting, or erosion. If
mechanical, biological, or chemical processes act on the material, dislodging it from a larger sample of material (e.g., separating a rock from a boulder), it is an
example of weathering, which is discussed later in this essay. Supposing that a rock has been broken apart by weathering, it may be moved further by mass-wasting
processes, such as creep or fall. Pieces of rock swept away by a river in a valley below the outcropping and small bits of rock worn away by high winds are examples of
erosion. Erosion and weathering are examined in separate essays within this book.
As for the relationships between erosion, weathering, and mass wasting, the lines are not clearly drawn. Some authors treat weathering and mass wasting as varieties
of erosion, and some apply a strict definition of erosion as resulting only from flowing media. (In the physical sciences, fluid means anything that flows, not just
liquids.) Weathering, mass wasting, and erosion also can be viewed as stages in a process, as described in the preceding paragraph. This broad array of approaches,
while perhaps confusing, only serves to illustrate the fact that the earth sciences are relatively young compared with such ancient disciplines as astronomy and
biology. Not all definitions in the earth sciences are, as it were, "written in stone."
Weathering
A mineral is a substance that occurs naturally, is usually inorganic, and typically has a crystalline structure. The term organic does not necessarily mean "living" rather,
it refers to all carbon-containing compounds other than oxides, such as carbon dioxide, and carbonates, which are often found in Earth's rocks. A crystalline solid is
one in which the constituent parts have a simple and definite geometric arrangement repeated in all directions.
Rocks, scientifically speaking, are simply aggregates or combinations of minerals or organic material or both, and weathering is the process whereby rocks and
minerals are broken down into simpler materials. Weathering is the mechanism through which soil is formed, and therefore it is a geomorphologic process essential to
the sustenance of life on Earth. There are three varieties of weathering: physical or mechanical, chemical, and biological.
The Three Types of Weathering
Physical or mechanical weathering involves such factors as gravity, friction, temperature, and moisture. Gravity, for instance, may cause a rock to drop from a height,
such that it falls to the ground and breaks into pieces. If wind-borne sand blows constantly across a rock surface, the friction will have the effect of sandpaper,
producing mechanical weathering. In addition, changes in temperature and moisture will cause expansion and contraction of materials, bringing about sometimes
dramatic changes in their physical structure.
Chemical weathering not only is a separate variety of weathering but also is regarded as a second stage, one that follows physical weathering. Whereas physical
changes are typically external, chemical changes affect the molecular structure of a substance, bringing about a rearrangement in the ways that atoms are bonded.
Important processes that play a part in chemical weathering include acid reactions, hydrolysis(a reaction with water that results in the separation of a compound to
form a new substance or substances), and oxidation. The latter can be defined as any chemical reaction in which oxygen is added to or hydrogen is removed from a
substance.
An example of biological weathering occurs when a plant grows from a crevice in a rock. As the plant grows, it gradually forces the sides of the crevice apart even
further, and it ultimately may tear the rock apart. Among the most notable agents of biological weathering are algae and fungi, which may be combined in a mutually
beneficial organism called a lichen. (Reindeer moss is an example of a lichen.) Through a combination of physical and chemical processes, organisms ranging from
lichen to large animals can wear away rock gradually.
Properties of Unconsolidated Material
Regolith is a general term that describes a layer of weathered material that rests atop bedrock. It is unconsolidated, meaning that it is in pieces, like gravel, though
much less uniform in size. Sand and soil, including soil mixed with loose rocks, are examples of regolith.
Every variety of unconsolidated material has its own angle of repose, or the maximum angle at which it can remain standing. Everyone who has ever attempted to
build a sand castle at the beach has experienced angle of repose firsthand, perhaps without knowing it. Imagine that you are trying to build a sand castle with a steep
roof. Dry sand would not be good for this purpose, because it is loose and has a tendency to flow easily. Much better would be moist sand, which can be shaped into a
sharper angle, meaning that it has a higher angle of repose.
A certain amount of water gives sand surface tension, the same property that causes water to bead up on a table rather than lying flat. If too much water is added to
the sand, however, the sand becomes saturated and will flow, a process called lateral spreading. Thus, to a point, the addition of water increases the angle of repose
for sand, which is only about 34° when the sand is dry. (This is the angle of repose for sand in an hourglass.) On the other hand, piles of rocks may have an angle of
repose as high as 45°. In practice, most aggregates of materials in nature have slopes less than their angle of repose, owing to the influence of wind and other erosive
forces.
Types of Mass Wasting
As noted earlier, there is some disagreement among writers in the geologic sciences regarding the types of mass wasting. Indeed, even the term mass wasting is not
universal, since some writers refer to it as mass movement. Others do not even treat the subject as a category unto itself, preferring instead to address related
concepts, such as weathering and erosion, as well as instances of mass wasting, such as avalanches and landslides.
For this reason, the classification of mass-wasting processes presented here is by no means universal and instead represents a composite of several schools of thought.
Generally speaking, geologists and geomorphologists classify processes of mass wasting according to the rapidity with which they occur. Most sources recognize at least
three types of mass wasting: flow, slide, and fall. Some sources include slump among the categories of relatively rapid mass-wasting process, as opposed to the
slower, less dramatic (but ultimately more important) process known as creep. Some writers classify uplift and subsidence with mass wasting; however, in this book,
uplift and subsidence are treated separately, in the Geomorphology essay.
Real-Life Applications
Creep
Creep is the slow downward movement of regolith as a result of gravitational force. Before the initiation of the creeping process, the regolith is in what physicists call
a condition of unstable equilibrium: it remains in place, yet a relatively small disturbance would be enough to dislodge it. Though it is slow, creep can produce some
of the most dramatic results over time. It can curve tree trunks at the base, break or overturn retaining walls, and cause objects from fence posts to utility poles to
tombstones to be overturned.
Changes in temperature or moisture are among the leading factors that result in the disturbance of regolith. A change in either can cause material to expand or
contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. In fact, some geomorphologists cite a distinct masswasting process, known as solifluction, that occurs in the active layer of permafrost, which thaws in the summertime. Water also can provide lubrication or additional
weight that assists the material in moving. One of the only causes of creep not associated with changes in temperature or moisture is the burrowing of small animals.
Slump and Slide
Slump occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl). The result is the formation of a small, crescentshaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Soil flow takes place at the bottom end of the slump. One is likely to see slumps in
any place where forces, whether man-made or natural, have graded material to a slope too steep for its angle of repose. This may happen along an interstate
highway, where a road crew has cut the slope too sharply, or on a riverbank, where natural erosion has done its work.
Often, slump is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat
or planar surface. These movements sometimes are called rock slides, debris slides, or, in common parlance, landslides. Among the most destructive types of mass
wasting, they may be set in motion by earthquakes, which are caused by plate tectonic processes, or by hydrologic agents (i.e., excessive rain or melting snow and
ice).
Flow
When a less uniform, or more chaotic, mass of material moves rapidly downslope, it is called flow. Flow is divided into categories, depending on the amounts of water
involved: granular flow (0-20% water) and slurry flow (20-40% water). Creep and solifluction often are classified as very slow forms of granular and slurry flow,
respectively. In order of relative speed, these categories are as follows:
Granular Flow (0-20% Water)
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Slowest: Creep
Slower: Earth flow
Faster: Grain flow
Fastest: Debris avalanche
Slurry Flow (20-40% Water)
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Slow: Solifluction
Medium: Debris flow
Fast: Mudflow
Earth flow moves at a rate anywhere from 3.3 ft. (1 m) per year to 330 ft. (100 m) per hour. Grain flow can be nearly 60 mi. (100 km) per hour, and debris
avalanche may achieve speeds of 250 mi. (400 km) per hour, making it extremely dangerous. Among types of slurry flow, debris flow is roughly analogous to earth
flow, falling into a range from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Mudflow is slightly faster than grain flow. If the water content is more than
40%, a slurry flow is considered a stream.
Earth flows involve fine-grained materials, such as clay or silt, and typically occur in humid areas after heavy rains or the melting of snow. Debris flows usually result
from heavy rains as well and may start with slumps before flowing downhill, forming lobes with a surface broken by ridges and furrows. Grain flows can be caused by a
small disturbance, which forces the dry, unconsolidated material rapidly downslope. Debris avalanches are commonly the result of earthquakes or volcanic eruptions.
Seismic disturbances or volcanic activity may cause the collapse of a mountain slope, sending debris avalanches moving swiftly even along the gentler slopes of
the mountainside. Likewise, mudflows may be the result of volcanic activity, in which case they are known as lahars. In some situations, the material in a lahar is
extremely hot. Mudflows tend to be highly fluid mixtures of sediment (material deposited at or near Earth's surface from a number of sources, most notably
preexisting rock) and water and typically flow along valley floors.
Fall
Most other forms of mass wasting entail movement along slopes that are considerably less than 90°, whereas fall takes place at angles almost perpendicular to the
ground. Anyone who has driven through a wide mountain area, with steep cliffs on either side, has seen signs that say "Watch for Falling Rock." These warnings, which
appear regularly on the drive through the Rockies in Colorado or on highways across the Blue Ridge and Great Smoky mountains in the southern United States, indicate
the threat of rock fall.
The mechanism behind rock fall is simple enough. When a rock at the top of a slope is in unstable equilibrium, it can be dislodged such that it either falls directly
downward or bounces and rolls. Usually, the bottom of the slope or cliff contains accumulated talus, or fallen rock material. Freezing and thawing as well as the
growth of plant roots may cause fall. The latter is not limited to rock fall: debris fall, which is closely related, includes soil, vegetation, and regolith as well as rocks.
Mass Wasting and Natural Disasters
Among the most dramatic and well-known varieties of mass wasting are avalanches, a variety of flow, and landslides, which (as their name suggests) are a type of
slide. These can result, and have resulted, in enormous loss of life and property. Some notable modern occurrences of mass wasting, and the type of movement
involved, are listed below. With each incident, the approximate number of fatalities is shown in parentheses.
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China, 1920: Landslide caused by an earthquake (200,000)
Peru, 1970: Debris avalanche related to an earthquake (70,000)
Colombia, 1985: Mudflow related to a volcanic eruption (23,000)
Soviet Union, 1949: Landslide caused by an earthquake (12,000-20,000)
Italy and Austria, 1916: Landslide (10,000)
Peru, 1962: Landslide (4,000-5,000)
Italy, 1963: Landslide (2,000)
Japan, 1945: Landslide caused by a flood (1,200)
Ecuador, 1987: Landslide related to an earthquake (1,000)
Austria, 1954: Landslide (200)
The Role of Plate Tectonics
Note how many times an instance of mass wasting was either caused by or "related to" (meaning that geologists could not establish a full causal relationship) volcanic
or seismic activity. Both, in turn, are the result of plate movement in most instances, and thus it is not surprising that several of the locales noted here are either at
plate margins or in mountainous regions where plate tectonic and other processes are at work. (For more on this subject, see the entries Plate Tectonics and
Mountains.)
To set mass wasting into motion, it is necessary to have a steep slope and some type of force to remove material from its position of unstable equilibrium. Plate
tectonic processes provide both. Not only does an earthquake, for instance, jar rocks loose from the upper portion of a slope, but the movement of plates also helps
create steep slopes, for example, the collision of the Indo-Australian and Eurasian belts that produced the Himalayas.
Some of the most vigorous plate tectonic activity occurs underwater, and, likewise, there are remarkable manifestations of mass wasting beneath the seas. Off Moss
Landing, a research facility that serves a consortium of state universities in northern California, is an underwater canyon more than 0.6 mi. (1 km) deep. At one time,
Monterey Canyon was thought to be the result of erosion by a river flowing into the ocean; however, today it is believed to be the result of underwater mass wasting.
Read more: http://www.answers.com/topic/mass-wasting#ixzz1mFHKSUaJ