Soil Formation, Erosion and Mass

WastingMildred C. Galicia

IV-6 BEEdProf. Shila Rose Sia

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Factors of Soil Formation

Erosion

Mass wasting

Triggers of Mass wasting

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FACTORS OF SOIL FORMATION

TimeTopography

Living organismsClimate

Parent material

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Parent Material- refers to organic (such as fresh

peat) and mineral material in which soil formation begins. • Residuum - becomes the parent material of soil and

imparts some of the parent characteristics into the resulting soil profile.

• Colluvium - incoherent mass of material that generally accumulates on the lower portion of slopes and in depressions.

• Recent alluvium - these young soils have poorly developed profiles, and most of their character is inherited from the parent material.

• Old alluvium - Soils located on stream terrace positions that contain water worn coarse fragments have parent materials .

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Climate- Climate is a major factor in determining the kind of plant and animal life on and in the soil. It determines the amount of water available for weathering minerals, transporting the minerals and releasing elements. Climate, through its influence on soil temperature, determines the rate of chemical weathering.

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Living Organism- Plants affect soil development by supplying upper layers with organic matter, recycling nutrients from lower to upper layers, and helping to prevent erosion. -Animals living in the soil affect decomposition of waste materials and how soil materials will be moved around in the soil profile.

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Landscape Position

- Landscape position causes localized changes in moisture and temperature.

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Time- Time is required for horizon formation. The longer a soil surface has been exposed to soil forming agents like rain and growing plants, the greater the development of the soil profile. Soils in recent alluvial or windblown materials or soils on steep slopes where erosion has been active may show very little horizon development.

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Erosion- is the process that breaks things down. As far as we're concerned, erosion is the breakdown of the continents and the land around you.

-> Denudation - The overall effect of breaking down and weathering the land.

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Things don’t disappear!

• The masses of dirt and rock are moved to another form and place . Scientist call it mass wasting.

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Mass wasting• Mass-wasting is the down-slope

movement of Regolith  (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity without the aid of a transporting medium such as water, ice, or wind.

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Gravity• Gravity is a force that acts

everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth.

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On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting tangential to the slope.

Another force resisting movement down the slope is grouped under the term shear strength.

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The role of water• Dry unconsolidated grains will form a

pile with a slope angle determined by the angle of repose.

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• Slightly wet unconsolidated materials exhibit a very high angle of repose.

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• When the material becomes saturated with water, the angle of repose is reduced to very small values and the material tends to flow like a fluid.

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Mass wasting Process• The down-slope movement of material,

whether it be bedrock, regolith, or a mixture of these, is commonly referred to as a landslide.

Slope FailuresSediment

Flows

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• Slumps - types of slides wherein downward rotation of rock or regolith occurs along a curved surface.

Slope Failures - a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.

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Rock Falls and Debris Falls - Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, and rocks.

Rock Slides and Debris Slides 

- Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such as a bedding plane or joint surface.

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Sediment Flows - material flows down hill mixed with water or air.• Slurry Flows- are sediment flows that

contain between about 20 and 40% water.

• Granular Flows - are sediment flows

that contain between 20 and 0% water.

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• Slurry Flows

– Solifluction - These occur in areas where the soil remains saturated with water for long periods of time.

– Debris Flows- these occur at higher velocities than solifluction, and often result from heavy rains causing saturation of the soil and regolith with water.

–Mudflows- a highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete.

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• Granular Flows

– Creep- the very slow, usually continuous movement of regolith down slope.

– Earthflows - are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day.

– Grain Flows - usually form in relatively dry material, such as a sand dune, on a steep 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.

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• Shocks - A sudden shock, such as an earthquake may trigger a slope instability.

• Slope Modification - modification of 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.

Triggers of Mass Wasting

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• Undercutting - streams eroding their banks or surf

action along a coast can undercut a slope making it unstable.

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• Exceptional Precipitation - heavy rains can saturate regolith reducing grain to grain contact and reducing the angle of repose, thus triggering a mass-wasting event. 

• Volcanic Eruptions - produce shocks like explosions and earthquakes.

• Submarine Slope Failures - these can be caused by rapid deposition of sediment that does not allow water trapped between grains to escape, or by generation of methane gas from the decay of organic material, which increases pressure between unconsolidated grains and thus reduces grain to grain contact.

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References http://wps.prenhall.com/esm_tarbuck_escience_11/32/8320/2129922.cw/index.html

http://earthsci.org/education/teacher/basicgeol/masswa/masswa.html

http://teacher.scholastic.com/dirt/erosion/whateros.htm

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How to Prevent Landslides?

If you even have the slightest feeling that the place where you stay is vulnerable to landslides, it's better to get into action and put in all the efforts possible to avoid the certain disasters. So how can we prevent landslides? It isn't much difficult if you resort to presence of mind. As we mentioned earlier, soil erosion is one of the most common reasons of landslides, and therefore soil erosion prevention can help you reduce the risk of landslides by a significant extent, if not eliminating it all altogether.

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Prevent Soil Erosion to Prevent LandslidesThe foremost thing to do is to divert the

discharge water away from slopes by constructing gutters and using sandbags. If there is no scope for diverting - especially in case of natural sources of water, you can contain its speed by building small dams; so as to ensure that the amount of erosion is in check. The velocity of water determines how much soil is eroded, and hence containing its speed is bound to be helpful when it comes to prevention of landslides. It is important to make sure that you divert or contain the water flow - and not stop it altogether, as stopping it will result in pressure build-up over a period, which might give in at one point of time. Never redirect storm drain or street gutter down a slope, even if it seems to be an easy way out. Instead you can use flexible pipes and divert this water in a safe manner.

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Plant Vegetation

Yet another simple way to prevent landslides is to plant trees and small shrubs on the slope. As these trees and shrubs grow, their roots hold soil together, and help in reducing erosion of soil which is likely to make the slope unstable in course of time. In fact, there exist quite a few species of plants with shallow roots which are specifically used to protect the top layer of the soil in mountainous regions. You can inquire about these plants in the nursery in your neighborhood, and plant them along the slope. 

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Retaining Walls

Retaining walls can also help when it comes to prevention of landslides, but only when you make sure that you construct them properly. If you don't facilitate proper drainage of water, the chances are that the wall will come down with the entire slope after some time. You can resort to one of the different types of retaining walls available on the basis of geology of your region. While a concrete retaining wall is an ideal option, a wooden retaining wall or a crude stone retaining wall will also work fine. Similarly, building a sea wall will be an ideal option if the landform in question is subjected to sea waves.

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Altering the Slope Gradient

If possible, you should opt for grading of the slope - wherein you remove the material from upper part of the slope and put it near the base and reduce its gradient. As it is a bit tricky task, it would be wise to get help from someone who has a professional expertise in such work. At the same time, you should refrain from resorting to activities like digging and piling garbage on unstable slopes - both of which happen to be possible triggers of a landslide.

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That was a significant bit of information on how to stop landslides in mountainous regions. While these measures to prevent landslides are helpful in reducing the overall risk involved, one cannot rule out the occurrence of these natural disasters - even after taking all these precautionary measures. There exist several natural causes of landslides - including tectonic movements, ground water, snowmelt etc., which can unleash a havoc, and hence the best way out is to avoid building your dwelling in a region which is vulnerable to landslides, as well as other natural disasters which can trigger one.

By Abhijit NaikPublished: 4/29/2011