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SOIL Just tiny fraction of earth materials Vital resources Very foundation of human life support
system Can be damaged by carelessness Basic role in providing food, fiber and other
basic materials Most abused resources
SOIL EROSION Natural process Part of constant recycling of Earth materials
(Rock Cycle) The rates of soil erosion vary from one place
to another and depend on the soil’s characteristic
Incorporation and transportation of materials by a mobile agent usually water, wind and ice
WIND Result of material movement by wind Generally occurs in areas with little or no
vegetation To reduce
erosion causes by wind, windbreaks (big trees) are often planted
A rock formation in the Altiplano, Bolivia sculpted by wind erosion.
Two main effects: Deflation – wind causes small
particles to be lifted and therefore moved to another region
Abrasion – suspended particles may impact on solid objects causing erosion
WATER/RAIN
Raindrops hit bare soil. Their kinetic energy is able to detach and move soil particles in a short distance
Each drop acts like a tiny bomb, blasting movable soil particles out of their position in the soil mass. Then water flowing across the surface carries away the dislodged soil particles
Rainsplash
Water and soil splashed following a single raindrop impact.
Rill – small channel Gullies – larger channel That fraction of the rainfall which does not
infiltrate the soil will flow downhill under the action of gravity, it is then known as runoff or overland flow
Rill & Gully
Runoff may occur for two reasons:1. If rain arrives too quickly2. If the soil has already absorbed all the
water it can hold*Detention Storage/ Ponding – can protect the soil from
raindrop impact
Diffuse overland flow. Note the raindrop impacts.
ICE Caused by movement of ice, typically as
glaciers Glaciers erode predominantly by two
different process:1. Abrasion – debris in the usual basal ice scrapes along
the bed, polishing the underlying rocks, similarto sandpaper on wood
2. Plucking – in this process, glaciers can also causepieces of bed rock to crack off
Through those agents, it can result formation of:
Deep valley between cliff often carved from the landscape by a river
Most canyons were formed by a process of long time erosion
CANYON
Grand Canyon, Arizona
Landform that extends above the surroundings terrain in a limited area, with a peak
MOUNTAINMount Olympus in Greece
CAVE
Natural undergroundvoid large enough for a human to enter
SPELEOLOGY – science of exploration and study of all aspects of caves
Canyon passage in Mammoth Cave, the world's longest cave.
Underground layer of water-bearing permeable rock from which groundwater can be usefully extracted using a water well.
HYDROGEOLOGY – study of water flow in aquifers and the characteristics of aquifers
AQUIFER
Caused by:
Results in:
EROSION
WIND WATER ICE
SURFACE UNDERGROUND
CANYON MOUNTAIN CAVE AQUIFERForms:
MASS WASTING
MASS WASTING - it refers to the down slope movement of rock, regolith, and soil under the direct
influence of gravity. - it does not require a transporting medium
such as water, wind or glaciers of ice.
GRAVITY - controlling force of mass wasting. - it causes material to move down slope.
MASS WASTING
GENERAL MOVEMENTS OF MASS WASTING
SLIDE - Where the mass movement has a well-defined zone or plane of sliding.- is a movement of surface material down a slope.
- includes a wide range of ground movement.
CREEP - is a long term process.-The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope.
FLOWS - involves continuous movement of material as a fluid behavior.
FALLS - involves free fall of material.including rockfall, is where regolith cascades down a slope, but is not of
sufficient volume or viscosity to behave as a flow.
LANDFORM DEVELOPMENT
STREAM VALLEY
TRIGGERS OF MASS WASTING:
Trigger – it is an event that initiates down slope movement
Seismic shaking Increased overburden from
structures Increased soil moisture Reduction of roots holding the soil
to bedrock
COMMON FACTORS THAT TRIGGERS MASS WASTING:
Saturation of material with water Over steepening of slopes Removal of anchoring vegetations Ground vibrations by earthquake
Role of Water When heavy rains saturate surface
materials, pores in sediment become filled with water, the cohesion among particles is destroyed, allowing them to slide past one another with relative ease.
However, if enough water is added to fill the openings between the grains, the sand will ooze out in all directions.
Thus, saturation reduces the internal resistance of materials, which then are easily set in motion by the force of gravity.
Mudflow
Over-steepened slopes due to undercutting by waves,
streams, and human excavation, resulting in loss of support for materials higher up the slope
A slope whose material is stable at a fairly gentle slope angle may become unstable if its slope angle becomes steeper. This can occur where a stream cuts into a valley slope, or where ocean waves remove the base (toe) of a slope. Also, sometimes humans over-steepened slopes when constructing building sites, or roads in mountainous areas as shown in the image to the left.
Removal of Vegetation anchors soil and regolith with plant
roots; removing plants makes the slope unstable and susceptible to failure A slope denuded of vegetation loses
surface protection from the impacts of raindrops, which can mobilize sediment grains with water flowing down slope. The roots of plants on a slope can play a significant role in binding sediment together, reducing the likelihood of rapid or sudden mass wasting of a slope. Removal of the vegetation, due to human cutting or harvesting, or due to fire, reduces strength of the slope. The related photograph shows the effects of a fire that swept through the foothills of the San Bernardino Mountains, California in October of 2003. Intense rainfall two months later produced a debris flow that killed 13 people in Waterman Canyon down slope.
Earthquakes can dislodge rock and
unconsolidated material, resulting in landslides
The vigorous shaking of an already-unstable slope by seismic waves may cause it to fail. Typically, the higher the magnitude of an earthquake, the more mass wasting will occur. Click on the USGS image to the left to see the effects of slope failure due to the Northridge, California earthquake of 1994.
CLASSIFICATION OF MASS-WASTING PROCESSES:
1. Slope Failures - a sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping.
A. Slump - form of mass wasting event that occurs when loosely consolidated materials or rock layers move a short distance down a slope. Slumps frequently form due to removal of a slope base, either from natural or manmade processes. Stream or wave erosion, as well as road constructions are common instigators for slumping. And Earthquakes also trigger massive slumps.
It divides into two broad categories and further subdivides these categories:
The Thistle town in Utah was destroyed by a massive landslide (slump) around 1983
B. Rock Falls and Debris FallsRock 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. A rock fall may be a single rock, or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff.
C. Rock Slides and Debris SlidesFrequently occur in high
mountain areas. It result when rocks or debris slide down a pre-existing surface, such as a bedding plane or joint surface.
A rock slide on Washington State highway 123 closed Cayuse Pass in Mount Rainier National Park.
2. Sediment Flows - material flows down hill mixed with water or air. A sediment flow is a mixture of rock, regolith with some water. They can be broken into two types depending on the amount of water present.2 types of Sediment Flows:
1).Slurry Flows - are sediment flows that contain between about 20 and 40% water. As the water content increases above about 40% slurry flows grade into streams.
2).Granular Flows - are sediment flows that contain between 20 and 0% water. Note that granular flows are possible with little or no water. Fluid-like behavior is given these flows by mixing with air.
Under the Slurry Flows are: Solifluction - produces distinctive lobes on hill slopes.
These occur in areas where the soil remains saturated with water for long periods of time.
Debris Flows- these occur at higher velocities, 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. debris flows are characterized by a mixture of sediment and water where the flow becomes slurry similar to wet concrete.
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.
This photo was taken ½ mile south of Genoa
This photo was taken in South of Genoa on Foothill Road.
Under the Granular Flows are:
Earth flows - are usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day. 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.
Creep - the very slow, usually continuous movement of regolith down slope. 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
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.
The toe of an avalanche in Alaska's Kenai Fjords.
TYPES OF MOTION: Falls - are common on slopes that are too
steep for loose material to remain on the surface. Many falls result when freeze and thaw cycles or the action of plant roots loosen rock to the point that the gravity takes over.
Slides – many mass-wasting processes are slides, which occur whenever material remains fairly coherent and moves along a well-defined surface. Sometimes the word slide is used as a synonym for the word landslide.
Flow- occurs when material moves down slopes as a vicious fluid.
RATE OF MOVEMENT:During events called Rock Avalanches, rock
and debris can hurtle down slope at speeds exceeding 200 kilometers (125miles) per hour. That is, high velocities result when air becomes trapped and compressed beneath the falling mass of debris.
But most mass movements, however, do not move with the speed of a rock avalanche. In fact, a great deal of mass wasting is imperceptibly slow like the creep. Its usually measured in millimeters or centimeter per year.
Slump is a form of mass wasting event that
occurs when loosely consolidated materials or rock layers move a short distance
down a slope.
Slumped chalk slopes at Mupe Bay, Dorset
Debris Flows are often referred to as mudslides,
mudflows, or debris avalanches. consist primarily of geological material
mixed with water. may also occur when strong rains on hill or
mountain slopes cause extensive erosion and/or what is known as "channel scour“.
can be as thick as wet mud mixed with rock, and can transport very large boulders.
may also eventually become thinner muddy flood waters as they deposit their heavier components.
Scars formed by debris flow in great Los Angeles during the winter of 1968-1969. The photograph was taken within several months of the debris flows occurring.
Lahars a type of mudflow composed of pyroclastic
material and water that flows down from a volcano, typically along a river valley
The term 'lahars' is originated in the Javanese language of Indonesia
can be extremely dangerous, because of their energy and speed
can flow several dozen meters per second and can flow for many kilometers, causing catastrophic destruction in their path
Lahar from a March 1982 eruption of Mount St. Helens.
Lahar from June 1991 eruption of Mt. Pinatubo in Philippines
Earth Flow is a down slope viscous flow of fine
grained materials that have been saturated with water, and moves under the pull of gravity.
Downhill Creep is the slow downward progression of
rock and soil down a low grade slope it can also refer to
slow deformation of such materials as a result of prolonged pressure and stress.