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All About Soil: Soil Formation: According to the Natural Resource Conservation Service, “soil refers to the loose surface of the earth as distinguished from rock”. Soil is formed when weathering continuously breaks down rocks into pieces. These pieces are broken down into even smaller pieces to form soil. Soil Profile: Soil is made up of many layers called horizons, such as the humus, topsoil, subsoil, weathered rock fragments and bedrock. Leaching (drainage of minerals/chemicals away from soil, ash, or similar material by the action of percolating liquid) can occur from the humus to the bedrock.
Factors Affecting Soil Profile (Layers), Soil Formation and eventually the Soil Composition:
Soil Composition:
• Soil is composed of organic matter, mineral and nutrients. • An average soil sample is 45% mineral, 25% water, 25% air and 5% organic matter (material that was once
living or was formed by the activity of living organisms) • Soil texture comes from different size mineral particles such as sand, silt and clay • Pieces of weathered rock and organic material, or humus (Decaying organic material; becomes mixed with
the top layers of rock particles, and supplies some of the needed nutrients to plants; also creates crumbly soil which allows adequate water absorption and drainage.), are the two main ingredients of soil.
Factor: Explanation: Time
-‐ The longer a rock is exposed to the forces of weathering, the more it is broken down. -‐ Mature soil is formed if all three layers have had time to develop.
Climate -‐ In areas with heavy rainfall and warm temperatures, weathering takes place more rapidly. -‐ Heavy rainfall may wash much of the topsoil away. -‐ But since organisms are more plentiful these areas, the soil is quickly replaced. -‐ They speed up the chemical and mechanical weathering of rocks.
Type of rock
-‐ Rocks that do not break down easily do not form soil rapidly. -‐ In some climates it takes along time for granite to break down. So soil formation from
granite is slow. -‐ Sandstone can break easily and forms soil quickly.
Surface features of the region
-‐ The surface features of the region also determine the speed at which soil is formed. -‐ On very steep slopes, rainwater running off the land erodes the soil and exposes rock to
weathering. -‐ The presence of vegetation also speeds up biological weathering of rock.
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• Rock particles form more than 80% of soil. • The composition of soil varies from place to place. The type of rock broken down by weathering determines
the kinds of minerals in the soil. The type of weathering also affects the composition of soil. Mechanical weathering produces soil with a composition similar to the rock being weathered. Chemical weathering produces soil with a different composition.
• Properties of Soil: Property: Description & Trend: Texture - Determined by the size of mineral particles within the soil.
- Too many large particles leads to extreme leaching, too many small particles lead to poor drainage.
Colour - Generally speaking, the darker a soil is, the more nutrient rich it is. The darker color often indicates
an increase in decomposed organic matter known as humus. - Gray soils often indicate poor drainage, while red soils can indicate very poor soils. - These general rules about soil colors can however be misleading. Under certain conditions, a very
poor soil can appear as dark black, while a rich healthy soil can appear as red. pH (Acidity/ Alkalinity)
- If a soil has too much acid in it, the nutrients in the soil will be dissolved too quickly, and leached away as the water drains.
- If a soil is too alkaline, or in other words, if there is not enough acid, than nutrients will not dissolve quickly enough.
- Thus, a neutral soil, which is neither too acidic, nor too alkaline is the preferred type of soil for plant life to thrive.
Structure - Dirt clods (small clump of soil that has formed a rock-‐like clump) are called peds by geologists. - Many soils form peds, as soil fragments bond together. These peds effect how easily water is able
to move through the soil, as well as plant roots, and other organisms. (Related to particle size) - Peds tend to form more often in wet locations, and less frequently in drier locations. Sandy
locations such as deserts do not form any peds at all. Types of Soil: Type: Description: Residual Soil:
Sometimes soil remains on top of its parent rock, or the rock from which it was formed. This is called residual soil. Residual soil has a composition similar to that of the parent rock it covers.
Transported Soil:
Some soil is removed from the parent rock by water, wind, glaciers and waves. Soil that is moved away from its place of origin is called transported soil. Transported soil can be very different in composition from the rock it covers.
SOIL EROSION & MASS WASTING 3 Soil Erosion: Erosion is the process of detachment and transport of soil particles by erosive agents (Ellison, 1944), and is a natural geologic process. It is a global problem -‐ 1/3 of the world’s arable land has been lost since 1950, mostly in Asia, Africa and South America at a rate of 13-‐18 t/a/yr (tons per year I think, I don’t know what the a stands for). In the USA, 30% of the farmland has been abandoned through erosion, salinization and water-‐logging. 90% of USA’s cropland is also losing soil faster than it is replaced, at a rate of >1 t/a/yr. Types of Soil Erosion: Type What it does:
4 Stages/Types:
1st Stage: Raindrop erosion -‐ Begins with raindrops striking bare soil, hence dislodges particles -‐ Intense rains seals off the surface -‐ When the rainfall exceeds the maximum infiltration capacity, water is stored in small
depressions on the surface of the soil. (Give a dimpled appearance to the surface) -‐ Once the depressions are filled, surface runoff begins, bringing soil particles with them.
2nd Stage: Sheet erosion -‐ Removal of a layer of exposed soil by the action of raindrop splash and runoff. -‐ The water moves in broad sheets over the land and is not confined in concentrated flow paths. -‐ Amount of erosion increases with slope and distance.
3rd Stage: Rill and gully erosion -‐ Occurs after runoff flows concentrate into rivulets, cutting into the soil surface. -‐ Amount of erosion increases with slope and distance. -‐ Rills => Narrow and shallow incisions into topsoil layers, will evolve into larger fluvial features -‐ Gullies => Gullies are larger than rills and cannot be fixed by tillage (agricultural preparation of
the soil by mechanical agitation of various types, such as digging, stirring, and overturning.). Gully erosion is an advanced stage of rill erosion, just as rills are often the result of sheet erosion.
Water Erosion
4th Stage: Streambank and bed erosion Occurs with an increase in the volume and velocity of runoff. Water is a great erosional agent:
-‐ Corrasion/Abrasion: grinding against surface by materials -‐ Attrition: colliding together to become smaller rocks -‐ Solution: Dissolve in carbonic acid. (Eg. Limestone – calcium carbonate dissolves) -‐ Hydraulic Action: water hitting against the rocks, loosening.
Wind Erosion -‐ Wind erosion may occur on any soil whose surface is dry and unprotected by vegetation.
-‐ Saltation (the movement of hard particles such as sand over an uneven surface in a turbulent flow of air or water) detaches particles, and smaller particles are suspended while larger particles creep.
-‐ Sandy and silty soils are the most susceptible -‐ This process causes soil accumulation in ditches and fence rows.
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-‐ It cannot be divided into such distinct types. Surface texture is the best key to wind erosion hazard potential.
Gravitational Erosion
-‐ Gravity is the principal force acting to move surface materials such as soil and rock.
Tillage Translocation
-‐ Net downhill movement by tillage (basically, LAND) results in the smoothing of surface. -‐ Combined with water erosion, it increases relief intensity and soil variability. -‐ Boundaries stop the movement
Mitigation Measures: Measure Description Picture: Contour Farming
-‐ Tilling at right angles to the slope of the land. -‐ Rows formed slows water run-‐off during rainstorms,
acting as a small dam, to prevent soil erosion and allows the water time to settle into the soil.
Strip Farming
-‐ Alternating strips of closely sown crops to slow water flow.
Terracing -‐ Level areas constructed at right angles to the slope to
retain water -‐ good for very steep land.
Windbreaks -‐ Planting of trees or other plants that protect bare soil
from full force of the wind. -‐ Planting sufficiently dense rows or stands of trees at
the windward exposure of an agricultural field subject to wind erosion. Evergreen species are preferred to achieve year-‐round protection; however, as long as foilage is present in the seasons of bare soil surfaces, the effect of deciduous trees may also be adequate.
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Mass Wasting: What is Mass Wasting?
-‐ Also known as slope movement or mass movement -‐ Geomorphic process by which soil, regolith (loose uncemented mixture of soil and rock particles that covers
the Earth's surface), and rock move downslope under the force of gravity -‐ Not accompanied by a moving agent such a river, glacier or ocean wave. -‐ Comprehensive term for any type of downslope movement of Earth materials (rock or soil) as a coherent
mass which is distinct from erosional processes and follows weathering -‐ Includes landslides, earthflows, mudflows, rockfalls, debris and snow avalanches, and subsidence -‐ Force of gravity dominates, and is often initiated by heavy rainfall or earthquake
Classification of Mass Movement: Important variables in classifying downslope movement of Earth materials are: 1. Types of mass movements (slide, slump, fall, flow, subsidence, or complex) 2. Slope material 3. Amount of water 4. Rate of movement 5. Sudden failure or flow Speed Diagram: DRAW YOURSELF!! Types of Mass Movement: Type Description Slide - Rock or debris slides down pre-‐existing surface e.g. bedding plane
- Downslope movement of coherent block of Earth material - A sheet of material that slips over a failure plane ending anywhere from a meter to a kilometer
down slope, and maintains its shape and cohesion until it impacts the bottom of a slope - Slides produce concave scars (a steep high cliff or rock outcrop) while slumps tend to produce a
scarp (a very steep bank or slope) or cliff exposure. Trees are broken and bent and the slide can bury the soil down slope.
- Note the concave scar typically produced by a slide.
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Subset of slides: Slumps
-‐ Also known as rotational slides as they have a rotational movement -‐ Involve the movement of relatively intact masses of rock or sediment downslope along a curved,
concave upward, failure plane. -‐ The material involved in the slump rotates along the failure surface as it slides down the slope. -‐ Occur on weaker rocks e.g. clay and the material is saturated. -‐ Usually undercutting has weakened the base or the pressure on the rocks is intensified by human
activity. -‐ In many cases small flows of unconsolidated sediment or soil move out in front of the slump. Slumps
usually occur in rock units where there are some unconsolidated or weak rock layers.
Flow -‐ Liquidy mass, highly fluid and lacks cohesion, saturated with water.
-‐ Moves quickly down a well-‐defined channel, the toe (end) of the flow spreads out -‐ Leaves a scar, and over 33% of the material is fine-‐grained -‐ Down slope movement of water-‐saturated soil, regolith, weak shale, or weak clay layers. -‐ TYPES OF FLOWS
o Earthflows (the contents are made of soil) and Mudflows (clay-‐sized particles, suspended particles and silt)
o Earthflows have a fairly slow speed, occurring over a few hours or so slow that they are almost imperceptible.
o Earth flows are accompanied with slumping, but unlike slumping, there is no backward rotation.
o Earth flows differ from mudflows in that they (1) tend to be slower, (2) are not confined to
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channels, (3) are more common in humid areas than dry, and (4) have a lower water content.
o A mudflow is the rapid down slope movement of water-‐saturated soil, regolith. The higher water content creates a flow rapid enough to be perceptible to the eye.
o Conditions favorable for the development of mudflows are: (1) unconsolidated surface materials, (2) steep slopes abundant but intermittent precipitation, and (3) sparse cover of vegetation. Mudflows tend to be more prevalent in dry regions where vegetation is sparse and heavy rains may form. When set in motion, they occupy stream-‐cut channels rushing along in a torrential flow of mud.
Subset of Flow: Creep
-‐ Is the slow downslope movement of soil, sediment or rock along more gentle slopes than give rise to the more rapid forms of mass wasting.
-‐ The surface itself gives little indication that creep is occurring, however features above the surface of the slope provide evidence that slow downslope movement is occurring.
-‐ Trees display bent after being tilted by the creep movement. Trees tend to grow vertically, towards the sun, and so when downslope movement tilts them, the trunks bend to grow vertically again.
-‐ Gravestones and fences in areas of creep are also are tilted downslope. -‐ Rock layers near the surface are also tilted downslope.
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Subset of Flow: Soilfluction
-‐ Freeze thaw cycles is the agent. -‐ Occurs in cold regions in which the water near the surface freezes and thaws repeatedly. -‐ Relatively slow form of mass wasting. -‐ When the water near the surface freezes the soil and rock is moved upward and in the downslope
direction due to the expansion of the water. The underlying frozen ground acts as sliding plane. -‐ When the surface of permafrost melts creating mobile water saturated layer, soil moves vertically
back to the surface -‐ Although the downslope motion may only be a fraction of an inch for each freeze and thaw cycle,
the repeated cycles each day result in significant downslope movement over time.
SOIL EROSION & MASS WASTING 9 Falls - Occur because of
o Excessive vibration o Freeze thaw action o External stresses o Jointing and faulting in the rock mass o Sudden shock e.g. earthquake o Slope modification
- Rock faces or vertical outcrops are prone to rock falls, it usually occurs on steep slopes >70 degrees - Rock falls occur when weathering or frost action weakens the rock in an outcrop along joints or
fractures, and the rock material breaks away, falling rapidly to the ground under the influence of gravity.
- This is the most rapid form of mass movement and is particularly a problem in road cuts in mountainous areas.
- Areas prone to rock fall often display accumulation of small rock fragments called talus slopes, if the fall is short, it produces a straight scree, if it is long, it forms a concave scree.
- Topples: Blocks of rock pivot and fall away from a slope
Avalanches - Rapid movements of snow, ice, rock or earth down a slope
- Common in mountainous areas, occur on steep slopes >22 degrees - Dry avalanche (newly fallen snow falls off older snow) and wet avalanche (partially melted snow
moves), often triggered by skiing - Debris avalanches moves sediments and organic material, and are often associated with saturated
ground conditions.
-
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- Trigger Factors for Mass Wasting Events: Trigger Factor How does it work: Sudden Shocks A sudden shock from an earthquake may release so much energy that slope failures are
triggered. Slope Modification When a slope is modified through human activity such as road cuts into regolith, the angle of
repose for these artificial slopes may be exceeded, and hence trigger a landslide. Undercutting by streams or larger bodies of water (e.g. streambank erosion) may result in this as well.
Exceptional Precipitation
This causes saturation of the ground and hence making it unstable. It decreases the shear strength of an oversteepened slope, causing failure.
Volcanic Eruptions Stratovolcanoes have inherently unstable piles of lava flows, rubble and pyroclastics such as ash. When an eruption occurs, slope failure is common, causing pyroclastic flows and landslides. If the volcano supports glaciers or snowfields, melting of snow and ice can cause avalanches and various flows when combined with unconsolidated deposits on the slopes.
Problems with Mass Wasting: - Caused by instability of a slope under the force of gravity
o 2 Forces of gravity: Tangential (pulls material down slope) and Perpendicular (keeps material on slope) - When tangential force > perpendicular force and friction, material falls. This is the same as tangential force
overcoming shear strength. Material is mobilized when the shear stress imposed on a surface exceeds the shear strength.
- Shear strength (= perpendicular force) is a measure if the resistance of earth materials to be moved. The interlocking of soil particles increases the ability of material to stay in place. Plant roots also help bind soil particles together.
- Shear stress (=tangential force) is a function of the force exerted by the weight of the material under the influence of gravity acting in the down slope direction. The slope of the surface determines the amount of stress that occurs on earth materials.
- Water destabilizes hill slopes by creating pressure in the pore spaces of earth materials. (Factor = Pore Water) Water infiltrating into slope materials saturates the soil particles at depth by filling the pore spaces between. The weight of water lying above creates water pressure that drives soil particles apart. This lessens the friction between them and enables them to slip past one another.
- The movement, especially in the case of slides and slumps, is along a failure plane. The failure plane may be a well-‐defined layer of clay or rock upon which sets the destabilized surface material.
- Slope instability is when the angel of repose is reached, where material cannot lie on the slope without falling. - Humans induce mass movement when subjecting a slope to a load that exceeds its ability to resist movement.
(Exceed the angle of repose). People building houses on scenic hill slopes often find their homes threatened by a landslide. Undercutting of hillsides during road construction commonly creates unstable slopes making them prone to failure. Removal of vegetation can too.
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Causes & Mitigation! Cause Measure Steepened slope Oversteepened slopes could be graded to reduce the slope to the natural angle of
repose. Added weight 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.
Decreased shear strength Increase shear strength • Use iron rods and anchors • If the slope is made of highly fractured rock, rock bolts may be emplaced
to hold the slope together and prevent failure. Saturation of soil Prevent Saturation:
• Re-‐compact soils • Prohibit over-‐irrigation • Install surface drains • Install subsurface drains
Reworking of fill Construct retaining wall with anchors 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. Steep slopes can be covered or sprayed with concrete covered with a wire mesh to prevent rock falls.
Devegetation Re-‐vegetate soil slopes Sediment flows: Slurry flows: - Soilfluction (waterlogged) - Debris flows (Material more coarse) - Mudflows (High water content) - NEED WATER Granular flows: - Creep - Debris avalanches - Flowing mass supported by grain to grain contact or collision