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Transcript
Weathering and Soils and Mass Movement Weathering Earth’s surface is constantly changing. One of the main reasons is our planet’s weather. Weathering breaks down rocks and converts them into soil. Weathering 1. 2. There are two types of weathering: Mechanical weathering Chemical weathering Types of Mechanical Weathering Mechanical weathering occurs when physical forces break apart rock. *does not change the rocks chemical composition. Examples: 1. Frost wedging 2. Unloading 3. Biological Activity 4. Abrasion Frost Wedging The freezing and thawing of water in cracks of rocks - expansion of water splits rocks apart - most common in the mountains of the middle latitudes - accumulation of material is called talus. Sometimes geologists refer to these piles as talus slopes Unloading When large masses of rock are exposed through uplift and erosion - Pressure on the underlying rock is reduced – called unloading - Unloading causes rock at the top to expand more than rock at the bottom. - Expansion causes large pieces to peel off as sheets of rock this is called exfoliation Exfoliation Biological Activity Caused by: - Plants - roots grow into the cracks of rocks and force them apart. - Animals - burrow and bring rock to the surface – weathering happens faster Abrasion - Wind and water can cause abrasion as rock fragments bounce off each other Chemical Weathering Chemical weathering is defined as the transformation of rock into new compounds. - water the most important agent Examples 1. Solution 2. Hydrolysis 3. Oxidation Chemical Weathering Solution -the dissolving of solid rock material in water. Water often contains substances such as acids that dissolve other minerals. Chemical Weathering Oxidation - Also known as rusting - oxygen combines with iron to form iron oxide - greatly speeds up in the presence of water and warm temperatures Chemical Weathering Hydrolysis - a permanent chemical combination with water - the rock's minerals are chemically altered by reacting with water and acid - resulting in expanded and weakened rock. Rates of Weathering Several factors effect the rates rocks weather. - Intensity (amount of acids) and duration play pivotal roles in the rates of weathering 1. Surface area – more area exposed the greater the rate 2. Climate – the warmer and wetter an area the greater the rate 3. Parent Material – different material weather at different rates Erosion - Erosion involves removal and transport - Rain, wind, and rivers are all agents of erosion. Soil The natural resource soil is one of the most important. Weathering creates layers of rock and mineral fragments called regolith. Soil is the part of the regolith that support plant life. The Components of Soil 1. 2. 3. 4. Soil is made of four things: Broken rock and mineral matter – 45% Organic matter (humus) – 5% Water – 25% Air – 25% The percentages of each of these components varies widely in soils. Soil Texture Soils are made of three components: Sand - largest grain size Silt - medium grain size Clay - smallest grain size Soil Texture Diagram-pg 134 Soil Texture Texture strongly influences the soil’s ability to support plant life - Sandy soils – drain and dry out too quickly - Clay-rich soils – drain very slowly - Loam soils – best for plant growth The Soil Profile The composition of soil varies with depth. Color, structure and texture all change with depth. These differentiated layers are called soil horizons. Soil Horizons A Horizon: The “A” horizon consists of organic matter (decayed plant material) or humus. Insects, fungus and microorganisms teem here. The lower portion of the “A” horizon is a mix of mineral and organic matter. Called topsoil. B Horizon: Known as the subsoil, this layer contains clay washed from above. It is the lower limit for plant roots and burrowing animals. C Horizon: The “C” horizon consists of partially weathered rock known as “saprolite”. This horizon resembles the original or “parent” rock. Soil Types Pedalfers: The soils found around here and in other temperate regions that get significant rain. Their “B” horizons contain iron- and aluminum-rich materials giving us the red and orange clays so common in the Carolinas and the rest of the East Coast. Soil Types Pedocal: Are soil typical of dry climates like those found n the western United States. Chemical weathering is slower. Because of this pedocals tend to have less clay. Pedocals are rich in calcium based minerals (calcite, limestone) Soil Types Laterite: These soils form in hot, wet tropical areas. The large amount of precipitation carries these soils deeply. The precipitation also washes our much of the silicate material making these soils orange. These soils are organically poor. One major reason for rainforest deforestation is the periodic need to replace the land exhausted by farming. Soil Erosion Soil erosion is a natural part of the weathering and transport process. However, excessive erosion can damage or destroy a resource that took 1000’s of years to develop. Soil Erosion Agents of Erosion - Wind - Water - Gravity - Ice (glaciers) - Human activity - such as farming, logging and construction remove natural vegetation that slows and controls erosion. Rate of Soil Erosion Rate depends on: - Soil characteristics - Climate - Slope - Vegetation Water Erosion Wind Erosion Sediment Deposition One problem caused by erosion is that of sediment deposition. High rates of erosion dump huge amounts of soil in rivers and lakes. These sediments fill rivers, channels, deltas and reservoirs and have to be removed by dredging. Controlling Erosion Though erosion cannot be stopped it can be controlled. Windbreaks between fields reduce wind speed. Silt fences control runoff from construction areas. “No till” farming does not break the soil, but allows in to remain consolidated. Terraced plowing follows the contours of the farmland not allowing runoff to pick up too much energy. Mass Movement The transport of rock and soil downhill due to gravity is called mass wasting. Mass Movement The causes (triggers) of mass wasting include: Water: Water makes soils heavier and unconsolidates individual soil particles. Over-steepened slopes: Undercutting slopes removes their structural support. Removal of vegetation: Removing vegetation kills roots that hold soils together. Earthquakes: Earthquakes shake soils apart reducing their structural integrity. The process is known as “liquefaction”. Types of Mass Wasting Rockfalls: When rocks and rock fragments fall through the air. Slides: When blocks of material move along flat, inclined surfaces. Slumps: Slumps occur when large blocks of material move downward. slump Types of Mass Wasting Flows: Flows occur when material become saturated with water and move down slope as a thick fluid. Examples are mudflows and earth flows. Creeps: Creeps are the slowest form of mass wasting. Creeps occur when rock and soil move forward centimeters per year. Creeps move due to freezing and thawing cycles and cannot be directly observed.