Weathering and Soil

Weathering and Soil

Weathering and Soil Rocks and Weathering Uniformitarianism - The same geologic processes that work today worked in the past to change Earths surface What are these processes? Erosion- breaking of rocks into smaller pieces and removal of those rocks and pieces by wind, water, ice or gravity. Weathering-Chemical and physical processes that break down rock and other substances, heat, cold, water, ice and gases contribute Deposition- process in which sediment is laid down in new locations Weathering and Soil Two Types of Weathering 1.) Mechanical-rock is physically broken down into smaller pieces --usually works slowly --over long periods of time, can wear down mountains Freeze thaw- important in cold climates, H2O seeps into cracks in rocks, freezes and expands

which forces the rock apart this is called FROST WEDGING Release of pressure-erosions removes material from surface of a mass of rock, pressure on rock is reduced which causes the outside of the rock to crack and flake off Plant growth-roots enter cracks of rocks, as plant grows, so will roots causing cracks to get larger Action of animals-burrowing by moles. Gophers, prairie dogs and insects loosens soil and breaks apart rocks Abrasion-the grinding away of rock by other rock particles carried in water, ice or wind Weathering and Soil 2.) Chemical- process that breaks down rock through chemical processes --process works with mechanical weather --creates holes or soft spots in rocks --rock breaks into pieces creating larger surface area

for chemical weathering to work Water-dissolves rock and carries other substances that break down rock, ie O, CO2 and other substances Oxygen- combines with Iron to form rust, this is called oxidation --rust makes rocks soft and crumbly and causes red/brown color White lichens cover a blue granite gravestone like snow near Lake Champlain, New York. Lichens, symbiotic organisms that combine fungi and algae, can be powerful weathering agents, secreting chemicals called chelates that work to break down rock. Carbon dioxide- when dissolved in water forms carbonic acid which easily weathers some rocks like marble and limestone Living organisms-plant roots produce weak acid that breaks down rock, ie. lichens

Acid rain-burning of fossil fuels puts S, C and N in the air. These compounds react with water vapor, rain produced is more acidic than normal = acid rain. The effects of acid rain can be seen on this close up of this statues face. The acid rain has reacted with the mineral grain of the rock which has over time caused cavities to form as well wearing away the definition of the facial features. Weathering and Soil Rate of weathering depends on three things 1.) Type of rock-minerals determine how fast the rock weathers Minerals that are soft, dissolve easily = fast weathering 2.) Surface Area -spaces increase surface area of rock allowing weathering to occur more quickly -More surface area = more weathering Weathering and Surface Area Weathering breaks rock into smaller pieces. While the pieces are usually

irregularly shaped, you can model the process with cubes. The diagram shows what would happen if a rock cube broke into smaller cubes. 3.) Climate More moisture = increase physical and chemical weathering Warm/moist climate = increase chemical weathering **chemical reactions occur more quickly at high temperatures Cold/moist climate = increase physical weathering Human activities also can increase the rate of chemical weathering -more people in an area = more pollution = more acid rain (chemical weathering) Weathering and Soil Products of Weathering Solid sediments- (from largest to smallest) boulders, cobbles, pebbles , sand, silt, clay, colloids Dissolved minerals-make hard water, When water evaporates, dissolved minerals will precipitate out and settle to the bottom

Soils- the loose weathered material at Earths surface in which plants can grow -mixture of rock, minerals., decayed organic material, water and air -forms constantly wherever bedrock weathers, continues over a long period of time. Composition Bedrock-solid layer of rock beneath soil -exposed to air, H2O and living things it weathers Humus-dark colored, organic material in soil -creates spaces in soil for air and water, contains nutrients plants need Fertility Fertility- a measure of how well soil supports plant growth -rich in humus = high fertility -sandy soil = low fertility (low humus) Weathering and Soil Soil Formation Rock is broken down by weathering and mixes with other materials on surface--- soil is formed Soil Horizon

Horizon ATopsoil- dark, crumbly, mixture of clay, humus, and minerals formed from plants adding organic material and plant roots weather rock Horizon BSubsoil-clay, particles of rock, little humus Rain water washes Materials down from horizon A Horizon C- small particles of weathered bedrock Weathering and Soil Soil Texture depends on particle size Sandy soil = course, grainy Soil with lots of clay = smooth and silky

Soil Texture is important for plant growth Soil that is mostly clay holds a lot of water and little air = plant death Sandy soil = loses water quickly = plant Death Loam- rich fertile soil that is made up of equal parts clay, sand and silt Best for growing MOST plants Weathering and Soil Soils of North America Soil, fertility, and climate affect the types and numbers of plants that grow in a region. Soil profiles that form in different environments will have very distinct differences from

each other Weathering and Soil Soil Conservation Natural resource- anything naturally occurring in the environment that people use Soil = one of Earths most valuable resources everything that lives on land depends directly or indirectly on soil 1/8 of land on Earths land is farmable Soil takes a long time to form, few cm in hundreds of years Soil damage and Loss Loss of Fertility Loss of moisture and nutrients can occur when only one crop is grown Desertification- the advance of desert like conditions into areas that previously were fertile Loss of Topsoil Exposure to wind and water causes erosion Plant covering can protect Break force of falling rain

Hold soil together Sod- the thick mass of tough roots at the surface of the soil, holds soil in place Weathering and Soil Soil Conservation Three ways people use land that might cause changes 1.) farming 2.) mining 3.) development- construction of homes, roads and other structures Soil Conservation-the management of soil to minimize its destruction Crop rotation-the planting of different crops in a field each year; corn or cotton one year followed by oats, barley or rye Corn, cotton take up large amounts of nutrients Peanuts, alfalfa and beans restore nutrients Legumes contain pods of nitrogen fixing bacteria Contour plowing-fields plowed along curves of a slope instead of straight rows Slows runoff, prevents soil from washing away

Conservation plowing-dead weeds and stalks are plowed into the ground Returns nutrients to soil, retains moisture and hold soil in place Weathering and Soil Dust Bowl Parts of Oklahoma and surrounding states that lost soil is the 1930s were called the DUST BOWL Farmers plowed the land removing the sod and exposed soil, drought dried out the topsoil which turned to dust and blew away = Desertification Land Reclamation- the process of restoring an area of land to a more natural, productive state Erosion and Deposition Erosion - the process by which weathered sediments are carried/transported Agents of erosion are the materials or forces that move sediments from one place to another force that causes erosion is gravity

Agents of Erosion: Gravity (Mass Movements) Wind Running Water (Streams) Waves Glaciers Gravity is the underlying force behind all erosion may act alone or with a transporting agent causes water to flow downhill causes glaciers to flow down valleys causes winds by pulling heavier (more dense) cold air down beneath lighter (less dense) warm air Erosion and Deposition Gravity - pulls weathered sediments down steep slopes (called mass wasting) mass movements occur when the force of gravity is greater than the force of friction (keeps weathered sediments from moving)

Types of Mass Wasting: Fast landslides mudslides Slow soil creep, slump Factors that Effect Mass Wasting: Gradient (slope) of the land surface Temperature Moisture (amount of water in the soil/ground) Erosion and Deposition Wind - heavy winds can move sand, but rarely more than a meter above the ground and only where it is very dry light winds can only move the smallest sediments occurs in arid climates and coastlines where loose sediments are available Deflation - process where winds blow away loose sediments,

lowering the land surface Abrasion - winds blow sand against rocks and other objects causing them to be "sandblasted" Erosion and Deposition Wind deposits are sorted and layered occur in arid/dry climates and along coastlines sand dunes show the direction of wind movement Cross-Bedding occurs if the wind direction changes - sediments are deposited at different angles wind deposits have a pitted (frosted) and rounded appearance Erosion and Deposition Running Water -(streams, rivers, etc.) running water is the dominant form of erosion the amount (volume) of water in a stream is called the stream's discharge

Factor's affecting a stream's discharge: Season - discharge greatest in the spring Climate - greatest in humid climates Ground/Soil - greatest when soil is saturated Weather - increases after a period of precipitation Streams carry sediments by: 1.) Suspension - carried within the water column 2.) Bouncing/Rolling - larger particles along the stream bottom as sediments move in the water, it hits rocks, the stream channel, and other sediments - this causes the sediments to become rounded in a process called abrasion 3.) In-solution - minerals dissolved in the water **Particles transported by a stream become rounded, smoothed and polished. As the velocity of a stream increases, its kinetic energy increases and the amount of erosion it does will increase Solid sediments transported by a stream move more slowly than the stream itself

the greater the velocity of the stream, the larger the sediment particles it can carry Erosion and Deposition Running Water (cont.) Factors that Affect Stream Velocity: Gradient - as gradient (slope) increases, stream velocity increases Discharge - as discharge (volume of water) increases, stream velocity increases Channel Shape - the path that a stream follows a stream's velocity will change due to the curvature of the channel Abrasion will cause streams over time to carve deep channels (downcutting) Creating a characteristic V-shaped valleys Meander - bends in a streams channel stream moves fastest along the outside

of a curve; slowest along the inside erosion occurs where the stream is moving fastest - causes the shape of the Channel Fastest stream flow occurs at a point farthest away from the bottom and the sides where drag occurs. This means that the rate of flow is greatest in the center near the surface. Flow is fast if the channel is narrow. The rate slows if the channel widens or if the stream enters a lake or the ocean. Erosion and Deposition Water Deposition Stream velocity is faster on the outside of a meander and slower on the inside deposition occurs on the inside curve of a stream erosion and deposition cause the meanders

to "grow" results in the formation of oxbow lakes Vertical & horizontal sorting of materials are the hallmarks of stream deposition. Largest deposited first, closest to shore, smallest deposited later, fatrther from shore Streams deposit sediment when the kinetic energy (velocity) of the stream decreases occurs when a stream enters a large body of water (delta) or dry land (forms a deposit called an alluvial fan) Erosion and Deposition

Waves - caused by the wind size of waves depends on how long wind blows in one direction water particles rise and fall in circular paths over deep ocean water when wave reaches shallow water near shore, friction causes the bottom of the wave to move more slowly (breaks) waves usually hit the shore at an angle - this causes a flow of water called a longshore current sand moves along the beach in a zig-zag pattern creates sandbars Wave Deposition Waves along shorelines will deposit sand and form beaches water currents will create sand dune-like features called ripple-marks

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