Chapter 7: Weathering & Soil

Aim: How does weathering change Earth’s surface?

Weathering Is surface processes that work to breakdown rocks into smaller pieces Smaller loose pieces are sediments Gravel, Sand, silt and clay describe sizes of sediments Size order: gravel, sand, silt, clay (largest to smallest) Wears mountains down to hills Produces strange rock formations 2 Types of weathering: Mechanical Weathering Chemical Weathering

Aim: How do mechanical weathering and chemical weathering differ?

Mechanical Weathering Occurs when rocks are broken apart by physical processes Chemical makeup of the rock stays the same 2 ways this can happen: Plants and Animals Water and nutrients that collect in the cracks or rocks enable plants to grow As the roots grow they enlarge the cracks Ex: tripped on crack in the sidewalk near a tree

Burrowing animals loosen sediments and push them to the surface Sediments reach the surface and other weathering processes occur Ice Wedging Occurs in temperate and cold climates Water enters cracks in rocks and freezes Pressure builds up in the cracks causing them to expand and break apart the rock The ice melts and this happens all over again The process of freezing and thawing occurs over and over, especially in mountains

Ice wedging wears mountain peaks It can also break up roads and highways Mechanical weathering reduces rocks to smaller pieces giving the rock more surface area As the surface area increases more rock is exposed to water and oxygen which aids in chemical weathering

Chemical Weathering Chemical reactions dissolve the minerals in rocks or change them to different minerals Changes the chemical composition of the rock Naturally occurring acids, such as carbonic acid, react with calcite in limestone The acid weathers away the limestone to form caves Kaolinite clay is created when acids react with feldspar in granite Clay is an end product in weathering

Plant acids can dissolve minerals in rocks and breaks the rocks into smaller pieces This weathering also enables nutrients to be available for plants Oxygen also causes weathering Oxidation occurs when some materials are exposed to oxygen and water Ex: rust

Effects of Climate on Weathering Chemical weathering is more rapid in warm, moist climates Mechanical weathering is important in dry climates and in cold climates

Aim: How does soil form?

Formation of Soil Is a mixture of weathered rock, decayed organic matter, mineral fragments, water and air Can take thousands of years to form Climate, slope, types of rock, types of vegetation and length of time of weathering affect rock formation There are different kinds of soil all over the world based on these factors 4 steps

Weathering of rocks Natural acids in rainwater weather the surface of exposed bedrock Water can freeze in cracks to break them apart Plant Growth Plants begin growing in cracks of rocks As they grow they continue the process of breaking down rocks Then a thin layer of soil begins to form

Living Organsims Insects & worms live underground with the plant roots Their wastes and dead material add organic matter to the soil Organic Matter as organic matter increases, the bedrock continues to breakdown This causes the soil layer to thicken Rich topsoil supports trees and plants with large root systems

Aim: What are the characteristics of soil?

Composition of Soil Most organic matter in soil comes from plants Animals and microorganisms remains also add organic matter to the soil Decayed organic matter over time turns into a dark-colored material called humus Humus is a source of nutrients for plants Animals burrowing in the ground mix the humus with fragments of rocks Good quality soil have equal amounts of humus and weathered rock material

Layers of Soil Layers of soil are called horizons All soil horizons form a soil profile Most soils have 3 horizons A Horizon top layer-AKA topsoil In a forest it is covered with litter Litter helps prevent erosion and evaporation of water from the soil Topsoil has more humus and fewer rock and mineral fragments than the other horizons

Is dark and fertile B Horizon is below the A horizon Lighter in color (less organic matter) Less fertile Leaching moves minerals that have been dissolved in water from A horizon down into the B horizon

C Horizon Bottom layer Contains partially weathered rock Thickest horizon Not affected by leaching and doesn’t have much organic material Coarser sediments Most like the parent material, which is rock

Aim: What determines the different types of soil?

Types of Soil Many different types of soil exist Climate affects soil Desert soil has a small amount of organic matter and is thin Prairies have thick, dark A horizon soil due to the organic matter from the large amount of grasses present Forest soil has a thinner A horizon than prairies because trees and leaves don’t supply as much organic matter as grasses do

The type of rocks soil comes from affects the type of soil Sandstone turns into a sandy soil Clay soil comes from basalt Type of vegetation that grows in an affects soil formation Slope of the land affects soil development The soil on the slope is poorly developed Soil in a valley is rich in organic matter

Aim: How has human activity affected Earth’s soil?

Soil Erosion Soil erodes when it is moved from the place where it formed Water and wind move sediments and cause erosion More severe on steep slopes Humans cause erosion to form faster by removing vegetation (ground cover) which helps increase soil erosion Serious problem for agriculture Soil loses nutrients, which plants need to grow

When soil erodes farmers compensate for the nutrient lose by buying fertilizer Forest harvesting also causes soil erosion Removing forests creates severe problems for the environment, especially rain forests Forests are cleared for lumber, farming and grazing Overgrazing also increases soil erosion Sheep and cattle are grazed on land until no ground cover remains Without protection from plants that soil is carried away by wind and water

Too much soil erosion, sediments can damage the environment This can cause streams to be covered, eggs from organisms die, or reduce downstream water quality

Aim: How can humans prevent soil erosion?

Preventing Soil Erosion Plant shelter belts of trees-this blocks the strength of the wind No-till farming-farmers do not plow the fields This practice provides cover for the soil all year round and reduces water runoff and soil erosion Contour farming used on slopes-this slows the flow of water down the slope and reduces soil erosion Terracing-leveled areas are built on slopes (looks like steps)

This practice reduces runoff by creating flat areas and shorter sections of slope