Weathering and Erosion Chapter 14 Weathering and Erosion

Pressure/Cementation The Rock Cycle Pressure/Cementation Sedimentary Rocks Weathering/Erosion Sediment Weathering/Erosion Melting Heat and Pressure Weathering/Erosion Igneous Rocks Metamorphic Rocks Heat and Pressure Cooling Melting Melting Magma

14.1 Weathering Processes Weathering: change in physical form or chemical composition of rocks at the earth’s surface Mechanical Weathering: physically breaks rocks into smaller pieces without chemical change Chemical Weathering: breaks rocks down by changing chemical make-up

Rock structure and weathering When granite cools underground, it is under pressure. As rocks on top are removed, pressure lessens and the rock expands, causing cracks. These cracks or joints are often sites of weathering.

Types of mechanical weathering Exfoliation: When rocks break off into curved sheets (like layers of an onion), caused by cracks in the rock

Royal Arches, an example of exfoliation

Types of mechanical weathering Ice wedging: when water gets into cracks and freezes, it expands, breaking the rocks apart

Types of mechanical weathering Organic activity: plant roots and digging animals can break apart rocks Abrasion: when rocks rub against each other, breaking them apart (caused by gravity, running water, and wind)

Chemical weathering Causes: interactions between minerals and other chemicals such as water (hydrolysis), carbonic acid (carbonation), oxygen (oxidation), or acid (from acid rain or plants) Result: minerals are changed into a new substance, often that gets dissolved in water and carried away

14.2: Factors that affect the rate of weathering Rock composition: some rocks are either harder (resist mechanical weathering) or more chemically stable (resist chemical weathering) than others Slow to weather rocks: quartz Fast to weather: calcite, limestone, sandstone and shale

Factors that affect the rate of weathering Amount of exposure: surface area—the more of the rock that is exposed to weather, the faster it will weather! Small pieces will weather faster due to high surface area, just like small sticks burn easier than large logs

Factors that affect the rate of weathering Climate: areas with rainfall and alternating hot/cold seasons have the fastest weathering. Areas that are hot and dry have the slowest Why: cold + water leads to frost wedging, warm + water leads to fastest chemical changes

Factors that affect the rate of weathering Topography: slope and elevation of the land Steep slopes speed up weathering because gravity pulls rocks away from each other More frost wedging at high elevations because it is colder

14.3: Soil and Weathering Regolith: layer of weathering rock fragments, midway between rock and soil Humus: decaying organic matter (dead plants/animals) Soil: mixture of weathered minerals, water, humus and gases (air).

Soil Composition Soil texture: size of mineral particles that make up the soil Clay: particles smaller than .002 mm in diameter, result from weathered feldspar Silt: particles between .002 and .06 mm Sand: particles between .06 and 2.0 mm, result from weathered granite and other rocks high in quartz

Types of soils Loam: even mixture of all three particle sizes Transported soil: soil that forms from material that has been carried from one place to another by wind, gravity, or water. Residual Soil: soil that has formed in one place from the rock below it.

Soil Profile Soil Profile: Cross section of a residual soil, divided into layers called “horizons.” A horizon: top, called topsoil, consists of humus and small rock particles (clay, silt, sand) B horizon: called sub soil, consists of clay and chemicals that have been carried down by water (leached)

Soil Profile C Horizon: bottom layer consisting of partially weathered bedrock (regolith) Over time, C horizon grows into B than an A horizon. Biggest difference between A & B—A has humus Biggest difference between B & C—B is more weathered

Soil and Climate Different climates produce different types of soil because climate affects weathering. Tropical soils: thick due to rapid weathering low in minerals because rainfall leaches minerals deep into soil away from plant roots lots of humus due to rapid decomposition Not very fertile soils

Soil and Climate Desert soils Artic soils Soil is thin and rocky due to slow weathering Artic soils Temperate soils: soils in areas with moderate rainfall and variation between hot and cool temperatures, 2 main types

Types of temperate soils Pedalfer Found in eastern US, areas with higher rainfall High in clay, quartz and iron (reddish) Pedocal Found in western US, drier areas High in calcite, tends to neutralize acids in soil and makes the soil more fertile

Soil and topography Slopes have thin soil and flat areas have thicker soils Why: erosion by water and gravity carry soil down off slopes and into flat areas

14.4: Erosion Erosion: process that moves (transports) the products of weathering. Done by gravity, wind, glaciers, and water. Soil erosion: soil erodes quickly because the particles are light, can be a serious problem since soil is used to grow food, bad because it removes top layer where we find important plant minerals

Soil conservation Necessary to prevent soil erosion One method is to slow or reduce water movement through crop fields Contour plowing: place furrows perpendicular to slope rather than in straight lines Terracing: break slopes up into flat steps to slow water

Soil conservation Keep plants on soil so roots hold soil in place No-till agriculture: don’t remove old plants after harvest Strip cropping: alternate strips of crop that exposes soil (like corn) with crops that don’t (like alfalfa) Crop rotation: alternate crops from season to season, include crops that hold soil.

Mass movements Mass movements: movement of rock fragments down a slope due to gravity Talus: loose rock fragments at base of slopes, carried by mass movements

Exfoliation Talus

Talus

Rapid Mass Movements Rockfall: fall of rock from a steep cliff, fastest form of rapid mass movement Landslide: movement of loose rock and soil, often involve large pieces of loose bedrock Mudflow: movement of large masses of mud, what we usually call landslides