Unit One A Changing Earth Chapter Two Weathering and Erosion

Weathering and Erosion Weathering Rock Cycle Rocks are classified into three categories Sedimentary, Igneous (formed when magma cools), Metamorphic Rock Cycle – the changing and transforming of rock as it undergoes weathering, heat, and pressure Weathering – process that breaks rocks into smaller pieces or decomposes them through chemical activity Takes place at or near the surface of the earth Forms soil, replenishes minerals for plant growth

Weathering and Erosion Weathering Rock Cycle (continued) Two kinds of weathering Mechanical weathering – process of breaking down rocks into smaller pieces Chemical weathering – changing rocks into different substances

Weathering and Erosion Weathering Mechanical (Physical) Weathering The process of breaking down rocks into smaller pieces Factors that contribute to mechanical weathering Temperature, water, wind, plant and animal life Examples of mechanical weathering Frost wedging (frost action) – frozen water in a rock causes the rock to crack Frost heaving – frozen water under a rock expands and lifts the rock

Weathering and Erosion Weathering Mechanical (Physical) Weathering Examples of mechanical weathering (continued) Pressure release – great pressure on a rock is suddenly released, causing cracks and breaks in the rock Exfoliation – sheets of rock peel away like layers of an onion Abrasion – occurs when rocks rub against each other Caused by water and wind Hoodoos – form when soft rock abrades more quickly than the harder rock above it

Weathering and Erosion Weathering Chemical Weathering Changes the rock into a different substance Types of chemical weathering Oxidation – a process that occurs whenever a substance combines with oxygen to form a new substance. When oxygen combines with iron, iron oxide (or rust) is formed Reaction of acids with minerals Carbonic acid – a weak acid formed when water and carbon dioxide mix

Weathering and Erosion Weathering Chemical Weathering Types of chemical weathering (continued) Acid rain – rain that contains sulfuric acid, carbonic acid and other chemicals; cities are often exposed to acid rain because of industrial smoke and exhaust from cars; caused by burning fossil fuels such as coal and oil Lichens – secrete mild acids that dissolve the rocks and further break down the rocks into the soil

Weathering and Erosion Weathering Caves Crashing waves, wind and running water form caves by mechanical weathering Chemical weathering forms limestone caves as acidic water seeps into cracks in limestone, dissolving calcite in the limestone, which causes cavities to form Speleothems – beautiful formations in caves, formed as the dissolved calcite is deposited out of the water (chemical weathering) Drip curtain – a speleothem that forms when seeping water hardens along a crack, forming a thin curtain-like sheet

Weathering and Erosion Weathering Caves Stalactites – structures in caves that hang from the ceiling and look like stone icicles Stalagmites – structures in caves that “grow” up from the ground as a result of the dripping of dissolved calcite Column – formed when a stalactite and a stalagmite grow together Spelunkers – people who enjoy exploring caves

Weathering and Erosion Weathering Soil Soil – the loose material at the surface of the earth made up of weathered particles, decayed organic material (humus), air, and water

Weathering and Erosion Weathering Soil – Soil particles Pedologist – scientist who studies soil Three basic sizes of soil particles Sand – largest kind of particle in soil; drains quickly Clay – smallest kind of particle in soil; holds nutrients and water well; 100,000 particles of clay = 1 particle of sand Silt – particle that allows water and air to mix in the soil

Weathering and Erosion Weathering Soil – Soil texture and formation Texture – refers to how much of each kind of particle is in the soil sample Loam – a soil that contains equal parts of sand and silt with about half as much clay Texture is important to farming because certain crops grow best in certain textures Kinds of weathered rock, climate and vegetation determine the composition and fertility of soil

Weathering and Erosion Weathering Soil – Soil horizons Horizons – multiple layers of soil O Horizon – top layer of soil made of leaf litter and humus Topsoil – A Horizon; made of minerals from weathered rock and humus; plants germinate and roots grow in this layer Subsoil – B Horizon; contains weathered minerals and some nutrients from the humus

Weathering and Erosion Weathering Soil – Soil horizons (continued) C Horizon – consists mainly of larger weathered fragments of the bedrock; contains clay and sand particles, but very little organic material (humus); rich in minerals Bedrock – R Horizon; regolith; unweathered parent rock that influences the texture of the soil above it

Weathering and Erosion Weathering Diagram Topsoil Subsoil Bedrock 1 2 3

Weathering and Erosion Erosion Agents of Erosion Sediment – small particles produced by weathering Erosion – moves the weathered material from one place to another Weathering and erosion often occur together Weathering is the breaking down of rocks and erosion moves the broken-down material from one place to another

Weathering and Erosion Erosion Agents of Erosion (continued) The primary force behind erosion is gravity Agents of erosion – other factors involved in the transportation of weathered material such as water, wind, and ice Deposition – the dropping of sediment (small particles) and rocks in a new location Depositions often have a layered look because sediment settles according to its weight. The heaviest sediment drops first and the lightest drops last, causing layers

Weathering and Erosion Erosion Mass Movements Mass movement – erosion that is primarily caused by gravity Five kinds of mass movement Soil creep – gravity pulling soil slowly down the slope of a hill; results in leaning trees and fences Earth flow – gravity pulling rock and sediment down a hill Mudflow – when water and soil combine and gravity pulls them down a hill Rockslides – huge slabs of rock sliding down a mountain Avalanche – when gravity pulls huge amounts of snow down a mountain

Weathering and Erosion Erosion Stream Erosion Load – sediment carried by a stream Dissolved load – sediment that dissolves in the stream and is transported to larger bodies of water The faster a stream moves, the more sediment it can pick up and move Suspended load – sediment that is carried by a stream but is not dissolved

Weathering and Erosion Erosion Stream Erosion (continued) Floodplain – an area where a river or stream commonly floods Yearly flooding provides nutrients to some farmland Delta – area at the mouth of the river where there are deposits of sediment that are rich in nutrients Birdfoot delta – the Mississippi Delta - the current and waves in the Gulf of Mexico have caused it to be shaped differently.

Weathering and Erosion Erosion Wave Erosion Waves erode away the beach and deposit the sand in new places The shoreline constantly changes as wave erosion and deposition takes place Sand bars constantly shift positions and form new areas of land and shallow places along the coast The bigger and more powerful waves from storms, such as hurricanes, increase wave erosion and deposition

Weathering and Erosion Erosion Wind Erosion Wind is the agent of erosion in dry areas, such as deserts Deflation – wind picking up loose sediment and carrying it away Two kinds of wind erosion Dust storms – when wind blows small, loose particles such as clay and silt Sandstorms – when wind blows sand

Weathering and Erosion Erosion Ice Erosion Glaciers – formed when layers of snow that haven’t melted compact and turn to ice Plucking – large pieces of bedrock are pulled out of the ground by a glacier and carried along the mountainside Moraines – piles of rocks and soil left behind by a glacier

Weathering and Erosion Causes of Erosion Water, wind and ice are agents of erosion Water may cause some materials to weather However, most weathering is done by the sediment and loads carried by water, wind, and ice The agents of erosion are mainly the means to transport weathered materials