1. Unit One A Changing Earth
Chapter Two
Weathering and Erosion

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. Weathering and Erosion Weathering
Diagram
Topsoil
Subsoil
Bedrock 1 2 3

16. 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

17. 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

18. 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

19. 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

20. 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.

21. 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

22. 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

23. 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

24. 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