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Weathering and Erosion Study Guide
Test will be on _____________________________________________.
The BIG idea of this unit – Weathering, erosion, and deposition act together in a
cycle that wears down and builds up the Earth’s surface.
Weathering – the process by which rock is broken down very slowly (much slower
than earthquake activity, landslides, or volcanic activity)
Mechanical Weathering – breakdown of rock into smaller pieces by physical
means
 Ice Wedging – alternate freezing and thawing of water that seeps into the
cracks of rocks
 Abrasion – the grinding and wearing away of rock by wind, water, or gravity
 Plants – the roots of plants grow down into the rock and eventually bust it
apart (root pry is an example)
 Animals - can wear down rock as well
Chemical Weathering – rock breaks down as a result of a chemical reaction
 Acid precipitation – rain, sleet, or snow that contains a high concentration
of acids; acid rain can dissolve the limestone that is on some buildings.
 Plant acid – lichen, which is fungi and algae living together can break down
rock
 Oxidation – oxygen in the air can cause rust
 Carbonic or sulfuric acid – can dissolve rock
Erosion – the process in which weathered rock and soil (sediments) are
transported/moved from one place to another; Example – The Rocky Mountains
have sharp peaks and are younger than the Appalachian Mountains because they
haven’t been eroded as much.
4 Agents of Erosion
1. Gravity/Mass Movement – when gravity pulls rocks and soil downhill
a. Landslide – sudden mass erosion of rocks, trees, and soil downhill
b. Mudslide – sudden mass erosion of mud downhill
c. Creep – slow movement of soil, rocks, and plants down a slope
where plant roots and tree trunks begin to lean toward the soil as
erosion continues
2. Water – happens when water picks up materials and carries them away;
running water changes the Earth’s surface more than any other agent
a. Runoff – water flowing across the Earth’s surface; most sediment
washes or falls into a river
b. Rivers – V-shaped valleys and canyons can be caused by erosion from
running water
c. Glaciers – frozen “rivers of ice” that can make U-shaped valleys
3. Wind – occurs when wind picks up loose sediments and carries them in the
air
4. Waves – the force of waves striking the shoreline break rock into smaller
pieces (abrasion). The smaller pieces are then carried away by the waves.
Human Effects of Erosion
 Clear cutting – clearing all vegetation; deforestation can increase soil
erosion
 Construction grading
 Farming (example – if cattle are over grazing, the soil will be eroded more
quickly by wind and water)
 Strip mining – soil and rock overlying mineral or rock deposits are removed
Reducing Erosion
 Planting vegetation – ground cover, shrubs, trees; Planting trees on a flat,
open plain helps to protect soil from erosion due to wind.
 Installing drainage pipes
 Silt fencing
 Retaining walls
 Terracing – a sloped plane that has been cut into a series of flat surfaces or
platforms that look like steps
 Contour plowing – rows that act as a series of dams, not rivers
 Crop rotation
Deposition – process of dumping or putting down material eroded away from
another place, (Example – If you mixed a sample of soil with water in a jar and let
it settle for 24 hours, it would show the deposition of sediment in a lake bed.)
Deposition of sediments at the mouth of a river can create a landform such as a
delta, a triangle-shaped deposit of sand, clay, or silt. When the velocity of a river
changes as it enters a quiet body of water, sediment is deposited and forms a
delta.
Soil – a mixture of weathered rock (bits of rock, silt, clay, sand), mineral
fragments, water, air, and humus (decayed plants and animals)
 The soil profile is in layers called horizons.
 The law of superposition states that older stuff will be found in the bottom
layer of soil.
 Soon after a plant dies, it breaks down and becomes part of the soil.
 The upper layer of soil has the most humus and is affected the most by
weathering and erosion.