Physical Geography

Lauren Brinkworth

Mechanical Weathering (physical)

Exposure to forces such as wind, water, cold and heat lead to mechanical weathering.

Definiton: Physical breakdown of rock by environmental factors.

The Process

  • Constantly happening in nature
  • Happening slower than we can detect
  • We can't see a rock expanding and contracting due to temperature fluctuation

Thermal Expansion

  • Minerals expand and contract based on temperature.
  • These minerals expand and contract at different rates when exposed to rapid temperature fluctuations such as day-night cycles. This causes stress and small cracks within the rocks and the gradual breakdown of the rock.
  • Grus is an example of thermal expansion. Grus is the accumulation of coarse-grained and loose fragments left behind by weathering.
  • It is a direct result of physical breakdown and weakening of a rock overtime.

Frost Shattering

  • Breakdown of rock due to the expansion of ice
  • It can happen in the crevices between rocks
  • At night when temperature drops, the water freezes and expands
  • The cracks within the rocks widen and eventually shatter portions of the rock
  • In areas where temperature often hover around the freezing point, if water seeps into cracks within a road surface it can freeze and expand
  • Water that seeps into rocks may contain salt. The salt crystals expand when they are exposed to heat and can widen the cracks

Plant Activity

  • Little crevices can provide just enough room for living organisms to enter and grow
  • The roots of plants and trees grow into cracks within rocks
  • As the roots grow in size, they widen the cracks
  • Roots often win the battle and push the rock, causing pieces of the rock to fall away


  • Abrasion is the breakdown of rock by friction
  • Stationary rocks are scraped away by other loose rock and mineral fragments
  • These small fragments are carried by the air or water and come in direct contact with the stationary rocks
  • Stones within the riverbed are nice and smooth due to the constant weathering force of water running over their surfaces
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Thermal Expansion

Chemical Weathering

Chemical weathering is what happens when rocks are broken down and chemically altered. After chemical weathering, we end up with a different substance than the one we started with.


  • Chemical breakdown of a substance when combined with water
  • Chemical reaction happening between the minerals found in the rock and rainwater
  • When it rains, water seeps down into the ground and comes in contact with granite rocks
  • The feldspar crystals within the granite react with the water and are chemically altered to form clay minerals, which weaken the rock


  • Oxidation is the reaction of a substance with oxygen
  • Causes rust
  • Rocks can get rusty if they contain iron
  • When iron reacts with oxygen, it forms iron oxide, which is not very strong. So when a rock gets oxidised, it is weakened and crumbles easily, allowing the rock to break down


  • Carbonation is the mixing of water with carbon dioxide to make carbonic acid
  • Formation of caves
  • Dissolved carbon dioxide in rainwater of in moist air forms carbonic acid, and this acid reacts with minerals in rocks

Lichens and Acid Rain

  • Lichens are a combination of fungi and algae and grow on rocks and produce acids that break down the minerals within the rocks
  • When water mixes with the lichens, they produce a weak acid
  • This occurs near rivers and streams

Acid Rain

  • Acid rain is rain that has been made acidic by certain pollutants in the air
  • Pollutants produced by burning fossil fuels
  • These pollutants are put into the air,cohere they mix with moisture. This process produces acids which falls to the ground

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Mass Movement

The large movement of rock, soil and debris downward due to the force of gravity. These changes can happen suddenly, as in one minute or it can happen more slowly over time.

Causes of Mass Movement

  • Gravity is constantly trying to pull rock and debris down the slope of a mountain
  • At the same time, the resistive forces of the mountain including the cohesive strength and internal friction between the materials- the mountains shear strength pull back against gravity
  • Shear strength maintains the slope's stability and keeps the materials in place
  • Causes of mass movement occur when gravitational force overcomes the resistive forces of the mountain
  • Increased slope steepness increases mass movement because the gravitational force acting in a steep slope is greater than the force acting on a gentle slope
  • Increased water is where water can wash away small particles that help keep the mountainside intact
  • If an area has decreased vegetation it will be more prone to mass movement. Vegetation stabilises soil particles on the surface and anchors soil under the surface through its root system
  • Earthquakes play a role in mass movement because the violent shaking has the ability to break off sections of mountains or hills, causing them to slide down the slope

Slumps and Rockslides

  • A slump is the sliding of coherent rock material along a curved surface
  • A rockslide is the sliding of rock material down a mountain

Debris Flow and Earthflow

Downward movement of unconsolidated materials. These are called flows because the materials flow, like a fluid down a slope.

  • A debris flow is the movement of a water-laden mass of loose mud, sand, soil, rock and debris down a slope
  • An Earthflow is a flow of fine-grained material that typically develops at the lower end of a slope

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