WEATHERING
def. – a slow, continuous process that breaks down rocks at or near the earth’s surface in
response to air, water, living matter, and the force of gravity
2 types – physical/mechanical
chemical
Physical –process by which rock is progressively broken down into smaller & smaller
fragments as a result of physical forces
-ice wedging – water in cracks expands and wedges the rock apart upon freezing
ex. pot holes in pavement
-abrasion – caused when particles or other rocks collide into, scrape against, crush or
come into contact with the surface of a rock
ex. glaciers scoured and broke off pieces of rock
gravity causes loose soil & rock to fall downhill, colliding with each
other and breaking into smaller pieces
- exfoliation – as rocks become exposed to surface air, the pressure on the rock is
reduced
- reduced pressure on rock may cause rock to crack and “peel”
- thermal expansion – rapid heating/cooling of surface rocks from day to night or
summer to winter causes expansion/contraction of rocks’ outer surface
-individual mineral grains or rock fragments may crack or flake off
- organic activity – plant roots can create pressure on rocks causing them to crack
-burrowing animals can expose rock surfaces to air/water for further
erosion
-humans mine, drive over, push rocks
Chemical- when chemical reactions take place between rocks and water, CO2, O2, and acids
causing the internal structure of the original minerals in the rock to change
-results in chemical composition change and physical appearance change
-oxidation - O2 + water, any rocks containing Fe will be oxidized
ex. red brown stains on rocks/soil
-carbonation – CO2 in water = carbonic acid
- some minerals dissolve when in contact with acids
ex. limestone caves are a result of carbonation
- acid rain – nitric acid/sulfuric acid dissolves metals
-plants – lichens & mosses secrete weak acids and break down rock over time (soil
formation!)
Factors Affecting Rates of Weathering
particle size – greater surface area = greater exposure
mineral composition – minerals that formed at the highest temperatures & pressures tend
to chemically weather first
climate – moisture + warm temperatures speeds up chemical weathering
cool + dry conditions reduces chemical weathering effects
plant, animal & human activity – cause air/water/soil conditions to change that can speed
up/slow down weathering rates
ex. burning of fossil fuels = more SO & NO in air = more acid rain