Rocks and Weathering
EQ: What is chemical and mechanical
weathering?
Weathering
• Weathering – process that breaks up rocks
and other substances at the Earth’s surface
– Two types of weathering, mechanical and
chemical
– Heat, cold, water, ice, oxygen and carbon dioxide
contribute to weathering
• Erosion – movement of rock particles by wind,
water, ice or gravity
Mechanical Weathering
• Mechanical weathering – rock is physically
broken into smaller pieces
• Caused by heating and cooling, freezing and
thawing, growth of plants, actions of animals,
and abrasion
– Abrasion- grinding away of rock by rock particles
carried by water, ice, wind or gravity
• Works slowly , over long periods of time.
Eventually wears away whole mountains
Mechanical Weathering
• Ice wedging
– Occurs in cold climates
– Water seeps into a crack in the rock, freezes
(expands)and thaws (contracts) repeatedly
– This process expands the rock slowly and pieces
crack off
Chemical Weathering
• Chemical Weathering – process that breaks
down rock through chemical changes.
– Agents are water, oxygen, carbon dioxide, living
organisms, and acid rain
– Produces a different mineral makeup from the
rock they came from
i.e. – granite begins with minerals of feldspar, quartz and
mica. After chemical weathering, granite changes
feldspar to clay
http://www.classzone.com/books/earth_science/terc/co
ntent/visualizations/es1202/es1202page01.cfm
Chemical Weathering
• Creates holes or soft spots in the rock allowing
it to break apart more easily
• Chemical and mechanical weathering often
work together.
– Mechanical weathering breaks apart rock
– This process exposes more surface area of the
rock to chemical weathering
Five agents of Chemical Weathering
• Water – most important agent of chemical
weathering. Wears away rock by dissolving
the rock, mixing it uniformly to make a
solution
• Oxygen – combines with iron in the presence
of water. This is oxidation. Causes rust. This
process makes the rock crumbly and gives it a
red or brown color
Five agents of Chemical Weathering
• Carbon Dioxide – found in our air. Dissolves in
water and forms a weak acid called carbonic acid.
Weathers marble and limestone
• Living organisms – as plant roots grow they
produce a weak acid that slowly dissolves rock
around the roots
• Acid Rain – cause is burning fossil fuels that
pollute air with sulfur, carbon, and nitrogen
compounds. The compounds mix with cloud
vapor to form acid.
Rate of Weathering
• Two factors that determine the rate
– Type of Rock –
• Depends on the minerals that make up the rock
• Is the rock permeable – full of tiny, connected air
spaces that allow chemicals to seep through it
– Climate
• Average weather conditions of an area
• Hot and wet climates weather rock more quickly
Class Work and Homework
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Rock Shake Page G46
Read pages G 40 – G 45, #’S 1-5