Mechanicaland

Chemical

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• First step to forming soil and sedimentary rock

– Breakdown of rock and other materials into smaller pieces

– The smaller pieces do not move to a new location until erosion carries them away.

– There are two main types of weathering

Breakdown of rock into smaller pieces without any change in the chemical composition of its mineralsSometimes called “physical” weatheringRock is torn apart by physical force, rather

than by chemical breakdown.Smaller pieces do not move to a new location

until erosion carries them away.

Ice WedgingWater fills joints of rocks and freezes.Water expands 10% when it freezes, pushing

rock apart.Repeated freeze and thaw cycles over the

years cause rock to break along the joint.

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• Exfoliation or Unloading

--Rock breaks off into sheets along joints which are parallel to the surface

-Caused by expansion of rock due to uplift and removal of surface material that originally buried the rock

Mechanical - Exfoliation

http://commons.wikimedia.org/wiki/File:Exfoliation_and_mass_movement.jpg

Thermal Expansion Repeated daily heating and cooling of rock Heat causes expansion; cooling causes

contraction.Different minerals expand and contract at different

rates causing the rock to split.

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• Plant roots grow into cracks in the rocks and slowly spread the rock splitting it over time.– Plant roots act as a wedge and widen cracks.– Other causes of biotic weathering:

• burrowing animals • animals• humans

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‐ Abrasion ‐ The process of scraping or wearing away

(sandpaper). ‐ Both wind and water can cause abrasion as

rock fragments bounce off each other and break into sediments.

• Chemical reactions break down the bonds holding the rocks together, causing them to fall apart.

– Chemical weathering occurs in all types of rock.

– Rock reacts with water, gases. and solutions.

Oxidation - oxygen combines with other elements in rocks to form new types of rockNew substances due to oxidation:

– rock softer than original– easier for other forces to break rock apart– “rusting” of the rock – color change in the rock

http://commons.wikimedia.org/wiki/File:Sandsteinformation_in_garden_of_the_gods.jpg

• Carbonation – carbon dioxide (CO2) is dissolved in water making carbonic acid (acid rain).– ACIDS

– Weak acid is formed when CO2 in the air mixes with rain.

– Acid is too weak to harm plants and animals but slowly causes feldspars and limestone to decompose.

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• Biotic (living things) attach themselves to the rock and break it down via chemical processes.– Lichen– Algae– Fungi– Animal Waste

2. Particle Size – Larger particles weather slower and smaller particles weather at a faster rate.

There are 4 factors that effect the rate of There are 4 factors that effect the rate of weathering:weathering:

1. Surface Area (exposure) - Exposing more surface area will increase the rate of weathering.

3. Chemical Composition (what a rock is made of) – Certain rocks and minerals are naturally weaker than others, while others are more resistant (stronger).

4. Climate – Warmer, moister climates have the most weathering. Heat & Water speed up all chemical reactions. This is the most important factor in weathering.