Weathering
How does Rock become Exposed to
the Surface?
• Most rocks, like granite, form under earth’s
surface.
• The rocks uplift and eventually make their way to
earth’s surface.
• Conditions on the surface are much different
from inside earth.
• Rocks are not used to these conditions, become
unstable, and break down.
Weathering
Area= L X W
Surface Area: (Area) X (# of sides.)
• Once a rock weathers, its surface area increases.
• Increase in surface area exposed allows for more
weathering.
Thus,
weathering
speeds up
over time.
Surface area
goes up
weathering time
goes down!
1. Physical Weathering (Mechanical)
2. Chemical Weathering
• Physically breaks rock into smaller pieces.
• Rock actually disintegrates, but does not
change in composition.
• Example: like a jackhammer smashing up rock
or concrete.
Major types of mechanical weathering are:
a) Frost wedging
b) Exfoliation/Unloading
c) Organic Activity
d) Abrasion
a) Frost Wedging
• Water gets into fractures in rock.
• When the water freezes, it expands and breaks
the rock apart.
• Most often in regions of constant “freeze and
thaw” cycles.
b) Exfoliation
•
•
•
•
Unique to Granite
Rock Heats and Expands
Rocks Cools and Contracts
Causes Rock Surface to Peel
c) Organic Activity
• Plant roots grow into rocks and split it apart.
• Burrowing animals loosen sediments.
c) Abrasion
• Wind rounds out dry rocks (spheroidal
weathering)
• Water rounds out rocks in streams
• Ice Carves and breaks apart rock
• Change in the chemical composition of rock.
• Occurs when rock reacts to oxygen, carbon, water,
and other chemicals.
Major types of chemical weathering are:
a) Oxidation
b) Carbonation
c) Hydration
d) Living Organisms
e) Acid Rain
a) Oxidation
• When rocks containing iron are exposed to
oxygen a chemical reaction occurs.
• The rocks oxidize or “rust” producing a red color.
Example: a rusty nail or a rusty bike chain
Red Rock in Sedona Arizona – Oxidized Rock
b) Carbonation
• When water reacts with
carbon dioxide it forms
carbonic acid.
• The acid reacts with rocks
that contain calcium
carbonate (carbonate
materials CaCO3).
• Rocks like limestone will
dissolve from the acid.
• Creates Caverns and
sinkholes
• How Test: Placing diluted acid
on rock to test for calcite.
c) Hydration
• Water weathers rock by dissolving certain
soluble minerals found in rock.
• Turns hard minerals into soft substances
• Ex. Feldspar to clay
d. Living Organisms
• Lichens that grow on rocks produce weak acids
that chemically weather rock
e. Acid Rain
• Compounds from burning coal, oil and gas react
chemically with water forming acids. (nitric and
sulfuric acids)
• Causes very rapid chemical weathering.
Factors affecting weathering:
1. Mineral Composition of Rock
2. Climate of a region.
1. Mineral Composition of Rock
- Some minerals are less stable (dissolve faster)
than others.
- Differential Weathering
As Rock Hardness Increases….Weathering
Decreases!
2. Climate of a Region
• The warmer and wetter a region is, the more
weathering occurs.
• Climate affects both mechanical and chemical
weathering.
–Works better in
areas with
temperature
extremes
-Hot days and Cold
Nights
-Dry climate
–Works better in a hot
and moist
environment
–More water = more
weathering
• Physical and Chemical Weathering break apart
rock into smaller pieces.
• Erosion – transports sediments to other locations
• As soon as sediment (loosened by one of the 2
weathering processes) moves, it is called erosion.
• Mass wasting - is simply movement down slope
due to gravity.
• Examples: Rock falls, slumps, and debris flows
• We call it erosion if the rock particle is moved by
some flowing agent such as air, water, or ice.
• What causes erosion to be severe in some areas
and minor elsewhere?
1.amount and intensity of precipitation
2.the texture of the soil
3.the steepness of the slope
4.ground cover (from vegetation, rocks, etc.).
Wind Erosion
• Occurs in areas with low rainfall, high
temperatures, and minimal plant cover
• Picks up smaller, lighter sediments
• Removes from the field the most fertile portion of
the soil.
• Farmers often plant windbreaks to protect against
wind erosion
• Common Features: Rock Sculptures and Sand
Dunes
Water Erosion
• Rain, streams, rivers, and the ocean have the
ability to move large amounts of sediments
• Creates Sorted deposits when rivers flow into a
larger body of water
• Faster moving water can carry larger sediments
than slow moving water
• Creates v-shaped valleys, rill erosion, gully
erosion, and coastal erosion
Gully erosion
Ice Erosion
• Glaciers scrape and gouge out large sections of
Earth’s landscapes
• Capacity to carry huge rocks and piles of debris
over great distances
• Leaves unsorted deposits when glaciers retreat
• Creates u-shaped valleys with majestic
waterfalls, lakes, and variously-shaped deposits
of sediments.
Who else is an erosional agent?