Weathering

• 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.

How does Rock become Exposed to the Surface?

Weathering

• Once a rock weathers, its surface area increases.

• Increase in surface area exposed allows for more weathering.

Area= L X W Surface Area: (Area) X (# of sides.)

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.

• Wind rounds out dry rocks (spheroidal weathering)

• Water rounds out rocks in streams

• Ice Carves and breaks apart rock

c) Abrasion

• 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

• 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.

b) Carbonation

• Water weathers rock by dissolving certain soluble minerals found in rock.

• Turns hard minerals into soft substances

• Ex. Feldspar to clay

c) Hydration

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?