WEATHERINGChapter 5.1

What is Weathering? Weathering can be

defined as any chemical or mechanical process by which rocks exposed to weather undergo changes in character or are broken down into smaller pieces.

Mechanical Weathering Mechanical Weathering is a type of

weathering that breaks down rocks into smaller pieces without changing the chemical composition of the rock.

For example, Granite can be broken down into smaller pieces, but the smaller pieces are still Granite.

Agents of Mechanical Weathering Frost Wedging – When water freezes

in the cracks of rocks causing them to expand and break the rock.

The products of frost wedging are large piles of rocks called talus.

Frost Wedging

Frost Wedging

Talus produced by Frost Wedging

Agents of Mechanical Weathering Unloading and Exfoliation – When

large sections of rock buried underneath the ground are exposed at the surface, enormous amounts of pressure are released. This release of pressure is referred to as unloading.

After unloading, the rocks begin to flake off in sections like an onion. This is referred to as exfoliation.

Unloading and Exfoliation

Unloading and Exfoliation

Giant Slabs of rock flake offAs a result of Unloading and Exfoliation

Agents of Mechanical Weathering Biologic Activity – Weathering that

occurs as a result of the actions of living organisms like plants and animals.

Plants often cause mechanical weathering when their roots penetrate down into the cracks of rocks.

When the roots grow they expand. Animals burrowing into the ground can

also create cracks in the rocks.

Biologic Activity

Roots penetrating the cracks in rocks

Gophers and Earthworms BurrowingAlso contribute to Mechanical Weathering

Chemical Weathering When rocks that

weather change their composition into a new type of rock, this is referred to as chemical weathering.

For example, if limestone (calcite) dissolves as a result of acid rain, then it becomes carbon dioxide gas and various ions.

Chemical Weathering The three main types of chemical

weathering. Carbonation occurs when carbonic acid

(H2CO3) dissolves minerals in rocks. Hydrolysis occurs when hydrogen in

water dissolves minerals in rocks. Oxidation occurs when oxygen reacts

with minerals in rocks.

Chemical Weathering Examples of each type include. Carbonation occurs when acid rain

(carbonic acid) dissolves limestone bedrock to produce caves.

Hydrolysis transforms feldspar (in granite), to produce clay sediments.

Oxidation transforms rocks with iron (like basalt and gabbro) into hematite (rust).

Chemical WeatheringOxidation of Mafic/Basaltic Rocks To produce the mineral Hematite

Weathering of Granite to Produce clay sediments

Carbonic Acid dissolves limestoneTo produce caverns

Rate of Weathering The most important factors that affect

the rate (time) of weathering are Surface Area - More cracks means

more surface area, more surface area means more weathering.

Composition – Certain minerals are more resistant to weathering than others. Quartz for example is more resistant to weathering than calcite.

Rate of Weathering

How Weathering Affects the Surface Area and the Rate of Weathering

Rate of Weathering Climate – Hot, humid climates have a

fast rate of weathering. Cold, dry climates have a slow rate of weathering.

Vegetation – Places with more vegetation tend to have a higher rate of weathering because certain plants help produce acids that help speed up the rate of weathering.

Rate of Weathering Varies depending upon the Climate