Weathering

How are the earth’s surfaces changed?

Weathering is the breaking down of rocks and other materials on the earth’s surface.

There is no change in the rock’s chemical make-up or composition with this.

There are two types of weathering.

Types of Weathering

Mechanical or physical weathering ~ causes:

Temperature changes causing ice or frost wedging

Root, organic, or biological weatheringGravity Abrasion (scraping or sanding)Unloading or exfoliating (like peeling)

Skip ahead to details…

Second Type of Weathering

Chemical or physical weathering ~causes:

Water wears down rocksAcid dissolves Earth’s surfacesAir pollution Plant chemicalsOxidation (exposure of minerals to

oxygen)

Mechanical and chemical weathering can work together…

Advance to chemicalweathering

Causes of mechanical weathering include:

Temperature: which causes cycles of expansion and contraction

Mechanical or physical weathering

Ice or frost wedging (frost action, freeze-thaw cycle): when liquid water goes into cracks and then freezes causing the cracks to get wider

Mechanical/physical weathering

The power of the freezing and warming cycle

More mechanical orphysical weathering

Organic activity root-pry, burrowing animals, human activities

A hearty growing plant can expand a crack in a rock until it splits!

CRACK!

Roots can also raise up cement slabs, crack sewer lines, and house foundations. Don’t you think they could also crack a rock?

Gravity causes rocks to fall and collide with other rocks

Mudslides and sinkholes…

Landslides, rockslides,holes, and cracks

Abrasion: wearing away by solid particles

Wave rock, Australia – carved by wind blown sand.

Phoenix, AZ

Unloading: the expanding of high pressure rock when exposed to a lower pressure

Classic sheeted granite in Yosemite National Park. It is broken into gently dipping plates by unloading joints.

Exfoliation or unloading - rock breaks off by layers or sheets along joints parallel to the ground surface; caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial

Extreme exfoliation!

Contraction due to crystallization

The surface pattern on this pedestal rock is honeycomb weathering, caused by salt crystallization. This example is on the island of Taiwan.

Salt weathering of building

stone on the island of Gozo, Malta

Back to overview

The other kind of Weathering~

Chemical Weathering

The altering of the composition or the make-up of minerals within a rock that results in a reduction

in size.

Agents of chemical weathering include:

Water: Dissolves minerals out of rocks making them weaker.

Example:

Acid: Dissolves minerals in rocks (examples: carbonic acid, acid rain, and plant acid)

Cave wall

Chemical weathering

Lichens such as these growing on the rocks in the pictures can produce weak acids that react with the rock.

Air polllution can produce acid rain.

3000 year old Egyptian Obelisk

3000 year old Egyptian Obelisk after 100 years in NY

Picture it…

The ancient Parthenon in Greece shows discoloration and chemical weathering

effects just from air pollution and acid rain.

Oxidation: Oxygen combines with iron minerals and sulfur minerals changing the composition of the rock

More chemical weathering

Oxidation turned these rocks in Nevada's Valley of Fire red.

Look at factors that hasten weathering:

1. How much surface area is exposed to the elements?

2. Climate:

Warm, wetclimates +

chemical weathering breaks down rocks rapidly, too.

Mechanical

weathering

+ cold climates break down rocks rapidly.

3. Topography: the position of the rock (Is it in a position where gravity can pull it down?)

4. Air pollution

5. Exposure time (Will the elements have time to break down the surface?)

Mechanical and chemical weathering work together to break down rocks and our earth surface.