GL 107 Earth Systems Science

K. Hannula

Discussion questions for 10/12/07

1.  What is mechanical weathering?

The physical breakdown of rocks into smaller pieces of rocks, without changing the mineral content of the rock.

2.  List and explain three processes that are important in mechanical weathering.

Frost wedging:  water freezes in cracks in rock.  As it freezes, it expands, pushing the cracks open further.

Unloading:  as erosion removes the weight above rock underground, the rock expands and cracks.

Biological activity:  roots wedge cracks open.

Abrasion:  small particles of rock rub against one another and against bedrock, wearing it down.

3.  What is chemical weathering?

The chemical transformation of minerals found in rocks to different minerals stable at earth's surface.

4.  Explain the role of water, oxygen, and carbonic acid in chemical weathering.

Water: important for dissolution (dissolving minerals such as halite in water).

Oxygen: needed for oxidation; changes state of iron and sulfur.

Carbonic acid: important for hydrolysis (breakdown of silicate minerals into clays and dissolved ions.)

5.  List three minerals that remain as residual products of chemical weathering.

Quartz, hematite (or iron oxides in general), clay minerals (like kaolinite).

6.  Which rock types are most susceptible to mechanical weathering?  Why?

Any rock that cracks easily is highly susceptible to mechanical weathering.  These include shale, slate, and schist.  Sandstones with a lot of open space between sand grains are also susceptible to mechanical weathering.

7.  Which rocks types are most susceptible to chemical weathering?  Why?

Rock salt (halite), gypsum, limestone (calcite), and marble (calcite) are susceptible to dissolution because they are made of minerals susceptible to dissolution.

Mafic igneous rocks are susceptible to oxidation, and also to hydrolysis.

In general, mafic igneous rocks are more susceptible than intermediate igneous rocks, which are more susceptible than felsic igneous rocks, because they consist of minerals that are more susceptible to both oxidation and hydrolysis.