General intro to Sedimentology:

We now embark on a study of sedimentary rocks, rocks that are composed of the fragments of preexisting rocks. Before you can make a sedimentary rock, you must do four things to preexisting rocks:

Today, we will examine the first of these processes: Weathering

Weathering: The process by which rocks and minerals at the Earth's surface are physically and chemically broken down. All weathering involves the rock's reduction into smaller (sometimes molecule-sized) pieces.

Erosion: What weathering is not - physical removal of the weathered material.

Weathering is a combination of both chemical and mechanical processes:

Mechanical Weathering:

This occurs through a distinct sequence of events. First, joints (i.e. cracks) form in rocks at or near Earth's surface. Then, those joints are enlarged by a set of secondary processes.

Of course, the properties of the rock being weathered will strongly influence the character of the weathering:

Chemical weathering

If all weathering were mechanical, we would expect weathering products to resemble parent materials in miniature. Is this what we see? Consider my Hawaii trip. Lots of quartz sand beaches, little olivine.

What can account for this? Consider the following fact: Many minerals form under (are at home in) conditions very alien to the earth's surface. When exposed to surface conditions , they are rapidly altered chemically. (In contrast, those that are more nearly "at home" on Earth's surface tend to resist weathering longer. Thus, olivine doesn't last long but quartz hangs around.) This leads us to the topic of chemical weathering, the process whereby rock materials are decomposed by chemical alteration of the parent material.
Mechanisms of chemical weathering: Having said this, lets consider some specific ways in which minerals equilibrate with the environment.


An important consequence of the factors we just listed is that in some regions, weathering products hang around and continue to interact with weathering agents and parent material. The result is soil. Terms:

In the real world, soil profiles are influenced and constrained by their environments.

Soil depth varies with:

Soil types - Soil scientists have a complex soil taxonomy. For us, three major types will suffice:

Paleosols: A soil requires the constant input of organic material, so it can be "killed" if it is deprived of this. Climate change and burial by sediments can both kill soils. Buried ancient soils are called paleosols. These can be used to reconstruct ancient environments. Laterite paleosols can be commercial sources of aluminum.