Weathering

Sedimentary source material:

Before you can make a sedimentary rock, you must create sediment from preexisting rocks through 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 - the physical removal of the weathered material.

Weathering is typically a combination of both chemical and mechanical processes:

Mechanical Weathering:

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

Sources of jointing include:

Secondary processes: Once the joints are there, they can be enlarged:

Talus: In the absence of any further modification, the products of these forms of mechanical weathering often pile up in large cones of "talus" at the base of slopes mountainous regions.


Great Falls of the Potomac - Foliated schist weathers into tabular shapes.

Mossman Gorge, QLD, Australia - Isotropic basalt weathers into spherical boulders.


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

Chemical weathering


Kaimu, HI. Now buried under a lava flow.
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, but even on basaltic Hawaii, very few black-sand beaches like the one at Kaimu (right), with grains of olivine, pyroxene, and plagioclase. Apparently, different materials are broken down chemically at different rates.

Mechanisms of chemical weathering: Having said this, lets consider some specific ways in which minerals equilibrate with the environment.

Controls of chemical weathering:

In contrast to Earth, the surface of Mars is permanently frozen, largely bereft of liquid water, lacks a biosphere (?) and has a reducing atmosphere. No surprise that there, basalt is actually the most common constituent of sand.


Vernal pool at summit of Enchanted Rock, TX.

Physical parameters of weathering:


Composition and weathering:

We can describe lithologies in terms of their resistance to weathering. Two big factors influence: