Weathering & Soils

A. The physical breakdown of rock involves breaking rock down into smaller pieces through mechanical weathering processes. These processes include abrasion, frost wedging, pressure release (unloading), and organic activity.

1. Abrasion is the grinding of rock by impact and friction during transportation. Rivers, glaciers, wind, and waves all produce abrasion.

2. Frost wedging occurs when water penetrates cracks in rocks and then freezes, causing expansion of the crack.

3. Pressure release -- when a rock that forms under great pressure is brought to the surface and overlying rock is eroded away, fractures will form parallel to the outer surface of the rock. This is common in granitic plutons, like Half Dome in Yosemite.

4. Organic activity includes root growth in cracks, and burrowing animals.

B. Chemical weathering is the chemical decomposition of the minerals in a rock to other minerals. What affects chemical weathering?

1. Surface area -- if the rock is broken down into small pieces, it undergoes chemical weathering more readily than does one large piece. Smaller pieces have more surface area for water and gases to react with the rock. Mechanical weathering is effective at increasing surface area.

2. Mineral stability -- minerals form at certain temperatures and pressures (e.g., olivine at hi temperature) because they are most comfortable (or stable) at those temperatures and pressures. When conditions (temperature and pressure) change (for instance, after a pluton cools, and is then exposed at the surface), those minerals become unstable and will tend to react chemically into other more stable minerals. Example: Because the minerals at the top of Bowen's Reactions series (olivine, pyroxene, Ca-plagioclase) form at the highest temperatures, they tend to be the most unstable at the surface, compared to minerals that form at the bottom of the Series (like quartz & potassium feldspar). Many of the minerals in Bowen's reactions series will weather to oxides or clays.

3. Oxidation -- many elements, particularly metals, tend to combine with oxygen. This often breaks up the chemical structure of minerals and produces other minerals as a weathering product. Example: Iron (Fe turning to rust, or hematite)

4. Acid -- Acids are compounds that have excess hydrogen ions (H+). These ions substitute easily for other positive ions, like Ca++ and Na+.

5. Water -- can provide oxygen and hydrogen for oxidation and or acidization. Water also can dissolve minerals. Example: Slightly acidic water dissolves limestone to form caverns, and sink holes.

6. Temperature/climate -- weathering can occur faster in warm, wet climates, slower in drier, cooler climates.

III. Soils

A. What is in soils?

B. What are different layers of soil?