Part IV. Weathering, soils, sediments, and sedimentary rocks.

A.) Weathering: The process that converts minerals in rocks into sediment, new minerals, and dissolved ions.

            1.) Weathering helps to shape the earth’s surface.

            2.) Weathered rock produces soil.

            3.) Weathering of silicate minerals absorbs carbon dioxide (CO2) from the atmosphere

            4.) Weathering damages man made structures, such as buildings, bridges, etc.

                        a.) Acid rain enhances weathering

            5.) Controls on weathering.

                        a.) Climate (long-term average of weather, usually 30 years or more)

                                    i.) Controlled by atmosphere circulation, latitude, and topography, and precipitation.

                                    ii.) Plants enhance weathering

                  b.) Relief (topography)

i.) 5-10% of modern earth is covered by mountains, but these areas supply 80% of all solid fragments that become clastic sedimentary rocks.
            Fragments = sand, gravel, and mud

                 c.) Composition of the source area (rock types).

            6.) Types of Weathering

a.) Mechanical Weathering: Physical processes that break down rocks and  minerals.

            i.) Frost wedging:

9% expansion when water freezes Þ generates >2,000 PSI
        Granite has a tensile strength of 600 PSI

            ii.) Root Wedging: Roots grow into fractures and expand with up to 1500 PSI pressure

           iii.) Pressure release: High internal pressure of some rocks (especially intrusive igneous) causes fracturing when pressure is removed.

• Pressure release fractures may be:

  Joints (vertical or horizontal fractures across which there has been no movement)

                      OR
           Exfoliation fractures

Study Questions:

1. What role does carbon dioxide play in determining the Earth's climate?

2. Where is most of Earth's carbon?

3. Why is rain acidic?

4. The diagram below was used in class to summarize some of the processes, starting material and products of weathering:

<--- Click on this image to see a bigger version.

Next Topic: Sediments and Sedimentary Rocks.