Chapter 6: Weathering & Erosion

Breaking a single piece of rock into pieces increases surface area dramatically. Initial cube has 6 sides, surface area = 6 cm 2 Each of 4 cubes have 6 sides 2 new surfaces were created along each plane where separation occurred Surface area of 4, ¼ pieces is 10 cm 2 1 cm

How does frost wedging occur? Water is most dense at 4°C. Ice is less dense than water. Ice occupies a larger volume than water. When water freezes it expands! Water seeps down into a crack, when it freezes it expands forcing the crack to open wider. Failure may occur and cause the broken off materials to fall down slope. Frost wedging is most common in areas with frequent freeze/thaw cycles. (daily)

What happens to the rock materials that break off? What do the piece form?

What happens to igneous plutons, especially granitic plutons, when rock layers above them are worn away by erosion?

Examples of exfoliation on granitic exposures

How does mechanical weathering assist chemical weathering? Chemical weathering can only occur on the exposed surface area. Mechanical weathering increases surface area. More surface area allows for more chemical weathering.

What is the main difference between mechanical and chemical weathering? Chemical weathering is different from mechanical weathering because the result of mechanical weathering is particle size is reduced, while the result of chemical weathering is that composition is changed. This is an example of the traditional distinction between physical and chemical changes in chemistry.

The chemistry of water Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge near the oxygen atom due the unshared pairs of electrons. Water has a partial positive charge near the hydrogen atoms.

The dissolution of halite shows the general process for mineral dissolution by aqueous solutions. Write the steps in the dissolution process. The ability of ions and other molecules to dissolve in water is due to polarity. NaCl, sodium chloride is shown in its crystalline form and dissolved in water. The positive end of the water molecule attracts the Cl. The negative end of the water molecule attracts the Na. The ionic bond between Na and Cl is stretched until broken.

HOW DOES ACID RAIN FORM?

What are typical pH values of acid rain?

acid rain effects soils & trees Diagram of the situation in a typical forest before acid rain (~1880) and after acid rain has become important. Note that on the right panel the soils are now releasing aluminum and hydrogen ions, and the pH of the exported water is very acid (pH = 4.6).

How does acid rain contribute to the chemical weathering of soils? Conifers appear to be particularly affected, with needles dropping off, and seedlings failing to produce new trees. The increased acidity changes the chemistry of the soil by leaching out some elements such as calcium, magnesium and potassium, which starves the trees. The trees are then much more susceptible to other forms of damage, such as being blown down, or breaking under the weight of snow.

List the main soil horizons and their components and characteristics.

Make a general list of the process of soil formation. Soils initially develop from undifferentiated parent materials. Over time, changes occur from the surface down through accumulation of organic matter and downward leaching of materials. These changes result in horizontal layers. The steps of soil formation are: 1. Parent materials are weathered mechanically. 2. Chemical weathering accelerates due to increases in surface area. 3. Organic materials are added. 4. Chemical and mechanical weathering continue producing horizons dependent on parent materials and environment.

List the dominant soil types found in the U.S. Very generally, two major soil types dominate in the United States. The eastern half of the country is predominantly covered by pedalfers. (blue regions) The western half of the United States is covered by pedocals. (green regions) Pedalfers are characteristic of regions with greater rainfall. The al in pedalfers stands for Al, the fe for Fe. Pedocals are characterisitc of regions with low rainfall amounts. The cal in pedocals stands for calcite.

How do precipitation amounts contribute to this soil distribution and development? Soil moisture maps show typical soil moistures nationwide. Soil moisture, when combined with differences in parent materials and amount of time for soil formation, account for major differences in types of soils observed in different regions.

What type of soil is dominant in tropical rainforests? Laterite soils are dominant in tropical rainforest environments. Laterites form thin layers covering rainforest floors. Laterites do NOT support agriculture! Organic material accumulates rapidly on forest floors. Hot and wet conditions greatly accelerate chemical weathering of debris. Nutrients are rapidly absorbed by nearby plant materials. Any other nutrients are quickly leached from the soil layer.

World-wide distribution of laterite soils

How do clay minerals effect plant growth? The surface layer of clay minerals generally carries a negative charge. Water molecules are polar. The positive end of the water adheres to the negative clay surface. The water is held there loosely and is available for uptake by plant roots. Plant roots also exchange Ca and K from clay surface layers for H. -Clay layers appear light gray in color