Weathering and Soils Lesson Outline / Study Guide

Weathering and Soils ? Lesson Outline / Study Guide

1. Introduction Weathering is the physical breakdown (disintegration) and chemical alteration (decomposition) of rock. Mass wasting is the collective term for rock and soil moving downslope under the influence of gravity only. Erosion is the physical removal of geologic material by water, wind, ice, or gravity.

2. Types of Weathering There are 2 types of weathering: Mechanical and Chemical. - Mechanical weathering involves the physical breaking of rocks into smaller pieces. Mechanical weathering dominates in cold, dry climates. Mechanical weathering includes: o Frost wedging. o Unloading (or exfoliation). o Thermal expansion. o Biological activity, such as tree roots, burrowing animals, humans. - Chemical weathering involves breaking down rock components and internal structures of minerals. Chemical weathering dominates in wet, tropical climates. o Water is the most important agent in chemical weathering. Water helps to transport ions and molecules involved in the chemical processes. o Chemical weathering includes: Dissolution: Minerals (e.g., calcite and halite) dissolve in water with the help of weak acids (i.e., carbonic acid in rainwater). Oxidation: Electrons are lost from an element. For example, iron minerals combine with oxygen to form rust. Hydrolysis: Minerals react with water; hydrogen atoms force their way into the crystal lattice structure of minerals which speeds up decomposition. o Alterations caused by chemical weathering include: Decomposition of unstable minerals. Formation or retention of stable materials. Physical changes, such as the rounding of corners or edges.

3. Rates of Weathering Factors affecting the rate of weathering include: - Surface area. The greater the exposed surface area on a rock, the faster the rate of weathering. - Rock characteristics. o Rocks containing calcite (limestone and marble) readily dissolve in weakly acidic solutions (such as rainwater). o Note: Silicate minerals weather in same order as their order of crystallization. The first minerals to crystallize on Bowen's Reaction Series are the first minerals to weather.

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- Climate. Temperature and the amount of moisture will dictate whether mechanical or chemical weathering will dominate with the weathering of rocks. Again, chemical weathering will dominate in warm, moist climates.

- Differential Weathering. Rocks do not weather uniformly due to regional and local factors. Differential weathering can cause unusual and spectacular rock formations and landforms.

- Spheroidal Weathering is the phenomena that rounds off sharp corners and edges to form weathered rock that is spherical in shape.

4. Soils Soil is composed of minerals, organic matter, water, and air. Regolith is composed rock and mineral fragments only. Humus is decayed animal and plant remains; humus is a small, but essential component of "good" soil. The composition of "good" soil is 45% mineral matter, 25% water, 25% air, and 5% organic matter. Factors controlling soil formation: - Climate is the most influential control of soil formation. Key factors are temperature and the amount of precipitation. - Parent material. Soils may be developed from weathered bedrock or particles can be transported and deposited to form soil. - Time. The amount of time necessary for soil formation varies depending on geologic and climatic conditions. - Plants and animals. Plants will add organic matter to soil and can mechanically weather soil. Animals can burrow into soils and add organic matter. - Topography. Optimum terrain is a flat to an undulating upland surface; steep slopes often have poorly developed soils.

5. The Soil Profile Soil-forming processes operate from the surface downward. Differences between the characteristics of different soil layers allow us to categorize soils into soil "horizons," which consist of: - O Horizon - Composed almost exclusively of organic matter. - A Horizon - Composed of organic and mineral matter. Together, the O and A Horizons form topsoil. - E Horizon - Contains little organic matter; this soil horizon is a zone of leaching. - B Horizon - Zone of accumulation. - C Horizon - Composed of partially altered bedrock. - Together, the O, A, E, and B Horizons make up the solum, or "true soil."

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6. The 3 Major Soil Types The 3 major soil types include: - Pedalfers: Fertile soils in high rainfall areas. This soil contains abundant aluminum and iron. - Pedocals: This type of soil is developed in dry, warm climates and contains abundant soluble minerals, such as calcite. Occasional rainfall causes precipitation of calcite in the B Horizon, which may cause formation of caliche. This type of soil is less fertile than pedalfers. - Laterite: Deep red soil developed in tropical climates. All silicate minerals are altered, leaving mostly aluminum and Fe oxides. Excessive rainfall causes this type of soil to be leached of almost all minerals except for the most insoluble ones. These soils are infertile and can only be used to grow crops for a few years before they must be abandoned.

7. Soil Erosion Erosion is part of the natural recycling of all Earth materials. The natural rates of erosion depend on: - Soil characteristics. - Climate. - Slope of the ground surface. - Type of vegetation growing in the area.

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