Weathering and Soil Chapter 7

Weathering Weathering is the breaking down of rocks and other materials on Earth’s surface. 2 Types: Mechanical Chemical

Mechanical Weathering Involves only physical changes, such as size and shape. Some causes of mechanical weathering are: Temperature changes Ice wedging Root action

Ice Wedging Water expands as is freezes (from 39F to 32F) which breaks rocks apart

Frost Wedging (Changes in Temperature)

Root action/wedging

Chemical Weathering When chemical changes occur when minerals in rocks are broken down into other substances. Causes of chemical weathering are: Oxidation (reaction with oxygen) Water Acids

Chemical Weathering: Oxidation Oxygen combines with another substance to form new substances called oxides. Rust is an iron oxide. Pyrite and magnetite contain iron and can undergo this type of chemical weathering.

Chemical Weathering: Water Most chemical weathering caused by water. Water chemically reacting with a mineral with little water content is called hydrolysis. Feldspar chemically combines with water to form clay.

Chemical Weathering: Acids Carbonation Biological Acids Carbon dioxide dissolves in rain to form a weak acid called carbonic acid. Carbonic acid comes into contact with certain minerals and carbonation occurs. Example: mineral calcite (in limestone and marble) Some green plants produce weak acids, such as mosses. Decaying organisms also produce acids.

Factors that affect the rate of weathering: Climate (hot wet = fast weathering) (hot dry = slow weathering) Surface area (more surfaces = faster weathering) Rock composition (rocks that contain quartz will weather slowly, rocks that contain feldspar will weather easily) Acid rain (pollution adding carbon dioxide, sulfur and nitrogen to the air speeds up the process of weathering from acid rain made up of sulfuric and nitric acid)

Soil Formation Soil forms when bedrock is continuously broken down over time by weathering. Humus (decaying remains of plants and animals) as well as living animals in the soil help speed the break down of rock into soil.

What is soil? Soil is a mixture made up of: Silt Sand Clay Minerals Humus Air Water

Why is soil important? To plants? To animals? To humans?

How do soils differ? By: Texture (particle size) Place (transported soil) Minerals within it Climate (tundra, northern forest, prairie, mountain, desert, southern forest, tropical)

Soil Profile Soil layers are called horizons and differ in color and texture. (layers labeled by O, A, B, C, bedrock) All the layers together are called a soil profile.