Types of Weathering

• Mechanical weathering breaks up rocks but doesn't change the chemical makeup---the rock remains rock. Mechanical weathering includes gravity, friction, frost, temperature and organic activity.
  
  Chemical weathering changes the chemical composition of rocks and makes them decompose into small particles. Chemical weathering occurs when the chemicals composing rocks are mixed with water, sulfuric and plant acids, oxidation and carbon. Their chemical reactions cause rocks to decompose more rapidy. These materials are found in the atmosphere, rain, soil and plants.
  
  

Mature Soil

• What is called mature soil takes thousands of years of weathering to form. It consists of three layers called horizons. The top horizon is topsoil. Topsoil is fertile and contains the nourishment needed for plant growth. The next horizon is called subsoil. It is formed by water soaking through the topsoil carrying some minerals with it, and takes a long time to form. The third layer is unweathered rock, which is not exposed to the atmosphere.
  
  

Need for Soil

• Soil is necessary for the life cycle. Soil provides food for plants that grow in it, and plants provide food for humans and animals. As plants and animals die, they lie on the soil, and bacteria in the soil cause them to decay. The decayed materials not only provide food for plants, but they help decompose rock, which forms new soil.
  
  

Soil Formation

• Soil is composed of 80 percent weathered rock and 20 percent decayed animals and plants. As bugs, moles and earthworms travel from the surface of the ground and underneath it, they carry rock particles and decayed material with them. As they travel, they leave underground layers of soil and plant nourishment. The breaking up of the soil also allows water to soak into the soil, carrying more nutrition underground.
  
  

Variations in Soil

• The different composition of rocks, and whether they were weathered by mechanical or chemical means, determines the type of soil in a given area.