Kimia's+Notes+Chapter+2+Section+2

__**Rocks and Weathering**__

Weathering and Erosion-The process of mountainbuilding thrusts rock up tot the surface of Earth.Heat,cold,ice,and water all contribute to weathering.Rainwater can dissolve minerals that bind rock together.The forces of weathering break rocks into smaller and smaller pieces of sediment.Then the forces of erosion carry the pieces away.The weathering and erosion that geologists obsevre today also shaped Earths's surface millions of years ago. Mechanical Weathering-Like a hammer,some forces of weathering break rock into pieces.These smaller pieces of rock have the same compostion as the rock they came from.Mechanical weathering works slowly,but over very long periods of time,it does more than wear dowm rocks.The causes of mechanical weathering include freezing and thawing,release pressurre,plant growth,actions of animals,and abrasion.In cool climates,the most important force of mechanical weathering is the freezing and thawing of water.Water seeps into cracks in rocks and then freezes when the temperture drops.Water expands when it freezes.Ice therefore acts like a wedge that forces things apart.Wedges of ice in rocks widen and deepen cracks.

Chemical Weathering-The causes of chemical weathering include the action of water,oxygen,carbon dioxide,living organisms,and acid rain.Each rock is made up of one or more minerals.Chemical weathering can produce new minerals as it breaks down the rocks.Chemical weathering creates holes in rocks.so the rocks break apart more easily.Chemical and mechanical weathering work together. Water-Water is the most important casue of chemical weathering.Water weathers the rocks by dissolving them.When a rock or other substances dissolves in water,it mixes uniformly throughout the water to make solution.Over time,many rocks will dissole in water. Oxygen-The oxygen gas in the air is an important cause of chemical weathering.The product of oxidation is rust.Rocks that contain iron also oxidizes,or rusts.Rust makes rock soft and crumbly and gies it a red or brown color. Carbon Dioxide-Another gas found in air,carbon dioxide,also causes chemical weathering.Carbon dioxide dissoles in rainwater and in water that sinks through air pockets in the soil.

Rate of Weathering Type of Rock-The minerals that make up the rock determine how fast it weathers.Rock made of minerals that do not dissolve easily in water weathers slowly.Some rocks weather easily because they are permeable.Permeable rock weathers chemically at a fast rate. Climate-Climate refers tot the average weather weather cinditions in an area.Both chemical and mechanical weathering occur faster in wet climates.Rainfall provides the water needed for chemical changes as well as for freezing and thawing.