Weathering and Erosion Weathering Weathering is the process that breaks down rock and other substances at Earth’s surface.It is a set of physical, chemical and biological processes that change the physical and chemical properties of rocks and soil at or near the earth's surface. More about Weathering • Definition – the breakdown of rocks and other substances at Earth’s surface to form sediment **Sediment is very small pieces of rock. – Weathering happens to rocks that are NOT MOVING – Weathering is part of the Rock Cycle Weathering and Erosion • The forces of weathering break rocks into smaller and smaller pieces. The forces of erosion carry the pieces away. • Erosion is the removal of rock particles by wind, water, ice or gravity. • Weathering and erosion work together continuously to wear down and carry away the rocks at Earth’s surface. Uniformitarianism • The weathering and erosion that geologists observe today also shaped Earth’s surface millions of years ago. • How do geologists know this? • Geologists make inferences based on the principle of uniformitarianism. This principle states that the same processes that operate today operate in the past. There are three types of weathering • Mechanical [sometimes called physical] • Chemical • Biological Mechanical Weathering • Mechanical weathering is the type of weathering in which rock is physically broken down into smaller pieces. • These smaller pieces of rock have the same composition as the rock they came from. • The cause of mechanical weathering include freezing and thawing, release of pressure, plant growth, actions of animals of abrasion. – Abrasion refers to the grinding away of rock by rock particles carried by water, ice, wind, or gravity. Types of Mechanical Weathering – Frost heaving and Frost wedging – Plant roots – Friction and impact – Burrowing of animals – Temperature changes Ice Wedging Water seeps into the cracks of rocks. When the temperature decreases, the water freezes and expands. The ice then acts as a wedge and forces things apart. Wedges of ice in rocks widen and deepen cracks. Ice Heaving Plant Roots Friction and Repeated Impact Burrowing of Animals Temperature Changes Chemical Weathering • The process that breaks down rock through chemical changes. • The agents of chemical weathering – Water – Oxygen – Carbon dioxide – Living organisms – Acid rain Water • Water weathers rock by dissolving it. Oxygen • Iron combines with oxygen in the presence of water in a processes called oxidation. • The product of oxidation is rust. Carbon Dioxide • CO2 dissolves in rain water and creates carbonic acid. • Carbonic acid easily weathers rocks such as limestone and marble. Living Organisms • Lichens that grow on rocks produce weak acids that chemically weather rock Acid Rain • Compounds from burning coal, oil and gas react chemically with water forming acids. • Acid rain causes very rapid chemical weathering. Biological weathering is the breakdown of rock caused by the action of living organisms, including plants, burrowing animals, and lichens. A lichen is a combination of fungus and algae, living together in a symbiotic relationship. • Lichens can live on bare rock, and they break down rocks by secreting acids and other chemicals. Rate of Weathering • The most important factors that determine the rate at which weathering occurs are the type of rock and climate. • The minerals that make up the rock determine how fast it weathers. Rock made of minerals that dissolve easily in water weathers faster. • Some rock weathers more easily because it is permeable. Permeable means that a material is full of tiny connect air spaces that allow water to seep through it. • Climate refers to the average weather conditions 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.