Forces That Shape Earth’s Surface 6.E.2.3 Explain how the formation of soil is related to the parent rock type and the environment in which it develops.

Weathering Weathering is the breaking down of the materials of Earth’s crust into smaller pieces. There are two types of weathering: 1. Physical weathering 2. Chemical weathering

Weathering and erosion work together continuously to wear down and carry away the rocks at Earth’s surface. The weathering and erosion that geologist observe today also shaped Earth’s surface a long time ago. Geologists make inferences based on the principal of uniformitarianism. This principal states that the same process that operate today operated in the past.

Physical weathering – is the processes that mechanically break up rocks into fragments, such as the movement of water, wind and ice, and frost action. Physical weathering is also known as mechanical weathering.

Mechanical Weathering The causes of mechanical weathering include freezing and thawing, release of pressure, plant growth, actions of animals, and abrasion. Abrasion refers to the grinding away of rock by rock particles carried by water, ice, wind or gravity. Ice wedging is the process that splits rock when water seeps into cracks, then freezes and expands. Look on pages 110 & 111 at the different types of mechanical weathering.

Chemical weathering – is the breaking down of the materials in rock through chemical alteration of those minerals. Chemical weathering involves reactions between air and water and minerals in rocks. An example of chemical weathering occurs when certain metallic elements combine with oxygen to form oxides. This is known as oxidation. The most familiar oxidation form is the “rusting” of iron in rocks or soil that produces a reddish brown stain of iron oxide.

Causes of chemical weathering: * Water is the most important cause of chemical weathering. * Oxygen gas in air is an important cause of chemical weathering. Iron combines with oxygen in the presence of water in a process called oxidation. * Carbon dioxide dissolves in rainwater and creates a weak acid called carbonic acid. * Living organisms, such as lichens, are plantlike organisms that grow on rocks and produce weak acids that chemically weather rock. * Acid rain causes very rapid chemical weathering.

Weathering The most important factors that determine the rate at which weathering occurs are the type of rock and the climate.

Erosion Erosion is the carrying away of pieces of weathered rock by gravity, water, wind, and ice. Piece by piece erosion can carry away a bolder, a hill, or even a whole mountain range!

Gravity Glaciers Wind Water Erosional Forces :

What is the greatest agent of erosion? Water

Erosional Forces These agents of erosion can only move material when they have enough energy to move the object. Another thing they have in common is they will deposit, or drop, the material they carry when they have lost the energy.

Gravity as a force: Gravity is the force which Earth has on all objects. It pulls boulders, soil and everything else to the lowest point possible. When gravity moves materials downslope this is then called mass movement.

Some mass movements are slow:

Some mass movements are fast:

Erosion by slump Slump - a type of mass movement that occurs when a mass of material moves down a curved slope.

Erosion by Slump

Erosion by Creep Creep is the slow movement of soil downslope that causes fence poles, telephone poles to tilt in the direction of movement.

Erosion by Rock Slides Rock slides happen when layers of rock slip downslope suddenly. Rock slides can be very fast and very dangerous Very common in mountainous areas.

Erosion by Rock Slides

Erosion by Rockfalls Rockfalls occur when material free falls through the air.

Glaciers as Agents of Erosion A moving mass of ice and snow is called a glacier. Glaciers form when snow and ice build up faster than they melt. There are two types of glaciers: –Continental glaciers and –Valley glaciers

Continental Glaciers Continental glaciers cover large parts of continents such as Antarctica.

Valley Glaciers Valley glaciers are found in mountain regions all around the world. They begin at the top of the mountain where it is snows all year and the temperatures are always below freezing.

Glaciers About 18,000 years ago glaciers covered 28% of the Earth’s surface. Since that time they have been melting and now cover about 10% of the Earth. Greenland and Antarctica are almost completely covered in ice.

Glacier Erosion Glacial plucking is a process that occurs when moving glacier picks up loosened rock particles.

Glacial Plucking of Rocks

Wind Erosion Wind is a very powerful force in erosion. Through blowing away silt and clay by deflation or by eroding down rocks by abrasion.

Deflation or blowing away of other material by wind.

Abrasion: Wind-blown sediment strikes a surface.

Sandstorms Are strong winds that carry sand particles through the air.

Dust Storms A strong wind storm that fills the air with thick dust.

Windbreaks Hedge or fence of trees designed to lessen the force of the wind and reduce erosion

Roots Vegetation is planted to reduce soil erosion. Plants with fibrous root systems, such as grasses, work best at stopping wind erosion.

Dust Bowl of the 1930’s

Questions What is the breaking down of rock? A. Weathering B. Erosion C. Deposition D. None of the above

What is the carrying away of pieces of weathered rock? A. Weathering B. Erosion C. Deposition D. None of the above

What is the slow movement of soil downslope? A. Mass movement B. Slump C. Creep D. Rock slide

The End