12.3 Mass Movements and Erosion Key Idea: Mass movement and erosion carry away weathered materials and reshape earth’s surface.

Learning Objectives: Give examples of mass movement Explain how erosion reshapes Earth’s surface Understand how different forces, like gravity, friction, and upward lifting are involved in shaping the Earth.

Mass Movement and Erosion Mass movement is the downward transportation of weathered materials due to gravity. Imagine a pile of crumbled rocks on a mountain slope; certain causes, such as an earthquake, heavy rain, or accumulation of too much material will initiate the movement of the weathered material downhill. Erosion, is the removal and transport of materials by natural agents, such as wind, and running water.

What Generates Mass Movements? Soil protects the bedrock beneath it from weathering. By removing soil and loose rock materials, mass movement thus speeding up the weathering processes continually expose fresh bedrock to weathering, thus speeding up weathering process. The rock fragments weathered off a cliff and pulled down by gravity, forming piles of debris called talus.

Talus

Talus

Types of Mass Movements Landslide: this is commonly used term for the movement of a mass of bedrock or loose soil down the slope of a hill, mountain, or cliff. Landslides are more likely to occur after heavy rains, or in the spring time, when large amounts of snow are melting. Geologists use various names to describe different types of landslides.

Types of Landslides The landslides are categorized based on the type and amount, of material that moves down hill. The speed at which landslides take place is also a factor: some are slow, other happen very quickly. Most common types of landslides aare: Creep Slump Earthflows Mudflows

Creep Creep is a slow, imperceptible movement of soil down a slope. Its effects are noticeable , because creep causes fence posts, poles, and other objects fixed in the soil to lean downhill. Creep is created by the freezing and thawing of water in the soil.

Creep Illustrated

Creep Affects Trees

Slump A slump is a type landslide that takes place along a surface that curves into a slope. For example, if the rock and soil at the bottom of a slope become worn away, the top of the slope becomes unstable and slumps downward.

Slump

Another Image of a Slump

Earthflows During and earthflow, a mass of weathered material that has been saturated with water flows downhill. The speed at which the materials flows downhill depends on the following: - the amount of water present -the composition of the soil -the steepness of the slope Some earthflows last days or months, other can last years.

Earthflow

Mudflows A mudflow is a rapid movement of water that contains large amounts of suspended clay and silt. While the earthflows contain more solid materials than water, the mudflows are basically water loaded with solid particles. The mudflows can move as fast as 60 miles per hour on a steep slope. Mudflows are capable of moving big boulders, trees, houses.

Mudflow

Landslides and mudflows on steep slopes

Erosion and Landforms The main agents of erosion are: - rivers and streams -glaciers -wind - ocean waves and currents By removing and transporting earth materials, these agents play an important role in shaping the surface of the Earth.

Other Factors Influencing Erosion Topography The word “topography” comes form the Greek words “topos”, which means place, and “graphein”, meaning writing. Topography is the study of landforms at any scale. Climate Composition of the rock

Topography The topography (all the landforms at the surface of the Earth) of a region depends on the balance at any given time between forces that uplift the land and agents of erosion which wear down the land. As the rocks are uplifted (by subduction, for example) weathering and erosion are action on the rocks. Sometimes the uplift is dominant, therefore the mountains’ height increases; Himalaya Mountains are a good example in this regard.

Climate The effect of erosion on topography is also influenced by climate. In regions with humid climates water is the primary agent of erosion. Because water is such an aggressive factor of erosion, areas with humid climates tend to have more rounded topography.

Rounded mountaintops

The topography of dry and cold areas is jagged and sharp

Composition of the Rock The composition of the rock influences topography greatly, because it affects the rates of erosion. Volcanic rocks, containing silica are resistant to erosion, because silica is a very hard and chemically stable mineral. Carbonated rocks, such as limestone or dolomite, could be easily dissolved by acid rain.

Volcanic Rocks Resist Erosion Stone dolls in Macedonia

Erosion of Volcanic Rocks Cappadocia-Turkey

Erosion Landforms in Carbonated Rocks (Karstic Landforms)