Erosion by Wind, Ice and Gravity

Section 1

Daily Dose of Destruction

cliff collapses video.jpg

Using the reading in this section as well as the internet, complete the following assignment.

SUSD5 Student Version of the Mass Movement Research assignment.

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Weathering is the process of breaking rocks into sediment while erosion is the process that wears away this sediment and other materials and moves them from one location to another.  Agents of erosion include gravity, glaciers, wind, and water.  Erosion only occurs when these agents have enough energy.  The moment agents of erosion lose their energy and momentum they deposit their load.  This process is known as deposition.  The load is what the agent is carrying.  As agents of weathering lose their energy, the heavier sediment is dropped.  As the energy is lost, gravel, then sand, then silt, and then clay are deposited, due to their respective masses.  Think back to the soil type lab.

Agents of Erosion

Mass movement, sometimes referred to as mass wasting, is what we call the movement of surface material due to gravity, water, wind, and ice.  There are five main types of mass movements: falls, creep, flows, slides, and slump.

Slump occurs when soil and rock slip down slope in one large mass. It happens when water penetrates the upper layers of a land mass, but doesn't penetrate the lower layers causing the ground to move outward and on top of the lower layers.  Evidence of slump having happened is upturned curved scars in the side of a hill or bank of a river.  Earthquakes can also cause slump as the ground is shaken, the top layers can fall and slide out overtop of the bottom layers.  Below are some images demonstrating slump.

Mass Movement Slump

slump diagram

Creep happens when sediments slowly move downhill.  This one is easy to remember because the sediment creeps downhill.  You can tell creep is happening when you see power lines, trees, fences, ending up curved or tilted.



When rock falls, crumbles, or rolls quickly downhill we call it a slide or a fall, and are commonly called landslides.  These two types of mass movement happen in mountainous areas where the Earth contains steep cliffs and mountain slopes.  You can easily tell that a landslide has happened when you see rocks and other sediment gathering at the base of a slope or mountain.  This pile of rock, at the base of a slope is called talus.


rock fall.jpg





rock slide video.jpg

Flows are the most devastating type mass movement.  Imagine a thick wall of mud, rock and debris coming toward you at high speeds, moving like a river of water, except in this case swimming would be nearly impossible.  Flows happen due to an accumulation of weathered material in dry areas.  Piles of debris and sediment are present. When it rains in this area, because it is loose, the water has an easy time penetrating it, and then gravity forces all of this sediment down hill where it picks up other sediment on its way.  When hits the base of the slope it spreads out in a conical shape called an alluvial fan.  A special type of mudflow is a lahar.  A lahar happens when a volcano begins to heat up snow filled slopes and the ice and snow melt.  This causes very quick and dangerous mudflows that can travel many miles destroying anything in the way.

mudflow diagram






volcanic lahar video.jpg

gigantic mudflow video.jpg

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Section 2

Daily Dose of Destruction

landslide caught on tape video.jpg

Mass movement or mass wasting happens after the angle of repose has been breached.   The angle of repose is the steepest angle at which a sloping surface made out of a specific material is stable.  The moment the angle of a slope has become too steep, due to circumstances like undercutting and erosion, the slope with that material will collapse until it is stable once more.  Once a material like sand has hit the angle of repose, a mass movement event is inevitable.  All it will take is a slightly steeper angle and then BOOM, the slope collapses.  Adding a solvent like water can change the angle of repose as well. Below is a video experimenting with two types of sand to demonstrate that the angle of repose is different based on the material types.

angle of repose video.jpg

In this next lab you are going to calculate the angle of repose of a sandy soil.  

SUSD5 student version of the Angle of Repose Lab.

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