Transcript Weathering
Erosion and Deposition Agents, Forces, and Results What Caused This? What is Erosion? • Erosion - moving of rock material from one place to a new location – For erosion to occur three processes must take place: detachment, lifting of the particles and transport – Many agents of erosion - flowing water, moving ice, waves, gravity, or wind – Sand consists of small pieces of rock that have been weathered from a parent rock and eroded and deposited somewhere else What Is Wind Erosion? • Wind - responsible for wearing away rocks and creating great deserts like the Sahara Desert and Gobi – Most effective in moving loose material – Two main effects: (1) Wind causes small particles to be lifted and moved to another region. (2) Suspended particles may impact on solid objects causing erosion by abrasion. – Occurs in areas where there is insufficient rainfall to support vegetation What Is Water Erosion? • Water - most influential force in erosion – Ability to move materials from one location to another over long distances – The faster water moves in streams the larger objects it can pick up and transport – Responsible for wearing away of rocks in rivers, lakes, and the oceans What Is Wave Erosion? • Waves - relentless pounding – Erodes the softer, weaker parts of the rock first, leaving harder, more resistant rock behind – Can take over 100 years to erode a rock to sand – Energy of waves along with the chemical content of the water erodes the rock off the coastline What Is Gravitational Erosion? • Mass movement - downward movement of rock and sediments, mainly due to the force of gravity. – Moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can pick up the material and move it to lower elevations – Process is occurring continuously on all slopes, some act very slowly while others occur very suddenly until equilibrium is reached What Is Glacial Erosion? • Ice - moves and carries rocks, grinding the rocks beneath the glacier – Glaciers pluck and abrade to cause erosion – Plucking occurs when water enters cracks under the glacier, freezing, and breaking off pieces of rock that are then carried by the glacier. – Abrasion cuts into the rock under the glacier, smoothing and polishing the rock surface What Is Bioerosion? • Bioerosion - erosion of ocean rock by living organisms – Caused by mollusks, polychaete worms, sponges, crustaceans, and fish – Mechanisms of bioerosion include biotic boring, drilling, rasping, and scraping – Bioerosion of coral reefs generates fine and white coral sand. The coral is converted to sand by internal bioeroders such as algae, fungi, bacteria, sponges, bivalves; external bioeroders include urchins and chiton. – The actions of these organisms cause a great deal of material to be available for erosion. What is Deposition? • Deposition - laying down of sediment that could have been transported by wind, water, or ice – Process of erosion stops when the transported particles fall out of the transporting medium and settle on a surface – Ability to carry material depends on an a balance of forces within the transporting medium – If the velocity of the medium reduces or the resistance of the particles increases, the balance changes and cause deposition. – Velocity can be reduced by large rocks, hills, vegetation,etc. Deposition - Wind • Wind velocity reductions can be related to variations in heating and cooling – Wind can transport fine materials in suspension hundreds of km from its original source in the desert. – Heavier material may be pulled along the ground. – Material is eventually deposited when the wind changes direction or loses its strength. – Obstacles, whether natural of man-made, will often decide where the deposition occurs and the nature of the feature formed. Deposition - Water • Stream water enters the lake and its velocity decreases. – As velocity decreases, water's ability to carry sediments decreases – Sediments carried by the stream are deposited where the slowing water can no longer move them. Largest particles are deposited near the shore. Increasingly smaller particles settle out farther from the shore where the water is calmer – Occurs in streams, rivers, oceans, etc. Deposition - Ice Glacial flows of ice - become slower if precipitation – – – – input is reduced or when the ice begins melting Deposits left by glaciers are called moraines and outwashes. Moraines are large chunks of broken rock left at the base and sides of the glacier as it melts and recedes. Finer material is carried in the rivers that form when the glacial ice melts. The deposits of these rivers look similar to normal river deposits and are called outwashes. Glacial rivers are usually white with fine silt known as "glacial milk".