Weathering & Erosion
Weathering is the process by which rocks are broken down. Weathering agents can be wind, water, freezing & thawing (see the Exfoliation video on the right), glaciers, chemicals (such as acid rain), avalanches (gravity) animals, plants & man - even meteors striking the earth!
Erosion is the process by which rocks are transported or moved. Erosion agents often are the same agents that weather rock. These include wind, water, glaciers, avalanches (gravity), animals and man. Tsunamis, volcanoes, earthquakes can also be eroding agents. You may remember doing a weathering lab when you were a UP. Or, remember the Magic School Bus video (you are not assigned to watch this video, but if you have time, you may reminisce about weathering and erosion when you were an UP!) about the statue on the mountain? You may also remember learning about density and how a substance can change density with changes in thermal energy. Go back to the Intermediate page on density and look for how changes in density can make a road buckle! |
The video above shows a eyewitness account of "exfoliating" where granite (which is an intrusive igneous rock) pops (weathers) as it expands under heat. This occurred at Twain Harte Lake in the Sierra mountains of California.
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Wind, water & ice weathering
Every now and then, Lake Superior freezes over sufficiently for the sea caves (located in Northern Wisconsin on the Bayfield Peninsula) to be viewed in the wintertime. Ordinarily, they can only be reached by boat. In the summer view below, you can see the sandstone (a clastic sedimentary rock) cliffs weathering from the wind and wave action. You can also see evidence of weathering from the tree roots in the rock. But in the wintertime, the water freezes in the cracks. Remember that water is wonky, so ice is less dense than water. These means that the same amount of water will take up more space when it freezes. So, as the water freezes, the ice expands in those cracks, it creates a force strong enough to break rock. The beautiful scenery you see has everything to do with weathering and erosion!
How glaciers break down & transport rock
Here's an interesting form of erosion!
In Death Valley, on the Racetrack Playa (playa means dried lake), there are rocks that move - BIG rocks! Some of the largest rocks weigh over 700 lbs. Yet no one has ever witnessed their movement, although the evidence is clear ..... as mud???? Some popular hypotheses are described here and here, and only recently was the mystery of these "racing rocks" finally solved (see below).
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Since the 1940's, evidence showed that large rocks and boulders moved on Death Valley's Racetrack Playa, but no one ever witnessed it happening in this remote place on earth, let alone understood how or why. In August 2014, researchers from the Scripp's Institute revealed evidence supporting one of two hypotheses. Watch the video below to find out how this mystery was finally solved.
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And yet another interesting form of weathering and erosion!
California was hit by twin disasters in late 2017 and early 2018 when wild fires burnt drought-stricken hills for weeks. With the vegetation gone, the soil was not held together by the roots, which allowed torrential winter rains to wash away (erode) the topsoil and, along with it, the dead and burnt trees and shrubs in devastating mudslides. Avalanches and mudslides are responsible for both weathering and erosion.
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