5.5 Diastrophism and Folding
Diastrophism
Diastrophism means the deformation of the Earth's crust. Rocks may be bent or broken in response to great pressure exerted either in the mantle or the crust. Tectonic activity is clearly one important source of pressure that deforms the Earth's crust. Sometimes the rise of molten material from below can cause deformation. Even erosion, deposition and continental scale glaciation can cause the Earth's surface to deform.
How do erosion, deposition and large-scale glaciation cause the Earth to deform? The relationship of Earth’s crust to the mantle is similar to the relationship of a raft on a sea of peanut butter. The raft with one person on it floats comfortably high. Even with three people on it the raft is less dense than the peanut butter, so it floats, but it floats uncomfortably low for those three people. When weight is added to the crust, through large scale glaciation or deposition, it slowly sinks deeper into the mantle like the raft in the peanut butter and the mantle material that was there is pushed aside When that weight is removed by erosion over tens of millions of years, the crust rebounds and the mantle rock flows back. This process is called isostasy.
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Thick accumulations of glacial ice added weight to the crust during the ice ages, and as the mantle beneath is squeezed to the sides, the crust subsided. The Greenland Ice Sheet in some locations is over 8,000 feet thick, and the crust beneath the thickest part has been depressed to the point where it is below sea level . When the ice eventually melts, the crust and mantle will slowly rebound, but full rebound will likely take more than 10,000 years.
Large parts of Canada and New England are still rebounding as a result of the loss of glacial ice since the last ice age ended. Other parts of the world are also experiencing isostatic rebound. The highest rate of uplift is near the Hudson Bay in Canada, which is where the Laurentide Ice Sheet was the thickest (over 9000 feet). Ice finally left this region around 8,000 years ago, and the crust is currently rebounding at a rate of nearly 1 in/year. Strong isostatic rebound is also occurring in northern Europe where the Ice Sheet was thickest, and in the eastern part of Antarctica, which also experienced significant ice loss during the Holocene.
When weight is added to the crust and it sinks as it does under the weight of large glaciers, we call this down warping.When that weight is removed as it was when the ice from the last ice age melted, the crust rebounds and we call that up warping. The east coast of the United States experienced down warping during the last ice age due the weight of the over lying ice. The crust sank and river mouths drowned creating a complex coastline full of bays and estuaries.
Folding
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When crustal rocks are subjected to great force and pressure over time they can deform rather than break. This deformation can cause rocks to fold. Folding can take place in any kind of rock but its easiest to recognize in sedimentary rock. Folding can our at any scale - from a rock you can hold in your hand to a mountain range. And the folds can be simple or highly complex.
A simple fold is a monocline, which is a one-sided fold. A simple upfold (like a bowl) is a syncline and a simple downfold is an anticline. If an anticline is pushed very hard from one side it becomes an overturned fold.
