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Question
i heard that the earth is not exactly a rigid body- my question is whether the outer shell of the earth changes its shape (even very small changes or waves) and if so what is the magnitude of these changes, wave length etc.

Answer
Hi Itamar,
Interesting question.  It turns out that the surface of the Earth (the Crust) does indeed change shape.  But, perhaps, not as you might imagine.

Earthquakes can cause "ground waves" that look much like the waves on water, they can also push sections up, down, or sideways. Earthquakes can, temporarily, liquify the ground or create large fractures.  Water, wind, rain, and ice all erode the surface changing or removing structures.  Volcanoes both add material and create structures.  That doesn't even speak of human intervention and actions.

But the rocks that make up the crust have a plasticity all their own.  Every rock will flow if enough pressure and temperature is applied.  Even solid rock will change shape if it's buried, deep enough, on a hillside.  The relentless pressure and gravity will deform the rock.  However, this process, like most geological processes, takes thousands of years, or more.

There is another force that causes even greater movement of the crust; Plate Tectonics.  This topic is far too complex for this venue but I can give you an idea what's going on.  The surface of the Earth is made up of more than 20 crustal plates.  They are rigid and slide around on a very thick layer of rock made plastic by the great heat of the Earth's interior.  The mechanism for that movement is not fully understood but heat currents in the rock below the crust are thought to be the prime mover.  As the plates are moved, they slide under others or push up against each other and bend.  Those that slide under can generate volcanism while those that bend are the mountains we see.  There is much more but if you visit these sites you'll get a better understanding:

http://www.pbs.org/wgbh/aso/tryit/tectonics/#
http://pubs.usgs.gov/gip/dynamic/dynamic.html
http://pubs.usgs.gov/gip/dynamic/developing.html

The surface does move in a variety ways and for a variety of reasons.

Hope this helps.
Bob  

Geology

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C. Robert Reszka, Jr.

Expertise

I can answer any general geology question (rocks, minerals, stratigraphy, geomorphology etc.). My expertise is in the geology of the Michigan Basin, PreCambrian, Paleozoic and Recent. I can answer questions concerning mining and petroleum exploration and production and the laws concerning those activities. I can also answer questions concerning stratigraphy of the Michigan Basin. I will also answer questions about mineral and rock collecting in the Basin. I won`t be able to answer many specific questions on hydrology, geophysics or geochemistry. I may be able to answer very general questions in those venues.

Experience

I have been working for the State of Michigan for 36 years as a Geologist and a Resource Analyst. I have experience with Subsurface Geology and Petroleum Geology, mining in Michigan, and Sand Dune Mining and Protection issues.

Organizations
Michigan Basin Geological Society

Publications
Decade of North American Geology.
Bedrock Geology of Michigan

Education/Credentials
BS Wayne State University

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