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Name: Kathy
Status: Educator
Grade: 6-8
Location: IL
Country: United States
Date: February 2009

Our class was talking about rock types and a great question came up. We wanted to ask about how long does it take for sediments to be compressed to form a sedimentary rock?

Wow. Good question. That depends, as they say.

Many clastic sedimentary rocks (those made out of grains or pieces of other rocks), are held together by minerals deposited between the rock pieces (called cements). How long it takes a rock to cement varies widely with local conditions and deposition. If deposited in very lime rich waters, the grains could cement together in a matter of years, not centuries. If there is no source of cement the grains may be packed together by pressure after being buried, but rocks like that you can break up in your bare hands. They are a pretty poor excuse for a solid rock. In some cases, pressure and hot water cause the grains to merge together. This, of course, would probably take many thousands of years, not counting the time it took to bury the sediments in the first place.

In drilling oil wells one can encounter sedimentary rocks that simply wash away as the fluid from the drilling hits them. Other times the same type of rock can be so hard that it must be drilled with a different, harder bit.

Cave formations which are a chemical sedimentary rock (limestone or aragonite) can form over decades or centuries (depending on size) as can most chemical sedimentary rocks. Flowstone in hot springs can coat objects in a matter of years. In some areas of the country it used to be considered quite fun to put common articles, even felt hats, in the runoff of these springs and to return several years later to retrieve the "petrified" objects.

I know this is not a firm answer, but the question is a very broad one. Hope this helps.

Bob Avakian
Oklahoma State University Institute of Technology

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