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Andrew Alden

Limestone Types

By April 15, 2011

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I've been doing a little research into one of my local bodies of limestone. I didn't quite realize that limestone has more than one way to form. We're all taught that limestone forms in shallow seas. Most of the time during geologic history, the sea has sat much higher than it does today (when the Antarctic and Greenland ice caps hold large amounts of water out of the ocean). And so it is that huge tracts of the continents are covered with flat beds of limestone, laid down in the wide, shallow epicontinental seas of past times.

We're taught that shallow tropical nearshore environments like the Bahamas and the Great Barrier Reef are where limestone is forming today. Thus it's easy to picture every body of limestone that way. But my local limestone turns out to be pelagic limestone, which forms way out in the open ocean.

Deep-sea limestone is rare because the tiny calcium carbonate shells, or tests, of plankton don't make it down to the deep sea; they dissolve at what's called the carbonate compensation depth. That's why most of the deep seafloor is covered with clay or bare rock. The only ways to form a pelagic limestone are around an ocean island—your typical coral atoll—or on large volcanic plateaus that lie scattered below the ocean.

My limestone is the plateau kind. It has no corals, no crinoids, none of that stuff, only plankton tests with some diatom tests, which are silica, mixed in. This is not the kind of thing that's obvious to the eye. It took the attentions of microfossil experts to figure this out.

How such a limestone makes its way to the continents is a separate story. As part of plate tectonics, the seafloor is carried one way or another until eventually it runs into a continent and is subducted. My plateau rode from the tropical Pacific north and east as far as California, where it got stuck. Eventually it became part of the Coast Range, where it caught the attention of quarriers, and now it's gradually being mined away and turned into concrete.

Details:
Limestone versus marble
Limestone picture gallery
About subduction
Cement and concrete

Comments

April 15, 2011 at 7:32 pm
(1) Eric Logan says:

Thanks for posting on the limestone topic. Since I began learning about the geology here in the Klamath region of Northern California I have wanted to have a clearer concept of origins and history of the extensive limestone and marble deposits and smaller isolated bodies here. I do wonder if some were originally associated with oceanic island arcs that were supposed to have been accreted to the region. By the way, literature mentions that some isolated limestone masses in, I think, the Eastern Hayfork terrain of this region contain Tethyan fossils. I guess that’s about right for the reported mesozoic age of the geological belts of the area. The reconstructed map of the continental arrangement of that time is so different from the present. Everything has been rearranged!

April 18, 2011 at 3:52 pm
(2) Keith Campbell says:

A lot of limestone and kilned limestone (quick lime) is used in Flue Gas Desulfurization Systems used to clean sulfur dioxide out of flue gas from coal fired power plants.
keith campbell
Denver

April 20, 2011 at 6:53 am
(3) John Marshall says:

What about chalk? This soft limestone is formed in deeper than reef depth in open water. We have plenty in the UK and France.

April 20, 2011 at 8:28 pm
(4) Guide says:

Chalk is interesting, a relatively young and less lithified version of limestone. The big deposits of Europe, though, still formed near continents and upon (flooded) continents just as they do in America. My limestone, the Calera Limestone, formed far from land out in the ancient Pacific. But I thank you for bringing up chalk, because I’ve never actually seen chalk in person and I tend not to think of it.

May 3, 2011 at 12:05 pm
(5) lyonga david says:

thanks for the wonderful information on this particular lime stone.i will be very happy to share this update with my mates.

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