[MUSIC] We're going to finish our discussion of sedimentary rocks by looking at the carbonate rocks. Carbonates are a little bit different from a classic sedimentary rock, because while some of them are plastic in texture, in the sense that they're composed of shell fragments that are cemented together. Some are are actually formed directly by precipitation of water or by the growth of reefs and other materials by biogenic processes. In general, carbonate sedimentary rocks are composed of two minerals, either calcite, which is calcium carbonate, or dolomite, which is calcium magnesium carbonate. Dolomites don't precipitate or don't accumulate directly. They're usually the product of later alteration of Calcite by ground water that contains magnesium that has flowed through the rocks and has reacted with the rocks. Carbonate rocks that are composed dominantly of Calcite are called Limestone, Carbonate rocks they're composed primarily of Dolomite or called Dolostone. I mentioned that some carbonate rocks have clastic textures. These can form when a layer of shells or shell debris collects, say, on a beach, and then later gets buried and cemented together. Some carbonate rocks, however, form on carbonate reefs. These days, or in recent geologic times, reefs are often dominated by growth of corals. Earlier in geologic history, there were other organisms out of which reefs formed. Generally, reefs have a complicated structure. The reef itself will be somewhat of a narrow ridge. Inboard of the reef, there may be a lagoon composed of class of broken up carbonate and offshore, there may be slopes composed of more carbonated debris. All this material, when buried and turned into rock, becomes carbonated rock or limestone. Here's an example of a carbonate reforming today, right off the shore of Hawaii. You can see that in the near distance, before the water gets very deep, you can see a shelf that's somewhat irregular. The surface of that shelf is recently living coral, as well as various other kinds of living organisms, such as mollusks and gastropods, which also secrete carbonate shells. When the material or debris composed of that material gets buried deeply and turned into rock, it doesn't necessarily keep the same texture, entirely. That's because ground water passing through the material will dissolve some of the calcite and recipitate it, and and some of the calcite grains themselves will re-crystallize a little bit, change their texture. So typically, old limestone has this sort of gray, knobbly, blocky look, as exposed in this outcrop on the shores of Western Ireland. Here's another example of carbonate sedimentary rock. These are the white cliffs exposed along the southern coast of England. They're composed of chalk. Chalk the familiar substance that you might use to write on a blackboard with, is composed of the calcite shells, of very tiny organisms, plankton, that once lived in the sea. In this case, these rocks are cretaceous in age, the plankton lived in seas that no longer exist, it accumulated slowly over many, many millions of years on the bottom of the sea floor and later was turned into rock. Now, so far we've been talking about carbonate rocks that form primarily by processes similar to those that form clastic sedimentary rocks, except that instead of being composed of clay, or of silica sand, they're composed of carbonate minerals. We can also get crystalline sedimentary rocks in a variety of ways. One of them is by the precipitation of calcite directly out of water, such as may happen on the walls of caves. These kinds of carbonate rocks are called or dripstone. And they may accumulate over time and form a solid mass that later would be a rock. Similarly, we sometimes get precipitation of carbonate rocks by hot water springs. The result would be travertine, which is often used as a decorative stone. So far we've talked about two general categories, the classic sedimentary rocks and the carbonate sedimentary rocks. We'll finish by just mentioning one other type, evaporates. Evaporates are sedimentary rocks that are formed of salts that precipitate from saline water. Here's an example in Death Valley, California. At times when there are floods, this area gets submerged by a very salty lake. In between the floods, the water in the salt water evaporates, leaving behind the salt. And what we're seeing are crystals of newly formed salt on the surface. Now, we're seeing salt, actually a fairly thick layer of salt, that's all broken up as it's dried up and cracked. But basically all that material is mostly salt with just a little bit of mud mixed in, adding the color. So to complete this discussion, let's just remember, there are three fundamentally different kinds of rocks, Igeneous, Sedimentary and Metamorphic. We have seen that rocks, by and large, are aggregates of mineral crystals that are either cemented together or intergrown to create a crystalline texture. There are a few rocks in ingeneous that are basically masses of glass. But volume wise they're relatively insignificant on the planet. In the next discussion, we're going to turn our attention to how rocks can change over time. In some cases they change at the surface to become soil, and in some cases they change at depth to become metamorphic rock. [MUSIC]
