Tuesday, December 17, 2013

Geology 101: Sedimentary Rocks

Before we get too far into things, we ought to establish some background. If words like sandstone and limestone aren't part of your daily vocabulary, it wouldn't be very fair of me to go on about them without letting you in on why they are important, would it?

First of all, rocks fall into three broad groups: igneous, metamorphic, and sedimentary. The first two are not of pressing concern at this time and so will only be mentioned briefly, although at some point we will return to them. Igneous rocks form by cooling from a molten state, i.e. magma or lava. They include rocks like granite, one of the patron rocks of countertops, and basalt, a common dark stone which is often what you get from flowing lava. Metamorphic rocks are produced by subjecting rocks to heat and/or pressure. The minerals of a rock are altered by these factors, and sometimes banded textures appear. Metamorphic rocks include marble, the other patron rock of countertops, and schist (foliated; sheet-like thin layers) and gneiss (banded), beloved for their pun value (many geologists are addicted to terrible puns). Of course, if you ramp up the forces of metamorphism too much, the rock melts, and you're back around to igneous processes.

Sedimentary rocks form by deposition: deposition of eroded fragments of older rocks, of minerals crystallizing out of water, of shells and skeletons, of ash and other volcanic debris, and so forth. You've got a few basic divisions:

Clastic sedimentary rocks result from the deposition of eroded rock fragments, or clasts ("grain" is also sometimes used, usually with sand). These rocks are your sandstones, your shales, your conglomerates, etc. On Earth, the backbone of clastic rocks is silica and minerals that form from silica, like quartz, which makes sense because of the abundance of silica in the Earth's crust. Presumably you could have a planet where something else is more important, but we'll leave science fiction world-building for another time. The size of the clasts provides a basis for classification: sand, for example, is between 2 mm and 1/16 mm in diameter, or about 0.079 to 0.0025 inches. Clay is smaller than 1/256 mm in diameter, and the human eye cannot discern individual particles. A USGS chart can be found here, if you want all of the details (and I mean all). The most important types of clastic rocks for the Twin Cities are sandstones, which are composed of sand-sized clasts, and shales, which are composed of clay-sized clasts, but there are others; conglomerates, for example, are rocks with clasts larger than sand, and siltstones are made of grains larger than clay but smaller than sand. Rock names are intended to be descriptive of general characteristics, but the rocks themselves can be mixed, say a shaly siltstone or a silty sandstone. Clasts are sometimes cemented together by various minerals, which can make the rocks quite durable.

Clasts can provide a lot of information about how they were deposited and what the wider environment was like. The size of clasts in a particular deposit or rock was determined by the power of the process that deposited them, which we usually think of in terms of speed; a rushing mountain river can move much larger objects than a sluggish creek, for example. It is not uncommon for terrestrial rock formations to have a fine-grained component, like siltstone, and discrete areas of coarser rocks: the siltstone represents floodplains and the coarser rocks are the rivers. Rocks from ancient coasts become finer-grained farther away from the shore, away from the mouths of rivers and the energy of the beach. Speed isn't everything, though: glaciers can move immense objects, but if we relied on them for mail service, package arrivals would be measured in years (and they would just destroy the mailbox anyway). They get things moved by throwing their weight around. The shape of clasts provides a clue to how much wear they have undergone. If they are jagged and irregular, they have not undergone much transport, but if they are rounded, they have seen a lot of tumbling, and may well have gone through previous cycles of erosion and deposition. Sand grains that have undergone a lot of transport by wind often end up with "frosted" surfaces, due to collisions with other grains. The minerals present give clues to the source rocks; for example, an eroding granite mountain will provide different clasts than an eroding marble mountain. Sorting of clast sizes can provide information about the depositional process, such as the duration or persistence of flow; a setting with constant movement, like a beach, will sort clast size better than, say, a stream that only flows a couple of times a year. Finally, currents often deposit clasts in distinctive structures like dunes and ripples, which we can recognize in rocks. The orientation of the structures tells us about the direction of flow, and the size and type of structures give us an idea of how fast it was moving.

Chemical sedimentary rocks are the other major category of sedimentary rocks. They form by minerals coming out of solution. You can approximate this by dissolving salt in a glass of water and then letting the water evaporate: you will be left with a salt deposit in the glass. Salt, gypsum, and some other minerals are known as evaporites because of this characteristic, and they show where ancient lakes and shallow seas used to be. More widespread is limestone, which forms from the compound calcium carbonate (sorry, there's some chemistry here). Limestone can be produced inorganically or organically; calcium carbonate is what sea shells are made of, as well as the protective coverings of many microscopic organisms. It can indicate an environment where there is very little input of clasts (for example offshore marine), or the presence of abundant life (for example limestones made of coral reef fragments, shell fragments, or some kinds of plankton "skeletons").

Rocks that once were limestone are very common in the Twin Cities. Much of the limestone has been chemically altered to dolomite (if you'll forgive the chemistry, some of the calcium is displaced by magnesium). You may have learned that acids cause limestone to fizz. Dolomite, however, reacts very weakly to acid. For paleontology, the alteration of limestone to dolomite often dissolves or destroys fossils, because many fossils are also made of calcium carbonate. The process may also create natural molds and casts of the original fossils. There can be overlap with clastic sedimentary rocks: in the metro, we have examples of sandy and shaly limestone and dolomite.
Dolomite/limestone over shale over sandstone, Lock & Dam 1

Finally, volcanic eruptions can also produce deposits of rock fragments that become sedimentary rocks. The ghosts of volcanic eruptions can be observed at some places in the Twin Cities in the form of thin bentonite layers. Bentonite is a clay that results from the decomposition of volcanic ash, and the beds in the metro are evidence of some robust eruptions, but that story will be for another time.

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