Friday, December 30, 2011

Rocks - a good place to start

It's probably a good idea to start with the fundamentals of modern geoscience: ROCKS and their derivatives. There are three main kinds of rock, and a lot of subdivisions within each. To explain them all would require a third of a book by itself, and a lot of pictures and diagrams, so I'll just summarize them here.

Dear Geologist,
Do you know who named these rocks, Igneous, Metamorphic and, Sedimentary?
3rd grade

There are three main rock types:
"Igneous" appears to be first used in the 1660's, but it's not clear who started using it. It derives from a Latin expression that means "is fire." Igneous rocks include volcanic lavas and intrusive rocks that came up from great depth, but didn't quite reach the surface. Because lava reaches the surface and cools rapidly, it doesn't have time to form visible mineral grains. Intrusives, however (like "granites", "monzonites, and "gabbros"), are insulated by the earth they intrude into and have time to cool more slowly - and grow crystals. Sometimes these can be huge - individual crystals in a "pegmatite" can be 5 cm or more on a side. Pegmatites are almost always gorgeous to look at. Kitchen counters are often made of beautiful granites or monzonites; mine is a Labradorite Anorthosite" - a dark gabbro-like pegmatite with single iridescent Labradorite crystals as long as 10 cm on one axis. I was lucky to find this already in the house I bought when I moved to the Pacific Northwest.

"Metamorphic" derives from Greek, and was used first sometime before 1810. It comes from a word meaning "after" or "beyond" (meta), plus a word meaning "shape" (morph). Old-time miners described these rocks as "being stewed and cooked" - because they sort of look that way, like molasses and water stirred together. They form from igneous, sedimentary, or even other metamorphic rocks during deep burial in the Earth, where pressure and heat can cause the rocks to deform plastically. A "gneiss" is one example - a coarse-grained rock that frequently has the original sedimentary layers still crudely visible in it... if you step back far enough. A "schist" on the other hand may be finely layered from original freshwater mud or seafloor dead-creature goop, and can glitter in the sunlight from all the tiny grains of muscovite (white mica) that form in it during the pressure-heat cycle. These rocks are usually classed by the degree of metamorphism - how much the original rock has been compressed, stewed, and cooked - with gneiss found on the extreme end and something called "greenschist" on the near end. Metamorphic rocks are usually moderately ugly-looking - but interesting to the discerning eye of an experienced geologist. They commonly draw the eye to their curious character.

There is a place in the Catalina Mountains north of Tucson, Arizona, where from a distance you can see the original sedimentary layers in the gneiss that makes up the front (south end) of the range. If you get up close, you can't see this very well at all - you just see the coarse recrystallized final product of the metamorphism. There is actually an even more final product: as you move northward in the range, the depth of burial of this uplifted and tilted block of sediment came from ever deeper depths, until the gneiss grades smoothly into granite. A graduate student mapping this complex once came across a shape of a crinoid - a 400 million-year-old fossil - from the original sedimentary rocks. For geologists, to see all this happening in one place as a continuum of change (or metamorphism) is indescribably cool. 

Sediments.The word "sedimentary" seems to have been first used around 1820-30, but again, it's not clear where it came from or who first used it. It relates to sediment - rock that has formed from loose sand, gravel, or mud deposited by water or air. Eventually it gets compressed and cemented by other minerals in the fluids that percolate through it. Sedimentary rocks often look sort of "grungy" - some can be ultra fine-grained and very hard, like "limestone" (which typically has fossils in it), or hard and coarse-grained, like "sandstone", or thinly layered ("laminated") like "shale."

Rock Derivatives:
If you did a trench in the ground from the leaf cover on top to the bedrock below it, you will see gradually-changing textures ranging from something you can scratch with your fingers to something that you can't. The soft stuff on the top are the soils, and often have organic content from dead vegetation in them (typically this makes them dark to black from the broken-down carbon content of the organic matter). These soils come from weathering of these main bedrock types described above. The rock breaks down over long periods of time for many reasons, including freeze-thaw cycles, abrasion from sand in wind or water, or the acids from plant roots. Sometimes it can be carried in on wind or water from long distances away. Given time, soils can accumulate, build up (for instance in a large basin or valley) until the overlying weight of the younger sediments compresses the underlying material into a new sedimentary rock. Groundwater slowly filtering through these materials help cement them into rock by depositing minerals between the soil grains.

Altered Rocks:
I used the expression "stewed and cooked" above to describe metamorphic rocks, but there are rocks that are literally stewed and cooked. These are the "altered" rocks - rocks in which the original mineral grains are changed by heat and fluids moving through them. The heat source can be an intrusive punching up from the upper mantle; this heat sets the overlying groundwater in motion into circulating loops called hydrothermal cells. This water moving through the vast bodies of surrounding rock can "grab" minerals from a distant source and draw them towards the heat source. An intrusive can break the rocks it is pushing up against, and these fractures can quickly fill with minerals that "drop out" of solution as they move up into the colder fractures and cracks closer to the Earth's surface. When the cracks finally fill with these minerals, they become "veins". Gold and silver and copper hunters look for evidence of this kind of circulatory concentration mechanism to home in on where the rich mineral deposits are hidden. If they find altered rocks - the most grungy-looking of all rocks you will ever see - then they know they are homing in on the Mother Lode.  So: ugly can be beautiful in yet another place on this planet of ours.

I'll talk later about mineral deposits and how the form, but this will serve as a quick introduction.


By the way, you probably noticed that I gave the size of those pegmatite crystals in centimeters, and not in inches. As far as I know, the United States is one of only three countries on Earth that still use the archaic English system, along with Myanmar (formerly called Burma) and Liberia. Not even the English use the English system anymore. The rest of the world moved many decades ago to the metric system, of which there are two inter-related versions: the SI (the Standard International system that uses kilograms, meters, centigrade, and seconds) and cgs (centimeters, grams, seconds and centigrade) systems. In order to teach correct principles, I feel I need to teach you what all the rest of the world - and also the science agencies of the US Government and virtually all American universities teach and use. 


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