alpacalypse wrote:This is my first year doing this event and I was wondering what should I know about the Sedimentary, Metamorphic, and Igneous rocks? I'm not really sure how to study for those.
In terms of identification:
Rocks are different than minerals. Minerals always have a defined chemical formula, which gives them consistent properties. Rocks are made of minerals and therefore can have very different properties, so rocks are identified using different criteria.
Igneous rocks are classified based on two things: composition and grain size. Composition is basically just color. Light colors (white, gray, and pink) are found in felsic rocks. Dark colors (black, gray, and green) are found in mafic rocks. Intermediate is somewhere between the two. Grain size refers to the size of the crystals in the rock. They can be very coarse (very big), coarse (big), fine (small), or glassy (nonexistent). Crystals form over time, so the longer the magma had time to cool, the larger the crystals will be. Because of this, crystal size reflects where the rock formed; intrusive rocks (very coarse and coarse) formed inside the mantle while extrusive rocks (fine and glassy) formed from lava flows or eruptions above Earth's surface. All igneous rocks can be described by their color and composition: for example, a fine, mafic rock is basalt, while a coarse mafic rock is gabbro. There are charts you can find online that show everything in a simple way.
Sedimentary rocks are also classified by composition and grain size, but in a different way. Sedimentary rocks are made up of small rock fragments that have been cemented together rather than being made of crystals. The main thing that determines sedimentary rocks is grain size. From smallest to largest, the sizes are clay, silt, sand, pebbles, cobbles, boulders (the last two aren't important for sedimentary rocks, they're too big). You can also describe the clasts (the sediments) in terms of roundedness, or sphericity. And lastly, you can also talk about sorting, which describes how similar the clasts are to each other in size and shape (important in geology, less important for this Scio event). What I just described were clastic rocks, but there are two other types of sedimentary rocks. Bioclastic rocks are similar, but they are made up of material that came from living organisms. They're divided into coals (plant remains), limestones (calcium carbonate shell remains), and diatomite (made from siliceous diatom remains). Chemical rocks form when you have chemical rich fluid evaporite, leaving the chemicals behind (for example, you have hot springs rich in calcium carbonate. The water evaporates, leaving the calcium carbonate behind, forming travertine).
Metamorphic rocks are rocks that have been changed by heat and pressure. The foliated rocks form a progression, going from slate to phyllite to schist to gneiss, increasing in foliation. Gneiss has extreme foliation, and shows banding (alternating light and dark bands). Non-foliated rocks can be a bit hard to identify, in my opinion. Marble and quartzite can look similar, but marble will fizz in acid and quartzite will scratch glass. Luckily, those are the only non-foliated rocks you have to know, amphibolite and hornfels are never on the list.
I hope that helped.
M