| MadSci Network: Earth Sciences |
Hello! We are a class of fourth graders who have been studying rocks and minerals for some time.
Hi! My name is Steve and I study rocks and minerals also. I'm a graduate student in college (the University of Illinois) studying for a PhD in geology. I enjoy collecting minerals as well as studying them!
We have a few questions. They are:
Is the crust the continents or below sea level?
The "crust" of the earth is the hard, brittle, outer layer of rocks. The crust of the continents, which is composed of quartz-rich granite-like and metamorphic rocks, differs from the crust under the oceans, which is composed of the igneous rock basalt. Continental crust is also thicker, around 35-45 km (but thicker under mountains like the Himalaya is Asia), than oceanic crust which is about 7 km thick (to convert kilometers to miles, multiply km by 0.6214). Another difference between continental crust and oceanic crust is that continental crust is less dense than oceanic crust (do you know what density is?). This is important when you learn about plate tectonics which is a theory which explains how pieces of the earth's crust move around on the earth's surface through time.
Also, how far below the surface is it?
The crust is at the surface when it's not covered by soil. Here in Illinois, where I am, the crust is buried under 60-120 m (to convert meters to feet, multiply m by 3.281) of material deposited by glaciers some 18,000 years ago. In other places, like the Grand Canyon in Arizona, the earth's crust is exposed for all to see.
What rocks have quartz in it?
Many rocks have quartz in them because quartz is a very common mineral. Many sedimentary rocks have quartz in them because quartz is very hard and doesn't weather away like other minerals. Here are some common rock types which contain quartz:
Also, what tests can a collector do to determine if you have quartz?
The easiest thing to do is to look at the rock under a magnifying glass. Geologists always carry a 10x magnifier with them when they go out into the field to look at rocks. Quartz looks glassy and has what's known as conchoidal fracture instead of the property of cleavage that many other minerals have. In many rocks quartz is clear like glass but in some rocks, such as granite, it appears to be smokey-colored.
Sometime's quartz looks similar to the common mineral calcite. One way to distinguish quartz from calcite is to drop some weak acid on the mineral. calcite will fizz like soda pop when you drop acid on it and quartz will not. Many geologists carry small bottles of weak (10%) hydrochloric acid (also called HCl) with them to test between quartz and calcite.
Another test is to look at the mineral's hardness. Quartz is harder than glass and will scratch it. When I'm in the field, I carry a small pocket knife and if I think something might be quartz, I try to scratch it with the knife blade. If it doesn't scratch, it's probably quartz. If it does scratch, it's some other mineral like calcite or feldspar.
What do you know about weathering and erosion?
Weathering is the chemical alteration (chemical weathering) and mechanical breakdown (mechanical weathering) of rock when it's exposed to air, moisture, and organic matter (like plants). An example of mechanical weathering is called frost wedging. When water seeps into cracks in rocks it can freeze in the winter. When water freezes into ice, it expands and cracks the rock apart. An example of chemical weathering is when groundwater flows through limestone. Groundwater is slightly acidic because it contains acids from decaying plant materials and from dissolved carbon dioxide gas. Remember I said that a test for calcite was to drop HCl on it? Well, the weak acid in the groundwater has the same effect on the calcite which makes up limestone and slowly (over geologic time) dissolved it away. That's how underground caves are formed.
Erosion is just a general term referring to all of the processes by which rock is broken down physically and chemically.
What makes rocks and minerals fluoresce?
Fluoresence is a neat phenomenon!
Minerals are composed of atoms which have a nucleus and electrons orbiting the nucleus. The electrons in some types of atoms, when they're struck by ultraviolet light, will move into orbits further away from the nucleus. When the electrons fall back down into their old orbits, they release energy.
For a rough analogy, think of it as lifting a barbell. If the barbell is sitting on the floor, then it's at a stable state (it will stay there unless it's disturbed). When you lift the barbell away from the floor (like an electron going to a higher orbit away from the nucleus), you have to add energy (your muscles). You can't hold it forever (because your arm gets tired!) so you place it back on the floor and then you have the energy back (you can do other things).
Anyway, when electrons drop back down to a lower orbit, the energy they release is in the form of visible light. The reason you see visible light from the electrons, is because some of the energy in the initial ultraviolet light is lost as heat which lengthens the wavelength (decreases the frequency) of the emitted light.
What is the most common Igneous rock?
I'm not sure...
All of the earth's ocean floors are covered with basalt so I guess that this might be the most common type of igneous rock (continental crust is thicker but a lot of that is metamorphic rock).
If you want to consider rocks making up the entire earth, the most common type is probably something like peridotite (bet you've never heard of it!). Peridotite is made up mostly of the minerals olivine and pyroxene and probably composes most of the earth's mantle (the layer beneath the earth's crust that extends down to the earth's core!).
Hope this helps answer your questions!
Steven Schimmrich
s-schim@uiuc.edu
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