To prepare for this lab, you should review the background information about the three rock types on p.52-58. You already should know something about igneous and sedimentary rocks from last week. This lab will start by review sedimentary and igneous rocks, introducing metamorphic rocks, and then moving into an activity involving all three rock types.
Please review the information about igneous rocks from last week's lab notes.
This story activity will set you up for the rock gallery walk next week.
Sedimentary rocks are made from the stuff weathered at the Earth's surface (sediment). A sedimentary rock by the following processes:
Weathering of the source rock to create detrital and chemical sediment.
Transportation of the sediment away from the source rock.
The higher the energy of the transporting medium, the large the grain size it can carry.
Deposition of the sediment in a variety of environments.
For example, mud is deposited in quiet calm water.
Compaction: the grains are pressed close together as more and more sediment is buried on top.
Cementation: the grains are "glued" together by minerals that precipitate out of the ground water that fills the pore spaces between the grains.
Best cements are: quartz, calcite and hematite.
Metamorphic rocks form deep underground by taking a pre-existing rock (the parent rock) and changing the temperature, pressure, or fluid activity on the rock - this causes the rock to change into a new (sometimes more beautiful) rock.
The temperature of metamorphism generally controls what minerals form in the metamorphic rock and to what size the newly forming minerals grow. Different minerals form at different temperatures and they generally get bigger with increasing temperature. As temperature increases, the minerals in the parent rock become unstable and the elements begin to recombine to form new minerals that are stable at the higher temperatures of metamorphism. For example, the clay minerals in shale will destabilize at higher temperatures and the elements in the original clay minerals may recombine to form muscovite. Muscovite Schist is an example of a metamorphic rock that contains a lot of muscovite and forms from the parent rock, shale.
You should be thinking of metamorphic rocks (and their parent rocks) in terms of composition - the parent rock provides the starting materials (chemical components in the form of the elements found in the minerals of the parent rock) and the heat from metamorphism drives the reactions that cause the components to either recrystallize or recombine to make the new minerals in the metamorphic rock. If you are aware of what the various minerals are made of, then you will be better able to predict what might form in when a parent rock is metamorphosed. Some common parent rocks include:
Shale (made mostly of clay minerals and silt-sized quartz grains): Clay minerals are quite variable in their composition (containing a whole host of elements, such as aluminum, silica, iron, potassium, magnesium, calcium, etc.) The one thing that characterizes ALL clay minerals is that they are rich in aluminum and silica. So, you would expect the metamorphic rocks that form from shale to contain aluminum-rich minerals. The incredible chemical variabiltiy of clay minerals allows a big range of different minerals to form depending on the temperature of metamorphism. Metamorphic rocks that come from shale hold the most information about the temperature and pressure of metamorphism. Consequently, metametamorphic petrologists spend A LOT of time looking for metamorphic rocks that were once shale.
Quartz Sandstone (made of sand-sized grains of quartz): Quartz sandstone is probably the least interesting of all the potential parent rocks because it is made of only one mineral that has only two elements in it (SiO2). When you heat up quartz sandstone, the only thing that you can make is more quartz. So the grains simply recrystallize so that the texture of sand grains cemented together is lost and you get a texture of interlocking quartz grains that are so tightly knit that you can't really see individual quartz minerals when you hold that rock in your hand. Usually quartzite is an off-white color, but impurities (such as iron) may make it red (oxidized iron) or green (reduced iron).
Limestone (made of chemical or biochemical calcite sediment): Like quartz sandstone, limestone doesn't have much variability (unless some clay particles get mixed in with the calcite sediment). Calcite (CaCO3) simply recyrstallizes to more calcite, sometimes making large visible grains of interlocking calcite.
Basalt / Gabbro (mafic igneous rocks that are enriched in iron, magnesium, and calcium). The chemical variability in the starting minerals of a mafic igneous rock, allows a variety of minerals to form through recombination, depending on the temperature. The lower temperature end of metamorphism tends to create a variety of green minerals that you did not see in the mineral lab (such as chlorite, actinolite, and epidote). So lower grade metamorphic rocks that form from mafic parent rocks tend to be a greenish color.
Anyhow, my point is that if you know the basic chemistry of the parent rock and the basic chemistry of the rock-forming metamorphic minerals, you will be better prepared to understand metamorphic rocks.
This activity will give you a framework for using what you have learned about rocks and for learning to identify some of each type of rock - read about the gallery walk on p.50.
You should go to the media center in the library and check out the box of rocks on reserve there (call number WT8001) or you can go to the library reserve room where another box is on reserve (call number YD9342 pc). In class we looked at these igneous rocks: R1, R2, R4, R6, R8, R11, and R12 (if you use the chart on p.55, ignore the row for R10), these sedimentary rocks: R14, R15, R18, R19, R20, R21, and R23 (if you use the chart on p.56, ignore R24), and these metamorphic rocks: R25A, R26, R28, R29, R31, and R33 (if you use the chart on p.57, ignore R25) - if R33 is not in the box, please let me know.
Click here for the syllabus information about missing class. Worksheets collected for this lab will be listed on the Lab Schedule page and must be turned within a week. You have to look at some samples in order to complete the worksheets, I will have the samples available in my office. Look here for my schedule to find out when I am on campus and not in class, then e-mail me to arrange a time look at the samples. Remember, if you do not turn the worksheets, you will receive a zero for them. Contact me about taking the quiz that you missed (this must be done sometime before the next class).
Quiz 3 is next week on Resources and Igneous Rocks - be sure to look at the quiz info on the class website's Lab Schedule page.
Notes about the worksheets I collected: