Lab 3 - Answers
A. A pocket of magma, which when it cools will most likely form large-crystalled, felsic rocks.
B. Area immediately adjacent to magma pocket which will experience high-heat and pressure and possibly undergo contact metamorphism. High-grade metamorphic rocks will be produced.
C. Oceanic crust, which is composed primarily of basalt and other mafic rocks.
D. Sedimentary rocks in the forearc basin. The deeper layers are likely to undergo regional metamorphism due to pressure and heat.
E. Volcanic vent. Fine-grained felsic rocks most likely to be produced here.
1. This is marble, identifable by its light, felsic color, and its fine-grained texture. Marble is a metamorphic rock that normally forms from limestone or some other carbonate rock.
D is the best choice for where marble might form because it is a potentially metamorphic region where there are sedimentary rocks. B is a plausible answer, but not as good as D.
2. This rock is granite, identifiable by its coarse-grained, light colored texture.
Granite is most likely to form in area A because the coarse grains require slow, subsurface cooling.
3. This glassy, black rock is obsidian.
Of the choices in the diagram, area E is the most likely place for this rock to have formed because it usually forms from volcanic activity. Area C was a plausible answer because it is an area of felsic rock below water. However, the oceanic crust is nearly all basalt, so E is a better answer (and, in fact, obsidian does form at volcanic vents).
4. This dark, asphalt-like rock is basalt. It is primarily fine-grained, though does have some large crystals in it.
Area C is the most likely place to find basalt because the oceanic crust is dominantly composed of it. Area E was a plausible answer, but not as good as C because basalt is less likely to be found at a volcanic vent than in the ocean crust.
5. This dark, hard, layered rock is slate. It is a metamorphic rock that is derived from shale or siltstone.
Area D is the most likely place to find slate because regional metamorphism tends to produce lower grade metamorphic rocks.
6. This shiny, layered rock is schist. It is a high grade metamorphic rock that is also derived from shale or siltstone.
Area B is the best answer for where schist might form because the proximity to the pluton is likely to produce a high grade metamorphic rock. Area D is plausible because of the obvious presence of sedimentary rock in the diagram and because regional metamorphism can produce some high grade rocks.
