Geology: Earth as a Planet
- Two materials as different in appearance as garnet-mica schist and granite are almost identical in chemical composition, and can be transformed from one to another by ordinary processes within the rock cycle. Describe each of the steps (there are several) that would result in the production of garnet-mica schist from a starting igneous granite. Direct metamorphism of the granite does not commonly yield garnet-mica schist, so think of some other way, going around the rock cycle in the clockwise direction, as it appears in your text. (Hint: try approaching the process backwards, that is, try thinking about what starting rock garnet-mica schist usually comes from, and back-tracking from there.)
- Absolute ages (in numbers of years) can only be obtained directly for igneous and some metamorphic rocks, while sedimentary rocks can normally only be given relative ages (like Cretaceous or Cambrian) based on the fossil life they contain, and on their positions relative to each other. However, in investing events such as the mass extinction that ended the Cretaceous (which killed off dinosaurs, along with about 60-90% of all other living species), and the theory that it was caused by a large meteorite impact on Earth, it is crucial to be able to correlate biological events (and their relative ages), with physical events (and their associated absolute ages). A) Explain in general terms how absolute ages may be estimated for sedimentary rocks that contain fossils, and which therefore have specific relative ages in the geologic time scale. B) Also, discuss at least two sources of uncertainty in the resulting ages. (Hint: you will find it helpful to refer back to the homework and handouts on geologic time, especially “Naming and Numbering the Geologic Time Scale”, in addressing both parts of this question.)
- In mountainous regions around the world, we often find a close association between signs of large scale compression of the rocks, such as folding and thrust faulting, and the presence of large batches of rock which have been transformed by regional metamorphism, or where igneous intrusions have been formed and are now exposed at Earth’s surface. Explain this association in terms of plate tectonics. In other words: A) describe how mountain ranges on continent edges form according to plate tectonics; B) discuss why these plate tectonic processes lead to compression, metamorphism and melting of some of the rocks involved in mountain-building processes; and finally C) describe how this combination of processes, coupled with isostasy and erosion, allows us to see, at Earth’s surface, plutonic igneous and regional metamorphic rocks that originally formed deep inside Earth.
- Comparing them to each other, which of the general classes of destructive processes (chemical weathering physical weathering, mass movement, and erosion) would be more effective, and which would be less effective on the Moon, where there is no liquid water, no atmosphere, and rather low gravity? Explain why you think so for each of the four kinds of processes, based on what really does happen on the Moon.