This large outcropping of rock just south of the TMCC campus and DRI is just one of many that together tell the careful observer something about the volcanic and hydrothermal history of this area during Miocene geologic epoch, about 16 million years ago. At that time, large amounts of andesite lava were erupted near what would later become Reno and Virginia City when this area was part of the Cascade volcanic chain. As the San Andreas fault migrated northward with time, so did the southern end of the Cascades. Relatively minor volumes of basalt (dark volcanic rock low in silica) and rhyolite (light volcanic rock rich in silica) were erupted in the area during later Quaternary time. The area remains tectonically active, as evidenced by numerous Quaternary faults, frequent small and infrequent large earthquakes, and movement of the ground as measured with high-precision GPS receivers.
There are a few predominant rock types in the area surrounding the campus, although the contacts between the different rock types in the area are often covered with sediment, obscuring relationships and making them difficult to determine. This is one of several rock types found in the square mile or so surrounding the TMCC/DRI campus and briefly described in the list below:
1. Unaltered dark gray volcanic andesite with visible millimeter-size plagioclase feldspar crystals and abundant magnetite, unaltered except for a rind due to weathering.
2. Dark gray volcanic andesite with millimeter-diameter crystals of epidote (a green mineral characteristic of propylitic alteration near ore deposits) and abundant magnetite (a metallic black mineral easily attracted to a high-strength magnet hung from a string).
3. Greenish-gray, epidote-chlorite-altered andesite with abundant tiny pyrite grains (metallic brassy yellow mineral) and trace amounts of magnetite (chlorite is a greenish mica).
4. Gray silicified andesite with some quartz veins. Hydrothermal (hot water) alteration has converted much of the rock to silica (the mineral quartz). Some of the rocks contain abundant, red hematite, which probably formed from the oxidation of pyrite.
5. Light-colored clay-altered andesite with white clay minerals that may include kaolinite. These clays are typical of argillic alteration associated with many hydrothermal ore deposits of gold, silver, and copper.
6. Breccia consisting mostly of angular fragments of silicified andesite cemented together by silica (quartz) – into a hard silica ledge with some quartz veins. Many surfaces coated with brownish goethite or reddish/purplish hematite.
7. Epidote-altered and clay-altered andesite with trace amounts of magnetite. This rock has a considerable amount of clay alteration.
In order to log this Earthcache as a find, go to the posted coordinates and look closely at the rocks within a few meter radius of the coordinates.
1. Carefully note the color, texture, resistance, etc. of the rock and decide which of the seven general rock types listed above it most resembles.
2. Briefly describe a likely or probable scenario or earth process that could possibly form a rock like the one you see in the outcropping. At least describe the sequence of events that must have happened to form the rock type that you see in the outcropping at the posted coordinates. THE ROCKS HOLD THE CLUES! OBSERVE CAREFULLY.
E-mail your answers to these two questions to the cache-owner in a separate e-mail. DO NOT POST YOUR ANSWERS IN YOUR LOG OR IT WILL BE DELETED AND NOT COUNTED!!
More thorough descriptions of the rock types and processes which form them may be found in the field trip guide for the 2013 Nevada Bureau of Mines & Geology Earth Science Week field trip at the following link:
http://www.nbmg.unr.edu/dox/e53.pdf