grandcanyon_names.jpgSelf test: Geological time

Answers at the bottom of the page. You will be able to read them by swiping your mouse over them as I have made the text colour the same colour as the background.

The diagram on the left shows a wall on the Grand Canyon, with some of the names that geologists have given to different rock formations they have identified.

1. There is an intrusive rock called the Zoroaster Granite in the area. Why does the Zoroaster Granite intrude only the Vishnu Schist and not the Bass limestone?
2. Three types of intrusive rock are granite, gabbro and diorite. In what way are these rocks different?
3. Granite and rhyolite are formed from the same magma. Why are they different, and how would you be able to tell which you had if you were given a piece?
4. There is an unconformity between the Bass Limestone and the Tapeats Sandstone.
a. What is the evidence for it?
b. Discuss the geological events that would have led to this happening.
5. Name the formations older than the Redwall Limestone.
6. Discuss how the Vishnu Schist could have formed from sand and mud deposited on the sea floor.
7. Discuss how a limestone such as the Redwall Limestone is formed.

1. Because the Bass Limestone was deposited after the Zoroaster was intruded - you dan tell this because the dyke is cut off along with the Vishnu at the base of the Bass (i.e. an unconformity).

2. They have different silica contents and therefore different proportions of light and dark minerals. Granite is richest in silica and gabbro is poorest; diorite is in-between.

3. Granite cools deep underground, so has no bubbles and large evenly sized crystals. Rhyolite cools at the surface, so has mostly small crystals or glass and may include bubbles. This is because it cooled more quickly, so crystals didn't have the time to grow.

4a The fact that the layers in the Bass are at an angle to those above, and are cut off by the boundary which is irregualr.

4b. The Bass Limestone would have been laid down as horizontal layers of shells etc. under the sea, on top of the Vishnu Schist, which must be much older. Over time it became rock. Later, both the Bass and underlying units were uplifted, tilted and eroded off to form the irregular surface you see at the top of both the Bass and the Vishnu. This must have sunk below the sea again, and sand deposited by underwater currents which became the Tapeats Sandstone.

5: From oldest to youngest: Vishnu Schist, Zoroaster Granite, Bass Limestone, Tapeats Sandstone, Bright Angel Shale, Muav Limestone and Temple Butte Limestone.

6. The sand and mud must have been buried under even more sediment, sinking into the crust, (possitly aided by plate tectonic action as evidenced by the granite). At first it became sedimentary rock, but when it got down to about 8 or 9 km the heat and pressure caused the minerals to start to change and it became a schist. It then must have been uplifted too rapidly for the minerals to change back. It was also intruded by the granite at some point before it was eroded off again.

7. Limestone is formed from fragments of shells, coral and so on which are made of calcium carbonate. These are the skeletons of marine organisms which usually live in shallow, biologically rich seas. They form limestone where there is a 'sediment starved' environment where there is no other sand or mud coming in to mask the shell component. After being buried, the shell fragments cement together and recrystallize to make limestone.