FIELD TRIP #1: A-MOUNTAIN

Transcription

1 Geosciences 251 FIELD TRIP #1: A-MOUNTAIN Why look at rocks on a field trip when we could examine them in air-conditioned comfort in the lab? In other words, is there any additional information to be gained by taking the trouble to "go to the field?" The answer is yes! Given a certain rock type, the context in which it occurs can be as informative as the rock type itself. There is critical information that can be obtained from the orientation of the rock, the type of rocks surrounding it, and the contact between the different sorts of rocks. These rock properties reflect the manner in which the rock formed, and are used to construct the geologic history of an area. What to look for in the field... There are two scales that rock units should be observed. Start by observing the rock unit as a whole make observations regarding overall appearances. After you have described in detail what you see, move closer to the rock unit. Act as if you are looking at the rock in hand sample describe fine-scale details, rock texture, composition, etc. Keep in mind the following: 1) What type of rock(s) is (are) present; e.g., igneous, metamorphic, sedimentary? What characteristics of the rock help you to make the identification? a) If sedimentary, what is the grain size or sizes? What is in between the grains (matrix)? b) If igneous, what kind of minerals are present? What is the texture? c) If metamorphic, are the rocks foliated or non-foliated? What types of minerals are present? d) Is there any layering? Can you trace the layers and, if so, how thick are they? Does the thickness of the layers stay the same across the unit? Are the layers flat, tilted, curved, contorted? e) Are the contacts abrupt or gradational? f) Look for structures, fabric, or fossils, which help in rock identification. 2) Break off a piece of the rock. Does the "inside" look the same as the "outside?" Describe the differences, if any.

2 Geosciences 251, A-Mountain, p. 2 ROCK UNIT 1 Field Notes on Rock Units Color [Does the "inside" look the same as the weathering "outside"?] Hardness Grain size(s) and matrix Mineral or clast composition, size and shape Texture Rock type Mode of formation Other observations and sketches [e.g., contact relaitonships with other rock units, orientation of rock layers, etc.] Rock

3 Geosciences 251, A-Mountain, p. 3 ROCK UNIT 2 Field Notes on Rock Units Color [Does the "inside" look the same as the weathering "outside"?] Hardness Grain size(s) and matrix Mineral or clast composition, size and shape Texture Rock type Mode of formation Other observations and sketches [e.g., contact relaitonships with other rock units, orientation of rock layers, etc.] Rock

4 Geosciences 251, A-Mountain, p. 4 ROCK UNIT 3 Field Notes on Rock Units Color [Does the "inside" look the same as the weathering "outside"?] Hardness Grain size(s) and matrix Mineral or clast composition, size and shape Texture Rock type Mode of formation Other observations and sketches [e.g., contact relaitonships with other rock units, orientation of rock layers, etc.] Rock

5 Geosciences 251, A-Mountain, p. 5 ROCK UNIT 4 Field Notes on Rock Units Color [Does the "inside" look the same as the weathering "outside"?] Hardness Grain size(s) and matrix Mineral or clast composition, size and shape Texture Rock type Mode of formation Other observations and sketches [e.g., contact relaitonships with other rock units, orientation of rock layers, etc.] Rock

6 Geosciences 251, A-Mountain, p. 6 ROCK UNIT 5 Field Notes on Rock Units Color [Does the "inside" look the same as the weathering "outside"?] Hardness Grain size(s) and matrix Mineral or clast composition, size and shape Texture Rock type Mode of formation Other observations and sketches [e.g., contact relaitonships with other rock units, orientation of rock layers, etc.] Rock

7 Geosciences 251, A-Mountain, p. 7 A-MOUNTAIN FIELD TRIP REPORT 1) Using the column below, sketch a stratigraphic column for the upper part of A-Mountain. Place the oldest rock on the bottom and the youngest rock on the top. Describe each rock unit in great detail. Your description may include color, texture (grain size), mineralogy, hardness, inclusions, weathering properties, etc. Identify the rock type in each unit. If you are not sure what rock name to assign, take an educated guess, based on your descriptive analysis. (4 points). Diagrammatic STRATIGRAPHIC COLUMN ROCK DESCRIPTIONS UNIT NAME (Your Observations/ Youngest Empirical Data) Oldest

8 Geosciences 251, A-Mountain, p. 8 2) Referring to your STRATIGRAPHIC COLUMN, describe in detail the geologic history represented by this sequence of rock layers. The best way to do this is to make a list of what happened first, second, etc. In other words, what geologic processes (circumstances) are represented by each rock unit? What can you infer from the changes in the types of rock formations? What was the style of volcanic eruption which formed a particular rock unit (layer)? Was it explosive or docile? How does it compare with the other volcanic units present? Make sure to include information inferred by the contacts. (8 points).

9 Geosciences 251, A-Mountain, p List the three main types of volcanoes and briefly describe them (3 points). Is A-Mountain a volcano? What evidence suggests whether it is or it isn t? (2 points) 5. Describe the erosion/deposition cycles found throughout the Basin and Range province. Given enough time, what would be the end result of these cycles? (2 points)

10 Geosciences 251, A-Mountain, p If time allows, we will look at the rock units on the north side of A-mountain. These are basically the same rock units that we looked at during our first stop of this trip. Describe the differences in appearance of these rock units and determine why the differences exist. (5 points)