GEOL 111 Grossmont College Metamorphic Rock Classification Lab Report

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GEOL 111 – Physical Geology Laboratory METAMORPHIC ROCK CLASSIFICATION AND IDENTIFICATION

I. Introduction & Purpose:

The purpose of this laboratory exercise is to become familiar with identifying common metamorphic rocks and understanding their depositional origin. In this lab you will learn to identify metamorphic rocks in hand samples from their physical properties. You will become familiar with the common metamorphic rock-forming minerals and processes. The nature and origin of metamorphic rocks, the major types of metamorphic rocks, and their structures, and the connection between plate tectonics and metamorphic rocks in the rock cycle will be explored.

II. General Overview and Classification of Metamorphic Rocks

A. Defining Metamorphism:

Directions: Answer the following:

1. Define “metamorphism” ____________________________________________________________
2. Every metamorphic rock has a______________________________ rock (or protolith) – the original rock type that was metamorphosed into the resultant metamorphic rock, e. the source rock.

B. Conditions of Metamorphism:

Metamorphic rocks form as a result of changing crustal conditions, e.g. increasing pressures and/or temperatures, that are between that of igneous and sedimentary rock-forming environments.

1). The four major agents of change that cause rocks to metamorphose:

1. a) ______________________________

b) ______________________________

c) ______________________________

d) ______________________________

C. Classification and Identification of Metamorphic Rocks

• Classification and identification of metamorphic rocks are based upon two major physical criteria:
  1. a) ______________________________, and b) ______________________________
• Metamorphic rocks are divided into two major groups based on whether the rock has a layered

versus non-layered texture. Special alternative names for these are:

a. Layered = ______________________________ and b. Nonlayered = ______________________________

D. Foliated Metamorphic Rocks

• Metamorphic rocks that possess a foliated and/or layered fabric have a crystalline texture consisting of elongate and/or platy crystals that all share a preferred orientation within the rock. Metamorphic rock s that have foliated/layered fabrics originate in two types of metamorphic environments: Regional Metamorphism (RM) and Dynamic Metamorphism (DM).
• There are four common types of metamorphic rock s that have foliated-layered textures. Each foliation type is unique and represents a progressive increasing scale of intensity of regional metamorphism as go from slate to gneiss. The textural character of foliated and/or layered metamorphic rock can vary greatly as a function of 1) grain size, 2) degree of preferred orientation of the rock’s mineral crystals, and 3) degree of layered segregation of light minerals from dark minerals.
• The foliated/layered metamorphic rocks are classified primarily upon texture, with mineralogy secondary criteria. There are four visually distinctive types of foliated rocks (listed below).

Directions: Describe the texture, mineralogy, uses, and likely parent rock for the four foliated rock types.

Texture: Foliated or Non-foliated? Fine-Grained or Medium-Gr. Or Coarse-Gr.?

Mineralogy: Quartz; Feldspar; Mica; Amphibole; Carbonate; Garnet; Serpentine; None Obs.

Rock Name	Grain Size and Layering Features	Rock Mineralogy
24. Slate
25. Phyllite
26. Schist
27. Gneiss

Please note that there are infinite shades of gray between these four types – no sharp divisions

Rock Name	Uses	Parent Rock
24. Slate
25. Phyllite
26. Schist
27. Gneiss

Exercise 2 – Compare and contrast the foliated metamorphic rocks.

1. Question: How does slate differ from both phyllite and schist?

1. Question: How does gneiss differ from the other three foliated rocks?

1. Question: What are the main criteria you use to distinguish between these four rock types?

E. Non-Foliated-Non-Layered Metamorphic Rocks

• Metamorphic rocks that have non-foliated (non-layered) textures originate in two types of metamorphic environments: regional metamorphism (RM) and contact metamorphism (CM). Note that the nonfoliated metamorphic rocks that form by regional metamorphism RM are mostly mono-mineralic (mostly of one mineral type), having mineral crystals that are neither platy nor tabular, such as quartz and Note that amphibolite and serpentinite may exhibit foliation.
• As noted above, the non-foliated (non-layered) metamorphic rocks consist of equant shaped mineral crystals that have no preferred orientation or These rocks have a massive, homogenous “crystalline” texture, much like that of either, phaneritic granite, or aphanitic basalt. The non-foliated metamorphic rocks are classified primarily upon mineral composition.
• There are four common types of metamorphic rock s that have non-foliated/ non-layered fabrics.

Directions: Describe the texture, mineralogy, uses, and likely parent rock of the four non-foliated rock types.

Rock Name	Grain Size and Layering Features	Rock Mineralogy
28. Marble
29. Quartzite
30. Serpentinite

Rock Name	Uses	Parent Rock
28. Marble
29. Quartzite
30. Serpentinite

Exercise 2 – Compare and contrast non-foliated metamorphic rocks.

1. Question: How does quartzite differ from marble? How are they similar?

2. Question: What is/are the main criteria you use to distinguish between these four rock types?

3. Question: Limestone and Marble are composed of what mineral? What test could you perform on the rocks to be sure?

III. Metamorphic Grades and Facies

A. How much a parent rock (protolith) is metamorphosed is called its metamorphic grade and varies from low grade (low temperature and pressure) to high grade (high temperature and pressure). British geologist George Barrow mapped rocks in the Scottish Highlands that were metamorphosed by granitic igneous intrusions. He discovered that as he walked away from the granitic intrusive igneous rock, there was a sequence of mineral zones from the high grade to the low grade of metamorphism. He defined the following sequence of index minerals, which represent degrees of metamorphism along a gradient from low grade to high grade:

• • Chlorite (lowest grade), biotite, garnet, staurolite, kyanite, sillimanite (highest grade).

1. Question: In the geologic map above, what is the zone of maximum metamorphic grade?

B. Finnish geologist, Pentti Eskola, first recognized 1921 that basalt volcanic rock could be metamorphosed to distinctly different metamorphic facies (unique assemblages of several minerals) under changing conditions of temperature and pressure (depth).

• Amphibolite facies: low pressure, high temperature (black hornblende amphibole, sillimanite)
• Greenschist facies: low pressure, low temperature (green actinolite amphibole and chlorite)
• Eclogite facies: high pressure, high temperature (red garnet, green pyroxene)
• Blueschist facies: high pressure, low temperature (blue amphibole)

1. Question: Match the metamorphic facies with where they would occur in this pressure-temperature diagram.

a. ________________________________

b. ________________________________

c. ________________________________

d. ________________________________

C: Determine where Eskola’s facies would occur related to the Theory of Plate Tectonics.

The diagram below shows a convergent plate boundary where the oceanic edge of a plate subducts beneath the continental edge of another plate. In the diagram below, notice how the geothermal gradient (rate of change in temperature with depth) varies relative to the subduction zone and the volcanic arc.

Directions: Using the geothermal gradient and estimating pressure by depth, match the numbers with where Eskola’s facies (amphibolite, Greenschist, Eclogite, and Blueschist) would occur in the diagram below:

1. ________________________________

2. ________________________________

3. ________________________________

4. ________________________________

IV. METAMORPHIC ROCK LABORATORY REFLECTION

Directions: Write a reflection of the lab activity, explaining its purpose, the methods used, the results obtained, and a brief personal reflection of what you enjoyed and learned about doing this lab.

• What was the purpose of this lab? What did you actually discover and learn during this lab?

• What did you enjoy most about this lab? Also, what was challenging or thought-provoking?