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Name, Class______

Metamorphic Rocks

Background Information

Materials in the Earth’s crust and mantle are subjected to a constantly changing environment in which they undergo metamorphism, or changes in structure and mineral content. Elevated temperatures and pressures within the Earth’s crust may cause some or all of the minerals in a pre-existing rock to become unstable and change. Chemically active fluids such as water help to promote changes by adding or subtracting ions. Minerals recrystallize and reorient their internal arrangement in response to the agents of metamorphism. The rock texture and mineralization that result are the basis of metamorphic rock identification.

Metamorphic rock texture is divided into two general groups: foliated and non-foliated. Foliation is the arrangement of minerals in parallel layers of flat or elongated grains. Foliation causes varying degrees of rock cleavage. Slaty cleavage planes ( flat, layered effect,) are separated by microscopic dimensions; the edges of a piece of slate look almost like the edges of the pages of a closed book. Schistose cleavage is easily seen, as the minerals have recrystallized into flaky, parallel layers. Gneisses show a characteristic banding, or “broken stripe” effect that is related to their cleavage properties. In contrast, nonfoliated metamorphic rocks show little or no structure, having a homogeneous or massive appearance.

Metamorphism may occur in a limited or contact fashion when magma intrudes into pre-existing rock on a local basis. Widespread or regional metamorphism is often associated with orogeny, or mountain building. Metamorphic rocks are commonly found in areas that are the weathered, exposed “roots” of old mountains and the Precambrian shields of the continents. Heat and pressure cause varying levels of metamorphism. Zones and grades of metamorphic activity may be identified by the presence of index minerals that reflect the environment at the time of their formation. Figure 1(a) lists some of the indicator minerals.

Figure 1(b) shows the correlations between original rock type – what the rocks were before metamorphism – and what they became afterward. The degree of foliation is also shown.

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Indicator Grade of

Mineral Metamorphism

______

( Increasing Chlorite Low

Biotite Low

Garnet Middle

Staurolite Middle

Kyanite Middle

Sillimanite High

Metamorphism)

Figure 1(a)

Original Rock Type

Metamorphic
Texture / Sandstone / Shale / Limestone,
Dolomite / Igneous
Non-foliated / Quartzite / Rare / Marble / rare
Very finely
foliated / rare / Slate / rare / rare
Finely
foliated / rare / Schist / rare / Schist
Coarsely
foliated / Gneiss / Gneiss / rare / Gneiss

Note: Level of metamorphism increases from top to bottom on the chart.

Figure 1(b)

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Figure 2: Metamorphic Textures

Deformation, Banding, Intergrown Crystals

Deformation Banding

Intergrown Crystals

There are two ways that metamorphic rocks can form. The two types of metamorphism are Contact and Regional.

Contact Metamorphism occurs when magma comes in contact with an already existing body of rock. When this happens, the temperature of the existing rocks rises and there is infiltration with fluid from the magma. The area affected by the contact of magma is usually small, from 1 to 10 kilometers. Contact metamorphism produces non-foliated (rocks without any cleavage,) such as marble, quartzite, and hornfels.

Regional Metamorphism occurs over a much larger area. This metamorphism produces rocks such as gneiss and schist. Regional metamorphism is caused by large geologic processes such as mountain-building. Regional metamorphism usually produces foliated rocks such as gneiss and schist. The huge forces of heat and pressure cause the rocks to be bent, folded, crushed, flattened, and sheared.

Metamorphic rocks are almost always harder than sedimentary rocks. They are generally as hard, and sometimes even harder than igneous rocks. They form the “roots” of many mountain chains and become exposed to the surface after the layers of rock above them are eroded away. Many metamorphic rocks are found in mountainous regions today and are a good indicator that ancient mountains were present in areas that are now low hills, or even flat plains.

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Problem: What are the characteristics of metamorphic rocks?

Materials: set of metamorphic rocks, hand lens, large specimens of metamorphic rocks

Procedures and Observations:

1. Review the background information section of this investigation.

2. Look at the set of metamorphic rocks that you have been given. Each specimen is numbered. Enter the number of each specimen that you have been told to examine in the Data Chart on the next page of this lab. Use your observations and the Scheme for Metamorphic Rock Identification from the Earth Science Reference Tables, page 7 (also shown here for your convenience,) to help you to identify each of the specimens.

3. Complete the Data Table. Be sure to use the hand lens for a better look at the texture and the minerals present in the samples.

Analysis and Conclusions:

1. How is “layering” in sedimentary rock such as sandstone different from the “banding” characteristic of the metamorphic rock gneiss?

2. Why are fossils generally not present in metamorphic rocks, not even in metamorphic rocks that were originally sedimentary?

3. How could an igneous rock turn into a metamorphic rock?

4. Which would you expect there to be more of in the rock record: rocks that underwent contact metamorphism, or rocks that underwent regional metamorphism? Why?

Data Table: Metamorphic Rocks

Specimen
Number / Minerals Visible
(Yes/No) / Foliated
(Yes/No) / Kinds of Minerals
Visible (if any) / Describe other features & draw map symbol for this type of rock / Name of Rock