Download Lab - Metamorphic Ro..

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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 preexisting 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.
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
Sandstone Shale
Limestone,
Metamorphic
Dolomite
Texture
Non-foliated
Quartzite
Rare
Marble
Very finely
rare
Slate
rare
foliated
Finely
rare
Schist
rare
foliated
Coarsely
Gneiss
Gneiss rare
foliated
Note: Level of metamorphism increases from top to
bottom on the chart.
Figure 1(b)
Igneous
rare
rare
Schist
Gneiss
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Figure 2: Metamorphic Textures
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.
Deformation, Banding, Intergrown Crystals
Deformation
Banding
Intergrown Crystals
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 mountainbuilding. 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?
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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