Download Metamorphic Rock Lab

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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
produce 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
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
Metamorphism increases from top to bottom on the chart
Figure 1(b)
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Figure 2: Metamorphic Textures
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 nonfoliated (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.
Deformation
Banding
Intergrown Crystals
There are two ways that
metamorphic rocks can form.
Metamorphic rocks are almost
always harder than
sedimentary rocks. They are
as hard (or harder) as 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:
Review the background information section of this investigation.
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 to help you to
identify each of the specimens.
Yes
Does the
rock react
to acid?
Yes
Mica Schist
No
Yes
No
Does the rock
have a sparkly
substance in
it?
Go back to the beginning
and start again!
Yes
Does the
rock
scratch
glass?
Shale (layers) or Meta-Conglomerate (fragments)
Slate
No
Does the
rock
contain
dark/light
colourded
layers or
fragments?
Go back to the beginning and start again!
Yes
No
Is the
rock a
dark
color?
Gneiss
Marble
No
No
Is the
rock black
and white
in color or
multicolored?
Yes
Yes
No
Does rock have visible bands, texture or fragments?
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.
Quartzite
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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
Minerals
Visible
(Y/N)
Foliated
(Yes/No)
Names of
Minerals
Visible
Special features & draw
sample
Name of Rock