GLS 100 Lab 11- Metamorphic Rocks
Physical Geology Lab – Dr. Lindley S. Hanson
Traveling beyond Rt. 128 you'll discover that the bedrock of New England is largely metamorphic. The
contorted gray, white, and black rocks are marine sedimentary and volcanic rocks that were thoroughly
recrystallized and altered during the orogenic events that built the Appalachian Mountains.
Metamorphic
rocks are relics of past tectonic activity, when continents and volcanic arcs collided and sediments from
intervening ocean basins were piled and deformed into alpine mountains. By studying these rocks
geologist can determine a great deal about the tectonic history of a region, ancient environments, and the
size and extent of mountains removed by erosion. In this lab you will learn to identify metamorphic rocks
and interpret their meaning.
Objective

Describe what is meant by a “solid state” transformation of one mineral into another.

List and explain the agents that cause metamorphism and the environment where they prevail.

Explain the meaning of “regional metamorphism” and “contact metamorphism.”

Explain why some metamorphic rocks are foliated and others are not.

List four minerals commonly developed by the metamorphism of a pelitic (clay-rich) rock and give
the metamorphic grade they represent.

Discuss how foliation and crystal size changes with increasing regional metamorphism.

Identify the sedimentary or igneous protoliths of the following rocks: marble, muscovite garnet
schist, amphibolite, serpentenite, quartzite, chlorite schist, slate, and hornfels.

Discuss the information about orogenic processes, uplift and erosion, and ancient environments
revealed to geologists through the study of metamorphic rocks.
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Notes:
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Processes of metamorphism
The different kinds of metamorphism listed below are defined by the principal process(es) involved.
1. Contact or thermal metamorphism: Metamorphism of rocks surrounding igneous intrusions. Heat
is the principal agent of metamorphism.
2. Dynamic or cataclastic metamorphism: Metamorphism resulting from the grinding and
recrystallization of rock in shear zones (i.e. faults). The principal agent of metamorphism is
shearing (pressure)
3. Regional or dynamothermal metamorphism: Regional metamorphism associated with deep burial
and large-scale tectonic activity (plate collisions). The principal agents of metamorphism are heat
and pressure
4. Hydrothermal metamorphism: Occurs locally in a wide variety of environments. The principal
agents of metamorphism are chemically active fluids. These fluids may be released from
magma or chemical reactions occurring deep within the crust.
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Textures of metamorphic rocks
Like all rocks, metamorphic rocks are classified by their texture and composition.
Definition: Texture refers to the size, shape, orientation and distribution of mineral grains in a
rock. In metamorphic rocks texture is determined by the type and grade of metamorphism, and
the composition of the original rock, or protolith. There are two basic metamorphic textural
categories: foliated and non-foliated.
Foliated metamorphic rocks:
These rocks have a planar or flaky fabric. This appearance is caused by the parallel arrangement
of platy minerals (mostly micas) and some needle-like minerals (amphibole/hornblende). The
development of foliation is dependent on the mineralogy of the rock and the metamorphic process
that created it.
If a rock is foliated it may have one of the following types of foliation. The name of a foliated rock
is based on the type of foliation it exhibits.
1. Slaty foliation (slaty cleavage)
Microscopic foliation resulting from the parallel alignment of microscopic clays and micas.
Slaty rocks exhibit a closely spaced rock cleavage. Rock fragments are flat, smooth and
have a dull luster. Rock name = slate
2. Phyllitic foliation
Microscopic foliation similar to slaty foliation. However surfaces may be bent, and have a
slightly shining luster. Rock name = phyllite
3. Schistose foliation
Macroscopic foliation consisting almost exclusively of platy or needle-like crystals, which
have a parallel or sub-parallel orientation. Rock name = schist
4. Gneissic foliation
Macroscopic texture consisting of alternating bands of schistose material and granular
(non-foliated) layers. Bands are commonly between .5 - 2 cm wide and the granular
component typically composes more than 40% of the rock. Rock name = gneiss
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Non-foliated rocks:
Non-foliated rocks are massive with no definable fabric. Rocks composed entirely
of equidimensional mineral, such as quartz, feldspar, or calcite and rocks
metamorphosed by heat under low-pressure conditions are typically not foliated.
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NOTES:
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EXERCISE 1: Identification of texture.
Listed below are four rocks created by the metamorphism of a mudstone/shale. If regionally metamorphosed a
rock of this composition will develop micas that exhibit a strong foliation. However, if contact metamorphosed, the
rock will contain randomly oriented minerals and lack foliation.
In the table below state whether the rock is foliated or non-foliated (column 2), and the type of foliation (column 3).
In column 4 state if the rock was regionally metamorphosed (by heat and pressure) or contact metamorphosed
(by heat alone).
Table 1: Identifying textures
#
Category
foliated
non-foliated
Type of foliation (if applicable)
slaty
cleavage
phyllitic
schistose
12
13
14
15
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NOTES:
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gneissic
Type of metamorphism
regional
contact
Using Texture and Composition to Classify Metamorphic Rocks
Classification of foliated rocks (Table 2.)
Microscopic, foliated rocks:
1.
Slate: slate, which is microscopic and dull, is composed of clay minerals realigned by pressure.
2.
Phyllite: Phyllite is shiny and may be perceptibly coarser but still largely microscopic. The shiny luster is
caused by the breakdown of clay minerals and their replacement by mica which is more stable.
Macroscopic, foliated rocks:
1.
Schists: The principle minerals in the rock serve as an adjectives to the rock name (i.e. muscovitegarnet schist, talc schist, graphite schist, etc.)
2.
Gneisses: The dominant mineral in the schistose layer serves as an adjective to the rock name. (e.g.
biotite gneiss)
Table 2. Classification of Foliated Metamorphic Rocks
TEXTURE
FOLIATION
COMPOSITION
METAMORPHIC ROCK
Slaty Rock Cleavage
(breaks into smooth, flat
fragments and has a dull
Micro-scopic
microscopic clay minerals and
incipient micaceous minerals
Slate
luster)
Phyllitic Foliation
(breaks into smooth, flat
microscopic micaceous minerals
Phyllite
or wavy fragments and
has a shiny luster)
Schist
Macro-scopic
(Examples: biotite schist, chlorite
Schistose Foliation
Variable
(composed predominantly
Contains one or more of the
garnet schist, mica schist,
of tabular or needle-like
following minerals: biotite, chlorite,
hornblende schist, amphibolite
minerals. Rock cleavage muscovite, talc, garnet hornblende,
is fair to good.)
quartz, feldspar, etc.
schist, talc schist, muscovite
etc.)
Gneiss
(Examples: biotite gneiss,
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Gneissic Foliation
hornblende gneiss, mica gneiss,
amphibolite, etc.)
(contains dark-colored
schistose layers or lenses
that alternate with nonschistose granular layers
of quartz and feldspar.)
i
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Classification of non-foliated rocks (Table 3.)
Non-foliated rock, which have granular or randomly oriented crystals, are largely classified by variations in
composition.
Table 3. Classification of Non-Foliated Metamorphic Rocks
TEXTURE
COMPOSITION
ROCK
Randomly-oriented crystals.
Talc and Green Amphibole
Soapstone
May be microscopic.
(white-green, soapy feel, H<2.5)
Serpentine
Serpentinite
(shades of green, waxy luster, commonly microscopic)
Original bedding may be visible.
Variable
Hornfels
(quartz, feldspar, andalusite, mica, etc.)
Composed of granular crystals.
Calcite
Marble
(white-to-brown, H < 5.5)
Quartz
(white-to-brown, H > 5.5)
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NOTES:
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Quartzite
EXERCISE 2: Fill in Table 4 and identify the rocks using tables 2 and 3.
TABLE 4.
#
Foliated or non-
Macro- or
foliated
microscopic
Visible minerals
1
2
6
9
10
9
Rock Name
#
Foliated or non-
Macro- or
foliated
microscopic
Visible minerals
Rock Name
11
12
13
14
15
Interpreting Metamorphic Rocks - Metamorphic rocks and their protoliths
Heat and pressure change preexisting rocks into a metamorphic rock. Its texture and mineral composition is
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governed by the type and grade of metamorphism (fig. 1), and the composition of the original protolith (Table 5).
Table 5. Common metamorphic rocks and their protoliths
Metamorphic Rock
Composition: dominant
mineral(s)
Protolith
marble
calcite
limestone
quartzite
quartz
quartz sandstone
serpentinite
serpentine
ultramafic rock
amphibole (e.g. hornblende)
basalt or gabbro
chlorite, green amphibole
mafic rock
muscovite, garnet
mudstone or shale
orthogneiss1
quartz, feldspar, mica, etc.
granite or other igneous rock
paragneiss2
quartz, feldspar, mica, etc.
amphibolite
(hornblende schist)
greenstone/green
schist
muscovite (garnet)
schist
1 Orthogneiss
layered detrital rock (e.g. shale with
interbedded sandstone
contains thin wavy discontinuous schistose lenses resulting from the alignment of micaceous
minerals when an igneous rock is regionally metamorphosed.
2 Paragneiss
Metamorphosed layered sedimentary rock. Typically exhibits well-defined schistose and non-
schistose layers.
EXERCISE 3 PROTOLITHS. Now that you've classified the samples, use table 5 to find what
they were prior to metamorphism and fill out table 6.
Table 6: Identifying sedimentary and igneous protoliths
#
Metamorphic Rk
Protolith
2
12
6
11
9
10,14,15
Interpreting Metamorphic Rocks - Metamorphic grade
Metamorphic grade refers to the intensity of the metamorphic conditions experiences
and is a function of temperature and pressure. Metamorphic grades are defined as
follows:
Table 6. Metamorphic Grade
Approximate range of temperature and pressure conditions for the grades of metamorphism related to regional
metamorphism. Diagenesis refers to the range of conditions still considered sedimentary . One kilobar of
pressure is approximately equal to an approximate depth of 3 miles.
Grade
Temperature
Pressure
Diagenesis (sedimentary conditions)
0-200oC
0-2 kb (<3 km)
Low grade
200-300oC
1-4 kb (3-12 km)
Medium grade
400-600oC
4-8 kb (12-25 km)
high grade
600-800oC
>8 kb (>25 km)
Identifying metamorphic grade: The intensity of metamorphism can be inferred from both texture and
mineralogy. Fine-grained rocks, such as slates and phyllites, are low-grade metamorphic rocks. Coarser-grained
schists and gneisses are intermediate to high grade. Some metamorphic minerals have a narrow stability range
and are used to determine pressure and temperature conditions, as shown in Figure 1 on the next page.
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Figure 1. Changes in mineralogy and rock type caused by the progressive metamorphism of a mudstone or
shale. Note that the clay minerals, which are formed in a sedimentary environment, are the least stable when
exposed to increases in temperature and pressure
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EXERCISE 4: Determining metamorphic grade from mineral assemblage and texture.
Instructions: Fill out table 7. Write the rock name for each sample in column 1.
Using the criteria in Figure 1
(texture and composition) determine the appropriate metamorphic grade (low, medium, or high). Estimate the
depth of metamorphism and fill out column two.
Table 7. Identification of metamorphic grade
Depth (km) of
Sample number and rock name
metamorphic grade
metamorphism*
Not
low
med
metamorphosed
grade
grade
8. _____________________
.
10. _____________________
.
14. _____________________
.
15. _____________________
.
high grade
*Check table 6 for approximate depths of metamorphism.
EXERCISE 5: A final question
Sample # 10 is one of the most common rocks in central Massachusetts. What does the rock tell geologists
about the depositional environment preceding the Appalachians, and the amount of erosion that has occurred
since the mountains were created nearly 400 million years ago?
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NOTES:
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