Page 213 - Laboratory Manual in Physical Geology

Deformed and intruded rocks
in mountain belt

Folding and shearing
(Fig. 7.4)
Fractures along
which hydrothermal
metamorphism may occur
Limestone Marble (Fig. 7.5)
(Fig. 7.2)
Marble
Shale
Marble
Slate
Sandstone Phyllite
(Fig. 7.3)
Conglomerate Schist
Gneiss

Quartzite

Mafic or ultramafic Granite
igneous rocks Metaconglomerate intrusion
Serpentinite

Confining pressure Directed pressure Contact
lithifies sediment into (differential stress): Metamorphism
sedimentary and low-grade regional metamorphism (hornfels)
metamorphic rocks
FIGURE 7.1 Generalized diagram of metamorphism. This hypothetical diagram shows how heat (from a body of granitic magma),
directed pressure (as in a mountain belt at a convergent plate boundary), and the chemical action of watery hot (hydrothermal) fluids drive
the process of metamorphism. Parent rocks far from the intrusion and directed pressure remain unchanged. In the region of folding and
igneous intrusion, mafic igneous rocks were metamorphosed to serpentinite. Sedimentary conglomerate, sandstone, and limestone parent
rocks were metamorphosed to metaconglomerate, quartzite, and marble. Shale was metamorphosed to slate, phyllite, schist, and gneiss
depending on the grade (intensity) of metamorphism from low-grade (slate) to medium-grade (phyllite, schist), to high-grade (gneiss).
Contact metamorphism occurred in narrow zones next to the contact between parent rock and intrusive magma. Hydrothermal metamorphism
occurred along fracture systems along which the fluids migrated through the rocks.

Every metamorphic rock has a parent rock (or ACTIVITY
protolith ), the rock type that was metamorphosed. Parent
rocks can be any of the three main rock types: igneous rock, 7.1 Metamorphic Rock Inquiry
sedimentary rock, or even metamorphic rock (i.e., metamor-
phic rock can be metamorphosed again), and the degree that
a parent rock is metamorphosed can vary. As temperature THINK What do metamorphic rocks look like, and
and pressure increases, so does the metamorphic grade. About it how can they be classified into groups?
Metamorphic grade refers to the intensity of metamor-
phism, from low grade (least intense metamorphism) to high OBJECTIVE Analyze and describe samples of
grade (most intense metamorphism). metamorphic rock, then infer how they can be

FIGURE 7.1 is a highly generalized illustration of meta- classified into groups.

morphism at part of a convergent plate boundary, where PROCEDURES
rocks were highly compressed at great depths within a 1. Before you begin , do not look up defi nitions and
mountain belt. A body of granitic magma also intruded information. Use your current knowledge, and
part of the region. Note how the rocks were folded and complete the worksheet with your current level of
changed. Mafic and ultramafic igneous rocks were meta- ability. Also, this is what you will need to do the
morphosed to serpentinite. Sedimentary conglomerate, activity:
sandstone, and limestone parent rocks were metamor-
phosed to metaconglomerate , quartzite , and marble . Shale ____ Activity 7.1 Worksheet (p. 199 ) and pencil
was metamorphosed to slate , phyllite , schist , and gneiss , ____ optional: a set of metamorphic rock samples
depending on the grade of metamorphism from low-grade (obtained as directed by your instructor)
(slate) to medium-grade (phyllite, schist), to high-grade 2. Analyze the rocks, and complete the worksheet
(gneiss). Hornfels formed only in a narrow zone of “contact” in a way that makes sense to you.
metamorphism next to the intrusion of magma. Watery 3. After you complete the worksheet , be prepared
hot fluids, called hydrothermal fluids , traveled along faults to discuss your observations, interpretations, and
and fractures, where they leached chemicals from the rocks inferences with others.
while hot and deposited mineral crystals as they cooled.

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