Page 214 - Laboratory Manual in Physical Geology

ACTIVITY Agents of Metamorphism
Temperature, pressure, and hydrothermal fluids (watery
7.2 Metamorphic Rock Analysis
hot fluids) are known as agents of metamorphism. Wher-
and Interpretation ever metamorphism is occurring, one or more of these
agents is involved in the metamorphic process.
THINK What are metamorphic rocks composed
About it of? How is composition used to classify Pressure Effects on Rocks
and interpret igneous rocks? Confining Pressure is pressure (stress) applied equally in

all directions ( FIGURE 7.2 ). When you jump into a swim-

OBJECTIVE Be able to describe and interpret textural ming pool you feel the confining pressure of the water
and compositional features of metamorphic rocks. pushing on every part of your body with equal force. If
you dive down deep under the water, the pressure increases
PROCEDURES
all around you. The same thing happens with rocks. Con-
1. Before you begin , read about Metamorphic fining pressure increases with depth below Earth’s surface
Processes and Rocks below. Also, this is what you and is equal in all directions. The deeper the rocks, the
will need :
greater the confining pressure. This is what compacts rocks
___ Activity 7.2 Worksheets (pp. 200–201 ) and pencil from sediment into sedimentary rock. The rock gets more
___ optional: a set of mineral samples (obtained as dense because it is squeezed into less space. Unequal-sized
directed by your instructor)
___ optional: a set of mineral analysis tools
(obtained as directed by your instructor) FIGURE 7.2 Effects of confining pressure (equal stress). As
rocks get buried, they experience confining pressure that is equal
2. Then follow your instructor’s directions for
in all directions (equal stress). The rocks become more dense as
completing the worksheets. pore space is squeezed, may recrystallize to crystals of equal size,
and remain nonfoliated.

EFFECTS OF CONFINING PRESSURE (EQUAL STRESS)

Undeformed
strata Increasing
confining
pressure

Sedimentary
rock: Sand grains
sandstone
Pore spaces Foliated
metamorphic
rock: quartzite

Sand grains fused,
no pore space

Metamorphism
5 30
As the sedimentary rocks get buried,
they are affected by confining pressure.
The rocks get squeezed equally in all
directions. Pore space is reduced, and the
sandstone becomes more dense quartzite.
30 1 , See Figure 7.13

Metamorphic Rocks, Processes, and Resources ■ 189