Page 234 - Laboratory Manual in Physical Geology

C. Finnish geologist, Pentti Eskola, first recognized in 1921 that basalt volcanic rock could be metamorphosed to distinctly

different metamorphic facies (unique assemblages of several minerals) under changing conditions of temperature and
pressure (depth).
• Amphibolite facies (low pressure, high temperature): black hornblende amphibole, sillimanite

• Greenschist facies (low pressure, low temperature): green actinolite amphibole and chlorite

• Eclogite facies (high pressure, high temperature): red garnet, green pyroxene
• Blueschist facies (high pressure, low temperature): blue amphibole (glaucophane, riebeckite)

1. Write the names of these metamorphic

Temperature (°C)
facies where they would occur in this
0 500 1000
pressure-temperature diagram. 0 0
Pressure (kilobars) 6 20 Depth (kilometers)

12 42

2. At the time that Pentti Eskola discovered these metamorphic facies, the Plate Tectonics Theory had not yet been
developed. Geologists now realize that volcanic arcs develop at convergent plate boundaries where the oceanic edge of one
plate subducts beneath the continental edge of another plate. Notice (below) how the geothermal gradient (rate of change
in temperature with depth) varies relative to the subduction zone and the volcanic arc. Place letters in the white spaces on
this illustration to show where Eskola’s facies should occur: A = Amphibolite, G = Greenschist, E = Eclogite,
B = Blueschist.

Subduction zone:
Subducting sediments
and seafloor basalt
are metamorphosed due Volcanic (magmatic) arc:
Shallow crustal rocks
to increase in pressure
are metamorphosed by
of subducting plate and heat from rising magma
increasing temperature
with depth
Igneous
intrusions

300°C 300°C
600°C 600°C
900°C
900°C
100
km Subducting oceanic lithosphere

200
km

3. REFLECT & DISCUSS Based on what you learned in this activity, write a generalization that expresses how pressure,
temperature, metamorphic environment, and metamorphic grade are related.

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