Ch. 7: Metamorphic Rocks

Question 1

Processes of metamorphism

Answer 1

Protolith → metamorphic rock

Question 2

Recrystallization

Answer 2

Original grain boundaries migrate into a new mosaic
pattern
Tiny clasts to large grains

Question 3

Neocrystallization

Answer 3

Driven mostly by elevated T; NEW metamorphic
minerals (porphyroblasts) may grow:
Clay and quartz to quartz garnet and mica

Question 4

Causes of metamorphism

Answer 4

Heat
Pressure
Directed pressure

Question 5

Heat

Answer 5

produced by geothermal gradient (~25°C/km) and intrusions;

metamorphic T between 200° and ~ 800°C

Question 6

Pressure

Answer 6

due to burial: geobaric gradient, 300 bars (or 0.3 kb) per kilometer

Question 7

If a rock has experienced 3 kbars of pressure, how deeply was it buried?

Answer 7

10

Question 8

Directed pressure

Answer 8

stress; produces fabrics in metamorphic rocks

Question 9

Index minerals

Answer 9

chlorite→ mica→ garnet→ staurolite→ kyanite [only from shale protoliths]
low to high all come from protoliths chlorite being first

Question 10

foliated rocks

Answer 10

foliated: slate; phyllite; schist; gneiss

Question 11

non-foliated rocks

Answer 11

non-foliated rocks: quartzite; marble; amphibolite; hornfels

Question 12

other rocks

Answer 12

other: migmatite, mylonite, blueschist, eclogite

Question 13

Types of metamorphism

Answer 13

Regional: majority
Contact: hornfels
Dynamic/shear: mylonites
Subduction: produces blueschist & eclogite

Question 14

Blueschist

Answer 14

high P subduction zone metamorphism

Question 15

Eclogite

Answer 15

very high P subduction zone

Question 16

Prograde metamorphism

Answer 16

typical sequence of changes from
low to high metamorphic grade; if protolith is shale, result is
sequence of index minerals (chlorite, etc.)

Question 17

Retrograde metamorphism

Answer 17

may produce

chlorite rind on blueschist blocks, garnet

Question 18

Metamorphic isograds

Answer 18

lines of equal metamorphic grade that separate zones

Question 19

Metamorphic zones

Answer 19

named for index minerals found within them; separated by isograds
Regional patterns of metamorphic zones are delineated by the isograds
—indicate thermal structure of ancient mountain belt
Changes in texture and minerals from shale to migmatite:

Question 20

Maryland Piedmont
What did the pattern of metamorphic isograds
reveal?
What metamorphic rocks are found in the local
area?

Answer 20

Kyanite is the hottest because closest to center of mountain belt where rocks are hotter

Gneiss, quartzite, shist, muse

Question 21

FOLIATION

Answer 21

All metamorphic rocks are RECRYSTALLIZED.
Some may also have aligned platy or elongate minerals
that form a planar alignment called FOLIATION
FOLIATED METAMORPHIC ROCKS by increasing metamorphic grade:
slate->phyllite->schist->gneiss

Question 22

SLATE

Answer 22

compact, very fine-grained, metamorphic rock
with a welldeveloped cleavage; freshly cleaved surfaces
are dull
low grade

Question 23

PHYLLITE

Answer 23

a rock in which very fine-grained micas impart a silky sheen to the cleavage surface
low grade

Question 24

SCHIST

Answer 24

a metamorphic rock exhibiting schistosity, i.e.,
visible micas
med-high grade

Question 25

GNEISS

Answer 25

a metamorphic rock displaying gneissic banding,
with alternating felsic and darker mineral layers.
high grade

Question 26

NON-FOLIATED METAMORPHIC ROCKS:

Answer 26

QUARTZITE – sandstone
MARBLE - Limestone
AMPHIBOLITE - Basalt or gabbro protolith
HORNFELS - contact meta
MIGMATITE- partially melted geniss part igneous and meta

Question 27

QUARTZITE

Answer 27

metamorphosed sandstone

Question 28

MARBLE

Answer 28

Limestone

Question 29

AMPHIBOLITE

Answer 29

Basalt or gabbro

Question 30

HORNFELS

Answer 30

Alteration by
heating
Associated with contact
metamorphism around
plutonic intrusions
Finely crystalline
May have reaction
“spots” (spotted
hornfels)

Question 31

MIGMATITE

Answer 31

is a partially melted gneiss that has features
of igneous and metamorphic rocks.
Light-colored (felsic) minerals melt at lower T
Dark-colored (mafic) minerals melt at higher T

Question 32

REGIONAL

Answer 32

Plate tectonic collisions deform huge mountain belts
 Rocks caught up in mountain-building are…
Heated and squeezed by deep burial (geothermal
gradient and plutonic intrusions)
Sheared by directed pressure (stress)
 Creates most
metamorphic
rocks

Question 33

CONTACT

Answer 33

Due to heat from
magma invading
country rock
 Creates “baked”
zones in the country
rock protolith
around the plutonic
intrusion
 Reaction zones
around minerals
may create
“spotted” hornfels

Question 34

DYNAMIC OR SHEAR

Answer 34

At plate boundary or
deep fault -- grinding
Produces the
metamorphic rock
“MYLONITE”

Question 35

Hydrothermal

Answer 35

Alteration by hot, chemically-active water
 A dominant process near mid-ocean ridge:
Cold ocean water seeps into fractures
Reacts with oceanic basalt
The hot water rises and is ejected via black smokers

Question 36

Shock

Answer 36

Rarely, Earth is struck by a large meteorite (bolide)
 Impacts generate a compressional shock wave
Extremely high pressure
Heat vaporizes or melts large masses of rock
 These conditions generate high-pressure minerals

Question 37

Subduction

Answer 37

Subduction creates the unique blueschist facies
 Trenches and accretionary prisms have…
Relatively low temperatures
High pressures
 High P – Low T creates
glaucophane, a blue
amphibole (the blue
in blueschist)

Question 38

Metamorphic FACIES:

Answer 38

a set of co-existing minerals that
indicate metamorphic rock reached a certain range of P
and T conditions

Question 39

SHIELDS

Answer 39

Large regions of ancient high-grade metamorphic rocks
eroded remnants of mountain belts exposed in continental
interiors; form the basement under sedimentary cover