Chapter 7: Metamorphism

Question 1

Metamorphic rock

Answer 1

a rock that forms when a pre-existing rock, or protolith, undergoes a solid-state change in response to modification of its environment

Question 2

What kind of change is meant in the context of metamorphism?

Answer 2

a metamorphic rock contains new minerals that did not occur in the protolith, and/or a new texture (arrangement of mineral grains) that differs from that of the protolith.

Question 3

protolith

Answer 3

original unmetamorphosed rock

Question 4

Metamorphic minerals

Answer 4

new minerals that grow in place within solid rock during metamorphism

Question 5

metamorphic texture

Answer 5

defined by the arrangement of mineral grains, can be manifested by metamorphic foliation

Question 6

processes that take place during metamorphism

Answer 6

• recrystallization; shape and size of the crystals change without changing the identity of the mineral
• phase change; changes one mineral into another
• metamorphic reaction or neocrystallization; growth of new minerals that differ from those of the protolith
• pressure solution: pressure of grains agains one another with water between them, grains dissolve, ions migrate away
• plastic deformation; rock behaves like soft plastic so minerals can change shape without breaking

Question 7

differential stress

Answer 7

when material undergoes squeezing or stretching unequally from different sides

Question 8

preferred orientation

Answer 8

the alignment of grains within a rock

Question 9

foliation

Answer 9

the parallel surfaces or layers that can occur in a metamorphic rock, can give the rock a striped or streaked appearance or it can give it the ability to split into thin sheets

Question 10

examples of foliated metamorphic rocks

Answer 10

• slate: metamorphism of shale or mudstone
• phyllite: a fine grained rock with a silky sheen (phyllitic luster), white mica
• schist: medium to course grain rock, defined by the preferred orientation of large mica flakes (muscovite or biotite), forms at higher temps than phyllite
• gneiss: composed of alternating dark and light colored layers, dark are more mafic, light are more felsic

Question 11

how do non-foliated metamorphic rocks occur?

Answer 11

either metamorphism occurred in the absence of compression or shear, or most of the new crystals are equant

Question 12

examples of non-foliated metamorphic rocks

Answer 12

• hornfels: a fine grained non-foliated rock which contains a variety of metamorphic minerals
• quartzite: formed by metamorphism of pure quartz sandstone, looks glassier than sandstone and does not have the grainy, be a variety of colors
• marble: metamorphism of limestone

Question 13

metamorphic grades

Answer 13

indicates the intensity of metamorphism, meaning the amount or degree of metamorphic change. Depends on the temperature and pressure at which metamorphism takes place.

Question 14

shearing

Answer 14

stretches the rock and flattens mafic components; smears rocks into parallel lines

Question 15

protolith of quartzite

Answer 15

quartz sandstone

Question 16

protolith of marble

Answer 16

limestone

Question 17

metamorphism of basalt from low grade to high grade

Answer 17

greenschist, amphibole, mafic granulite

Question 18

metamorphism of shale from low grade to high grade

Answer 18

slate, phyllite, schist, gneiss

Question 19

at an igneous pluton, where do the highest grade metamorphic rocks form?

Answer 19

In the wall rock of the pluton, also known as metamorphic aureole or contact aureole, the grade decreases further away from the pluton

Question 20

Since metamorphism in the wall rock of a pluton (contact aureole) takes place without application of differential compression or shear, what type of rock is formed?

Answer 20

hornfels

Question 21

dynamic metamorphism

Answer 21

occurs as a consequence of shearing under metamorphic conditions without requiring change in temperature or pressure, forms mylonite, occurs at plate boundaries, rifts, and collision zones

Question 22

dynamothermal metamorphism

Answer 22

involves heat and compression and shearing

- occurs beneath mountain ranges

Question 23

where is blueschist (blue colored amphibole) found?

Answer 23

only under accretionary prisms at subduction zones, forms as a result of compression and shearing

Question 24

shield

Answer 24

a broad region of long-lived, stable continental crust where sedimentary cover either was not deposited or has been eroded away so that precambrian basement rocks are exposed.