Geology Chapter 8

257 shield

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

contact metamorphism (p.250)

develops in response to heat without a change in pressure and without differential stress

metamorphic rock (p.234)

one that forms when pre-existent rock, or protolith, undergoes a solid-slate change in response to the modification of its environment at depth in earth.

238 preferred orientation

pancake shaped grains become roughly parallel with one another, and cigar-shaped grains become aligned with one another.

What two features characterize most metamorphic rocks?

possession of metamorphic minerals and

metaconglomerate (p.241)

pressure and cobbles of the rock into pancake-lie shapes.

How does slate differ from a phyllite? How does phyllite differ from a schist? How does a schist differ from a gneiss?

slate/phyllite: slate is phyllite when slate is subjected to a temp high eough to poroduce a new assemblage of metamorphic minerals out of clay. phyllite/schist: schist formed at even higher levels than phyllite.

244 quartzite

???

234 protolith

A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed. For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths.

Why does metamorphism happen at the site of meteor impacts and along mid-ocean ridges?

Because water permeates the earth's crust and hot water chemically reacts with rock

gneiss (P.241)

a compositionally layered metamorphic rock, typically comprised of alternating dark-colored and light colored-layers that range in thickness from millimeters to meters.

hornfels (p. 243)

a fine-grained nonfoliated rock that contains a variety of metamorphic minerals (some equant and some inequant).

index mineral (p.249)

a rock indicates the approximate metamorphic grade of the rock

metamorphic facies (p.248)

a set of mineral assemblages inductive of a certain range of pressure and temperature.

metamorphic grade (p.245)

a somewhat informal way to indicate the intensity of metamorphism, meaning the amount of degree of metamorphic change.

253 shock metamorphism

changes in rock due to the passage of a shock wave

metamorphic texture (p.235)

defined by the arrangement of mineral grains

241 phyllite

fined-grained metamorphic rock with a foliation caused by preferred orientation of very grained white mica

hydrothermal metamorphism (P.253)

fluid then rises through the crust, near the ridge, causing hydrothermal metamorphism of ocean-floor basalt.

metasomatism (p.240)

hydrothermal fluids passing through a rock during metamorphism pick up ions of one element and drop off ions of another, thereby changing the overall chemical composition of the rock.

differential stress (p.238)

if a material is squeezed (or stretched) unequally from different sides, we say that it is subjected to differential stress.

mylonite 250

is extremely fine grained and has a strong foliation that roughly parallels the fault.

How is hornfels nonfoliated?

it does not display a preferred orientation. lack of foliation means either metamorphism occurred in the absence of compression and shear or that most of the new crystals are equant.

What is metamorphic foliation, and how does it form?

layering of metamorphic rocks. the parallel alignment of platy minerals (such as mica) and/ or the presence of alternating, light-colored and dark colored layers.

250 thermal metamorphism

local metamorphism caused by igneous intrusion

241 schist

medium to coarse grained metamorphic

burial metamorphism (p.250)

metamorphism due only to the consequences of very deep burial

How are metamorphic rocks different from igneous and sedimentary rocks.

metamorphism produces new minerals that did not occur in the protolith, and/or produces a new texture that is distinct from that of the protolith.

dynamothermal metamorphism (p.252)

metamorphism with heat, compression, and shearing

240 vein

mineral-filled cracks, commonly consist of milky-white quartz

metamorphic mineral (p.235)

new minerals that grow in place within the solid rock only under metamorphic conditions.

dynamic metamorphism (p.250)

occurs as a consequence of shearing alone under metamorphic conditions

exhumation (p.256)

process where deeply buried rocks end up back at the surface

metamorphic zone (p.249)

regions between isograds;

metamorphic aureole (p.250)

the distinct belt of metamorphic rock that forms around an igneous intrusion

241 slate

the finest grained foliated metamorphic rock

metamorphic foliation (p.235)

the parallel alignment of platy minerals (such as mica) and/ or the presence of alternating, light-colored and dark colored layers.

Describe the geologic settings where thermal, dynanmic, and dynamothermal metamorphism take place?

thermal: develops in response to heat without a change in pressure and without differential stress. dynamic: shearing alone under metamorphic conditions without requiring a change in temperature or pressure. dynamothermal: involves heat, compression and shearing

migmatite (p.242)

under certain conditions, gneiss may begin to melt, producing felsic magma and residual, still-solid mafic rock. If ther melt freezes again before flowing out of the source area, a mixture of igneous rock and relict metamorphic rock forms.