metamorphism and metamorphic rocks

metamorphism and metamorphic rocks

the rock cycle

metamorphism• high enough temperature & pressure to “change” rocks

but not high enough to melt rocks…changes to rocks occur in the solid-

state…• hot, reactive fluids also contribute

• old minerals, unstable under new P, T conditions, re-crystallize into

new minerals

• metamorphism occurs at depth; cannotsee metamorphic rocks unless

they are uplifted

• new rocks are metamorphic rocks

metamorphic rocks: controlling factors• parent rock composition (also called protolith)

• temperature and pressure during metamorphism

• tectonic forces

• fluids

no new material is added to rock during metamorphism

if parent material contains only one mineral

limestoneQuickTime™ and a

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marble (CaCO3)

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limestone (CaCO3)

parent rock composition

metamorphic rock will have similar composition to parent rock

resultant metamorphic rock will only have one mineral--mineral will be recrystallized (texture changes)--

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limestone under microscope(stained)

(note fragments of shells)

marble under microscope(note interconnecting grains)

texture changes

if parent material contains many minerals…

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garnet growing garnet schist (metamorphic rock)

…old minerals will recombine to form new minerals

clay, quartz, mica, and volcanic fragments in a sandstonewill combine to form new metamorphic minerals

example is garnet: which grows during metamorphism

heat is essential

temperature during metamorphism

• heat from Earth’s deep interior

• all minerals stable over finite temperature range

• higher temperatures than range cause melting(and therefore generates igneous rocks)

think about mixing flour, yeast, water, salt….

….nothing happens until they have a heat source and then they make bread

pressure in the Earth acts the same in all directions

pressure is proportionalto depth in the Earth

look at example with deep water

pressure increaseswith depth

volume decreaseswith depth

pressure during metamorphism

increases at ~1 kilobar per 3.3 km

high pressure minerals: more compact and dense

--grains pack together--

consequence on cube is squeezing into smaller cube

tectonic forces - driven by plate motion!lead to forces that are not equal in all directions (differential stress)

compressive stress (hands squeeze together)causes flattening at 90° to stress

shearing (hands rubbing together) causes flattening parallel to stress

flattened pebbles in metamorphic rock

fluids• hot water (water vapor) most important

• heat causes unstable minerals to release water

• water reacts with surrounding rocks and transports dissolved material and ions

time• metamorphism may take millions of years

• longer times allow new minerals to grow larger--coarser grained rocks

metamorphic rocks: basic classification

foliated (layered)

non-foliated (non-layered)

type of foliation -- e.g. slaty

composition -- e.g. marble

based on rock texture

foliation

foliated (layered) metamorphic rocksresults from differential stress (not equal in all directions)

non-foliated foliated

appearance under microscope

foliated metamorphic rock: slate

foliated metamorphic rock: slate

foliated metamorphic rock: phyllite (higher T, P than slate)

garnet

foliated metamorphic rock: schist (higher T, P than phyllite)…new minerals grow -- garnet (large, roundish grains)

schist under microscope

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garnet

banding of quartz/feldspar and ferromagnesian minerals

foliated metamorphic rock: gneiss (higher T, P than schist)

non-foliated (non-layered) metamorphic rocksresults from pressure: equal in all directions

named on the basis of their composition

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limestone (CaCO3) marble (CaCO3)

Carrara Marble Quarry Michelangelo’s David

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non-foliated metamorphic rocks: quartzite

metamorphosed quartz sandstone

Photo credit: R. Weller

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non-foliated metamorphic rocks: hornfels

metamorphosed basalt

types of metamorphismcontact metamorphism

• occurs adjacent to magma bodies intrudingcooler country rock -- “contact”

• produces non-foliated metamorphic rocks• happens in a narrow zone of contact

(~1 to 100 m wide) known as aureole• forms fine-grained (e.g. hornfels) or

coarse-grained (e.g. marble) rocks

types of metamorphismregional metamorphism

• occurs over wide region and mostly in deformed

mountain ranges

• produces foliated metamorphic rocks

• happens at high pressures and over a range of temperature

• increases in pressures and temperatures forms rocks of

higher metamorphic grade

other types of metamorphism (less common)

• produces migmatites, which have both intrusive and

metamorphic textures

• occurs during impact events

partial melting during metamorphism

shock metamorphism

• yields very high pressures

• forms “shocked” rocks around impact craters

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migmatite

igneous and metamorphictextures

hydrothermal alteration along mid-ocean ridge

cold sea water encounters hot basalt, forms steam, alters minerals

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black smoker offshore Pacific Northwest

hot steam/sea water cools as it emerges into ocean andprecipitates metals

temperatures cooler in down-going (subducting) plate(dashed purple line is isotherm -- line of equal T)

plate tectonics and metamorphismregional metamorphism associated with convergent boundaries

• pressure increases with depth

• temperature varies laterally

• different P, T conditions yield different degrees

of metamorphism