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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 - PowerPoint PPT Presentation
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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