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; cannot�"see metamorphic rocks unless�" "they are uplifted�

• new rocks are metamorphic rocks�

Metamorphism: Fundamental Questions�

•  What are the subsolidus changes in fabric and composition that occur during metamorphism?�

•  What transfers mass and energy to cause these changes?�

•  In what tectonic settings does metamorphism occur?�

•  How does the study of metamorphic rocks and processes help us understand plate tectonics and Earth’s evolution?�

Metamorphism: Simple Definitions �

•  Metamorphism is defined as physical and chemical changes that occur in pre-existing rocks (igneous, metamorphic, or sedimentary) in the solid state that yield in a lower free energy state as a result of changes in conditions (e.g. T and P).�

–  Prograde: Increasing T and P�–  Retrograde: Decreasing T and P�

•  Diagenesis occurs at relatively low T and P and grades into metamorphism.�

•  Melting or migmatization occurs at the most intense, i.e. highest T an P limits of metamorphism and therefore grades into igneous processes.�

General P/T Conditions of Metamorphism �

From Spear, 1993

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�

limestone

marble (CaCO3)�

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)--�

limestone under microscope�(stained)�

(note fragments of shells) �

marble under microscope�(note interconnecting grains) �

texture changes�

if parent material contains many minerals…�

garnet growing � garnet schist (metamorphic rock)�

…old minerals will recombine to form new minerals�

clay, quartz, mica, and volcanic fragments in a sandstone�"will 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 proportional�to depth in the Earth�

look at example with deep water�

pressure increases�with depth �

volume decreases�with depth �

pressure during metamorphism�

Increases about 1 kilobar per 3.3 km�or 30 MPa/km (megapascals/km)�

high pressure minerals: more compact and dense�

--in rocks, 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�

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 �

Controlling Factors in Metamorphism �Note that metamorphic�equilibration is also�strongly affected by�kinetic factors, which �are not illustrated.�

Four Factors:�1) Temperature�2) Pressure�3) Fluid activity �4) Deviatoric stress �

metamorphic rocks: basic classification�

foliated (layered) �

non-foliated (non-layered) �

type of foliation -- e.g. slaty �

composition -- e.g. marble �

based on rock texture�

flattened pebbles in metamorphic rock�

foliation�

foliated (layered) metamorphic rocks�results 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�

garnet �

banding of quartz/feldspar and ferromagnesian minerals�

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

non-foliated (non-layered) metamorphic rocks�results from equal pressure (or stress) in all directions�

named on the basis of their composition �

limestone (CaCO3)� marble (CaCO3)�

non-foliated metamorphic rocks: quartzite�

metamorphosed �quartz sandstone�

Photo credit: R. Weller

non-foliated metamorphic rocks: hornfels�

metamorphosed �basalt�

types of metamorphism�contact metamorphism�

• occurs adjacent to magma bodies intruding�"cooler 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 metamorphism�regional 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�

Shock Metamorphism�•  Related to impacts of objects on planetary

surfaces (asteroids and meteorites).�•  Deformation (metamorphism) is extremely

rapid and transient.�•  Achieve much higher peak metamorphic

temperatures and pressures than commonly observed in terrestrial environments.�

•  Products include impact breccias, melts (tektites), and planer deformation features in quartz and feldspar.�

Canyon Diablo Iron Meteorite

Planetary Impact Structures

Apollo 11 Image: The Moon Galileo Image: Ganymede

From: http://neo.jpl.nasa.gov/neo.html

Impact Simulation - Gravity Dominated Model

Simulation by: James Richardson and H. Jay Melosh, Univ. of Arizona

Comet Impact Body: 1.5 g/cc 6 x 3 km

Shock Metamorphism P/T Conditions �

PDS in Shocked Quartz�

From: http://www.impact-structures.com/

Planar Deformation Structures�

migmatite�

igneous and metamorphic�textures�

hydrothermal alteration along mid-ocean ridge�

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

black smoker offshore Pacific Northwest�

hot steam/sea water cools as it emerges into ocean and�precipitates metals�

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

plate tectonics and metamorphism�regional metamorphism associated with convergent boundaries �

• pressure increases with depth�

• temperature varies laterally�

• different P, T conditions� yield different degrees�

"of metamorphism�