Metamorphic: "to change form"

Normal depths for metamorphism: below sedimentary depths (~2 km) and above igneous depths (~50 - 250 km)

Definition of metamorphism - a solid-state change in which the minerals within a rock are changed without melting it. Note that the rock doesn't change composition. If that were to occur, then the result would be metasomatism.

Conditions promoting metamorphism

Rock stability related to environment of formation

Metamorphism does not include melting

• Heat
• Necessary for chemical changes
• Geothermal gradient - temperature increases with depth at a rate of 20 - 30 *C/km in the crust.
• Sources of heat for metamorphism
• Radioactive isotopes
• Intruding magma
• Friction between rock bodies
• Pressure
• Measured in bars - 1 kilobar (1 kb) is approximately 3 km depth. 1 kb = 14,700 pounds/square inch.
• Lithostatic (confining) pressure - equal in all directions.
• Directed pressure - applied in one plane. Directed pressure will result in a foliation (from Latin, foilatus - leaf-like)
• Results of pressure
• Compaction
• Recrystallization
• Foliation
• Fluids - will cause metasomatism to take place; the bulk composition of the rock changes.
• Often necessary for chemical reactions
• Sources of fluids
• Surface infiltration
• Pores in sedimentary rocks
• Cracks in subducting plates
• Magmas
• Heated minerals
• Magmatic gases
Parent rocks (protoliths)

Common metamorphic rocks

• Foliated rocks derived from shale
• Mud (sediment)
• Shale (sedimentary rock or protolith)
• Slate (metamorphic rock; forms at relatively low temperature and requires directed pressures)
• Slaty cleavage (rock cleavage vs. mineral cleavage)
• Slate colors
• red: iron oxides
• green: chlorite
• purple: manganese oxides
• black: carbon
• Uses of slate
• blackboards
• roofing materials
• floor tiles
• pool-table tops
• Phyllite (requires low to intermediate temperatures and directed pressures)
• has a characteristic sheen to it.
• Schist (requires intermediate to high temperatures and directed pressures)
• develops a strong foliation and a distinct cleavage known as schistosity
• Gneiss (high temperatures and directed pressures)
• How gneiss forms - through metamorphic differentiation in which the lower temperature minerals get to the point of almost melting, leaving the higher temperature minerals in a more undeformed form.
• Where gneiss forms - often in the core of ancient mountain ranges such as the North Cascades.
• Migmatite - if the rock is heated to temperatures at which the more felsic minerals begin to melt (600 - 800°C), the rock is in a gray area where part of it can be called metamorphic and part is igneous - that is a migmatite.
Foliated rocks derived from igneous rocks
• Basalt metamorphoses to greenschist
• Granite and diorite metamorphose to gneiss
Nonfoliated rocks
• Pure limestones and dolostones form marble which has recrystallized calcite crystals.
• Silica-rich limestones and dolostones form skarn
• Sandstone metamorphose to quartzite
• Shale and basalt form hornfels

Types of metamorphism

Contact metamorphism
• Affected by magmatic heat and fluids, not by pressure
• Metamorphic aureoles
Regional metamorphism affects vast regions
• Burial metamorphism
• Occurs at depths greater than ~10 km
• Confining pressure and geothermal heat
• Produces nonfoliated metamorphic rocks
• Dynamothermal metamorphism
• Occurs between converging plates
• Directed pressure, heat, and confining pressure
• Produces foliated metamorphic rocks
• Other types of metamorphism
• Hydrothermal metamorphism
• Continental processes
• (More common) oceanic processes: serpentinization
• Fault metamorphism
• Directed pressure and frictional heat
• Can result in foliation and recrystallization
• Large, flattened, rotated recrystallized grains: augen
• Shock metamorphism
• From meteor impacts
• Produces the minerals stishovite and coesite
• Minerals are distinctive of impact sites
Metamorphic grade and index minerals
Metamorphic grade - based on the occurrence of diagnostic minerals that form at particular temperatures and pressures.
Low-grade metamorphic rocks
• Retain some of their original characteristics
• Low temperatures (200 - 400 °C)
• Pressure (1 - 6 kilobars)
High-grade metamorphic rocks
• Lack original characteristics
• High temperatures (500 - 800 °C)
• Pressure (12 - 15 kilobars)
E.g., increasing metamorphic grade with the protolith being a shale:
shale -> slate -> phyllite -> schist -> gneiss -> migmatite
Mineral zones - characterized by index minerals which are only stable over restricted temperature/pressure ranges.
Metamorphic index minerals and mineral zones act as geothermometers and geobarometers for the temperatures and pressures to which the rock was subjected.

E.g., kyanite, andalusite, and sillimanite



Metamorphic facies
• Metamorphic facies: consistent temperature and pressure conditions
• Greenstone facies: low temperatures and confining pressures
• Greenschist facies: low temperatures and directed pressures
• Amphibolite facies: intermediate temperatures and pressures
• Granulite facies: high temperatures and pressures
• Eclogite facies: extremely high temperatures and pressures
• Blueschist facies: low temperatures and high pressures
Once metamorphic facies develop they may later be folded or deformed. Mapping the metamorphic facies will result in isograds being made. Isograds are lines of equivalent metamorphic grade.
Plate tectonics and metamorphic rocks
• Regional dynamothermal metamorphism develops directed pressure at plate boundaries.
• Contact metamorphism and regional metamorphism develops from magma generated at plate boundaries.
• Subduction zone metamorphic environments.
• Trench: low temperature, high pressure.
• Close to surface: high temperature, low pressure.
• At greater depth: high temperature, high pressure.
Metamorphic rocks in daily life
• Strong and weather-resistant - slate for roofing
• Valued for appearance - garnet (January's birthstone), verde antique, Carrara marble
• Used for buildings, sculpture, and common commodities - slate, jadeite, talc, asbestos
• Sites used for some valuable mineral deposits - hydrothermal metamorphism will concentrate precious metals such as gold, platinum, as well as others such as copper and lead.
• Potential hazards from metamorphic rocks - since some metamorphic rocks develop a strong foliation they may be a hazard, say for landslide potential, if the foliation is oriented in the wrong direction.