Metamorphic Rocks: Comprehensive Study Notes
Metamorphic Rocks: Key Concepts
Metamorphic rocks are those changed from an original parent rock (protolith) through metamorphism.
Etymology: Meta = Change, Morph = Form or shape.
Primary effects: Changes in mineralogy and texture.
Metamorphic Character
Metamorphic texture: Intergrown and interlocking grains and foliation; a planar fabric from aligned minerals.
Metamorphic minerals: Some minerals are formed only during metamorphism, e.g., Chlorite, Staurolite, Andalusite, Kyanite, Sillimanite, Garnet.
Causes of Metamorphism
Heat (T)
Pressure (P)
Compression and/or shear stress
Hot water (fluids)
Heat (Temperature, T)
High temperatures without melting; heat energy breaks and reforms atomic bonds.
Depth is governed by the geothermal gradient: 20-60^{\circ}\mathrm{C}\,\mathrm{km}^{-1}.
Can be enhanced by magmatic intrusions.
Note: Compression can also contribute to metamorphism in conjunction with heat.
Pressure (P)
Temperature (T) and pressure (P) both change with depth; P increases by 270-300\ \mathrm{bar/km}.
Mineral stability depends on both T and P; stability fields can be plotted on a phase diagram.
As T and/or P change, mineral assemblages change.
Differential Stress
Pressure that is greater in one orientation (common in tectonic settings).
Normal stress: operates perpendicular to a surface (tension and compression).
Shear stress: operates parallel/slantwise across a surface.
At higher T and P, differential stress deforms rock.
Deformation produces specific mineral shapes and aligns minerals; inequant (platy/elongate) grains show preferred orientation.
Hydrothermal Fluids
Metasomatism: hot water with dissolved ions and volatiles.
Fluids facilitate metamorphism by accelerating chemical reactions.
Fluids can add or remove elements from rocks, altering composition.
Metamorphic Processes
Change occurs slowly in the solid state.
Phase Change
New minerals form with the same chemical composition but different crystal structure.
Example: Andalusite (low T, low P) → Kyanite (low T, high P) → Sillimanite (high T, high P).
Pressure Solution and Recrystallization
Mineral grains partly dissolve where surfaces press together; ions dissolve into water and migrate.
Neocrystallization
New minerals form due to changes in temperature and pressure.
Original protolith minerals are digested; elements restructure to form new minerals.
Example: shale can transform into garnet-mica schist.
Plastic Deformation
At elevated temperatures, mineral grains soften and deform without breaking, allowing rock to be squeezed or sheared.
Rocks behave plastically, changing size/shape instead of fracturing.
Metamorphic Structure: Folia
Foliation: preferred alignment of platy minerals (e.g., mica, chlorite) or alternating light/dark mineral layers.
Develops perpendicular to the direction of compression.
Minerals flatten, recrystallize, and rotate to form a layered or banded appearance.
Mafic and felsic minerals can separate into distinct bands.
Preferential vs Random Orientation
Metamorphic rocks can show preferentially oriented minerals (foliation) or random orientations depending on stress history and fluid content.
Direction of maximum stress influences the foliation direction.
Metamorphic Rock Types
Two broad groups: foliated and non-foliated.
Foliated rocks have a linear appearance due to foliation from differential stress.
Non-foliated (granoblastic) rocks form in rocks of a single composition and lack foliation; they are typically denser and harder than the parent rock.
Stress state: Confined stress (equal in all directions) tends to produce non-foliated textures.
Foliated Rocks: Examples and Textures
Slate: Fine; preferred orientation of clay flakes; reorientation and recrystallization; slaty cleavage.
Phyllite: Fine to medium; preferred orientation of white mica; neocrystallization; slight sheen.
Schist: Medium to coarse; visible large mica flakes showing strong foliation; platy minerals dominate.
Gneiss: Coarse to very coarse; mineral layers (mm to m) of alternating dark and light minerals; mica may be rare.
Slaty Cleavage: Characteristic feature in slate (and related rocks).
Non-Foliated Rocks
Hornfels: Fine-grained assemblage formed by heat from nearby magma; varied mineralogy.
Amphibolite: Metamorphism of mafic rocks; hornblende (amphibole) and plagioclase dominate; little to no quartz or muscovite.
Marble: Metamorphism of limestone or dolostone; calcite/dolomite is the primary mineral.
Quartzite: Metamorphism of pure quartz sandstone; quartz-dominant.
Migmatite
Partially melted gneiss; exhibits features of both igneous and metamorphic rocks.
Mineralogy controls behavior: light-colored (felsic) minerals melt at lower temperature (igneous), dark-colored (mafic) minerals melt at higher temperature (metamorphic).
May contain metamorphic veins.
Texture, Grain Size, and Composition: Identification Scheme
Scheme for Metamorphic Rock Identification involves:
Rock Name
Map Symbol
Type of Metamorphism (e.g., Regional, Contact)
Foliation and banding characteristics
Mineral alignment and grain size (Fine to coarse)
Notes on mineral assemblage (mica, quartz, feldspar, amphibole, garnet, pyroxene, etc.)
For non-foliated rocks, texture is granoblastic rather than foliated.
Examples from the identification scheme:
Slate: regional metamorphism of shale; foliated; banding; mineral alignment; slate rock symbol.
Phyllite: regional metamorphism; fine to medium grains; mica-rich; neocrystallization.
Schist: regional metamorphism; medium to coarse grains; large mica flakes; schistosity.
Gneiss: regional metamorphism; banding of dark/light minerals.
Migmatite: high-grade/metamorphic with partial melting; features of both igneous and metamorphic rocks.
Hornfels: contact metamorphism; nonfoliated; fine grains.
Marble: contact/region metamorphism of limestone; nonfoliated.
Quartzite: contact/regional metamorphism of sandstone; nonfoliated.
Metaconglomerate: conglomerate parent may show pebbles distorted or stretched.
Practice Questions (From Transcript)
Question 1: Which of the following is not a characteristic of metamorphism?
A. development of foliation texture
B. growth of new minerals under high temperature
C. existing phenocrysts are changed to platy or elongated
D. development of fracture
Question 2: According to the causes of metamorphism, metamorphism is least likely to occur in which setting?
A. subduction zone
B. 10 km deep beneath the Appalachian mountain
C. on the sandy beach of South Carolina
D. rocks forming the magma chamber of Mountain St. Helens
Questions 3 & 4:
The parent rock of quartzite is:
A. granite
B. shale
C. sandstone
D. limestone
Which of the following metamorphic rocks is foliated?
A. hornfels
B. marbles
C. quartzites
D. schists
Metamorphic Intensity (Grade)
Intensity is measured by grade, indicated by different minerals present.
Temperature ranges:
Low grade: T\in [\,250, 400^{\circ}\mathrm{C}\,]
High grade: T>600^{\circ}\mathrm{C}
High-grade rocks tend to be drier than low-grade rocks.
Foliated Rocks and Intensity of Metamorphism
As metamorphic intensity increases, crystal size and coarseness of foliation also increase.
Typical progression with increasing grade:
Low grade: Slate
Intermediate grade: Phyllite, Schist
High grade: Gneiss, Migmatite
Key textures: Slaty cleavage, Schistosity, Banding.
Example sequence reference: Slate, Phyllite, Schist, Gneiss, Migmatite (with Migmatite incorporating partial melting).
Note: Citations in figure credits for images of slate, phyllite, schist, gneiss, migmatite (e.g., John Grotzinger, Ram\'on Rivera-Moret, Kip Hodges).
Index Minerals and Metamorphic Zones
Index minerals record metamorphic grade; different minerals have limited P–T ranges.
Rocks of the same metamorphic grade define metamorphic zones.
Isograds are the boundaries between metamorphic zones.
Isograds and Zones (Examples)
Isograds can be named by minerals such as Chlorite zone, Biotite zone, Garnet zone, Staurolite zone, Sillimanite zone.
In some regional maps, you might see zones labeled for specific geographic regions (e.g., Canada, NY, VT, ME, NH, MA, CT, RI) with color-coded isograds.
Important concept: Isograds help map the degree of metamorphism across regions; high-grade zones contain minerals like Sillimanite and/or Garnet, while low-grade zones contain Chlorite.
Question prompt: Can high-grade metamorphic rocks be changed to low-grade metamorphic rocks? (Isograds address this concept by showing distinct mineral assemblages corresponding to grade.)
Metamorphic Environments
Metamorphism occurs in a wide range of environments with different conditions.
The primary controlling relationship is between temperature (T) and pressure (P).
Not all metamorphic conditions occur in every environment.
Depth and Environments (Illustrative Sketch)
Depths and settings (conceptual):
0 to ~35 km: crustal environments; inclusion of surface processes and shallow metamorphism.
35 to ~75 km: deeper crustal environments; regional metamorphism becomes more widespread.
Asthenosphere and mantle lithosphere contexts: high-pressure metamorphism in subduction zones; shock metamorphism in impact events; seafloor (hydrothermal) metamorphism in oceanic settings.
Major metamorphic environments (summary):
Shock metamorphism (impact events)
Regional metamorphism (large-scale, associated with mountain-building)
High-pressure metamorphism (subduction zones)
Contact metamorphism (heat from intruding magma)
Burial metamorphism (deep burial under sediments)
Water-seafloor metamorphism (hydrothermal alteration at seafloor)
Continental mantle lithosphere involvement (deep crustal to mantle contexts)
Take-Home Messages
Understand how rocks are modified by metamorphism.
Recognize how metamorphic character differs from the parent rock.
Know the two groups of metamorphic rocks (foliated vs non-foliated) and their differences.
Be able to classify metamorphic rocks based on the presence/absence of foliation, texture, and rock composition.