Chapter 8
Metamorphism: A Process of Change
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphism: A Process of Change
Prepared by
Ron Parker
Earlham College Department of Geosciences
Richmond, Indiana
Introduction

Metamorphic – Changed from an original “parent.”
Meta = Change.
 Morph = Form or shape.

Parent rocks are called “protoliths.”
 Metamorphism can
occur to any protolith.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Introduction

Protoliths undergo pronounced changes in…
Texture.
 Mineralogy.


Due to change in physical or chemical conditions.
Burial.
 Tectonic stresses.
 Heating by magma.
 Fluid alteration.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphism
Metamorphism occurs in the solid state.
 It doesn’t include weathering, diagenesis, melting.
 Metamorphics often look totally unlike protoliths.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Character

Metamorphic rocks have distinctive properties.
Unique texture – Intergrown and interlocking grains.
 Unique minerals – Some that are only metamorphic.

Staurolite, Kyanite, Sillimanite, etc.


Unique foliation – A planar fabric from aligned minerals.
These transformations can change the rock utterly.
Fossiliferous limestone
Red mudstone
Garnet gneiss
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Marble
Chapter 8: Metamorphism: A Process of Change
Metamorphic Processes
Metamorphic change is slow and in the solid state.
 Several processes are at work.


Recrystallization – Minerals change size and shape.

Phase change – New minerals form with…
Same chemical formula.
Different crystal structure.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Kyanite
Chapter 8: Metamorphism: A Process of Change
Metamorphic Processes

Several processes may operate at the same time.

Neocrystallization – New minerals with P-T changes.
Initial minerals become unstable; change to new minerals.
Decomposition of the original minerals in the protolith.
Chemical reaction of the elements to form new minerals.
In this way, a shale can transform into a garnet mica schist.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Processes

Several processes may operate at the same time.

Pressure solution – Mineral grains partially dissolve.

Plastic deformation – Mineral grains soften and deform.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Causes of Metamorphism

The agents of metamorphism are…
Heat (Temperature – T).
 Pressure (P).
 Differential stress.
 Hydrothermal fluids.

Not all agents are required; they often do co-occur.
 Rocks may be overprinted by multiple events.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Heat (Temperature)

Metamorphism occurs as the result of heat.

Temperature (T) ranges between 200oC and 850oC.
The upper T limit is…melting. It varies based upon
rock mineral composition and water content.
 Heat energy breaks and reforms atomic bonds.
 Sources of heat.

The geothermal gradient.
 Magmatic intrusions.
 Compression.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Pressure (P)

P increases with depth in the crust.
270 to 300 bars per km (1 bar is almost 1 atm = 14.7 psi).
 Metamorphism occurs mostly in 2 to 12 kbar range.

T and P both change with depth.
 Mineral stability is highly dependent upon T and P.

This stability can be graphed on a “phase diagram.”
 Changes in T and P lead
to changes in minerals.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Differential Stress
Pressure that is greater in one orientation.
 A commonplace result of tectonic forces.
 Two kinds of differential stress: Normal and shear.


Normal stress – Operates perpendicular to a surface.
Tension – Pull-apart normal stress.
Compression – Push-together normal stress.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Differential Stress

Two kinds of differential stress: Normal and shear.

Shear stress – Operates sideways across a surface.
Causes material to be “smeared out.”
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Differential Stress

At higher T and P, differential stress deforms rock.

Rocks change shape slowly without breaking.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Differential Stress

Deformation acts on minerals with specific shapes.
Equant – Roughly equal in all dimensions.
 Inequant – Dimensions not the same.

Platy (pancake-like) – 1 dimension shorter.
Elongate (cigar-shaped) – 1 dimension longer.
Differential stress causes these minerals to align.
 Alignment fabric records stress trajectory.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Differential Stress

Preferred platy mineral alignment is called foliation.
Foliation imparts a layered or banded appearance.
 Rocks commonly break parallel to foliation planes.


Foliation develops perpendicular to compression.


Minerals flatten, recrystallize, and rotate.
Inequant grains align by rotation and new growth.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Hydrothermal Fluids
Hot water with dissolved ions and volatiles.
 Hydrothermal fluids facilitate metamorphism.

Accelerate chemical reactions.
 Alter rocks by adding or subtracting elements.


Hydrothermal alteration is called metasomatism.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rock Types

Two major subdivisions of metamorphic rocks.

Foliated – Has a through-going planar fabric.
Subjected to differential stress.
Has a significant component of platy minerals.
Classified by composition, grain size, and foliation type.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rock Types

Two major subdivisions of metamorphic rocks.

Non-foliated – No planar fabric evident.
Crystallized without differential stress.
Comprised of equant minerals only.
Classified by mineral composition.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Slate – Fine clay, low-grade metamorphic shale.

Has a distinct foliation called slaty cleavage.
Develops by parallel alignment of platy clay minerals.
Slaty cleavage oriented perpendicular to compression.
Slate breaks along this foliation creating flat sheets.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Phyllite - Fine mica-rich rock.
Formed by low – medium grade alteration of slate.
 Clay minerals neocrystallize into tiny micas.
 Micas reflect a satiny luster.
 Phyllite is between slate and schist.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Schist – Fine - coarse rock with larger micas.
Medium-to-high-grade metamorphism.
 Has a distinct foliation called schistosity.

Parallel alignment of large mica crystals.
Micas are visible because they have grown at higher T.

Schist often has other minerals due to neocrystallization.
Quartz.
Feldspars.
Kyanite.
Garnet.
Staurolite.
Sillimanite.

Large non-mica minerals are called porphyroblasts.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Gneiss – Has a distinct banded foliation.
Light bands of felsic minerals (quartz and feldspars).
 Dark bands of mafic minerals (biotite or amphibole).

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Compositional banding develops in several ways.
Original layering in the protolith.
 Extensive high T shearing.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Compositional banding

Solid-state chemical differentiation.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Migmatite
Migmatite is a partially melted gneiss.
 It has features of igneous and metamorphic rocks.
 Mineralogy controls behavior.

Light-colored (felsic) minerals melt at lower T.
 Dark-colored (mafic) minerals melt a higher T.


Felsics melt first; mafics remain metamorphic.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Non-foliated rocks lack a planar fabric.

Absence of foliation possible for several reasons.
Rock not subjected to differential stress.
Dominance of equant minerals.
Absence of platy minerals.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Amphibolite – Dominated by amphibole minerals.
Basalt or gabbro protolith.
 Usually not well foliated.


Hornfels – Alteration by heating.
Associated with plutonic intrusions.
 Finely crystalline.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Quartzite – Almost pure quartz in composition.
Forms by alteration of quartz sandstone.
 Sand grains in the protolith recrystallize and fuse.
 Like quartz, it is hard, glassy, and resistant.

Metamorphic Alteration
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Marble - Coarsely crystalline calcite or dolomite.
Forms from a limestone or dolostone protolith.
 Extensive recrystallization completely changes the rock.
 Original textures and fossils in the parent are obliterated.
 Used as a decorative and monument stone.
 Exhibits a variety of colors.

Metamorphic Alteration
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Rocks

Type depends on protolith.
Minerals contribute elements.
 Some protoliths yield specific rocks.


Broad compositional classes.
Pelitic.
 Basic (or Mafic).
 Calcareous.
 Quartzo-feldspathic.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Classes

Pelitic – Shale protoliths.
Al-rich clay minerals yield micas.
 Rock type depends on grade.

Slate.
Phyllite.
Schist.
Gneiss.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Classes

Mafic – Basalt or gabbro protolith.
Rich in Fe and Mg and poor in Si, Al, Na, and K.
 Turn into biotite- and amphibole-dominated rocks.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Classes

Calcareous – Carbonate protolith.
Rich in Ca (limestones) and Ca and Mg (dolostones).
 Recrystallize into calcite and dolomite marbles.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Classes

Quartzo-feldspathic – Granitic protolith.
Quartz + feldspar in granites stable under metamorphism.
 Recrystallize and become foliated into granitic gneisses.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Intensity
Different minerals are stable as T and P changes.
 Grade is a measure of metamorphic intensity.

Low grade – Slight.
 High grade – Intense.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Intensity

Specific minerals typify particular grades.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Grade

Prograde – Metamorphism via increasing T and P.
Common in rocks that are buried in orogenic belts.
 Progressive changes.

Recrystallization causes mineral growth.
Neocrystallization results in new mineral assemblages.
Mineral changes release water.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Grade

Example: Prograde metamorphism of a pelitic rock.

Low grade – Shale protolith.
Clays recrystallize into larger, aligned clays to yield a slate.
Clays neocrystallize into tiny, aligned micas in a phyllite.

Intermediate grade –
Micas recrystallize and grow large to form a schist.
New minerals grow in the schist.

High grade –
Micas decompose; elements recombine into new minerals.
Neocrystallization yields quartz and feldspars in a gneiss.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Grade

Retrograde – Metamorphism via decreasing T and P
Common in rocks that are brought from depth by erosion.
 Accompanied by addition of H2O by hydrothermal fluids.


Many prograde rocks aren’t “retrograded.”

Rocks at the surface can preserve prograde conditions.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Contact Metamorphism
Certain minerals have a limited P-T range.
 These “index minerals” record metamorphic grade.
 Index mineral maps.

Define metamorphic zones.
 Grade boundaries called
isograds.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Facies
Metamorphic facies – Mineral assemblage from a
specific protolith at specific P-T conditions.
 The same minerals result from the same…

Protoliths.
 T and P conditions.


Named for dominant
mineral.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Environments
Metamorphism occurs in different settings.
 Different settings yield different effects via…

Geothermal gradient.
 Differential stresses.
 Hydrothermal fluids.


These characteristics are governed by tectonics.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Metamorphic Environments

The types (and settings) of metamorphism are...
Thermal – Heating by a plutonic intrusion.
 Burial – Increases in P and T by deep burial in a basin.
 Dynamic – Shearing in a fault zone.
 Regional – P and T alteration due to orogenesis.
 Hydrothermal – Alteration by hot water leaching.
 Subduction – High P–Low T alteration.
 Shock – Extreme high P attending a bolide impact.
 Mantle – Extreme high mantle P causes phase changes.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Contact Metamorphism
Due to heat from magma invading host rock.
 Creates zoned bands of alteration in host rock.

Called a contact (or metamorphic) aureole.
 The aureole surrounds the plutonic intrusion.

Zoned from high (near pluton) to low grade (far from pluton).
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Contact Metamorphism

Grades of alteration form bands around the pluton.
Bands range from highly altered to slightly altered.
 Analogous to changes in pottery with increased heating.


The width of each aureole zone is due to…
The size of the plutonic intrusion.
 The degree of metasomatism.


The dominant rock is hornfels.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Burial Metamorphism

As sediments are buried in a sedimentary basin…
P increases because of the weight of the overburden.
 T increases because of the geothermal gradient.


Requires burial below diagenetic effects.

This is ~ 8–15 km depending on the geothermal gradient.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Dynamic Metamorphism
Breakage of rock by shearing at a fault zone.
 Fault location determines type of alteration.


Shallow crust – Upper 10-15 km.
Rocks behave in a brittle fashion.
Mineral grains crush and pulverize forming fault breccia.

Deeper crust – Below 10-15 km.
Rocks behave in a ductile manner.
Minerals smear like taffy to form mylonite.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Regional Metamorphism
Tectonic collisions deform huge “mobile belts.”
 Directed compression thickens mountains.


Rocks caught up in mountain building are…
Heated via the geothermal gradient and plutonic intrusions.
Squeezed and heated by deep burial.
Smashed and sheared by differential stresses.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Regional Metamorphism
Regional metamorphism creates foliated rocks.
 This type of metamorphism is, by far, the most
important in terms of the amount of rock altered.


Collisional belts are often…
1000s of km long.
100s of km wide.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Hydrothermal Metamorphism
Alteration by hot, chemically aggressive water.
 A dominant process near mid-ocean ridge magma.

Cold ocean water seeps into fractured crust.
 Heated by magma, this water then reacts with mafic rock.
 The hot water rises and is ejected via black smokers.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Subduction Metamorphism
Subduction creates the unique blueschist facies.
 Trenches and accretionary prisms have…

A low geothermal gradient – low temperature.
 High pressures.


High P – Low T favor
glaucophane, a blue
amphibole mineral.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Shock Metamorphism
Rarely, Earth is struck by a comet or asteroid.
 Impacts generate a compressional shock wave.

Extremely high pressure.
 Heat that vaporizes or melts large masses of rock.


These conditions generate high-pressure minerals.
Coesite.
 Stishovite.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Exhumation
How do metamorphic rocks return to the surface?
 Exhumation is due to...

Uplift – Compression squeezes deep rocks upward.
 Extensional collapse – Uplifted range spreads outward.
 Erosional unroofing – Weathering and erosion removes
vast amounts of rock.

Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
Finding Metamorphics
Large regions of ancient high-grade rocks – called
shields – are exposed in continental interiors.
 Shields are eroded remnants of orogenic belts.


Shield rocks form the basement under sedimentary cover.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change
This concludes the
Chapter 8
Metamorphism: A Process of Change
LECTURE OUTLINE
earth
Portrait of a Planet
Third Edition
©2008 W. W. Norton & Company, Inc.
Earth: Portrait of a Planet, 3rd edition, by Stephen Marshak
Chapter 8: Metamorphism: A Process of Change