THE LANGUAGE OF THE EARTH – PART III

METAMORPHIC

ROCKS

Rock Cycle

DefinitionMetamorphism- changes in the mineral assemblage and

textures of igneous, sedimentary or other metamorphic rocks due to prolonged exposure to elevated temperatures and pressures (or increased grades of metamorphism)

Types of Metamorphism1) Burial metamorphism (also known as Diagenesis or Lithification) -

Low T, low-mod confining to differential P; original sedimentary/igneous features generally preserved; forms in deeper parts of thick sedimentary sequences in non-tectonic area (e.g., on passive continental margins)

Types of Metamorphism2) Contact (or Thermal) metamorphism - High T, low-mod.

confining P; forms adjacent to igneous intrusions.

Types of Metamorphism3) High-pressure metamorphism - High P (strongly

differential), low-mod T; forms at plate boundaries convergent boundary zones. Also know as blueschist metamorphism.

Types of Metamorphism4) Regional metamorphism -Variable T, mod-high differential

P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas

Types of Metamorphism5) Cataclasis metamorphism -Variable T, very high directed P;

typically localized to narrow zones of intense mechanical deformation (shear zones).

Types of Metamorphism6) Metasomatism -Variable T&P; distinguished from other forms of

metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss).

Controls on Rates of Metamorphic Change• Fluid content (particularly water) of

the rock. Water helps to catalyze the mineral transformations. Water gets “baked” out of rocks during prograde metamorphism.

• Temperature - chemical reactions occur faster at higher temperatures.

• Time - For a rock to develop a new metamorphic mineral assemblage corresponding to a particular P & T, it must exist under those condition for a sufficiently long period of time (generally tens of thousands to millions of years).

Why do we see high grade metamorphic rocks at the earth’s surface?

Isostatic Rebound

“Rapid” erosion of mountain belts of “dry” rocks

Mineralogic Response to Metamorphism

Minerals that form depend on: • T and P conditions• Bulk composition of the source

rock

Progressivemetamorphismof a graywacke

(dirty sandstone)

Textural Response to MetamorphismReflects the intensity and directionality of pressure (or stress).• Increased grain size - During prograde metamorphism or at a particular grade that is

maintained for a long period of time, minerals will tend to increase in size. • Foliation - As new platy minerals grow, they will align themselves perpendicular to the

maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosity to the rock.

• Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..

Development of Foliation in Bedded Sedimentary Rocks

Other Textural ResponsesPorphyroblasts

Some metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblasts

Alkali Feldspar Staurolite Garnet

Textural Response to Thermal Metamorphism Granoblastic TextureGranoblastic Texture

Creates a very dense, granular, fine-grained mineral texture called granoblastic texture

Metamorphic rocks displaying granoblastic

texture are called hornfels

basaltic hornfels sedimentary hornfels

Summary of Metamorphic Rock Types

STRUCTURAL DEFORMATIONLarge-scale Response to Directed Pressure

Dependent on Temperature and Rate of Stress

High Temperature, Low Rate of Stress → FOLDING

STRUCTURAL DEFORMATIONLarge-scale Response to Directed Pressure

Low Temperature, Rapid Rate of Stress → FAULTING

Rock Cycle

Basalt

Granite

MaficMafic

Quartz + Clay+Lithics

FelsicFelsic

Gabbro

Rhyolite

Clay+Lithics

GraywackeShale

Slate

Schist

GneissMigmatite

Qtz SandstoneLimestone

QuartziteQuartziteMarbleMarble

GreenstoneGreenstone

Amphibolite