metamorphism

Metamorphism – transformation of one rock type into another

• Metamorphic rocks – produced from pre-existing sedimentary, igneous and metamorphic rocks

• Parent rock – origin of a metamorphic rock

See next slides for samples of metamorphic rocks

Norway

China

Lake Huron, Ontario, Canada

Michigan, North America

California, North America

California, North America

Metamorphism leads to changes in:

• Mineral content

• Texture

• Chemical composition

Metamorphic agents are:

• Heat

• Pressure

• Chemically active fluids

Metamorphism occurs from a few km into the crust all the way down to the upper mantle.

Degrees of metamorphism

• Low-grade metamorphism : little change in the parent rock

• High-grade metamorphism : identity of parent rock becomes hard to distinguish

• Remember : parent rock should not melt, if it does, then igneous processes (not metamorphic) become involved

Example of low-grade metamorphism (from shale to slate)

Shale (sedimentary rock) Slate (metamorphic rock)

Example of high-grade metamorphism: from shale (top) to slate (top right) to gneiss (right)

Degrees of metamorphism

low-grade LOW SHORT

high-grade HIGH LONG

Heat

Pressure

Chemical activity

Agents of metamorphism Time

Heat as an agent of metamorphism

Heat provides the energy of activation for the chemical transformation in metamorphic rocks.

Chemical reactions result in:

• recrystallization of existing minerals

• formation of new minerals

Chemical changes are all based on the increased kinetic energies of the ions.

Temperatures increase with depth at a rate known as the geothermal gradient.

Hence, degree of metamorphism increases as depth increases.

Heat as an agent of metamorphism

The Western Deep Levels Mine in South Africa at 4 km is the deepest in the world.

At these depths, the rock is actually hot enough to burn human skin. Miners work in pairs- one digging, the other operating a large fan to keep cool.

The deepest mine in the world

• Primordial heat – trapped when Earth’s crust first cooled

• Radioactivity – energy released when subatomic particles are spontaneously emitted by radioactive elements

• Solar radiation – absorbed and converted to thermal energy by the Earth’s surface

What heats up the Earth?

Primordial heat

Earth initially formed as a molten ball of rock. When the surface cooled to become the crust, the heat became trapped.

This trapped heat now drives the convection cells in the mantle and the movement of the molten outer core round the inner core.

Solar radiation – the Earth’s crust absorbs about 50% of the Sun’s energy

The absorbed solar radiation is distributed unevenly. The greatest concentration occurs in the tropical belt (red zone).

Radioactivity

Radioactive elements contribute to Earth’s internal heat. Background radiation (not enough to harm) exists all around us. Shown at right is a geologist measuring background radiation with a portable Geiger counter.

Pressure as an agent of metamorphism

Two types of pressure:

• Confining pressure – experienced by buried rocks; forces are applied equally in all directions

• Differential stress – forces are unequal in different directions

Confining pressure is evenly distributed.

• squeezes out the spaces between mineral grains

• produces a more compact, denser rock

Confining pressure produces rock layers that are undeformed.

Directonial stress deforms rock layers as shown in C. (the series shows the deformation of sediments deposited in a river flood plain)

The layers of rock shown below have been deformed by directional stress. Such pressure is most active in convergent plate boundaries.

Rocks in shallow depths are pulverized when subjected to differential stress. At greater depths, on the other hand, rocks are more ductile and are compressed and elongated rather than crushed.

A sample of conglomerate becomes metaconglomerate when subjected to differential stress at depth (note the elongated rock fragments).

Heat and pressure

Heat increases with increasing pressure. This means that areas of mountain building (continental vs continental convergent plates) are hotter due to greater pressure.

METAMORPHIC ROCKS

• definition

• degrees of metamorphism

• agents of metamorphism

Where does metamorphism occur?

• contact metamorphism

• regional metamorphism

• dynamic metamorphism

Contact metamorphism

Contact metamorphism

Contact metamorphism

Contact metamorphism

Granite

Gneiss

Contact metamorphism

From limestone to marble

From igneous rocks to marble

Syenite

(igneous)

marble

From sediments to metamorphic rocks

Regional metamorphism

Mountain-building:

• plate tectonics (convergent boundaries)

• directional stresses are involved

• greatest volume of metamorphic rocks are produced in this way

Review of plate tectonics

Regional metamorphism

Regional vs contact metamorphism

Textural changes

Texture – size, shape, and distribution of particles that constitute a rock

• foliated

• non-foliated

Textural changes (foliation)

Types of foliation:

• rock or slaty cleavage – minute crystals

• schistosity – larger crystals

• gneissic texture – segregation of minerals

Rock or Slaty cleavage

Rock or

Slaty cleavage

Rock or Slaty cleavage

Schistocity

mica schist – most abundant schist type

Schistocity

mica schist

Schistocity

Schistocity

Schistocity

Schistocity

Schistocity

talc schist

Schistocity

talc schist

Schistocity

talc schist

Gneissic texture

Gneissic texture

Gneissic texture

Gneissic texture

Nonfoliated texture

marble

Nonfoliated texture

Nonfoliated texture

Nonfoliated texture

Nonfoliated texture

quartzite

Nonfoliated texture

quartzite