METAMORPHIC ROCK

Earth Science

Chapter 2.4

Objectives:

1. Describe two ways a rock can undergo metamorphism.

2. Explain how the mineral composition of rocks changes as the rocks undergo metamorphism.

3. Describe the difference between foliated and non-foliated metamorphic rock.

4. Explain how metamorphic rock structures are related to deformation.

Origins of Metamorphic Rock

Metamorphic rocks Rocks in which the structure, texture, or

composition of the rock have changed○ All three types of rock can be changed by heat, pressure,

or a combination of both

A rock’s texture or mineral composition can change when its surroundings changeIf the temperature or pressure of the new

environment is different from the one in which the rock formed○ the rock will undergo metamorphism

1. Contact metamorphism When magma moves through the crust, the

magma heats the surrounding rock and changes it.○ Some minerals in the surrounding rock are changed into

other minerals by this increase in temperature.○ The greatest change occurs where magma comes into

direct contact with the surrounding rock.

2. Regional metamorphism occurs when pressure builds up in rock that is

buried deep below other rock formations or when large pieces of the Earth’s curst collide

with each other.○ The increased pressure and temperature causes rock

to become deformed and chemically changed.

Composition of Metamorphic Rock As rocks undergo metamorphism

Original minerals in a rock change into new minerals that are more stable within the new pressure and temperature conditions.

Index minerals (form only in metamorphic rock)○ Are used to estimate

1. Temperature

2. Depth

3. Pressure ---- at which a rock undergoes meta-morphism.

calcite

quartz

++ +hematite

Heat and Pressure

Garnet

=

Texture of Metamorphic Rock

Two types:1. Foliated Metamorphic Rock

○ Mineral grains are arranged in planes or band○ Usually contains aligned grains of flat minerals, such

as biotite, mica or chlorite Metamorphic rocks can become other metamorphic rocks if

the environment changes again.

2. Nonfoliated Metamorphic Rock ○ Mineral grains are not arranged in planes or band○ commonly made of one or only a few minerals.

Recrystallization - Process in which crystals of these minerals may change in

size or the mineral may change in composition

This shows what happens to shale, a sedimentary rock, when it is exposed to more and more heat and pressure.

Shale Slate Phylite

SchistGneiss

Two examples of nonfoliated metamorphic rock

Marblelimestone

metamorphism

undergoes

Sandstone Quartzite

metamorphism

undergoes

Metamorphic Rock Structures Metamorphic rock has features that indicates its

history. These features are caused by deformation:Change in the shape of a rock caused by a force

placed on it.○ These forces may cause a rock to be squeezed or stretched.○ Folds, or bends, in metamorphic rock are structures that

indicate a rock has been deformed.