METAMORPHIC ROCKS (1)

Metamorphic RocksMetamorphic rocks are produced from preexisting igneous, sedimentary, or from other metamorphic rocks.. During metamorphism, rocks may be subjected to all three metamo

Metamorphic Rocks

Metamorphic Rocks

Metamorphic rocks are produced from preexisting igneous,

sedimentary, or from other metamorphic rocks Every metamorphic rock has a parent rock – the rock from which it was formed

Metamorphism, which means to “change form,” is a process that leads

to changes in the mineralogy, texture, and sometimes the chemical

composition of rocks Factors that might cause a rock to alter from one form to another includes changes in temperature, pressure (stress),

and the introduction to chemically active fluids

Metamorphism often progresses incrementally, from slight changes

(low-grade metamorphism) to substantial changes (high-grade

metamorphism) An example of low-grade would be shale turning into slate when put under pressure (stress) In high-grade changes, slight melting may occur, as well as folds or obliteration of fossils or vesicles

in the parent rock

Examples of Metamorphic Rocks

Marble

Quartzite

Phyllite

What Drives Metamorphism?

The agents of metamorphism include heat, pressure (stress), and

chemically active fluids During metamorphism, rocks may be

subjected to all three metamorphic agents simultaneously

Heat is the most important of metamorphism because it provides the

energy to drive chemical reactions that result in the recrystallization

of existing materials and/or the formation of new materials

Earth’s internal

heat comes

mainly from

radioactive

decay within the

Earth’s interior

What Drives Metamorphism?

Pressure, like temperature, increases as you get deeper into the

Earth Buried rocks are subjected to this pressure, called confining pressure, which causes the spaces between mineral grains to close, producing a more compact rock having a greater density This

pressure may ultimately cause minerals to recrystallize into new minerals that a display a more compact form

Unlike confining pressure, which

“squeezes” rock equally in all

directions (and does not fold or

deform them), differential stress,

where forces pushing on the rocks

are unequal, can create folds and

deformation This is especially

prominent at convergent plate

Metamorphic Rock Textures

Texture is used to describe the size,

shape, and arrangement of grains

within a rock

Most igneous and sedimentary rocks

consist of mineral grains that have a

random orientation By contrast,

deformed metamorphic rocks that

contain platy minerals (micas) and/or

elongated minerals (amphiboles)

typically display some kind of preferred

orientation in which the mineral grains

exhibit a parallel or specific alignment

This preferred orientation of a rock’s

minerals is called a foliated texture

Mica Schist

Examples of Foliated Textures

Various types of foliation exist, depending largely upon the grade of metamorphism and the mineralogy of the parent rock We’ll look at three main types of foliation: rock or slaty cleavage; schistosity; and gneissic texture

Rock or Slaty Cleavage:

This type of foliated texture

describes a rock’s tendency

to break or cleave along a

specific crystal plane Slate

is a rock with excellent rock

cleavage, as it breaks in flat

slabs

Examples of Foliated Textures

Schistosity: This type of

foliated texture describes a

rock created with large,

platy minerals (such as mica

and chlorite) that have

grown large enough to be

seen by the unaided eye.

In addition to platy

minerals, schist often

contains deformed quartz

and feldspar grains that

appear as flat, or

lens-shaped, grains hidden

among the mica grains

Examples of Foliated Textures

Gneissic Texture: During

high-grade metamorphism,

ion migrations can result in

the segregation of minerals

Although foliated, gneisses

will not usually split as

easily as slates and schists

Gneisses that do cleave tend

to break parallel to their

foliation and expose

mica-rich surfaces that resemble

schist

Foliated Metamorphic Rocks

Gneiss

Schist

Slate

Phyllite

Other Metamorphic Textures

Not all metamorphic rocks exhibit a foliated texture Those that do not

are referred to as nonfoliated Nonfoliated textures usually form in

environments where parent rocks are composed of minerals that

exhibit equidimensional crystals, such as quartz or calcite

Another texture common to

metamorphic rocks consists of

particularly large grains, called

porphyroblasts, that are

surrounded by a fine-grained

matrix of other minerals

Porphyroblastic textures develop

in a wide range of environments

and result in very large specimens

of certain minerals, such as

Nonfoliated Metamorphic Rocks

The exterior of the Taj Mahal is constructed mainly of the

metamorphic rock marble

Metamorphic Environments

Hydrothermal metamorphism occurs when hot fluids circulate

through fissures and cracks that develop in rock This hot fluid

Metamorphic Environments

There are a number of environments in which metamorphism occurs Most are in the vicinity of plate margins, and many are associated with igneous activity

Contact or thermal

metamorphism

occurs when rocks

immediately

surrounding a molten

igneous body are

“baked” and therefore

altered from their

original state

Metamorphic Environments

Regional metamorphism occurs where rocks are squeezed

between two converging lithospheric plates during mountain

The typical transition in mineralology that results from progressive

metamorphism of shale