GEL Metamorphic Rocks

Marble

A crystalline metamorphic rock whose parent is limestone or dolostone.

Hornfels

A fine-grained nonfoliated metamorphic rock, and unlike marble and quartzite, it has a variable mineral composition.

Index mineral

A mineral that is a good indicator of the metamorphic environment in which it formed. Used to distinguish different zones of regional metamorphism.

Migmatite

A rock exhibiting both igneous and metamorphic rock characteristics. Such rocks may form when light-colored silicate minerals melt and then crystallize, while the dark silicate minerals remain solid.

Gneiss

A texture of metamorphic rocks in which dark and light silicate minerals are separated, giving the rock a banded appearance.

Schistosity

A type of foliation that is characteristic of coarser-grained metamorphic rocks. Such rocks have a parallel arrangement of platy minerals such as the micas.

Slaty cleavage

A type of foliation that is characteristic of slates, in which there is a parallel arrangement of fine-grained metamorphic minerals.

Slate

A very fine-grained foliated rock composed mainly of minute chlorite and mica flakes that are too small to be visible.

Quartzite

A very hard metamorphic rock formed from quartz sandstone.

How are gneisses and migmatites related?

Both gneisses and migmatites contain quartz, potassium feldspar, biotite, and amphibole.

Contact metamorphism

Changes in rock caused by the heat from a nearby magma body.

Thermal metamorphism

Changes in rock caused by the heat from a nearby magma body; same as contact metamorphism.

Hydrothermal metamorphism

Chemical alterations that occur as hot, ion-rich water circulates through fractures in rock.

Distinguish between contact metamorphism and regional metamorphism.

Contact metamorphism (or thermal metamorphism) occurs in Earth's upper crust, when rock immediately surrounding a molten igneous body are "baked." Regional metamorphism is a common, widespread type of metamorphism typically associated with mountain building, where large segments of Earth's crust are intensely deformed by the collision of two continental blocks.

What are the agents that change rocks through metamorphism?

Heat, pressure, directional stress, and chemically active fluids.

What are index minerals and how are they used?

Index minerals are minerals that are good indicators of the metamorphic environment in which they are formed. Geologists distinguish among different zones of regional metamorphism using these index minerals.

How is the intensity or degree of metamorphism reflected in the texture and mineralogy of metamorphic rocks?

Index minerals are used to determine different zones of regional metamorphism, which is found from the texture and mineralogy of metamorphic rocks.

Schist

Medium- to coarse-grained metamorphic rocks in which platy minerals are dominant.

Nonfoliated texture

Metamorphic rocks that do not exhibit foliation.

What are the two textural divisions of metamorphic rocks and the conditions associated with each?

Metamorphic rocks with foliated textures contain platy and/or elongated minerals, and they display some kind of preferred orientation in which the mineral grains exhibit a parallel to subparallel alignment. Nonfoliated metamorphic rocks typically develop in environments where deformation is minimal and the parent rocks are composed of minerals that exhibit equidimensional crystals, such as quartz or calcite.

Phyllite

Represents a degree of metamorphism between slate and schist. Its constituent platy minerals are larger than those in slate but not large enough to be readily identifiable with the unaided eye.

Distinguish between slaty cleavage, schistosity, and gneissic textures.

Slaty cleavages exhibit an excellent splitting property. Schistosity are structures that result when platy minerals are large enough to be discernible with the unaided eye and exhibit planar or layered structures. Gneissic textures appear during high grade metamorphism, when ion migration can result in the segregation of minerals in which the dark biotite and amphibole crystals and light silicate minerals have separated.

Lithostatic pressure

Stress imposed on a layer of rock by the weight of overlying material.

Metamorphism

The changes in mineral composition and texture of a rock subjected to high temperatures and pressures within Earth.

In what two ways can the parent rock affect metamorphism?

The overall chemical composition and the mineral makeup of the parent rock.

Parent rock

The rock from which a metamorphic rock formed.

Briefly describe the three mechanisms by which minerals develop a preferred orientation.

The rotation of platy mineral grains shows that the new alignment is roughly perpendicular to the direction of maximum stress. Recrystallization that produces new minerals occur perpendicular to the direction of maximum stress. The newly formed mineral grains exhibit a distinct layering, and the metamorphic rocks containing them exhibit foliation. Flattening spherically shaped grains normally develop roughly spherical crystals and have a rather simple chemical composition.

Rock cleavage

The tendency of rocks to split along parallel, closely spaced surfaces. These surfaces are often highly inclined to the bedding planes in the rock.