Metamorphic Rocks

Question 1

Metamorphic

Answer 1

Derived from Greek - “of changed form.” Metamorphic rocks are changed from one form to another by intense heat, pressure or the action of watery hot fluids. The characteristics of the original rock, and the type and intensity of metamorphism, determine how drastic the change is.

Question 2

Parent rock

Answer 2

The rock type that was metamorphosed. Can be any of 3 main types: Igneous rock, sedimentary rock, or even metamorphic rock (metamorphic rock can be metamorphosed again).

Question 3

Where does metamorphism happen?

Answer 3

Metamorphism takes place at varying depts within the Earth’s crust, where temperature and pressure are higher than at the surface. The result of metamorphism includes the creation of new minerals (remember supernovas and the creation of terrestrial planets? Similar process), or the development of bands or layers of minerals (for example, layers of quartz or mica), and the parallel alignment of new and old mineral crystals.

Question 4

Contact metamorphism

Answer 4

Occurs locally, adjacent to igneous intrusions, and along fractures that are in contact with water hot (hydrothermal) fluids. Contact metamorphism is caused by conditions of low to moderate pressure, intense heating, and reaction with the metamorphosing magma or hydrothermal fluids over days to thousands of years. The intensity of contact is greatest next to the magma and then decreases rapidly over a short distance from the magma. Zones of contact metamorphisms are narrow, on the order of millimeters to tens-of-meters thick.

Question 5

Regional metamorphism

Answer 5

Occurs over very large areas, such as deep within the cores of rising mountain ranges, and is generally accompanied by folding of rock layers. Contact and regional metamorphism aren’t always distinct. For example major intrusions of magma are preceded by contact metamorphism and followed by regional metamorphism.

Question 6

Textures of metamorphic rock

Answer 6

There are two main types of metamorphic rock - Foliated and non-foliated

Question 7

Foliated metamorphic rock

Answer 7

exhibits foliations - layering and parallel alignment of platy (flat) mineral crystals, such as mica.

It can look like repetitive layering, each layer may be as thin as a sheet of paper or over a meter in thickness. The word comes from Latin folium, meaning “leaf” and refers to the sheet-like planar structure. It is caused by shearing forces (pressures pushing different sections of the rock in different directions than others). The layers form parallel to the direction of the shear or perpendicular to the direction of higher pressure.

Question 8

Foliated textures

Answer 8

Slaty, Phyllite texture, Schistosity, Gneissic banding. The names of these textures are closely related to the names of the metamorphic rocks that exhibit such texture.

The same parent material, if it is buried deeper will experience higher pressure and temperature and transform into higher grades of metamorphism.

Question 9

Slaty texture

Answer 9

Slaty (think slate) is the lowest grade texture. Means very flat foliation developed along flat, parallel, closely spaced shear planes in tightly folded clay or mica rich rocks. This low-grade metamorphism does not produce shiny mica, so slaty foliation is dull and not shiny. Slate can vary in color, but is always dull in luster and has slaty foliation

Question 10

Phyllite texture

Answer 10

Means wavy or wrikled foliation of fine grained “shiny” minerals like mica. The rock will have a satiny (shiny) luster compared to the dull luster of slaty foliation. Phyllite texture is shinier than slaty because mica crystals have formed at this higher metamorphic state.

Question 11

Schistosity

Answer 11

Means a glittery layering of visible (medium to coarse-grained) shiny minerals like mica and/or linear alignment along prismatic crystals. Schist can really vary in terms of what it looks like (coloring, type of mineral) but it will be layered and quite likely shiny with bigger grains than either phyllite or slate. Schist has large grains, very shiny and visible layers.

Question 12

Gneissic banding

Answer 12

Alternating layers or lenses of light and dark medium to coarse-grained minerals. It can also vary in terms of colors. As a rock gets buried deeper and deeper (but not deep enough to melt), it allows the minerals to migrate around and align themselves perpendicular to the stress from the local tectonic situation. Gneiss is an example of high-grade metamorphism. The layers or “stripes” can get wavy under stress.

Gneiss is the backbone of continental crust, and the oldest rocks on Earth are gneiss. On all continents where the overlying rocks are weathered and stripped away, gneiss is the ultimate “basement” of crustal earth. Gneiss is not defined by the minerals it contains but by it’s texture - coarse-grained with stripes. Stripes are made of dark and light minerals that are formed by internal flow within the rock at high temperatures and pressure (600-700 celsius). In these conditions, minerals tend to flow at their own speeds and similar minerals flow together into bands with similar properties. If Gneiss gets so deep it melts, it becomes an igneous rock (and will probably harden into granite). Of course that granite may turn back into gneiss if it hangs out deep under the earth for a long time.

Question 13

Nonfoliated metamorphic rocks

Answer 13

Have no obvious layering (no foliations), instead you need to identify patterns in grain size and luster.

Question 14

Crystalline texture

Answer 14

means a medium to coarse-grained aggregate of intergrown, usually equal-sized crystals. Marble is the most famous example of this texture.

Question 15

Microcrystalline texture

Answer 15

fine-grained aggregate of intergrown microscopic crystals. may look similar to a sugar cube. Tends to have a dull luster.

Question 16

Sandy texture

Answer 16

medium to coarse-grained aggregate of fused, sand-sized grains that resemble sandstone

Question 17

Glass texture

Answer 17

Homogeneous texture with no visible grains. The surface is glossy and may be as shiny as glass. Anthracite coal has a glassy texture.

Question 18

How to classify metamorphic rocks

Answer 18

The first key step is to figure out if the rock is foliated or nonfoliated. After you determine that you should look for other specific kinds of textural features. It is important to classify the grain size (fine, medium, or coarse) and also the luster fo the rock.

Question 19

Distinguishing between rock types

Answer 19

All rocks are classified as Igneous, Sedimentary, or Metamorphic based on their properties. Some properties are characteristic of more than one type of rock. For example, igneous, sedimentary, and metamorphic rocks can all be dark, light, or comprised of mineral particles. Therefore, it is essential to classify rocks from multiple properties.

Question 20

Components of rocks

Answer 20

Rocks are composed primarily of crystals or grains

Question 21

Crystals (components of rocks)

Answer 21

Individual minerals that have grown in place to form a rock. In igneous rocks they are minerals that crystallize from the solidifying magma/lava. In metamorphic rocks they are minerals that grow within the solid rock through chemical reactions between neighboring minerals. Thus identifying crystals in a rock means the rock must be either igneous or metamorphic. Because crystals are individual minerals, they will display distinct mineral properties. because minerals have regular, ordered arrangement of atoms, crystals tend to have regular, polygonal shapes. Adjectives such as blocky, bladed, needle-like, prismatic, rectangular, and hexagonal are commonly used to describe crystals. Many minerals also tend to display cleavage. On a broken surface of a rock, broken mineral crystals may exhibit cleavage which will be seen as flat, shiny faces.

Question 22

Grains (components of rocks)

Answer 22

Grains are pieces of pre-existing rock or organic material (e.g., shells) that have been cemented together to form a sedimentary rock. Thus identifying grains in a rock means that the rock must be sedimentary. Abrasion due to the transport of sedimentary grains by wind or water tends to wear down any sharp edges, producing rounded grains (i.e., shapes like balls, jellybeans, etc.) Sandstone is a sedimentary rock that is composed of small (<2mm diameter) rounded grains, whereas conglomerate is a sedimentary rock composed of rounded pebbles.

Question 23

Layering - a common rock fabric

Answer 23

Another easily definable attribute of rocks that will help you classify rocks is the presence or absence of some form of layering in rocks. Sedimentary rocks commonly exhibit layering that can be defined by changes in the color or size of the grains. This layering is called bedding. Changes in color of beds reflect changes in chemical or mineral composition.

Metamorphic rocks commonly contain minerals such as micas and amphiboles that are aligned parallel to each other, producing a form of layering (foliation). Extreme metamorphism can lead to the development of a gneiss, a rock characterized by the segregation of minerals into distinct layers, typically bands of light minerals and dark minerals.

Since layering is very rare in igneous rocks, the presence of banding in a rock is evidence that the rock is likely a sedimentary or metamorphic rock. Examining a layered rock to determine whether it is composed of grains or crystals will allow you to classify it as a sedimentary or metamorphic rock, respectively.

Question 24

Characteristic texture

Answer 24

The key differentiation between igneous, metamorphic and sedimentary rocks is understand the characteristic texture for each rock type.

Question 25

Igneous characteristic texture

Answer 25

Intrusive Igneous rocks (rocks formed underground and displaying crystals) will often be composed of different mineral crystals (usually of various colors). Examples include granite. They are not foliated.

Extrusive igneous rocks (rocks formed via volcanism) have very obvious and unique textures such as holes (vesicular basalt for example) or glassy texture (obsidian).

Question 26

Sedimentary characteristic textures

Answer 26

Most common sedimentary rocks have clastic textures meaning they are composed of particles/grains cemented together (sandstone is an example). Only sedimentary rocks have sedimentary structures such as mud cracks, fossils, raindrop impressions, ripple marks, etc. Any sedimentary structure visible means it must be a sedimentary rock.

Question 27

Metamorphic characteristic texture

Answer 27

Foliated metamorphic rocks look distinctly different from sedimentary and igneous rocks. Unfoliated metamorphic rocks will have crystals that are uniform in size and color; this will help distinguish them from igneous rocks.