Metamorphic Rocks Metamorphic rocks are formed from any rocks that have undergone “extreme” heat and/or pressure. That heat and/or pressure has caused the original rock to change form (metamorphosis). The “new” rock is usually much different from its original form.

Types of Metamorphism There are 2 types: 1. Regional Metamorphism 2. Contact Metamorphism

Regional Metamorphism Occurs when large areas of rock are under intense heat AND pressure. Most often happens during mountain building events (orogenies). It is important to keep in mind that the heat and pressure are VERY intense and this happens over VERY LARGE areas.

Regional Metamorphism of Shale shale is a sedimentary rock. During metamorphism, the pressure squeezes the mineral crystals closer together. Heat then rearranges the mineral crystals. The first level of metamorphism of shale results in the metamorphic rock slate. This “new” rock slate shows low-grade foliation – layers.

Foliation or Foliated Texture Parallel alignment or orientation of platy minerals. Occurs during directed stress. Can look like the strata (layering) in sedimentary rocks)

Foliation After heat and pressure, mineral crystals are aligned into layers - FOLIATION Before heat and pressure is applied – mineral crystals are random

Textural Changes Foliated Textures: Elongate minerals and structural features are forced into parallel alignment by directed (differential) stress.

Metamorphosed conglomerate becomes, wait for it… metaconlgomerate

Deformed foliation

Regional Metamorphism of Shale If further heat and pressure is applied to slate, the second level of metamorphism will result in the rock phyllite. Phyllite is shiny and has medium foliation. If more heat and pressure are applied to phyllite it will result in a third level of metamorphism and the rock schist. Schist has medium to high foliation and is very shiny.

Regional Metamorphism of Shale Finally, if more heat and pressure is applied to schist, it results in the highest grade of metamorphism and the rock gneiss. Gneiss shows mineral banding (high foliation). So much heat and pressure has been applied that the minerals have aligned themselves into bands of colors.

Phyllite (low to medium grade metamorphism) Shale (Sedimentary rock) Slate (low grade metamorphism) Gneiss (very high grade metamorphism – shows banding) Schist (medium to high grade metamporphism)

Honors level: Since metamorphic rocks can be created from other rocks, there is more than one way to end up at a metamorphic rock (especially higher levels of metamorphism. For that reason, they are sometimes referred to from their parent rock. A granite that becomes a gneiss can be referred to as a granitic gneiss.

Phyllite Slate

Schist

Gneiss

no noticeable “layers” Marble Quartzite Nonfoliated Rocks – no noticeable “layers”

Contact Metamorphism Contact metamorphism occurs on a much smaller level and ONLY includes heat. NO PRESSURE. This happens when hot magma comes in direct contact with existing rock and the heat changes the rocks form. Contact metamorphism is not strong enough to create foliation.

Contact Metamorphism

Regional or Contact Metamorphism Rocks Marble Quartzite Metaconglomerate Serpentine

The Limits of Metamorphism Metamorphism is actually WAY more complicated than what we’ve introduced in Honors Earth Science.

Oh, hey, LOOK! A reference table!

Foliation, or not?

or like full-on stripes? Is the foliation just alignment? or like full-on stripes?

grain size is old hat, we know this…

Know this – Anthracite is the harder coal that burns cleaner because it was metamorphosed (from the softer, bituminous coal.)

The rest is hit-or miss whether they decide to ask something about it or not.