Metamorphic Rocks

Metamorphic Rocks Rock changed from an existing type of rock to a new type. 1. Metamorphism 2. Temperatures 3. Pressure 4. Chemical reactions 1. Result of heat, pressure and chemical reactions. 2. Range from 200 to 800 degrees Celsius. The rock gets soft enough for minerals to change in size, shape or to form layers but not to melt. 3. Squeezes minerals together making them denser. 4. Amount of change varies with the amount of heat and pressure. If there is only a slight change then the original rock can be determined.

Types of metamorphism Contact Regional Rocks are heated by contact with intrusive magma. They are found along the edges of igneous rocks. Large areas, rock is buried until there are increases in heat and pressure.

Characteristics of the original rock affect the degree of metamorphism Fine-grained rocks are more easily changed than coarse-grained rocks.

Types of metamorphic rock: Mineral crystals are in parallel bands—they break in layers and are formed when crystals or original rock recrystallize, crystals flatten, and when minerals of different densities separate into different layers (alternating light and dark). Examples: Schist—may form from granite slate, basalt. Gneiss—forms from granite or diorite. Slate—formed from shale. No bands, do not break into layers. Examples: Quartzite—formed from sandstone Marble—formed from limestone. Anthracite coal—formed from Bituminous coal Soapstone—formed from talc Foliated Non- foliated

Usually black and white Foliation Banding of minerals Usually black and white

Distorted Structure Folded layers

Metamorphic Textures • • Foliated • Non-foliated Non-foliated • - - Folios = page or leaf-like - - Rock has distinct banding or layering > > Often not smooth like in sedimentary rocks - - Formed under directed pressure • Non-foliated Non-foliated • - - No distinct layering character - - Often a massive crystalline texture - - Formed under uniform pressures

Foliated Texture

Foliated Textures • • Slatey Schistose • • • • Phyllitic • • Gneissic - - Looks like blackboard - - Distinct bands of minerals > > Dull surface - - Visible mineral grains - - Smooth, thin layering > > Garnets, staurolites - - Breaks into flat slabs - - May have shiny > > Referred to as slatey cleavage appearance - - No mineral grains visible > > Due to mica minerals • • Phyllitic • • Gneissic Gneissic - - Looks like waxed surface - - :Larger grains > > Has a "sheen" to it - - May look like igneous rock - - May have little "waves" on - - May have crude banding surface surface > > Intensely distorted > > Referred to as crenulations - - Different minerals than - - Some small grains visible schistose

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 building.