Metamorphic Rocks & The Rock Cycle

Metamorphic Rocks Metamorphic rocks form from transformation of pre-existing rock by heat and/or pressure WITHOUT melting Metamorphism is derived from the Greek words meta, meaning change, and morphé, meaning form. During metamorphism, a rock changes form while remaining solid.

High Temperature and Pressure High temperature is ultimately derived from Earth’s internal heat, either through deep burial or from nearby igneous intrusions. High pressures come from deep burial or from compression during mountain building.

Metamorphic Textures Metamorphic rocks are classified into two textural groups: 1. Foliated Layers and bands of minerals High pressure during metamorphism causes minerals with flat or needlelike crystals to form with their long axes perpendicular to the pressure Increases with time and pressure (more time & pressure  more foliation) 2. Non-foliated Unlike foliated rocks, non-foliated rocks are composed mainly of minerals that form with blocky crystal shapes.

Foliation

Grades of Metamorphism Different combinations of temperature and pressure result in different grades of metamorphism Low Grade: Low temperatures and pressures and a particular suite of minerals Intermediate: between low- and high- grade metamorphism High Grade: High temperatures and pressures and a different suite of minerals

Grades of Metamorphism

Types of Metamorphism Regional Metamorphism When high temperature and pressure affect large regions of Earth’s crust Low to high grades of metamorphism Creates foliation Rocks are named for style of foliation Contact Metamorphism When molten material comes in contact with solid rock, a local effect of contact metamorphism occurs High temperature Moderate to low pressure

Regional Metamorphic Rocks Slate Platy minerals aligned on planes (microscopic) Splits along flat, nearly parallel planes Dull Sheen Low grade metamorphism **Often confused with shale. Shale makes a thud when hit on a table, slate makes a clink.

Regional Metamorphic Rocks Phyllite Finely crystalline, can have a bit of a shiny sheen Crinkly/Wavy layers Low to moderate metamorphic grade

Regional Metamorphic Rocks Schist Platy minerals are aligned on planes Crystals are larger with the grains aligned in parallel layers

Regional Metamorphic Rocks Gneiss (nice) Minerals separate into light and dark layers Coarsely crystalline High grade metamorphism

Contact Metamorphic Rocks Quartzite Metamorphosed Sandstone Composed of Quartz Sugary appearance

Contact Metamorphic Rocks Marble Metamorphosed Limestone Non-foliated Composed of calcium