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Metamorphic Rocks Metamorphism Metamorphic rocks form from igneous or sedimentary rocks which have been exposed to heat and/or pressure. Metamorphism occurs when heat and pressure break some of the bonds in the minerals, but not enough to melt the rock. There are 3 main agents that cause metamorphism: heat, pressure, and circulating fluids. Heat Metamorphism triggered by a change in temperature is caused when: A rock moves to a greater depth (and hence, warmer temps) by tectonic movement or deep burial under sedimentary deposits, or Magma rises up intrusively and “bakes” the surrounding rock Magma creates what is known as contact metamorphism Pressure Metamorphism is triggered by a change in pressure, caused when: A rock is deeply buried under sedimentary deposits, or Tectonic plates slide or collide, exposing rocks at the plate boundaries to extreme pressure This can create mountains and is known as regional metamorphism Pressure There are two kinds of pressure: Confining pressure pushes on rocks equally from all directions, compressing them and reducing their size, or Directed pressure pushes on rocks greater in one direction than the other. This tends to bend or stretch the rock and align its minerals in the direction of the pressure. This alignment is called foliation. Confining Pressure Circulating Fluids Fluids (usually water) circulate underground and drive metamorphism by either speeding up the movement of a rock’s own elements or by adding or removing ions to change the overall composition of the rock. Limestone is a sedimentary rock. When exposed to heat and pressure, limestone metamorphoses into marble. Shale is a sedimentary rock. When exposed to heat and pressure, shale metamorphoses into slate. Sandstone is a sedimentary rock. When exposed to heat and pressure, sandstone metamorphoses into quartzite. Coal is a sedimentary rock. When exposed to heat and pressure, coal metamorphoses into anthracite. Granite is an igneous rock. When exposed to heat and pressure, granite metamorphoses into gneiss. Talc is a silicate mineral When exposed to heat and pressure, talc minerals metamorphoses into soapstone. Basalt is an igneous rock. When exposed to heat and pressure, basalt metamorphoses into schist. 3 Main Metamorphic Environments High temperature/low pressure Low temperature/high pressure High temperature/high pressure