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Metamorphic Rocks Chapter 3, Section 3.4

Looking Back Rock Cycle Sedimentary Rocks Igneous Rocks Test

Let’s Review What 4 major processes form sedimentary rock: WEATHERING, wind/water/ice/gravity deposit sediments, compaction, cementation How do igneous rocks form? Igneous rocks form when magma or lava cools and hardens. What is the difference between Intrusive and extrusive igneous rock? Intrusive forms under the surface, extrusive forms on or at the surface.

Next Test sometime during the week of 9/23 Looking Ahead Monday we start Earth’s Resources Chapter 3 Quiz on Tuesday!! Late Work Policy Next Test sometime during the week of 9/23

Introduction Metamorphic rock forms when existing rock is changed by heat and pressure. Metamorphism is a very appropriate name for this because it means “to change form”. Rocks often look different than they originally did after metamorphism. Folds, different compositions.

Formation of Metamorphic Rock Most metamorphic changes occur at elevated temperatures and pressures. These conditions are found a few km below the earth’s surface and extend into the upper mantle. Occurs in one of two settings!

Types of Metamorphism – Contact Metamorphism When magma intrudes (forces its way into) rock, contact metamorphism takes place. What happens when you put cheese on a hot sandwich? During contact metamorphism, hot magma moves into rock. Produces what is called ‘low-grade metamorphism’. Changes in rocks are minor Marble is a common contact metamorphic rock Formed when magma intrudes into limestone.

Types of Metamorphism – Regional Metamorphism During the building of mountains, large areas of rock are put under high heat and pressure. The plates are pushing past one another; similar to how friction will burn you! The changes in rock from this kind of metamorphism produces BIG changes and is called “Regional Metamorphism” Regional Metamorphism results in large-scale deformation and high-grade metamorphism.

Agents of Metamorphism

Heat The most important component of metamorphism. Provides the energy needed to drive chemical reactions that cause minerals to crystallize. Heat for metamorphism comes from two sources: Magma Change of Temperature with depth Magma will “bake” any rock it comes into contact with Heat increases with depth; about every km down you go in the crust, the temperature increases 20-30 degrees Celsius.

Heat When buried at a depth of about 8km, clay minerals are exposed to heats of about 150 to 200 degrees C, which is 300-400 degrees F. Minerals become unstable and recrystallize to form new minerals that ARE stable at that temperature. But silicates (quartz, etc) are stable at that temperature, so it takes a lot more to deform them.

Pressure (Stress) Pressure, like temperature, also increases with depth. You have the entire atmosphere PLUS earth above you pushing down! Pressure on rocks causes the spaces between minerals to close, and you get a more COMPACT rock with a bigger density. Increases in pressure make rocks FLOW instead of fracture! Minerals tend to flatten and elongate. This is why mountains tend to have folded rocks! Pushing a stack of papers

Classification of Metamorphic Rock

Introduction How were other rocks classified? Texture and Composition Metamorphic rocks are also classified by texture and composition! Texture can be FOLIATED or NONFOLIATED.

Foliated Metamorphic Rocks When rocks undergo contact metamorphism, they become compact and dense. Slate is a good example; slate forms when shale is put under temperatures and pressures only a little bit higher than normal. Pressure means clay minerals become more compact and the pressure increase makes the clay minerals align in the SAME DIRECTION. Under more extreme conditions, certain minerals recrystallize in a preferred orientation Right angles to the direction of force. Gives it a ‘banded’ appearance Called a Foliated Rock.

Foliation Process

Nonfoliated Metamorphic Rocks A metamorphic rock that does not have a banded texture is called a nonfoliated metamorphic rock. Most of these only contain one mineral. Marble is nonfoliated rock made from calcite. When parent rock is changed, the crystals combine to form larger interlocking crystals.

Let’s Review Metamorphic rocks form…when? When existing rocks are changed by heat and pressure. What is the difference between contact metamorphism and regional metamorphism? Contact is when hot magma intrudes, regional is during mountain-building. What are the two classifications of metamorphic rock? Foliated and Nonfoliated.

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