Midterm: Metamorphism

Question 1

What are the three main drivers of metamorphism?

Answer 1

• Temperature
• Pressure
• Fluids

Question 2

How does temperature drive metamorphism?

Answer 2

Temperature increases with depth according to the geotherm, which varies with tectonic setting. Elevated temperature will break chemical bonds and alter the existing crystal structures of minerals. Atoms and ions will recrystallize, making new and often larger structures and minerals (e.g., clays to micas to garnets with increasing pressure/temperature).

Question 3

How does pressure drive metamorphism?

Answer 3

Pressure increases with depth about the same everywhere. Elevated pressure acts to change a rock’s chemical composition and mineralogy too but will also cause minerals to rotate such that they’re perpendicular to the principal stress direction (especially platy minerals like clays and micas), as well as recrystallize into larger crystals.

Question 4

How does fluid drive metamorphism?

Answer 4

Hydrothermal fluids carry dissolved CO2 and other substances (e.g., Ka, K, Cu, etc.). As these warm fluids percolate through rock, they accelerate metamorphism by modifying mineral compositions and sometimes completely replacing them. This is also called metasomatism.

Question 6

How are metamorphic rocks classified?

Answer 6

• Primarily by texture
• Secondarily by their composition

Question 8

What are metamorphic rocks divided between?

Answer 8

• Foliated
• Non-foliated
• Porphyroblastic

Question 9

What are foliated rocks like?

Answer 9

• If the parent is shale (sedimentary rock composed of mud/clay), the metamorphic sequence begins with slate and continues onward to gneiss with higher grade of metamorphism.
• If the parent is an igneous rock like granite, lower grades of metamorphism are resisted until the gneiss-stage is reached.
• Migmatites occur if the temperature becomes so high that the metamorphic rock begins to partially melt. These will often show ductile flow features!

Question 10

Non-foliated (granoblastic)

Answer 10

Classified by their composition. The parent rocks are composed of minerals that grow equant crystals (e.g., cubes), not platy shapes that can be oriented. Inferring a metamorphic grade can be difficult, as constituent minerals will still recrystallize and grow larger, generally larger crystals implies higher metamorphic grade. Some examples:
- Sandstones metamorphose into quartzite
- Carbonates (e.g., limestone) metamorphose into marble

Question 11

Porphyroblastic

Answer 11

Rocks with large mineral crystals set in a fine-grained matrix. As some metamorphic minerals are stable over a range of pressures and temperatures, they will continue to grow larger while the matrix recrystallizes with changing conditions. Common examples of these larger minerals are garnet and staurolite.