Igneous Rocks

Question 1

Describe ‘extrusive’ and ‘intrusive’ igneous rocks.

Answer 1

Extrusive: rocks formed when magma solidifies at the surface. Can be explosive or flowing.

Intrusive: rocks formed at depth when magma crystallizes slowly. If large enough, they’re called ‘plutonic’ (plutons).

Question 2

What are some intrusive igneous features?

Answer 2

Batholiths: regional scale intrusive body from main mantle plum.

Laccoliths: local intrusive body with other rocks below (fed by dykes).

Dykes: look like giant fences; lava has forced its way into cracks and tunnels and solidified.

Others include: sills, volcanic necks

Question 3

What are the three distinct components of magma?

Answer 3

Melt: liquid portion of mobile ions

Solids: silica materials crystallized from melt

Volatiles: gases like H2O, CO2, SO2 (make magma lighter so it rises)

Question 4

Melting of rocks begins if:

Answer 4

Confining pressure drops slightly.
Temperature increases slightly.
Volatiles are introduced.

Question 5

What is peridotite?

Answer 5

Iron and magnesium rich silicate minerals. They form from mafic (basaltic) magma.

Question 6

Describe and compare mafic and felsic magma/rocks.

Answer 6

Mafic (basaltic): DARK. Caused by direct melting of the mantle (usually from ocean ridges). Ferromagnetic silicates. (pyroxene, olivine, amphibole, basalt, gabbro, Ca-Plagioclase Feldspar).

Felsic…