Lecture 4: Igneous Rocks

Question 1

What is an igneous rock?

Answer 1

It is a rock that solidified from molten material (magma)

Question 2

What are the three main types of rocks?

Answer 2

Igneous
Metamorphic
Sedimentary

Question 3

Where does the magma come from that forms igneous rocks?

Answer 3

A magma body is formed and is risen to the surface because it is less dense than its surrounding rocks

Question 4

What are the two main textures of igneous rocks?

Answer 4

Phaneritic (large crystals)

Aphanatic (small crystals)

Question 5

What are the properties of a phaneritic texture?

Answer 5

• large crystals
• slow cooling
• intrusive

Question 6

What are the properties of an aphanatic texture?

Answer 6

• much smaller crystals, tiny & smooth texture
• rapid cooling at the surface
• extrusive

Question 7

What is extrusive and intrusive?

Answer 7

Extrusive - rock was formed and cooled slowly deep in the ground
Intrusive- rock was formed and cooled rapidly at the surface

Question 8

If a rock is Felsic, what does that mean? Give 3 examples.

Answer 8

• light coloured
• quartz, muscovite mica and potassium feldspar
• formed at a low temperature

Question 9

What does it mean if a rock is intermediate ?

Give 2 examples.

Answer 9

Equal parts of light and dark colours

- ex. Biotite Mica and Sodium Feldspar

Question 10

What does it mean if a rock is mafic? Give 3 examples.

Answer 10

• dark coloured
• olivine, pyroxene, amphibole and calcium feldspar
• formed at a high temperature

Question 11

Order of minerals on Bowens reaction series (top to bottom)

State:
- names of minerals in group
-

Answer 11

Olivine, pyroxene, amphibole (mafic, crystallizes at a high temperature)

Biotite mica, k-feldspar, muscovite mica, quartz (felsic, low temp)

(Up to down): Ca-rich plagioclase, Na-rich plagioclase

Question 12

What are most igneous rocks made of?

Answer 12

Silicate minerals, which have an abundance of silica (SiO4)

Question 13

Why are rocks important?

Answer 13

• contains minerals
• serves as aquifers (natural storage for groundwater)
• contains almost all of the worlds petroleum
• contains the worlds mineral resources (metals mainly)

Question 14

What is the rock cycle?

Answer 14

It's a fundamental concept that depicts the changes that take place on and within earth to transform one member of a rock class to another. 
- Earth's material is constantly being recycled.

Question 15

What characteristics of rocks be used to reconstruct its history?

Answer 15

• its composition (mineral content, fossil content, organic content)
• its arrangement of crystals (arrangement of atoms)
• geological structures within the rock (flow features, cooling features)

Question 16

True or false: Magma can either rise and extrude at earth’s surface (via volcanos) or it can be trapped below earth’s surface.

Answer 16

True:
Intrusive - magma cooled at earth’s interior
Extrusive - magma cooled at earth’s exterior

Question 17

If rocks have a higher content of ferromagnesian silicates, what does that imply?

Answer 17

It implies that the rock is dark coloured (mafic).

Magma formed and cooled at high temperatures tends to be enriched in ferromagnesian silicate minerals.

Question 18

If rocks have a lower content of ferromagnesian silicates, what does that imply?

Answer 18

It implies that the rock was cooled at low temperatures and is celtic (light coloured)

Question 19

Explain how glassy rocks obtained that specific texture.

Answer 19

Lava was immediately cooled at surface, there was no time for the crystals to grow.

Question 20

Name 3 common minerals of igneous rocks and give examples (bowens reaction series)

Answer 20

• Ferromagnesian silicates (Magnesium and Iron ions bonded with silica units)
  ex. Olivine, pyroxene, amphibole, biotite, muscovite mica)
• Feldspars (non-ferromagnesian silicates)
  ex. K-feldspar, Na-feldspar, Ca-feldspar
• Quartz (only consists of silica)
  ex. quartz