Igneous Rocks

Question 1

Igneous Rocks

Answer 1

Formed by minerals crystallising from a melt known as magma (or, when extruded at the Earth’s surface)
There are two types of igneous rock types, Intrusive/Plutonic and Extrusive/Volcanic.

Question 2

Intrusive/Plutonic

Answer 2

• Formed when magma solidifies deep underground.
• Includes granites, the main rock of continents.
• Intrusive/plutonic rocks cooled slowly deep within the Earth and are coarse-grained or phaneritic (most crystals > 1mm -bigger than 1mm)

Question 3

Extrusive/Volcanic

Answer 3

• Formed when magma solidifies at the Earth’s surface as lava
• Includes basalts, the main rock in the ocean floor
• Extrusive/volcanic rocks cooled quickly at or near Earth’s surface, giving crystals little time to grow. These rocks tend to be fine-grained or aphanitic (most crystals <1mm- smaller than 1mm)

Question 4

Name/describe 2 igneous rock textures (of igneous rock)

Answer 4

-Phaneritic (coarse) Texture
Large Crystals
Magma cooled & crystallised slowly

-Aphanitic
Small Crystals
Magma cooled & crystallised more quickly

Question 5

Extrusive rock examples

Answer 5

Volcanic (fine grained)

• Rhyolite (Acid- >65% Sio2)
• Andesite (Intermediate- 52-65% SiO2)
• Basalt (Basic- 45-52% SiO2)

Question 6

Intrusive rock examples

Answer 6

Hypabyssal (med. grained)

• Porphyry (Acid- >65% SiO2)
• Porphyrite (Intermediate- 52-65% SiO2)
• Dolerite (Basic- 45-52% SiO2)

Plutonic (coarse grained)

• Granite (Acid- >65% SiO2)
• Diorite (Intermediate- 52-65% SiO2)
• Gabbro (Basic- 45-52% SiO2)
• Peridotite (Ultra Basic- <45% SiO2)

Question 7

Sills and Dykes

Answer 7

(within the earth’s crust)
Sill- horizontal (or sub-horizontal) layer of solidified lava
Dyke- when injected vertically

Question 8

Identification Summary (examples)

Answer 8

Light coloured (Acid):
Coarse texture = Granite 
Medium texture = Micro-granite 
Fine texture = Rhyolite 

Dark Colour (Basic):
Coarse texture = Gabbro
Medium texture = Dolerite 
Fine texture = Basalt

Question 9

Felsic

Answer 9

• Light coloured minerals (quartz, alkali feldspar, and some plagioclase feldspar)
• Rich Si, Na, Al and K and poor in Fe and Mg
• Dark coloured minerals biotite and amphibole are present, but only in minor amounts

Example- Granite

Question 10

Intermediate

Answer 10

-Rocks contain roughly equal amounts of dark-coloured minerals and light-coloured minerals

Example- Andesite

Question 11

Mafic

Answer 11

• Abundant dark-coloured minerals (olivine, pyroxene, and plagioclase)
• These are compositionally rich in Fe, Mg and Ca.

Example- Basalt

Question 12

Ultramafic

Answer 12

• Fe and Mg-rich minerals from the mantle (olivine and pyroxene, but not plagioclase).
• Compositionally rich in Fe, Mg and Ca, but poor in Si

Example- Olivine