Igneous Rocks

Question 1

What is an igneous rock?

Answer 1

A rock that is formed from the cooling and crystallising of magma or lava

Question 2

What is an intrusive igneous rock?

Answer 2

A rock that forms from magma that has cooled within the earth’s crust

Question 3

What is an extrusive igneous rock?

Answer 3

A rock that forms from magma that has reached the earth’s surface and has cooled from lava

Question 4

What are the 4 categories of crystal size and what sizes are the crystals?

Answer 4

Glassy - no present minerals
Fine (aphanitic) - <1mm crystals
Medium - 1-5mm. Seen with naked eye
Coarse (phaneritic) - >5mm. Easily seen with naked eye

Question 5

What are phenocrysts?

Answer 5

Large crystals surrounded by smaller matrix

Question 6

How are phenocrysts formed?

Answer 6

Two stage cooling. First cools slower.
Porphyritic textures

Question 7

What is felsic?

Answer 7

A.k.a silicic
Felsic –> feldspar + silica

Question 8

What is the most common mineral on earth?

Answer 8

Feldspar

Question 9

What can be said about the appearance of quartz and feldspar in igneous rocks?

Answer 9

Only present in silicic and intermediate rocks

Question 10

What is mafic?

Answer 10

Contain magnesium and iron
Dark and dense
Mafic —> Magnesium + ferric (iron)

Question 11

What can be said about the appearance of hornblende in igneous rocks?

Answer 11

Mostly found in intermediate rocks.
Often confused with augite (a.k.a hard to identify)

Question 12

What can be said about the appearance of augite in igneous rocks?

Answer 12

Type of pyroxene
Found in mafic and ultramafic rocks

Question 13

What can be said about the appearance of olivine in igneous rocks?

Answer 13

Sometimes in mafic, definitely in ultramafic

Question 14

What is silica?

Answer 14

SiO²
Often described quartz or sand grains.
% silica in a rock includes the quartz (SiO²) and any other silicon and oxygen present

Question 15

What is the silica content required for a rock to be identified as silicic/felsic?

Answer 15

> 66%

Question 16

What is the silica content required for a rock to be identified as intermediate?

Answer 16

52-66%

Question 17

What is the silica content required for a rock to be identified as mafic?

Answer 17

45-52%

Question 18

What is the silica content required for a rock to be identified as ultramafic?

Answer 18

<45%

Question 19

How does the silica percentage relate to colour?

Answer 19

The more silica, the lighter the colour
(Except obsidian)

Question 20

How does the silica percentage relate to colour?

Answer 20

The more silica, the lighter the colour
(Except obsidian)

Question 21

In the igneous rock classification table, what rock can be found in ‘silicic’ ‘glassy’?

Answer 21

Obsidian

Question 22

In the igneous rock classification table, what rock can be found in ‘silicic’ ‘fine’?

Answer 22

Rhyolite
Pumice

Question 23

In the igneous rock classification table, what rock can be found in ‘silicic’ ‘medium’?

Answer 23

Microgranite

Question 24

In the igneous rock classification table, what rock can be found in ‘silicic’ ‘coarse’?

Answer 24

Granite

Question 25

In the igneous rock classification table, what rock can be found in 'intermediate' 'fine'?

Answer 25

Andesite

Question 26

In the igneous rock classification table, what rock can be found in 'intermediate' 'medium'?

Answer 26

Microdiorite

Question 27

In the igneous rock classification table, what rock can be found in 'intermediate' 'coarse'?

Answer 27

Diorite

Question 28

In the igneous rock classification table, what rock can be found in 'mafic' 'glassy'?

Answer 28

Scoria

Question 29

In the igneous rock classification table, what rock can be found in 'mafic' 'fine'?

Answer 29

Basalt

Question 30

In the igneous rock classification table, what rock can be found in 'mafic' 'medium?

Answer 30

Dolerite

Question 31

In the igneous rock classification table, what rock can be found in 'mafic' 'coarse'?

Answer 31

Gabbro

Question 32

In the igneous rock classification table, what rock can be found in 'ultramafic' 'coarse'?

Answer 32

Peridotite

Question 33

Which crystal sizes (G,F,M,C) can be found in which intrusion type (E,I)

Answer 33

Glassy & Fine are extrusive. Medium & coarse are intrusive.

Question 34

What is flow banding?

Answer 34

Formed by friction as the magma or lava slows down near an interface, aligning the minerals as it moves

Question 35

What is conchoidal?

Answer 35

A fracture which results in a curved surface

Question 36

What is vesicular?

Answer 36

Textural term for a rock containing vesicles. Vesicles were bubbles of gas which came out of solution as a result of pressure release

Question 37

What is porphyritic?

Answer 37

A texture where large crystals, called phenocrysts, are completely surrounded by smaller crystals

Question 38

What is equigranular?

Answer 38

When crystals in an igneous rock are approximately the same size

Question 39

What is amygdaloidal?

Answer 39

Where there are large vesicles which have been filled with a secondary mineral

Question 40

What are ophiolites?

Answer 40

Sections of the earth's oceanic cryst that have been tectonically moved (obducted) onto continental crust

Question 41

What are the formation and characteristics of silicic magmas?

Answer 41

>66% silica Generated by the melting of continental crust Magma is viscous Form at lower temperatures than other magmas (600-900°C)

Question 42

What are the formation and characteristics of intermediate rocks?

Answer 42

52-66% silica Most common is andesite Less viscous than silicic, but still a little

Question 43

What are the formation and characteristics of mafic magmas?

Answer 43

Contain pyroxene and sometimes olivine There will be no quartz 52-45% silica Low viscosity Starts solidifying at around 1000°C

Question 44

What are the formation and characteristics of ultramafic magmas?

Answer 44

<45% silica Pyroxenes and some olivine Extremely high melting points Peridotites

Question 45

What determines crystal grain size?

Answer 45

Rate of cooling. Volume of magma

Question 46

What is the formation of glassy texture?

Answer 46

Lava extruded onto the surface. Cools too quickly - no crystals. Forms volcanic glass. Cooling occurs in hours

Question 47

What is the formation of fine texture?

Answer 47

Hard to see with naked eye. Lava extruded onto the surface. Cooling time usually takes weeks, sometimes months. Can occur in intrusive settings, but only at chilled margins

Question 48

What is the formation of medium texture?

Answer 48

Easily seen with naked eye. Intrusive. Form in minor intrusions. A thousand years cooling required for these rocks

Question 49

What is the formation of coarse texture?

Answer 49

Major intrusions - batholithes and plutons. Millions of years for cooling

Question 50

What is texture?

Answer 50

General character or appearance of a rock shown by arrangement of minerals with crystal shape and size. It does NOT refer to the roughness or smoothness of the surface

Question 51

What is nucleation?

Answer 51

The growth of crystals from a magma building onto another piece of crystal growth

Question 52

What is euhedral and how is it formed?

Answer 52

Crystals that are well formed with crystals faces. Form at depth slowly, grew unimpacted

Question 53

What is subhedral?

Answer 53

Some well formed faces, some pooly formed faces

Question 54

What is anhedral?

Answer 54

Poorly formed crystal faces

Question 55

What is pegmatite?

Answer 55

Common plutonic rock Coarse crystals. Interlocking crystals. Form from waters that separate from a magma in the late stages of crystallisation. From slow cooling of a low viscosity fluid phase rich in water

Question 56

What is hypabyssal?

Answer 56

When igneous rocks form at relatively shallow depths below the surface

Question 57

What is equant?

Answer 57

Crystals have all axes the same length (cubey)

Question 58

What is prismatic?

Answer 58

Crystals have four or more sides but are elongated in one direction