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A-Level Geology (1st Year) > Igneous Rocks > Flashcards

Igneous Rocks Flashcards

1
Q

What is an igneous rock?

A

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

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2
Q

What is an intrusive igneous rock?

A

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

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3
Q

What is an extrusive igneous rock?

A

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

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4
Q

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

A

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

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5
Q

What are phenocrysts?

A

Large crystals surrounded by smaller matrix

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6
Q

How are phenocrysts formed?

A

Two stage cooling. First cools slower.
Porphyritic textures

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7
Q

What is felsic?

A

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

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8
Q

What is the most common mineral on earth?

A

Feldspar

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9
Q

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

A

Only present in silicic and intermediate rocks

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10
Q

What is mafic?

A

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

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11
Q

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

A

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

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12
Q

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

A

Type of pyroxene
Found in mafic and ultramafic rocks

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13
Q

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

A

Sometimes in mafic, definitely in ultramafic

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14
Q

What is silica?

A

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

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15
Q

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

A

> 66%

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16
Q

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

A

52-66%

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17
Q

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

A

45-52%

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18
Q

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

A

<45%

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19
Q

How does the silica percentage relate to colour?

A

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

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20
Q

How does the silica percentage relate to colour?

A

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

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21
Q

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

A

Obsidian

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22
Q

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

A

Rhyolite
Pumice

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23
Q

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

A

Microgranite

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24
Q

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

A

Granite

25
Q

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

A

Andesite

26
Q

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

A

Microdiorite

27
Q

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

A

Diorite

28
Q

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

A

Scoria

29
Q

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

A

Basalt

30
Q

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

A

Dolerite

31
Q

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

A

Gabbro

32
Q

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

A

Peridotite

33
Q

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

A

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

34
Q

What is flow banding?

A

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

35
Q

What is conchoidal?

A

A fracture which results in a curved surface

36
Q

What is vesicular?

A

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

37
Q

What is porphyritic?

A

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

38
Q

What is equigranular?

A

When crystals in an igneous rock are approximately the same size

39
Q

What is amygdaloidal?

A

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

40
Q

What are ophiolites?

A

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

41
Q

What are the formation and characteristics of silicic magmas?

A

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

42
Q

What are the formation and characteristics of intermediate rocks?

A

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

43
Q

What are the formation and characteristics of mafic magmas?

A

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

44
Q

What are the formation and characteristics of ultramafic magmas?

A

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

45
Q

What determines crystal grain size?

A

Rate of cooling. Volume of magma

46
Q

What is the formation of glassy texture?

A

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

47
Q

What is the formation of fine texture?

A

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

48
Q

What is the formation of medium texture?

A

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

49
Q

What is the formation of coarse texture?

A

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

50
Q

What is texture?

A

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

51
Q

What is nucleation?

A

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

52
Q

What is euhedral and how is it formed?

A

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

53
Q

What is subhedral?

A

Some well formed faces, some pooly formed faces

54
Q

What is anhedral?

A

Poorly formed crystal faces

55
Q

What is pegmatite?

A

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

56
Q

What is hypabyssal?

A

When igneous rocks form at relatively shallow depths below the surface

57
Q

What is equant?

A

Crystals have all axes the same length (cubey)

58
Q

What is prismatic?

A

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

A-Level Geology (1st Year) (11 decks)

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