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OCR A LEVEL GEOLOGY > Igneous rocks > Flashcards

Igneous rocks Flashcards

1
Q

What are the 8 igneous rock forming minerals in igneous rocks?

A
  1. Olivine
  2. Andesite
  3. Hornblende
  4. Biotite Mica
  5. Plagioclase feldspar
  6. K feldspar
  7. Muscovite mica
  8. Quartz

OLIVES
AND
HAM
BUT
PLEASE
KILL
ME
QUICK

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2
Q

Name 5 silicic igneous rocks that have a crystal size below 1mm

A

Pumice
Obsidian
Rhyolite
Ash/tuff
Agglomerate

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3
Q

Name a siliic igneous rock that has a crystal size between 1-5 mm

A

Microgranite

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4
Q

Name 3 silicic igneous rocks that have a crystal size greater than 5mm

A

Granite
Pegmatite
Granodiorite

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5
Q

Name 4 intermediate igneous rocks

A

Andesite (<1mm crystal size)
Microdiorite (1-5mm crystal size)
Diorite and Granodiorite (>5mm)

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6
Q

Name 4 MAFIC igneous rocks

A

Basalt and Scorcia (<1mm)
Dolerite (1-5mm)
Gabbro (>5mm)

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7
Q

Name an Ultramafic igneous rock

A

Peridotite (>5mm)

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8
Q

What is the composition of silicic igneous rocks?

A

25% quartz
45% K Feldspar
20% Plagioclase feldspar
9% Biotite Mica
1% Other

USE SIMU GRAPH

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9
Q

What is the composition of intermediate rocks?

A

2% Quartz
35% Plagioclase Feldspar
4% Biotite Mica
20% Hornblende
1% Other

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10
Q

What is the composition of MAFIC rocks?

A

40% Plagioclase Feldspar
59% Augite
1% Other

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11
Q

What is the composition of Ultramafic rocks?

A

10% Augite
89% Olivine
1% Other

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12
Q

What is percentage of silica in Silicic, Intermediate, Mafic and Ultramafic rocks?

A

Silicic = >66%
Intermediate = 66-52%
Mafic = 52-45%
Ultramafic = <45%

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13
Q

What is the colour index for Silicic, Intermediate, Mafic, Ultramafic rocks?

A

Silicic = 10
Intermediate = 25
Mafic = 60
Ultramafic = 100

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14
Q

Where do rocks with crystal sizes greater than 5mm cool?

A

Batholiths

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15
Q

Where do rocks with crystal sizes between 1 and 5 cool?

A

Sills, Dykes and laccoliths

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16
Q

Where do rocks with crystal sizes less than 1mm cool?

A

Lava flows
Bombs
Ash
On the surface

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17
Q

What is special about Granodiorite

A

Doesn’t contain K feldspar
Both can be silicic and intermediate

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18
Q

What is special about pegmatites?

A

They cooled quickly however contain large crystals. This Is because they were formed from late-stage hydrothermal fluids after most of the intrusion had crystallised.

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19
Q

What are vesicles?

A

Gas bubbles found in cooling magma/lava which tend to rise to the top creating bubbles when the magma/lava cools unless it is too viscous.

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20
Q

What is flow banding?

A

Streaks that remain in a cooled magama/lava flow when it was really viscous eg obsidian and ryholite?

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21
Q

What are the 7 igneous textures?

A

Glassy
Equicrystalline
Amygdalodial
Flow banding
Ophitic
Cumulate
Porphyritic

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22
Q

What is glassy texture?

A

Means that there are no crystals due to rapid cooling of magma/lava.

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23
Q

What is Equicrystalline (aka equigranular texture)

A

All the crystals in the rock are the same size.

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24
Q

What is amygdaloidal texture?

A

Where vesicles have been filled in with other minerals, usually from minerals dissolved in water then evaporated.

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25
Q

What is Porphyritic texture?

A

Where rocks have two stages of cooling resulting in two distinct crystal sizes. The large crystals are called phenocrysts and form first. They are cooled slowly and surrounded by a finer ground mass which cooled more quickly.

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26
Q

What is ophitic texture?

A

Forms when an elongate crystal is enclosed by another mineral. Common in dolerite and gabbro where Plagioclase is enclosed by augite.

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27
Q

What is cumulate texture?

A

When crystals settle out of magma typically on the floor/roof/walls of a magma chamber and accumulate. They grow after they settle

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28
Q

What is a batholith and a pluton?

A

Batholith - 10-100km thick intrusion of magma underground
Pluton - 1-10km thick intrusion of magma underground

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29
Q

What are sills and dykes?

A

Sills are horizontal tubes which magma flows through parallel between layers
Dykes are vertical tubes which magma rises through perpendicular to the layers.

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30
Q

What is a xenolith? And how are the assimilated?

A

Xenoliths are rocks that are not derived from the original. They form when they are assimilated (Melting rock becomes part of the magma). They melt due to the country rock being cut by sills and dykes (this is called stoping).

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31
Q

What is the geothermal gradient?

A

Is the change in temperature with increasing depth towards the centre of the Earth.

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32
Q

What are chilled margins?

A

This is the area where the cooler country rock surrounding the magma, cools it faster than magma that is not in contact with the country rock so will have different crystal sizes.

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33
Q

What are baked margins?

A

The area where the country rock has been heated when it has been in contact with magma.

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34
Q

What is magma like at fold mountains (Convergent plate margin, continental - continental)

A

Silicic magma, viscous so doesn’t rise from volcanos.
Forms granite batholiths
Partial melting of silicic continental crust below 35km due to increasing temperature >800 degrees
Felsic minerals melt first due to lower melting points

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35
Q

What is magma like a cordilleras (Oceanic - Continental, convergent plate margin)

A

Partial melting of subducted mafic oceanic crust (Flux melting due to water)
Contaminated with oceanic sediments
Batholiths of intermediate diorite/granodiorite
Violent volcano
Same magma reaches the surface heavily contaminated by crust to form andesitic volcanos.

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36
Q

What is magma like at an MOR (Oceanic - Oceanic, Divergent plate margin)

A

5% partial melt of peridotite in the mantle due to less pressure
Mafic magma rises to form gabbro, diorite dykes and basalt pillow lavas on the ocean floor

37
Q

What is magma like at hot spots

A

Mafic magma rises from the hot spot, erupting basalt lava
If the plate moves, the volcano will become extinct and a new one will from over the plume creating an island arc

38
Q

What is a concordant sill/dyke ?

A

Intrusions run parallel to the existing beds

39
Q

What is a disconcordant sill/dyke?

A

Intrusions cut across existing beds

40
Q

How do ring dykes form?

A

Form when blocks of country rocks above an intrusion sinks downwards as the magma chamber empties creating circular faults which fill with more magma.

41
Q

Explain how cone sheets form

A

Form when blocks of country rock above an intrusion are pushed upwards by pressure of the magma. This creates circular faults that fill with magma. Dykes will dip toward the source.

42
Q

How can we use the dips of dykes to tell apart cone sheets and ring dykes?

A

///// \\ (outward dipping) = Ring dykes
\\ //// (inward dipping) = Cone sheets

43
Q

What factors determine how viscous a magma is?

A

Temperature = hotter, less viscous
Composition = more silica, more viscous

44
Q

What factors determine the density of magmas?

A

Temperature - higher temps cause magma to expand causing the density to decrease (More buoyant)
Pressure - Higher pressure causes magma to compress which increases its density (Less buoyant)
Composition - Mafic magma is rich in iron so will make it more dense

45
Q

Why may magma not mix together very well?

A

Due to compositions (Chemical reactions)
Due to viscosity / density differences

46
Q

What might happen if different compositions of magma are forced to mix?

A

May trigger the sudden release of volatiles such as water and co2. This causes pressure to rise and can cause an eruption

47
Q

What creates hot springs?

A

Magma/hot country rocks heating groundwater.

48
Q

How do geysers erupt?

A

1- Water is heated over 100 degrees but cant boil due to confining pressure
2- Some water rises, reducing the pressure causing flash boiling (Water —> steam)
3- Steam expands and pushes out any remaining water
4- Water drains back into the ground, recharging the system.

49
Q

What are 5 ways to locate a magma chamber?

A

Seismic surveys - P waves slow in magma, S waves stop
Ground movement - Swelling shows magma is moving upwards. Tiltmeters + gps can measure this change
Gas emissions - gas escapes when magma nears the surface
Changes in ground water - Increases gas pressure causes water levels to rise
Dyke orientation - Radial dykes tend to form around a pluton

50
Q

What are 6 ways to distinguish sills from lava flows?

A

Sills will have two chilled margins (Above + Below), lava flows only have one (Below)
Sills have two bakes margins (Above and Below), lava flows have only one below.
Sills have xenoliths above and below, lava flows sometimes have some below
Vesicles are rare in sills, common in lava flows
Sills have medium crystals, lava flows have fine crystals
Sills may show differentiation of magma if the sill is thick and cools slowly, lava flows may have flow banding.

51
Q

What are “aa” and “pahoehoe” lava flows?

A

Aa - Lava flows with a blocky, jagged surface that is more viscous
Pahoehoe- flows with a smooth/ropy surface that is less viscous

52
Q

What are pyroclasts?

A

Fragments erupted during an eruption. Pyroclastic describes all the fragmented materials formed by explosive eruptions including bombs, blocks, lapilli and ash.

53
Q

Define Block, Bomb and Agglomerate

A

Block - Hard rock thrown from a volcano
Bomb - Lava thrown from a volcano
Agglomerate - Rock made from blocks and bombs

54
Q

Define Lapilli, Lapilli Tuff, Ash and Tuff

A

Lapilli - Particles 2-64mm in diameter that are ejected out of a volcano
Lapilli tuff - Rock made from lapilli
Ash - Particles < 2mm in diameter that are ejected out of a volcano
Tuff - Rock made from ash

55
Q

What are pyroclastic flows?

A

Hot mixture of pyroclastic material and gas

56
Q

What is a nuee ardente?

A

Is a type of pyroclastic flow of gaseous magma droplets and ash. When cooled and solidified the deposit is called ignimbrite.

57
Q

What is Stokes law?

A

A way of calculating viscosity

58
Q

What are shield volcanos?

A

A volcano found on oceanic crust/hot spots, producing mafic magma during fissure eruptions. Divergent plate margins (Common and frequent eruptions that are non explosive). Found in Iceland

59
Q

What is a strato-volcano (Composite volcano)

A

Huge but rare volcanos along destructive plate margins that produce silicic/intermediate magma. Pyroclastic flows are common but the volcanos set of in large intervals.

60
Q

If a volcano is dormant, what is happening?

A

Could be because a magma chamber is recharging, which builds up pressure. Once the pressure exceeds the mass of the overlying rock, it explodes.

61
Q

What is a caldera?

A

A large volcanic crater that has undergone collapse due to the magma chamber being emptied or the top of the volcano is blown off.

62
Q

What are 4 types of eruptions?

A

Hawaiian - Lots of fluid, basaltic magma, few pyroclastic flows/ material
Strambolian - More explosive, less fluidity, basaltic + andesitic lava, some pyroclastic material + explosive.
Vulcanian - Violent and viscous andesitic lava with large pyroclastic material.
Pinian - Extremely explosive, viscous gas-filled andesitic + ryholitic lava, lots of pyroclastic material blasted out

63
Q

What is the VEI?

A

Measures the relative size/magnitude of explosive eruptions 0-8 index of increasing explosivity.

64
Q

How can we measure the thickness and extent of a pyroclastic deposit?

A

Using isopachyte maps, These tell us where the majority (Thickness) of the material is and can show us the wind direction at the time of the eruption. Is determined by grain size/wind speed/direction and energy of blast.

65
Q

Why does Hekla (Volcano in Iceland) erupt intermediate lava when it should erupt only mafic lava?

A

The longer the time between erupting, the more acidic the magma is. The longer time between eruptions causes mafic minerals to crystallise at the bottom of the magma chamber, and silicic minerals at the top, making the magma more silicic.

66
Q

Why is magma more silicic (andesitic) at oceanic - continental convergent plate margins when oceanic crust is mafic?

A

Contamination of the magma due to the granitic (silicic) crust.

67
Q

What is the order in which igneous minerals cool?

A
  1. Olivine
  2. Augite, Calcium Feldspar
  3. Hornblende, Plagioclase feldspar
  4. Biotite Mica
  5. Sodium Feldspar, K feldspar
  6. Muscovite Mica
  7. Quartz
68
Q

What does Bowens reaction series show?

A

Describes the trend in which minerals cool
Also shows the increasing stability of minerals

69
Q

What minerals are in the discontinuous series of the Bowen reaction series?

A

Olivine
Augite
Hornblende
Biotite Mica

70
Q

What minerals are in the continuous series of the Bowen reaction series?

A

Calcium Feldspar
Plagioclase feldspar
Sodium feldspar

70
Q

What happens to minerals if there is a large difference between temperature of crystallisation and the temperature of the mineral is later on?

A

The mineral will become unstable

70
Q

What is the discontinuous series in the Bowens reaction series?

A

Shows the crystallisation of minerals that are rich in iron and magnesium with silica (mafic)

71
Q

How does zoned plagioclase form?

A
  1. Early formed plagioclase is calcium rich
  2. Later plagioclase will be increasingly Sodium rich and Calcium poor
  3. This means that as the crystal grows its composition changes from increasingly calcium rich in the centre and sodium rich on the outside due to the cooling changing.
72
Q

Why may a crystal have the same shape of Olivine but be different composition?

A
  1. The minerals in the discontinuous reaction series react with silica in the surrounding liquid magma
  2. A mineral can get altered and become the next mineral in the series
  3. This takes time so is only seen in plutonic rocks (volcanic/extrusive rocks cool too fast for this effect to happen

The crystal will have centre of olivine but reaction rims of augite where olivine has been altered by silica in the magma

73
Q

What is magmatic differentiation?

A

Different magmas evolving from a parent magma

74
Q

What are three ways that magmatic differentiation occurs?

A

Fractional crystallisation
Gravity settling
Filter pressing

75
Q

How do magmas usually mix?

A

Magmas of different compositions cannot usually mix but often they cannot (they are immiscible) but usually as country rocks are assimilated can alter the magma.

76
Q

What causes MAFIC magma to form at some places but silicic magma forming at other places?

A

Mafic magma usually forms where partial melting of the upper mantle occurs
Silicic magma is created when continental crust melts

77
Q

What is fractional crystallisation?

A

Means that a fraction/proportion of the magma crystallises at a certain temperature, taking the elements out of the magma to make certain minerals ei magnesium, iron and calcium will be used up at a high temperature to build olivine, augite and calcium rich plagioclase.

78
Q

What is gravity settling?

A

Early formed dense minerals may sink through the magma to accumulate in layers (process = cumulate layering) but usually doesn’t happen in silicic magmas because the liquid is too viscous.

79
Q

What is filter pressing?

A

The early-formed crystals build up a cumulate layer at the bottom of the magma. By their weight alone, they squeeze out the liquid from between the crystal. This liquid is poor in the elements needed to make the crystals but creates a layer above the cumulate layer of a different composition.

80
Q

What is an example of an intrusion where cumulate layering has occurred

A

The Skaergaard intrusion

81
Q

What is the palisades sill?

A
  1. Sill intruded, causes contact metamorphism of sandstone above and below to form metaquartzite
  2. Rapid cooling at top and base of the sill to form chilled margins of basalt
  3. Fractional crystallisation: Olivine crystallises, is denser so sinks through the low-viscosity magma (gravity settling) to create a cumulate layer.
  4. Remaining magma is depleted in iron, magnesium and so cools to form dolerite and gabbro (but higher plagioclase % than would be typical)
82
Q

What is reaction rim?

A

One mineral surrounding another, as a result of a reaction between the inner mineral with the melt, to form the outer mineral

83
Q

What is the eutectic system?

A

Crystallisation occurs simultaneously in both the discontinuous and continuous series, however the minerals crystallise in each are immiscible and from two solid phases, this is known as the Eutectic system and can be explained by the eutectic phase diagram.

84
Q

Explain why mafic rocks contain more calcium than sodium

A

Because calcium cools first in mafic rocks at higher temperatures

85
Q

What elements are in the platinum group?

A

Platinum
Palladium
Iridium
Osmium
Rhodium
Ruthenium

86
Q

How do magmatic sulphide deposits form

A
  1. Form during the fractional crystallisation process when mafic magma becomes saturated in sulphide.
  2. At this point the magma can no longer hold sulphur in solution
  3. Droplets of the immiscible sulphur are then fall as they are denser
  4. Platinum group elements usually bond with iron, as they are siderophiles but as there isn’t much molten iron in the magma outside the outer core, the behave like chalcophiles (ore-loving) and bond with sulphur
  5. They therefore gather up by the sinking sulphide droplets and removed from the magma to create layers.

OCR A LEVEL GEOLOGY (12 decks)

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