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Geology AS > Module 6 - Igneous Processes and Products > Flashcards

Module 6 - Igneous Processes and Products Flashcards

1
Q

What are igneous rocks?

A

Those that have cooled from magma

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2
Q

What is magma?

A

A molten rock

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3
Q

What is lava?

A

Molten rock that cools at the surface

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4
Q

What are silicic igneous rocks?

A

Have a composition rich in silica, more than 66% and are light in colour

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5
Q

What’s another way of saying light in colour?

A

Leucocratic

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6
Q

What are intermediate igneous rocks?

A

Have a silica content of 52-66% they are grey

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7
Q

What’s another way of saying grey?

A

Mesocratic

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8
Q

What are mafic igneous rocks?

A

Silica content of 45-52% and are dark in colour

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9
Q

What’s another way of saying dark in colour?

A

Melanocratic

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10
Q

What are ultramafic igneous rocks?

A

Silica content of less than 45%

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11
Q

What are felsic minerals?

A

Light coloured and silica rich

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12
Q

What percentage of the earth to igneous rocks make up?

A

95%

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13
Q

What’s the easiest way to classify igneous rocks?

A

By observing the Crystal grain size. The grain size can tell us a great deal about the cooling history of the rocks.

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14
Q

What size are fine-grained crystals?

A

Less than 1 mm in size so that individual crystals cannot be identified by eye

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15
Q

What size is a medium grained crystals?

A

1 to 5 mm. They can be seen by the eye

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16
Q

What size are coarse grain crystals?

A

Greater than 5 mm in size

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17
Q

What does the chemical analysis of the rock give?

A

Overall chemical composition of the rock

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18
Q

What is SiO2 like in igneous rocks?

A

Not present as quartz, but is contained within other silicate minerals such as feldspar and olivine

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19
Q

Which type of rock can Colour be misleading in?

A

Obsidian which is dark in color but is clear in thin sections as it is the silicic rock

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20
Q

What are felsic minerals?

A

Quartz and feldspar both rich in silica

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21
Q

Where does the term felsic come from?

A

Feldspar and silica

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22
Q

How is quartz formed?

A

The magma must be oversaturated with silica so that excess silica is left after the other rock forming minerals crystallize to form the quartz

Only found in silicic or intermediate rocks

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23
Q

What are feldspars?

A

Are the most common rock forming minerals in igneous rocks. They form about 60% of the minerals so are essential to igneous classification

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24
Q

What are the main types of feldspar?

A

Potassium feldspar and plagioclase feldspar

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25
Q

Where is potassium feldspar found?

A

Only in silicic and intermediate rocks. It is often found as large phenocrysts as well as in the groundmass of granite

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26
Q

Where does the word mafic come from?

A

Magnesium and ferric (iron)

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27
Q

Where are biotite and muscovite micas found?

A

Biotite - silicic and intermediate

Muscovite - silicic

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28
Q

What is hornblende?

A

One of the group of minerals called amphiboles which are particularly common in intermediate rocks.

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29
Q

What is augite?

A

It belongs to the pyroxene group of minerals and is one of the main minerals in mafic and ultramafic rocks

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30
Q

What are mafic minerals like?

A

Dark in colour, silica poor and rich in magnesium and iron

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31
Q

Where do igneous rocks cool?

A

At the surface or at great depth

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32
Q

What does extrusive mean?

A

It’s the general term for all the igneous rocks that cool at the surface - both lava and pyroclasts

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33
Q

What is intrusive the term for?

A

Igneous rocks that cool below the surface

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34
Q

What does hypabyssal describe?

A

When igneous rocks form at relatively shallow depths below the surface

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35
Q

What is plutonic describing?

A

Igneous rocks that form deep below the surface

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36
Q

What is the general rule for the Crystal size in rocks?

A

The slower the rate of cooling, the larger the Crystal size

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37
Q

How do glassy rocks form and where does this take place?

A
  • They have no crystals. •This is because they form very quickly - in hours, usually in the sea as water is the quickest way of cooling magma
  • Form at the surface - always volcanic extrusive
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38
Q

How and where do fine crystals form?

A
  • <1mm so can’t be seen by eye
  • forms due to rapid cooling over weeks or months
  • at the surface in extrusive volcanic rocks as lava flows. In chilled margins at the edges of minor intrusions
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39
Q

How and where do medium groaned rocks form?

A
  • 1-5mm in size can be seen by eye but are difficult to identify
  • Fairly slow cooling over a few thousand years
  • Below the surface, in minor intrusions, 1km down (hypabyssal)
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40
Q

How and where do coarse grained rocks form?

A
  • > 5mm can be seen and identified in samples
  • cools very slowly over millions of years
  • found in major intrusions at great depth >10km down as batholiths or plutonic rocks
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41
Q

Why does obsidian look black?

A

Because like cannot pass through it. In a thin section it is clear glass

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42
Q

What is a pegmatite?

A

An igneous rock that has exceptionally large crystals. Usually larger than a few centimeters or even meters long. They form in the last stages of crystallisation from the water-rich residual magma.

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43
Q

What does equigranular texture mean?

A

It means that all the crystals are of equal size

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44
Q

What is a vesicular texture?

A

It is where gas bubbles are trapped in lava as it cools rapidly, leaving holes where the gas was present. The holes, called vesicles, are usually oval or ellipsoid in shape and also elongated parallel to the direction of flow. The gas rises so most of the vesicles are found near the top of the lava flow or sometimes at the edge of an intrusion. Most common in basalt and pumice

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45
Q

What is Amygdaloidal texture?

A

When vesicles are later Infilled with minerals deposited from groundwater. A vesicular rock will be very porous so that ground water can flow through it. The ground water contains dissolved minerals such as calcium carbonate, so calcite will be precipitated into the holes. This can happen millions of years after the vesicular rock is formed. The most common minerals are calcite or quartz. Each infilled hole is called an amygdale. Common in basalt

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46
Q

What is flow bonding?

A

It occurs when layers of dark and light minerals form due to the separation of minerals within a silicic lava flow. They will be aligned parallel to the flow direction, though they’re often contorted because the lava was viscous so flowed slowly. Usually seen in ryhyolite.

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47
Q

What is porphyritic texture?

A

It forms where a rock has two stages of cooling and results in two distinct sizes of crystals. Large crystals are called phenocrysts and form first by cooling slowly. They are surrounded by a finer groundmass, which cooled faster. Most common in porphyritic basalt and porphyritic granite

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48
Q

How did porphyritic basalt form?

A

The phenocrysts - plagioclase feldspar form slowly in a magma chamber beneath a volcano. When an eruption occurs the magma and plagioclase crystals are erupted and cool quickly forming the groundmass

Sometimes the phenocrysts will be aligned parallel to the direction of the lava flow

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49
Q

How does porphyritic granite form?

A

Potassium feldspar phenocrysts form at great depth - 30km. The magma and feldspar crystals move up within the crust by diapiric action to form a batholiths at a depth of about 15km. The rest of the magma cools very slowly to form the coarse grained crystals of the groundmass

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50
Q

How is high-silica magma created?

A

Mainly generated by the melting of the Earths crust at convergent plate margins.
66-75% silica content
Tend to have lots of quartz and other high-silica minerals such as plagioclase and potash feldspar and micas.
Light colour
Viscous
The magma solidifies at a relatively low temperature - 600-900 degrees

Obsidian - pumice - rhyolite - granite - granodiorite

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51
Q

What is intermediate magma like?

A

52-66% silica
Most common rocks - andesite (fine grained) / diorite (coarse grained)
Contain little to no quartz but lots of plagioclase feldspar and ferromagnesian minerals such as pyroxene augite and amphibole hornblende
Magma is less viscous but still very sticky.
Solidifies at a higher temperature than a silicic magma.

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52
Q

Mafic igneous rocks?

A

Should be dark coloured
Minerals - pyroxene augite, sometimes olivine with ca rich plagioclase, no quartz
45-52% silica
Low viscosity - easily reaches the surface
Basalt is the most common mafic igneous rock
Solidifies at high temperature >1000 degrees

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53
Q

Ultramafic rocks?

A

<45%
Consist mostly of ferromagnesian minerals
Most common is peridotite
Forms much of the upper Mantle
Some ultramafic rocks are monomineralic (consist of only one mineral)
Have extremely high melting points
Ultramafic magma doesn’t exist on earth at the moment

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54
Q

What is a divergent plate margin?

A

Where two plates are moving apart and magma is rising up between them

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55
Q

What is a convergent plate margin?

A

Where two plates are colliding and magma is formed above a subduction zone deep in the crust

56
Q

What is a hot spot?

A

Formed by a fixed Mantle plume bringing magma to the surface

57
Q

What is partial melting?

A

Occurs where some of the minerals in a rock melt to form a magma

58
Q

What happens at Divergent Plate margins?

A
  • Origin of magma - Partial melting of peridotite in the asthenosphere.
  • Composition of magma - Mafic magma.
  • Extrusive volcanic activity - Frequent eruptions submarine, fissure or shield. Mainly non-viscous basalt lava, few pyroclasts. Effusive non-violent eruptions
  • Intrusive activity - Dolerite dykes and sills below volcanoes
59
Q

What happens at convergent ocean-ocean margins?

A
  • Origin of magma - Partial melting of subducting oceanic plate
  • Composition of magma - mafic to intermediate
  • Extrusive volcanic activity - Regular but infrequent eruptions. Strato volcano. Lots of basalt lava with some pyroclasts and andesite. Violent eruptions
  • Intrusive activity - Dolerite dykes and sills below volcanoes
60
Q

What happens at convergent ocean-continent plate margin?

A
  • Origin of magma - Partial melting of subducting oceanic plate and partial melting of continental crust and mixing of magma
  • Composition of magma - Intermediate and silicic
  • Extrusive volcanic activity - Irregular and infrequent eruptions. Strato volcanoes and lava domes. Little lava, mainly andesite with some rhyolite, large amounts of pyroclasts. Very violent eruptions of ash, agglomerate and ignimbrites
  • Intrusive activity - Dolerite dykes and sills below volcanoes. Batholiths.
61
Q

What happens at convergent continent-continent plate margins?

A
  • Origin of magma - partial melting of deep continental crust
  • Composition of magma - silicic
  • Extrusive volcanic activity - None
  • Intrusive activity - Batholiths
62
Q

What happens at Hotspots?

A
  • Origin of magma - partial melting of peridotite in asthenosphere
  • Composition of magma - Mafic
  • Extrusive volcanic activity - Frequent eruptions of shield volcanoes. Mainly non-viscous basalt lava. Effusive non-violent eruptions
  • Intrusive activity - Dolerite dykes and sills below volcanoes
63
Q

What happens to the magma at hotspots?

A

Mantle plumes rise from deep in the Mantle and partial melting of mafic material occurs. Convection in the Mantle transports heat from the core to the Earths surface. The Mantle plumes carry the heat upwards in narrow, rising columns of hot material, which spreads out when the plume meets the rigid lithosphere. The lower pressure here allows partial melting of Mantle peridotite to form enormous volumes of basaltic magma. Which may erupt at the surface to form flood basalts.

64
Q

How does a hot spot form?

A

If a Mantle plume provides a continuous supply of magma in a fixed location, a hot spot is formed.

65
Q

What type of magma is generated at oceanic-continental margins?

A

The oceanic plate subducts and partial melting produces magma. If the magma rises quickly at shallow depths then it will be basalt. If the magma rises through silicic continental crust the mafic and silicic magma mixes to give intermediate magma. Some of the magma will reach the surface to form intermediate volcanoes but most will be intruded to form granite batholiths

66
Q

What depth is the moho like at fold mountains?

A

The Moho is at its deepest at the highest fold mountains

67
Q

What is magma?

A

A mixture of minerals melted together with volatiles and water

68
Q

What happens in the discontinuous series of Bowens reactions?

A

All these minerals are mafic minerals rich in iron and magnesium with silica. Olivine is the first to form at >1500 degrees and as the temperature lowers pyroxene (augite), amphibole (hornblende) and biotite will form.

Left side

69
Q

What happens in the continuous series of Bowens reactions?

A

It shows the crystallisation of plagioclase feldspar. At high temperatures it as calcium rich and sodium rich at low temperatures.

70
Q

Which rocks is calcium rich plagioclase found in?

A

Mafic and ultramafic

71
Q

What type of rocks is sodium rich plagioclase found?

A

Silicic and intermediate

72
Q

What are the low temperature minerals?

A

When the two series converge at a low temperature the minerals crystallise to form felsic minerals rich in silica.
Potassium feldspar
Muscovite mica
Quartz at about 700 degrees

73
Q

What is magmatic differentiation?

A

A number of processes that cause magma to evolve into magmas of different compositions. This leads to different igneous rocks being produced from a single parent magma.

74
Q

What is fractional crystallisation?

A

As olivine and pyroxene form at high temperatures, they use iron and magnesium from the magma. The result is that the magma becomes depleted in iron, magnesium and in terms of the continuous side calcium.

75
Q

What is gravity settling?

A

Crystals will generally be denser than the liquid in which they are suspended and so will settle out
• early formed minerals with higher density than the surrounding (e.g. Olivine) sink
• gravity settling removes the crystals from the remaining liquid so that they do not react with the remaining magma.
• a layer of dense, early formed minerals at the base of an intrusion or magma chamber is a cumulate layer

76
Q

What is filter pressing?

A

The point where crystals and liquid exist together as a slushy mass. The weight of the overlying crystals causes the liquid to get squeezed out forming a separate layer on top.

77
Q

What happens when molten rock becomes solid?

A

As magma crystallises, the crystals that form will have different composition to the molten rock. When crystals separate from the rest of the magma as they grow, the chemistry of he magma changes. A single magma could form ultramafic, mafic, intermediate and silicic rocks if there is enough time for differentiation to take place.

78
Q

What is produced if the parent magma is mafic?

A

It will produce
• peridotite at the base of the intrusion, rich in dense olivine
• Gabbro forming the main part of the intrusion
•granite may form veins or a thin layer at the top of the intrusion
•diorite may exist toward the top of the intrusion

79
Q

What is country rock?

A

Any rock, whether sedimentary, igneous or metamorphic into which an igneous rock is intruded

80
Q

What is a minor intrusion?

A

They cool at hypabyssal depth below the surface and include sills and dykes

81
Q

What are major intrusions?

A

Plutonic and cool deep below the surface and include batholiths

82
Q

What is the contact?

A

The place where igneous rock meets the country rock

83
Q

What is the country rock?

A

Any rock that an igneous rock is intruded into

84
Q

What are intrusive igneous rocks?

A

Rocks that have crystallized below the earth surface. You can only see intrusive rocks after the rocks that used to lie above them have been eroded away. Most that we see are millions of years old.

85
Q

When are intrusions described as concordant?

A

When they are intruded along a bedding plane

86
Q

When are intrusions described as discordant?

A

When they cut across the bedding plane.

87
Q

What are most sills and Dykes made of?

A

Medium grained, mafic rock dolerite. Except along the margins where chilling by the country rocks produces a finer grained basalt.

Sills and dykes that have granite as a source will be made of porphyry or microgranite. Both medium grained silicic rocks

88
Q

What are sills?

A

Sills are sheet like intrusions that are concordant and parallel to the beds. Some sills occasionally cut across the beds in steps from one bed to another to form transgressive sills.
Sills are generally irregular with a variable width and sinuous outcrop and usually intruded along bedding planes.
Sills are intruded when the fluid pressure is so great that the underlying magma actually lifts the overlying rocks. Usually form at shallow depths.

89
Q

What are dykes?

A

Sheet like intrusions that are discordant and often vertical.
•range in size - cm - 100m thick
•most are vertical - form straight lines in outcrop forming long thin features
•often intruded along zones of weakness in the country rock, such as faults or joints
•common as feeder vents under volcanoes - have a radical pattern

90
Q

Major intrusion batholiths?

A
  • can be very large
  • one in the Andes 4500km long
  • most are discordant where they are intruded into the country rock
  • roughly circular outcrop with steep sides
  • usually granite in composition although they can be composed of granodiorite or diorite (coarse grained)
  • cool very slowly at depths 5km - 30km as plutonic rocks
91
Q

Where are most batholiths intruded?

A

At convergent plate margins where they form the core of fold mountains, which have resulted from plate collision.

92
Q

What type of magma is viscous and rises slowly?

A

Silicic magma. It is difficult to mix with other magmas because the viscosities are so different.

93
Q

What are batholiths?

A

Huge bodies of magma usually produced by repeated intrusion of magma in the same area. As the magma moves up it generates more magma by stopping and assimilation

94
Q

How are xenoliths formed?

A

When pieces of country rock are detached and settle into the magma

95
Q

What is a chilled margin?

A

Where the igneous rock has cooled rapidly so it has fine crystals

96
Q

What is a baked margin?

A

In the country rock where it was heated by the intrusion and altered

97
Q

What is a metamorphic aureole?

A

A large area around a batholith where the rocks have been metamorphosed.

98
Q

What’s the difference between intrusive and extrusive rocks?

A

Extrusive rocks cool at the surface whilst intrusive rocks actually cool below the surface.

99
Q

What type of characteristics do extrusive rocks have?

A

Fine grained crystals
Flow banding
Vesicular texture

100
Q

What are the characteristics of intrusive rocks?

A

They have medium and coarser grains.

They have chilled and baked margins.

101
Q

Chilled margin

A
  • edge of the igneous intrusion closest to the cold country
  • few cm to several m wide
  • fine grained crystal size (cooled quickly)
102
Q

Baked margin?

A
  • the area of country rock adjacent to a minor intrusion gets altered by the heat
  • recrystallised by the heat (contact metamorphosed)
  • have a sugary texture, become harder and lighter in colour
  • few cm to m (large intrusion = large baked margin)
103
Q

Features of sills?

A
  • Two chilled margins
  • Two baked margins
  • Xenoliths from above and below
  • May show differentiation
  • Medium - average crystal grain size
  • Dolerite rock is most common
  • Rare vesicles
  • No fragments
  • regular surfaces
  • No weathering
104
Q

Characteristics of lava flows?

A
  • Only one clear chilled margin below
  • One baked margin below and none above
  • No xenoliths from above but possibly some from below
  • May have flow banding or phenocrysts, may be aligned parallel to the direction of the flow
  • Fine average crystal grain size
  • Basalt most common
  • Vesicles common in the upper part and may become amygdales
  • Lava fragments in overlying sedimentary rocks as a result of erosion and redeposition
  • Irregular upper surface (scoria or rubble)
  • Reddened surface
105
Q

What is a volcano?

A

A vent at the surface of the Earth through which magma and other volcanic materials are ejected

106
Q

What are volcanoes proof of?

A

An active earth and that the plates are in constant motion

107
Q

What are the main gases released by volcanoes?

A

Water in the form of steam
CO2 (12%)
N (7%)
SO2 (7%)

108
Q

What is mafic lava like?

A

Low viscosity flows of basalt that are thin and widespread over many kilometers as they are fluid.
Cools to form…
aa lava - rough block jagged surface
pahoehoe - smooth ropy surface

109
Q

What are intermediate and silicic Lavas like?

A

Viscous flows of andesite are irregular and tend to be limited to the area around the cone.
Very sticky lava can form lava domes within the crater and often shows flow banding

110
Q

What are pyroclasts?

A

An individual fragment ejected during an eruption

111
Q

What does pyroclastic describe?

A

All the fragmental materials formed by explosive eruptions, including bombs, blocks lapilli and ash

112
Q

What are pyroclastic rocks?

A

Formed when the pyroclastic material is compacted into rocks

113
Q

What is a pyroclastic flow?

A

A hot mixture of pyroclastic material and gas

114
Q

What’s the difference between the pyroclasts?

A
  • Ash - <2mm forms tuff when consolidated
  • Lapilli - 2-64mm forms lapilli tuff
  • Bombs and blocks - coarsest particles >64mm form the pyroclastic rock agglomerate
115
Q

What is an isopachyte?

A

A line joining points of equal thickness of a deposit such as ash. An isopach map

116
Q

What is a lahar?

A

Mudflows of wet ash and volcanic debris that can flow rapidly down a mountainside

117
Q

What is viscosity?

A

A measure of a fluids resistance to flow and controls the stickiness of magma which depends on silicic content

118
Q

What are the variables of distribution of pyroclasts?

A

Energy of the volcanic blast, grain size of the pyroclasts, velocity and direction of wind, gradient of the volcano sides and magma viscosity

119
Q

How does distribution vary by grain size?

A

Coarse bombs and blocks are dropped closer to the vent. Finer material is carried further away.

120
Q

How does distribution of pyroclastic material vary due to wind?

A

The ash is deposited based on the direction the wind is blowing. The wind can carry ash clouds over huge distances

121
Q

Distribution by energy of blast?

A

The greater the energy, the further away from the volcanoes the material will be found. Depending on if the energy is blasted in a particular direction

122
Q

What does highly viscous rhyolite and andesite lava mean?

A
  • The volcanoes will have steep sides
  • Infrequent eruptions
  • Lava flows more slowly
  • Trapped gases may escape explosively
  • There are explosive eruptions
  • Tuff and pyroclastic flow forms
123
Q

What does basalt lava with low viscosity mean?

A
  • Volcano will have shallow sides
  • Frequent eruptions
  • Lava flows easily
  • Gases easily released
  • rarely pyroclastic
  • Quiet
  • Cooler basalt lava is quite viscous
124
Q

What are the characteristics of shield volcanoes?

A

Gentle slopes of less than 10 degrees and a roughly circular shape around a central vent.

125
Q

What are fissure eruptions?

A

Where magma reaches the surface along long, linear cracks or fissures

126
Q

What are submarine eruptions?

A

Where magma comes from a vent or fissure on the sea floor

127
Q

What is low viscosity?

A

Where magma is fluid and flows freely

128
Q

What does the term effusive describe?

A

The fluid, nonexplosive, basalt lava

129
Q

What do you mafic volcanoes have in common?

A
  • the viscosity of the magma is low because the silica content is low
  • the gas content of the magma is high but the low viscosity allows the gas to escape so the eruptions are quiet and nonexplosive
  • frequent
  • the activity is described as effusive
  • they occur at divergent plate margins or hotspots
130
Q

What are shield volcanoes?

A

Characterized by gentle slopes. Composed almost entirely of thin basalt lava flows built up from a central vent

131
Q

What is columnar jointing?

A

Where the lava flow is more than 3m thick, the inside of the flow will cool steadily and slowly whilst the outside cools quicker. Hexagonal columns form.

132
Q

Why can pillow Lavas be used as way up indicators?

A

Because they have a rounded top and sagging bottom

133
Q

What is the difference between Hawaiian eruptions, Strombolian eruptions, vulcanian eruptions and plinian eruptions?

A

H- few pyroclasts, basalt (large amounts of fluid)
S- more explosive, regular explosions of gas and pyroclastic material
V- violent with viscous andesite lava
P- extremely explosive, viscous gas filled andesite and rhyolite lava. Tremendous volumes of pyroclastic material

134
Q

Where are explosive volcanoes?

A

Convergent plate margins particularly around the edge of the pacific (the ring of fire)

135
Q

Features that are common for intermediate and silicate volcanoes?

A 
High viscosity of magma
Steep sides (30 degrees)
Explosive eruptions 
At least 50% of the material erupted in pyroclastic 
Infrequent eruptions
136
Q

Facts about strato volcanoes?

A

About 60% of volcanoes on earth
Ideally have a conical shape but often don’t
Lava rarely flows
Ash covers a wide area

137
Q

Eruptive cycle of a strato volcano?

A
  • period of no activity, seams dormant. Magma chamber fills with magma
  • pressure exceeds weight of overlying rock- blasts away top part of volcano
  • lava reaches the surface and forms a layer on top of the pyroclasts. As gas pressures decreases the lava supply goes down and plugs up the vent ready for the next cycle

Geology AS (8 decks)

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