Earth Materials Lecture 12: Igneous Petrology Flashcards

1
Q

What are the majority of igneous rocks composed of?

A

99% silicate-based – SiO4
.
Rare carbonatites – CO3
.
Composed of crystals +/- glass.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

List Intrusive types of igneous rocks/behavior

A

Shallow – hypabyssal
Deep – plutons (adj. plutonic)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

List Extrusive types of igneous rocks/behaviors

A

Effusive - lava flows
Pyroclastic products

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

List the IUGS classification of Igneous rocks (based on silica content(SiO2)

A

Subdivisions at:
<45% Ultrabasic / ultramafic
45 – 52% Basic / mafic
52 – 63% Intermediate
>63% – Acid / felsic / sialic.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Define and Describe Ultrabasic/Ultramafic rocks

A

<45 % silica.

Predominantly
Fe, Mg, Ca silicates.

+/- spinel / garnet.
Plagioclase rare.

Minor Cr, Fe, Ti oxides
accessory phases:
magnetite, chromite.

ltrabasic rocks are made up of mafic minerals, such as olivine, pyroxene, and amphibole
They also contain metal oxides and native metals
They are high in magnesium and iron

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

List the main Ultramafic rocks

A

Dunite: An ultrabasic igneous rock with a hexagonal crystal structure

Peridotite: A dark, heavy, and dense igneous rock that’s high in magnesium and iron, and low in silicon

Pyroxenite: A common ultramafic rock

Troctolite: A rare variety of ultramafic rock with a higher percentage of calcic plagioclase

Anorthosite: A rare variety of ultramafic rock

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Describe the occurance and distribution of ultramafic rocks

A

Found in Earth’s upper mantle.

➢ Nodules in volcanic rocks.
➢ Orogenic belts.
⚫ Cumulates; often layered rocks.
⚫ Ophiolites: mantle and cumulates.
⚫ Ancient lavas: komatiites.
⚫ Kimberlites and related volcanics.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe and define Mafic/basic rocks

A

45 – 52 % silica

Produced by partial melting of mantle.

Rocks / magmas / lavas of basaltic composition.

Fe, Mg, Ca, Al, main elements with traces of Na, K, Ti, P.

Below are plutonic rocks; volcanic equivalents are ~basalts

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

List the main Mafic rocks

A

Basalt: The most common igneous rock on Earth, basalt is formed when lava cools quickly on the surface.

Gabbro: A common mafic rock that may contain olivine.

Diabase: A common mafic rock.

Andesite: A chemically intermediate rock with a salt and pepper color and texture.

Diorite: An intermediate igneous rock that often contains hornblende

Common mafic minerals
Olivine, Pyroxene, Amphibole, Biotite, Magnetite, Ilmenite, and Apatite.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe the occurrence and distribution of Mafic/Basic rocks

A

Oceanic crust - intrusive and extrusive

Large Igneous Provinces -
Short-lived major outpourings of magma

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Describe and define Intermediate to acid rocks

A

Intermediate rocks are igneous rocks with a chemical composition between mafic and felsic rocks.

52 – >63 % silica

Generally fractionates of basic magma; crustal melts.

Decreasing Fe, Mg, Ca; increasing Na, K, P, trace elements.

Andesine, oligoclase, albite; orthoclase / microcline / sanidine.
Quartz. Fe-olivine (fayalite).
Augite +/- enstatite; hornblende; biotite; muscovite.
Minor zircon, titanite, apatite etc

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

List the main intermediate rocks

A

Diorite: An intrusive intermediate rock that looks like a blend of light and dark minerals.

Andesite: An extrusive intermediate rock.

Granodiorite: An intrusive intermediate rock that’s between diorite and granite.

Trachyandesite: A volcanic intermediate rock.

Quartz syenite: An intermediate rock that contains quartz.

Quartz monzonite: An intermediate rock that contains quartz.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Describe the occurrence and distribution of intermediate to acid rocks

A

Lots in UK
-Ordovician acid tuffs Snowdonia
- Devonian granites Shetland, Cumbria
-Carboniferous granites Devon and Cornwall

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the various properties for identifying igneous rocks in hand specimens

A

Colour of fresh and weathered surfaces.
⚫ Determine composition – mineralogy – depends on grain size.
⚫ Texture:
− Granularity – grain size, range, mode.
− Is there a groundmass? Is it porphyritic?
− Crystallinity – crystals/glass, mixture.
− Fabric – massive, lineated, layered?
− If lineated/layered, determine which mineral(s) define these
fabrics.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly