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CIVIL 220 > Introduction to Igneous Rocks > Flashcards

Introduction to Igneous Rocks Flashcards

1
Q

Magma

A

Molten rock, together with any suspended crystals and dissolved gases (volatiles), that forms when temperatures rise and melting occurs in the mantle or crust

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2
Q

Lava

A

Magma that has erupted on the Earth’s surface

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3
Q

Igneous rocks form…

A

… from the cooling of magma

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4
Q

What does rate of cooling depend on

A

The environment of cooling

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5
Q

Magma is formed by…

A

… the partial melting of source rocks deep within the Earth and as it is less dense than the surrounding rocks they rise buoyantly

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6
Q

3 ways magma forms

A
  • Decompression melting (pressure decrease)
  • Flux melting (addition of volatiles)
  • Heat transfer melting (by exisiting rising magma)
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7
Q

What is magma composed of

A
  • Liquid: melt composed of mobile ions of elements from the mantle/crust
  • Solid: silicate minerals already crystallised from the melt
  • Gas (volatiles) - H2O vapour, CO2, SO2
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8
Q

What are the three groupings of magma and its two sources

A

1) Mafic magma - most abundant; basaltic composition; generated in the mantle
2) and 3) Intermediate and silicic magmas - andesitic to rhyolitic composition; generated in the crust

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9
Q

Decompression Melting

A
  • from Solid to Solidus there is an adiabatic rising of the mantle
  • circulation causes part of the mantle to rise
  • Temperature stays around the same but pressure decreases and melting occurs
  • Basaltic (mafic) magma forms
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10
Q

Flux Melting

A
  • Volatiles are easily evaporated molecules e.g. H2O and CO2
  • At a subduction zone the subducting plate carries volatiles into the mantle
  • At depth, high T and P drive volatiles from the subducting plate as they help break molecular bonds and decrease melting temperature forming basaltic (mafic) magma
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11
Q

Heat Transfer

A
  • Rising basaltic magma ponds at the base of the crust
  • Heat transfer from hot magma melts cold rocks
  • Forms new magma of different composition:
    1) If mixing of basaltic magma and melted crust then andesitic (intermediate) magma forms and rises
    2) If melted crust alone rises then magma is rhyolitic (silicic)
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12
Q

Composition of Magma

A
  • Silica is the dominant component and their amount is used to classify igneous rocks
  • Basaltic (mafic) magma: Low viscosity; high T; poor volatility; mafic minerals dominate which leads to a dense magma and a dark coloured lava
  • Andesitic (intermediate) magma: Moderate viscosity; medium T; contains volatiles; mafic and felsic minerals present which lead to medium density and colour
  • Rhyolitic (silicic): High viscosity; low T; high volatile content; Felsic minerals which lead to low density and light colour
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13
Q

Magma to Rock

A
  • as magmas cool their chemical components reorganise into crystals
  • minerals form as it cools and they become saturated in their constituents
  • this means that minerals form in a particular order dependant on T
  • This is called BOWEN’S REACTION SERIES
  • According to Bowen’s Reaction Series: Mafic minerals crystallise at high T; Felsic minerals crystallise at a lower T
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14
Q

Bowen’s reaction series and silicate structure

A

As magma cools silicates generally crystallise in a particular order and a decreasing temperature means and increase in complexity - isolated tetrahedra -> single chain -> double chain -> sheet -> framework

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15
Q

What is classification of igneous rock based on

A
  • Rock Texture (determined by relative sizes of the crystals in the rock and reflects the mode of formation of the rocks)
  • Rock composition which is wither chemical (total amount of silica in the rock) or mineral (relative abundance of minerals in the rock)
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16
Q

How is crystal size controlled in igneous rocks

A

Rate of cooling

17
Q

How is crystal shape controlled in igneous rocks

A

Timing of crystallisation i.e. early crystals can grow freely in magma and later crystals have to fit into spaces between early crystals

18
Q

What are the three igneous textures

A

1) Aphanitic - fine grained (e.g. basalt)
2) Phaneritic - coarse grained (e.g. gabbro)
3) Porphyritic - large phenocrysts in fine-grained matrix (e.g. andesite)

19
Q

Aphanitic

A
  • crystals too small to see with the naked eye
  • formed by rapid cooling
  • Characteristics of some extrusive (volcanic) rocks
20
Q

Phaneritic

A
  • large interlocking crystals
  • slow cooling
  • characteristic of intrusive (plutonic) rocks
21
Q

Porphyritic

A
  • large crystals (phenocrysts) in fine-grained matrix (groundmass)
  • complex cooling history
  • slow then rapid cooling
  • characteristics of many extrusive (volcanic) rocks
22
Q

Three environments for igneous rock occurrence

A

1) Plutonic environment: High pressure; slow cooling; intrusive; examples include Gabbro, diorite and granite
2) Sub-volcanic or hypabyssal environment: transitional between plutonic and volcanic; intrusive; example is diabase (dolerite)
3) Volcanic environment: low pressure; fast cooling; extrusive; examples including basalt, andesite and rhyolite

23
Q

Deep Igneous Intrusions

A
  • form at the depth in crust
  • form plutonic rocks
  • cool slowly so large crystals grow
  • coarse grained
  • granite is the most common
  • Gabbro constitutes a significant part of oceanic crust
  • Plutonic rocks are exposed at the surface by uplift and erosion
24
Q

Types of Plutonic rocks

A
  • Peridotite (very low SiO2, phaneritic and generally dark coloured)
  • Gabbro (low SiO2, phaneritic and generally dark)
  • Diorite (medium SiO2, phaneritic and generally medium coloured)
  • Granite (high SiO2, phaneritic and generally light coloured)
25
Q

Shallow Igneous Intrusions (2 Types)

A
  • Discordant Intrusions (cross-cutting) which forms a dyke

- Concordant Intrusions (parallel to local structure) which forms a sill)

26
Q

Example of sub volcanic rocks

A
  • Diabase (low SiO2, medium grained and generally dark)
27
Q

2 types of volcanic activity

A

Effusive and Explosive (Pyroclastic) Activity

28
Q

Effusive Activity

A
  • forms crystalline igneous rocks
  • magma extruded at the ground surface (little or no gas or water involved)
  • not very violet: lava flows and lava domes
29
Q

Explosive Activity

A
  • forms clastic rocks
  • magma ejected into the air (gas and/or water involved)
  • Varying degrees of violence: Lava and Tephra which is fragments of solidified lava ejected during an eruption or molten lava solidifying in the air
  • Tephra comes in the forms of volcanic bombs, scoria/pumice, pyroclastic falls or flows
30
Q

Examples of Volcanic Rocks

A
  • Basalt (low viscosity lava, low SiO2, aphanitic/porphyritic and dark)
  • Andesite (medium viscosity lava, medium SiO2, Porphyritic and light in colour)
  • Dacite (medium-high viscosity lava, medium - high SiO2, porphyritic and light-medium colour)
  • Rhyolite (high viscosity lava, high SiO2, porphyritic and generally light coloured)
31
Q

2 types of Rhyolite

A
  • Obsidian (high viscosity lava, high SiO2, Aphanitic and dark coloured)
  • Pumice (high viscosity lava, high in SiO2, Aphanitic and light in colour)
32
Q

Ignimbrite

A
  • Pumice-dominated pyroclastic flow deposits
  • formed from cooling of pyroclastic material
  • at high temperatures deposits can weld into rock
  • degree of welding depends on weight of overlying material
  • vesicular and low density when there is less overlying material (poorly welded)
  • dense and non-vesicular at depth when crushed
  • Aphanitic or eutaxitic
  • generally light coloured
  • pumice clasts in glassy matrix

CIVIL 220 (9 decks)

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