Introduction to Igneous Rocks

Question 1

Magma

Answer 1

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

Question 2

Lava

Answer 2

Magma that has erupted on the Earth’s surface

Question 3

Igneous rocks form…

Answer 3

… from the cooling of magma

Question 4

What does rate of cooling depend on

Answer 4

The environment of cooling

Question 5

Magma is formed by…

Answer 5

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

Question 6

3 ways magma forms

Answer 6

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

Question 7

What is magma composed of

Answer 7

• 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

Question 8

What are the three groupings of magma and its two sources

Answer 8

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

Question 9

Decompression Melting

Answer 9

• 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

Question 10

Flux Melting

Answer 10

• 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

Question 11

Heat Transfer

Answer 11

• 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)

Question 12

Composition of Magma

Answer 12

• 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

Question 13

Magma to Rock

Answer 13

• 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

Question 14

Bowen’s reaction series and silicate structure

Answer 14

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

Question 15

What is classification of igneous rock based on

Answer 15

• 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)

Question 16

How is crystal size controlled in igneous rocks

Answer 16

Rate of cooling

Question 17

How is crystal shape controlled in igneous rocks

Answer 17

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

Question 18

What are the three igneous textures

Answer 18

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)

Question 19

Aphanitic

Answer 19

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

Question 20

Phaneritic

Answer 20

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

Question 21

Porphyritic

Answer 21

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

Question 22

Three environments for igneous rock occurrence

Answer 22

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

Question 23

Deep Igneous Intrusions

Answer 23

• 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

Question 24

Types of Plutonic rocks

Answer 24

• 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)

Question 25

Shallow Igneous Intrusions (2 Types)

Answer 25

- Discordant Intrusions (cross-cutting) which forms a dyke | - Concordant Intrusions (parallel to local structure) which forms a sill)

Question 26

Example of sub volcanic rocks

Answer 26

- Diabase (low SiO2, medium grained and generally dark)

Question 27

2 types of volcanic activity

Answer 27

Effusive and Explosive (Pyroclastic) Activity

Question 28

Effusive Activity

Answer 28

- 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

Question 29

Explosive Activity

Answer 29

- 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

Question 30

Examples of Volcanic Rocks

Answer 30

- 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)

Question 31

2 types of Rhyolite

Answer 31

- Obsidian (high viscosity lava, high SiO2, Aphanitic and dark coloured) - Pumice (high viscosity lava, high in SiO2, Aphanitic and light in colour)

Question 32

Ignimbrite

Answer 32

- 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