2.15 Igneous intrusions

Question 1

What is a minor igneous intrusion?

Answer 1

minor intrusions cool at hypabyssal (relatively shallow) depth below the surface and include sills and dykes

Question 2

What is meant by the term hypabyssal depth?

Answer 2

relatively close to the surface

Question 3

What is a concordant intrusion?

Answer 3

concordant intrusions are intruded along a bedding plane and are parallel to sedimentary beds - sills are concordant intrusions

Question 4

What is a discordant intrusion?

Answer 4

• discordant intrusions cross cut bedding planes in the existing (country) rock - dykes are discordant intrusions
• discordant intrusions will follow other lines of weakness such as faults or joints
• the intrusions will also create new channels as new fractures are created by the pressures of magma injection

Question 5

What is meant by the term country rock?

Answer 5

any rock into which an igneous rock intrudes (it is the older, existing rock in the area)

Question 6

What is a dyke?

Answer 6

a discordant minor igneous intrusion

Question 7

What is a dyke?

Answer 7

a discordant sheet-like minor igneous intrusion

Question 8

What is a sill?

Answer 8

a concordant sheet-like minor igneous intrusion

Question 9

What type of rock are most sills and dykes composed of?

Answer 9

medium-grained mafic rock (dolerite) except along the chilled margins where they are fine-grained (basalt)

Question 10

What is a transgressive sill?

Answer 10

a sill that cuts across beds in steps from one bed to another

Question 11

What is a dyke swarm?

Answer 11

dykes often occur in large numbers associated with crustal extension e.g. along a 25km stretch of the Arrang coast, 525 dolerite dykes can be seen all orientated in a similar direction

Question 12

What are ring complexes (ring dykes and cone sheets)?

Answer 12

dykes that are formed in curved, ring-like structures on the surface
- ring dykes form because of roof collapse when magma pressure decreases in the underlying pluton/magma chamber and magma oozes out into the fractures surrounding the collapse (they dip away from the centre of the underlying pluton)
- cone sheets form because of the pressure cretaed by the magma pushing up and fracturing the overlying roof rocks (they dip towards the centre of the underlying pluton)

Question 13

Which direction do ring dykes dip?

Answer 13

in a radial pattern away from the centre of the underlying pluton

Question 14

Which direction do cone sheets dip?

Answer 14

in a radial pattern towards the centre of the underlying pluton

Question 15

What is a volcanic plug?

Answer 15

a volcanic plug is an intrusion formed when magma crystallises in the conduits or vent of a volcano - over time, weathering and erosion removes the less resistant lava flows and volcaniclastic material making up the flanks of the volcano leaving behind the more resistant plug that formed

Question 16

Why are volcanic plugs left exposed?

Answer 16

over time, weathering and erosion removes the less resistant lava flows and volcaniclastic material making up the flanks of the volcano leaving behind the more resistant plug that formed

Question 17

What is a diapir?

Answer 17

a body of relatively low density material that pierces and rises up through overlying material of a higher density

Question 18

What is a chilled margin?

Answer 18

the large temperature contrast between the country rock and the magma results in more rapid cooling of the edges of the intrusion - the crystal size of the igneous rock will be finer

Question 19

What is a baked margin?

Answer 19

the large temperature contrast between the country rock and the magma results in low grade metamorphism of the country rock along the contact - the baked margin will have a sugary texture and become harder and lighter in colour

Question 20

What factors will determine the size of the chilled and baked margins?

Answer 20

• the size of the intrusion (larger intrusions cool more slowly and have wider baked margins)
• the temperature of the intruding magma

Question 21

What is a major intrusion?

Answer 21

major intrusions cool at plutonic depths (deep below the surface) and include batholiths

Question 22

What is meant by the term plutonic?

Answer 22

when igneous rocks form deep beneath the surface

Question 23

What is a pluton?

Answer 23

a pluton represents one magma body - a large igneous intrusive body less than 100km²

Question 24

What is a batholith?

Answer 24

• a large igneous intrusion greater than 100km ², which may be an aggregate of plutons
• a batholith represents a long period of repeated igneous intrusions

Question 25

How might a batholith be recognised in outcrop?

Answer 25

• batholiths are discordant and the outcrop pattern is roughly circular in shape with steep sides
• they are normally composed of granite (although they can be granodiorite or even diorite, all of which are coarse-grained)

Question 26

Why are plutons and batholiths coarse grained?

Answer 26

they cool slowly at depths of 5-30km as plutonic rocks

Question 27

What is a metamorphic aureole?

Answer 27

• batholiths heat a large zone of country rock creating an area of metamorphic rock, which can be hundreds or metres to several kilometres wide
• the rocks are altered by contact metamorphism

Question 28

What factors will affect the width of a metamorphic aureole?

Answer 28

• the size and temperature of the intrusion at the time of emplacement
• the dip of the sides of the intrusion
• the composition of the surrounding rocks (e.g. limestones are altered much more easily than sandstones)

Question 29

What is meant by the term stoping?

Answer 29

the process that accommodates the magma of a batholith or pluton as it moves upwards into the country rock by the mechanical fracturing of the surrounding country rock
- as the magma moves upwards it moves along joints, faults and bedding planes separating masses of country rock, which eventually become detached and settle into the magma as xenoliths

Question 30

What is a xenolith?

Answer 30

a clast or fragment of pre-existing rock contained within an igneous rock

Question 31

What is meant by the term assimilation?

Answer 31

the melting process that incorporates blocks of country rock, freed by stoping, into the magma by melting them

Question 32

Why do some xenoliths survive and not assimilate?

Answer 32

in order to assimilate country rock, enough heat must be provided by the magma to melt the country rock - granitic magma is cooler than mafic magma and is therefore less able to melt and assimilate the country rock