Igneous Rocks

Question 1

What do vesicles tell us?

Answer 1

Decompression history

Ascent in conduit

REPRESENT EXSOLVED WATER BUBBLES THAT WERE FROZEN INTO GROUNDMASS ON COOLING

Question 2

What does crystal shape and texture tell us?

Answer 2

T-P-t history in ground

Question 3

What does zoning tell us?

Answer 3

Recharge and emptying events in the chamber

Question 4

What do argon isotope ratios tell us?

Answer 4

When the volcano erupted

Question 5

Magma =

Answer 5

Molten rock beneath ground

Question 6

Intrusive rocks =

Answer 6

Igneous rocks that intruded and cooled beneath the ground

Question 7

Extrusive rocks =

Answer 7

Igneous rocks that extruded and cooled above the ground

Question 8

What is magma made up of?

Answer 8

1) silicate melt (l)
2) crystals (s)
3) volatiles (g)

Question 9

Nucleation is dependent on…

Answer 9

1) rate of undercooling
- cool below liquidus line
- form bonds
- release heat
- re melt nuclei
“NEED SIGNIFICANT UNDERCOOLING TO ALLOW NUCLEI TO PERSIST AND GROW AS CRYSTALS”

2) ion availability
- if one mineral contains more abundant ions than another it will grow faster
- ions fill spaces in a crystal lattice structure

3) ease of ion travel through melt
- viscosity (gas/temp/silica)
- network formers (Si/O/Al) and modifiers (Fe/Mg)

Question 10

Volcanic glass

Answer 10

Highly polymerised, high silica melts

Inhibits:

• physical flow
• chemical diffusion
• crystal growth

Therefore cooling rate > diffusion rate (since cooling rate increases with viscosity)

So crystallisation is limited

Forms an AMORPHOUS SOLID WHICH LACKS ORDERED CRYSTALLINE STRUCTURE

Question 11

Plagioclase continuous melt

Answer 11

Ca rich in basic melts

Na rich in acidic melts

Question 12

Basalt

Answer 12

Gabbro

Question 13

Andesite

Answer 13

Diorite

Question 14

Dacite

Answer 14

Granodiorite

Question 15

Rhyolite

Including igneous pumice

Answer 15

Granite

Question 16

Compatible elements

Answer 16

Zu/Co/Ni

Question 17

Incompatible elements

Answer 17

K/Rb/Sr/Ba/REE

Question 18

Anatexis =

Answer 18

Melting rock and creating magma…

1) liquid melt fraction enriched in lower T constituents
2) restite = residual rock, enriched in higher T refractory elements

MADE UP OF 1 AND 2 UNTIL THE TEMP IS GREATER THAN THE LIQUIDUS

Question 19

Subalkaline rocks

Answer 19

Majority of igneous rocks
Form at divergent/hot spots/convergent margins

Have more silica than Na/K in due to larger degrees of partial melting than alkaline (reduce concentration of them in the rock)

Question 20

MORB - subalkali basalt

Answer 20

1) ENRICHED
- 10-15% partial melting
- deeper undepleted mantle source
- result of magma mixing and assimilation
- enriched in incompatible trace elements e.g. K/Ba/Rb
- less partial melting has taken place deeper, so more enriched here

2) NORMAL
- 20-30% partial melting
- depleted mantle source

Question 21

Alkaline rocks

Answer 21

Rare

Silica undersaturated in comparison to Na and K

Smaller degrees of partial melting OR at greater depth than sub alkaline magmas

Slow moving divergent/rift valleys

Question 22

Bowen’s reaction series

Answer 22

1200 degrees
OLIVINE
PYROXENE
CA PLAGIOCLASE

800 degrees
AMPHIBOLE
CA/NA PLAGIOCLASE

600 degrees
NA PLAGIOCLASE
K FELDSPAR
MICAS
QUARTZ

Question 23

Harker diagrams

Answer 23

Wt % of different compounds

Will become enriched as other elements/compounds come out and then depleted when minerals with those elements/compounds in crystallise out

Question 24

Open system diversification due to:

Answer 24

1) assimilation

2) magma mixing and mingling (mafic + felsic = intermediate)

Question 25

A’a lava

Answer 25

Form when viscosity is high

Steep slopes

Surface of flows = clinker breccia
Hot, dense interior which flows
Basal breccia

Rough top surfaces

Question 26

Pahoehoe lava

Answer 26

Emplaced slowly

Endogenous growth of “lobes”

Flat surfaces and low gradient slopes

Question 27

Trachytic texture

Answer 27

Parallel alignment due to flow

Question 28

What do gases do?

Answer 28

Magma containing dissolved gas

Pressure gradient = rapid exsolution (VESICULATION) and expansion

Lowers density = acceleration

Explosive expansion of overpressurised bubbles = ejects material out of bent up to 600m/s

Magma&raquo_space;> pyroclastic jet

Liquid + bubbles&raquo_space;> gas + particles

Question 29

Continuous phase in magma

Answer 29

Liquid

Question 30

Continuous phase in pyroclastic jet

Answer 30

Gas

Question 31

Effusive eruptions =

Answer 31

Gases can escape easily from melt

Question 32

Silicic lava domes

Answer 32

Formed by viscous magma being erupted effusively onto the surface and then piling up around the vent

Often preceded by explosive activity - the top part of the magma chamber is the most gas rich

Question 33

Minerals in rhyolite/granite

Answer 33

K feldspar
Quartz
Na rich plagioclase
Mica
Amphibole

Question 34

Minerals in Dacite/granodiorite

Answer 34

K feldspar
Quartz
Plagioclase
Mica
Pyroxene
Amphibole

Question 35

Minerals in andesite/diorite

Answer 35

(K feldspar)
Quartz
Plagioclase
Pyroxene
Biotite
Amphibole

Question 36

Minerals in basalt/gabbro

Answer 36

Ca rich plagioclase
Pyroxene
Olivine

Question 37

Minerals in peridotite

Answer 37

Olivine

Pyroxene