1.2 Igneous Rocks And Processes

Question 1

How is crystal size in igneous rocks related to the cooling rate of magma? 1-5
What are the textures of an igneous rock? 1-3

Answer 1

1. Very rapid cooling = no chance for crystals to form - obsidian, glassy - pumice is volcanic froth
2. Rapid cooling = small irregular crystals = magma at or near surface
3. Medium cooling when crystals form in the middle of crust
4. Slow cooling = larger well formed crystals - deeply buried intrusion
5. Porphyritic - magma cools at 2 different rates - large phenocryst crystals surrounded by tiny groundmass crystals

Textures are:

1. Coarse grained - can see individual crystal eg granite and gabbro
2. Medium grained - speckled appearance, need lens to see - Dolerite
3. Fine grained - mostly uniform colour, need microscope to see crystals- basalt, rhyolite

Generally igneous rocks show crystalline texture with mosaic of interlocking crystals

Question 2

What rocks and minerals does viscous lava produce?

Answer 2

Viscous lava produces silicic rock like obsidian, granite, micro-granite and rhyolite with minerals like quartz, biotite, hornblende, muscovite, orthoclase
These are lighter in colour and density

Question 3

What rocks and minerals does intermediate lava produce?

Answer 3

Rocks - andesite, diorite, granodiorite
Minerals - biotite, augite, hornblende, plagioclase
Medium colour and density

Question 4

What rocks and minerals does non viscous lava produce?

Answer 4

Mafic and ultramafic composition
Rocks - basalt , dolerite, gabbro and mantle peridotite
Minerals - olivine, augite
Darker and denser

Question 5

How does viscous magma affect volcanic activity - explosive?

Answer 5

Viscous magma flows slowly 
Builds steep sided cones
Solidifies quickly to plug vent
Plug causes pressure to build in volcano
Gas can’t escape from viscous magma 
Eurputions are violent with lava droplets and vent rock blasted out - Nuee ardente or glowing cloud
Plug causes branch pipes to form
Eg mount pelee in Martinique 
Mount St. Helens in America
Krakatoa in Indonesia

Question 6

How does non viscous lava affect volcanic activity - passive?

Answer 6

Free flowing spreads over wide areas
It forms shield volcano and fissure volcanoes
Gas can escape
Lava doesn’t plug vent
More frequent but less explosive reactions
Mauna Loa cone 10km base 200km across

Question 7

How are igneous bodies distinguished?

Answer 7

Structures - columnar jointings and pillow lavas

Forms - lava flows, sills, dykes, plutons

Question 8

What is columnar jointing?

Answer 8

Formed during cooling and contraction of thick flows.
Lava cooling towards different cooling centres
Contraction and tension as lava cools in opposite directions and causes cracks and rocks to separate into 6 sided columns

Question 9

What is pillow lava?

Answer 9

Caused by underwater eruptions of lava.
Outside cools quickly to cause glassy skin
Inside stays molten and sinks and sinks to form saggy structure or tear drop

Question 10

What are lava flows?

Answer 10

Magma is erupted from volcanoes
Cools quickly at earth’s surface producing fine crystals
Baked margin caused as heat from lava causes country rock to recrystallise and form contact metamorphic rock
Chilled margin - lava flow in contact with air and country rock cools quickly forming small crystals

Question 11

What are sills?

Answer 11

Where magma intrudes and is parallel between layers of sedimentary rock.
They do not cut across country rock so usually horizontal
They are concordant
They have 2 chilled margins - rapid cooling so small crystals
They have 2 baked margins - contact metamorphism
Crystals larger in the middle of the sill where magma cooled slower

Question 12

What are dykes?

Answer 12

Intrusive igneous body where magma cools below surface
Cuts through fissures and goes across layers of country rock
They are discordant
Medium sized crystals 1-3mm
Contain microgranite, diorite, dolerite
2 baked margins
2 chilled margins

Question 13

What are plutons?

Answer 13

Form coarse grained crystals - large and can take years to cool
They are discordant as they are cutting across country rock
Granite, gabbro and granodiorite are formed
Metamorphic aureole - large zone of contact metamorphism

Question 14

What is the texture and mineralogy of mantle peridotite?

Answer 14

Coarse grained 
Ultramafic
Stays still in mantle - no partial melting
A part of mantle caught in magma rising and blasted out with lava
S - 2-6mm
O - random
S - subhedral
Mineralogy - olivine and augite
Colour - very dark

Question 15

What is the texture and mineralogy of basalt?

Answer 15

Ultramafic partially melted into mafic rock
Quite dark and dense
Cooled very quickly
S - <1mm
O - random
S - anhedral
M - augite and olivine

Question 16

What is the texture and mineralogy of granite?

Answer 16

Coarse grained
Partially melting, intermediate to silicic, deep in crust
Very light colour and density
S - 3-9 or 10mm
O - random
S - subhedral
M - quartz, orthoclase, muscovite, biotite, hornblende

Question 17

What is the texture and mineralogy of andesite?

Answer 17

Intermediate
Fine grained 
Quite light and bright
S - 1mm
O - random
S - anhedral
M - augite, hornblende, biotite, plagioclase

Question 18

What is an igneous rock?

Answer 18

Igneous rocks crystallise from molten rock
When they crystallise on the earth’s surface they are known as lavas
They are described using:
Size -> range + units
Orientation -> random or aligned
Shape -> euhedral, subhedral, anhedral