Section 4: Igneous Rocks And Volacanic Activity

Question 1

How do igneous rocks form

Answer 1

From the cooling and crystallization of molten rock (lava or magma)

Intrusive: formed underground
-cooled slow
-large crystals develop
-phaneritic texture - crystals so coarse individual minerals can be seen with naked eye

Extrusive: formed above ground
-cooled quickly
-smaller crystals
-aphantic texture -very fine grained rocks where individual crystals can only be seen with microscope

Question 2

Igneous rock texture

Answer 2

Phaneritic- large crystals, from intrusive rock
Aphanitic- small crystals from extrusive
Porphyritic- 2 stage cooling process, minerals crystallize in magma and are brought to surface
-magma been slowly cooling and crystallizing and suddenly erupted at the surface causing remaining uncrystallized magma to cool rapidly

Question 3

Magma generation and plate techtonics

Answer 3

1. divergent plate boudaries(decompression melting
   -decompression melting as convection moves rock towards the surface, minimal temp change but drop in pressure causes partial melting
   -mafic magma produced

2.convergent plate boundaries (flux melting)
-flux melting as an oceanic plate subducts under continental plate, water from oceanic plate drives down the melting point, as magma rises some of the oceanic crust melts producing felsic magma

3. other = mantle plumes(hot spots)
   - hot plastic rock rising from core-mantle boundary, decrompression as it rises leads to melting, accounts for magmatic activity NOT at plate boundary

Question 4

Mafic vs Felsic

Answer 4

Felsic (FelSic)
-contain felsic and silica
-light colour
-comes from high viscosity(sticky) magma
Mafic
-contains magnesium, iron
-dark colour
-comes from low viscosity (runny) magma

Question 5

Intrusive Igneous rocks
Dike and sill

Answer 5

Shallow
-depths<2km
-cool more rapidly
-fine grained
-follow fractures or other planes of weakness
-dike-sheet structure cross-cutting layers of existing rock
-sill- sheet structure which parallels layering in exisiting rock

Deep (pluton)
small plution=stock, large =batholith
depths >2km
-strong, good for foundation
-almost impermeable to water
can form extensive sets of fractures

Question 6

Volcanic activity explosivness

Answer 6

high viscosity/high gas = explosive
low viscosity/low gas=non-explosive

-high silica content (felsic) and low temps produce more viscous magma=more explosive

-viscous magma traps gas bubbles >pressure builds>explosive volcano
-non-viscous magma allows gas to escape>non explosive volcano

Question 7

Shield volcano

Answer 7

not very explosive
mafic magma flows
erupts often
lava erupts from fissure and runs down gentle slopes, cooling

Question 8

Stratovolcano

Answer 8

usually intermediate or felsic
frequently explosive, often viscous magma
may erupt many times and stay active for a 100,000 years

Question 9

Volcano steps (important?)

Answer 9

1.magma forms in teh mantle or crust (melting of pre-exisiting rock)
2. gas is dissolved in magma becasue of high pressure
3.magma rises becasue of difference in density
4.gas bubbles begin to form due to lower pressures

Question 10

Volcanos and plate tectonics

Answer 10

Shield volcanos found at hot spots
strato volcanoes found at convergent subduction zones

Question 11

Volcanic hazards

Answer 11

1. fluid material (lava flow)
   -low viscostity flow quickly and spread laterally
   -road and property damage common
   2.pyroclastic materials
   (ash)
   -form due to bubble formation and magma fragmentation
   (density current)
   -lateral flow of hot gases and pyroclastic material
   3.Lahars (volcanic debris flows)
   - water, pyroclastic material and debris, follow river channels