Lecture 10

Question 1

What is a volcano

Answer 1

A vent (fissure) in which a combination of melted rock, solid rock debris and gas is erupted

Question 2

Volcanic Settings

Answer 2

1. Hot spots under oceans (Hawaii) and continents (Yellowstone)
2. Divergent boundaries (spreading zones): mid ocean ridges
3. Young spreading zones: rifts in continents
4. Subduction and collision zones

OR anywhere where magma is being produced and there is a path for it to reach the surface

Question 3

Magma

Answer 3

Molten rock underground

Question 4

Lava

Answer 4

Molten rock on surface of planet

Question 5

Igneous rock underground

Answer 5

Plutonic (intrusive)

Question 6

Igneous rock above ground

Answer 6

Volcanic (extrusive)

Question 7

Crystal size in igneous rock depends on

Answer 7

How much time they have had to cool (more time=bigger crystals)

Question 8

Crystal size in plutonic rocks

Answer 8

Coarser grained crystals easily visible to the eye (Phaneritic)

Question 9

Crystal size in volcanic rock

Answer 9

Fine grained or no crystals (aphanitic or glass)

Question 10

How to identify igneous rocks

Answer 10

• silica content (<45% to >70%)
• mineral assemblages
• texture (crystal size, shape, etc)

Question 11

Mafic magma

Answer 11

• 45 to 50% silica
• typically formed by partial melting of the mantle
• low viscosity (runny)
• melt around 1200 degree Celsius
• forms rock that have abundant ferromagnesian minerals (dark colour as result)
• mafic lava solidifies to form basalt
• instructions of mafic lava cause gabbro

Question 12

Intermediate magma

Answer 12

• 50 to 70% magma
• typically formed at subduction zones
• intermediate properties
• moderate viscosity
• melt at intermediate temperatures
• ferromagnesian minerals, Na and Ca feldspar (plagiocase)
• little or no quartz
• green/grey/purple colour (depending on crystal size)
• intermediate lava solidifies to form andesite
• intrusion of intermediate magma form diorite

Also dacite (extrusive) and granodiorite (intrusive)

Question 13

Felsic magma

Answer 13

> 70% silica

• formed in association with continental crust
• high viscosity
• melt at relatively low temperatures (800 degrees Celsius)
• abundant K feldspar
• pale colour
• felsic lava solidifies to form rhyolite
• intrusions of felsic magma form granite

Question 14

Do we use grain size or crystal size or igneous rock

Answer 14

Crystal size

Question 15

Partial melting

Answer 15

• occurs at plate margins, rifts, hot spots etc and is how magma is produced (mafic)
• because rocks are made up of different minerals, and these minerals melt at different temperatures, the magma is often a different composition than the rock being melted
• magma type determines by what minerals are undergoing partial melting
• what melts is dependent based on temperature, composition, pressure, and presence of water
• on earth’s surface rocks melt between 800-1200 degrees Celsius

Question 16

Fractional crystallization

Answer 16

• opposite to partial melting

- changes the composition of the magma by the crystallization of minerals (basalt)

Question 17

Differentiation

Answer 17

The evolution of magma composition resulting in different igneous rock types

Review charts in lecture notes

Question 18

What and where

Answer 18

All starts with partial melting of mantle

End result depends on how much crust the magma needs to travel through

Question 19

Magma types by igneous setting

Answer 19

Review lectures slide!!!

Question 20

Composition and eruption style

Answer 20

SiO2 content is critical in determining how the molten rock behaves and it’s eruption style

Question 21

Behaviour of mafic magmas

Answer 21

Low silica means low viscosity (runny)

Gases in magma can escape easily

Quiet eruptions

Mainly lava flows

Fluid form of basalts tend to form shield volcanoes (e.g. Hawaii)

Question 22

Fissure eruptions

Answer 22

Flood basalt e.g. Deccan traps

Can cover huge areas (up to millions of km)

Continuous to semi continuous eruptions that last 10-1000 of years

Can also form in mid ocean ridges

Question 23

Behaviour of felsic and intermediate magmas

Answer 23

High silica so higher viscosities

Gases in magma cannot escape easily which causes explosive eruptions

Large amounts of tephra (ash)

Many hazards

Question 24

Stratovolcanoes

Answer 24

Also known as composite volcanoes (made of different rock types and different types of deposits)

The classic volcano shape, forms at subduction zones

Felsic and intermediate (mainly intermediates)

Question 25

Rhyolite caldera complexes

Answer 25

Big hole in the ground Hard to see without satellite imagery

Question 26

Volcanic hazards

Answer 26

``` Lava flows Temphra (ash) fall Pyroclastic flows Mudflows (Lahars) Tsunamis Gas emissions Climate change ```

Question 27

Lava flows

Answer 27

- large flows limited to fluid magma (I.e., basalt) - historical examples relatively small (Hawaii, Iceland) - past examples of massive fissure eruptions (Columbia basalts, Deccan traps) May have/ or did affect climate and cover 100s to 1000s of square km

Question 28

Tephra (ash) fall

Answer 28

Significant fall limited to explosive eruptions (e.g. dacite, rhyolite can be produced from other compositions in the right conditions) Major hazard to planes People with respiratory illness Mass loading on buildings Short term climate effects

Question 29

Pyroclastic flow

Answer 29

-Fluidized masses of rock fragments and gases that move rapidly in response to gravity -limited to explosive eruptions (dacite, rhyolite) -very fast, hot and dense ash flows Review examples

Question 30

Mudflow

Answer 30

Lahar -a debris flow composed of a slurry of pyroclastic material, rocky debris, and water

Question 31

Lahars

Answer 31

- water source (glacier, snow pack, heavy rainfall) - volcano triggered debris flows, far reaching, consistency of concrete - review examples

Question 32

Climate effects

Answer 32

1. Large explosive eruptions can cause short term cooling (strato/composite volcanoes) 2. long term (decades to many thousands of years), effusive eruptions can cause long term climate change (flood basalts) 3. Over the history of the earth volcanoes help balance co2 levels in the atmosphere

Question 33

Tsunami

Answer 33

Krakatau

Question 34

Gas emissions

Answer 34

Lake Nyos

Question 35

Short term climate effects

Answer 35

Big eruptions inject SO2 into the stratosphere which reacts with water to form sulfuric acid water droplets. These absorb and scatter incoming solar radiation and cause the average global temperature to drop by 0.6 degrees

Question 36

E.g. of short term climate effect

Answer 36

Tambora “The year without summer” which lead to largest famine in Europe >100 Tb of SO2 released Toba (biggest eruption in 2 million years) -released 1000 Tg of SO2

Question 37

Large igneous provinces (LIPS)

Answer 37

Essentially another term for flood basalt, but specifically refer to exceptional events Erupt for long periods of times (thousands to millions of years) Continuous injection of aerosols into the atmosphere rather than just one big injection over a short period of time Extinction events coincide with LIPs