Sept 21 lecture (volcanoes)

Question 1

magma sources tend to be ___- __km deep into crust and upper mantle

Answer 1

50-250km

Question 2

volcanoes are generated at: (list 3)

Answer 2

1. divergent plate boundaries
2. convergent plate boundaries
3. hot spots

Question 3

magma compositions vary in SiO2, iron, Mg, and volatile gases.
True/ false

Answer 3

true

Question 4

mafic magma= low in ___, but high in ___ and ____

Answer 4

SiO2
high in iron and Mg

*most common
basalt

Question 5

felsic magma= ____ levels of SiO2 ___levels of iron, Mg

• rare
• ___/___ rock

Answer 5

high

low

obsidian/ rhyolite rock

Question 6

intermediate magma= ___ levels of everything
This is the most __ type

___rock

Answer 6

intermediate

dangerous

andesite

Question 7

amount of SiO2 determines ____

Answer 7

viscosity

Question 8

viscosity determines ___ __ ____

Answer 8

style of eruption

Question 9

variations in lava are due to differences in:

Answer 9

• composition
• gas content
• lava temp

Question 10

the higher the silica content and lower the lava temp, the ___ viscous the lava is and the ___ is flows

Answer 10

more

slower

Question 11

the more __ a lava contains= more violent the eruption is

Answer 11

gas

Question 12

magma produced at a divergent plate boundary is typically melted ____ material, which is rich in _____ (ultramafic)
- a melt from it is mafic

Answer 12

asthenosphere

ferromagnetism

Question 13

at divergent plate boundaries, ___ is emplaced as new seafloor at the spreading ridge/ rift

Answer 13

basalt

Question 14

at divergent plate boundaries, rift systems in continental crust may melt granitic crust and produce ___/ ___ lavas

Answer 14

rhyolite or andesite

Question 15

what characterizes rift volcanoes?

Answer 15

a bimodal suite of extrusive igneous rocks

magma comes up through gap b/w diverging plates

Question 16

a fissure eruption=

Answer 16

low viscosity basalt, low volatile load

Question 17

True/ false

magmas associated with a hot spot volcanoes under continental crust generally will produce a basalt lava

Answer 17

FALSE

• produce felsic lava (often explosive)
• magmas associated with a hot spot volcano in a ocean basin will produce basalt lava

Question 18

magma at convergent boundaries:

Answer 18

very complex but:

• subducted continental crust may melt and produce rhyolite lava
• subducted oceanic crust may melt and produce basalt or andesite lava
• subduction of sediments derived from the top of the subducted slab may produce a variety of water rich and dangerous lavas

Question 19

composite volcano=

Answer 19

medium viscosity, but extra volatile (steep sided- mechanically unstable)

Question 20

any volcanic rock that’s ejected into the air is called a

Answer 20

pyroclast

Question 21

____ are typically characteristic of viscous, siliceous rhyolitic, and andesitic lavas, which typically trap more gas

Answer 21

pyroclasts

Question 22

pyroclasts eventually fall, often lithified together upon cooling, forming rocks knows as ___. Rocks resulting from smaller fragments= ___ ___ while larger fragments produce ___ ___

Answer 22

tephra
volcanic tufts
volcanic breccias

Question 23

volcanic bomb=

Answer 23

fragments ejected as blobs of lava that become rounded and cooled in flight
Or
chunks torn loose from previously solidified volcanic rock (very large)

Question 24

volcanic domes consist of viscous, ___-rich

Answer 24

silica

Question 25

in volcanic domes,

Answer 25

• viscous lava piles up close to vent –> build compact, high peak
• dome blocks vent, leads to a buildup of gas pressure before much material has emerged
• eg eruption of mount st helens

Question 26

____ ____ are mixtures of hot gases, ash, and rocks that form superheated, dense currents that can move very fast

Answer 26

pyroclastic flows

aka nuee ardente

Question 27

pyroclastic flows can be very devastating b/c:

Answer 27

• the solids are buoyed up on a layer of hot gas: little frictional resistance
• generated during some violent eruptions; molten material often cools before falling to earth

Question 28

combination of older volcanic ash and water can create ____

Answer 28

lahars (mud flow)

Question 29

where does water come from for lahars? Why do they go so fast?

Answer 29

• precipitation, melt from snow-capped volcanoes

- high, steep slope (unstable) mixed with water = flows fast down steep hill

Question 30

explain phreatic explosions

Answer 30

• volcanic islands are susceptible to seawater seepage, and when the water meets magma, it’s turned to superheated steam
• result of excessive subsurface pressure due to steam buildup

Question 31

how can volcanic eruptions induce tsunamis?

Answer 31

often stems from phreatic eruptions underwater, or very large explosions