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Atoms to volcanoes - 33739 > Extrusive processes > Flashcards

Extrusive processes Flashcards

1
Q

What is the volcanic spectrum?

A

Effusive to explosive

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2
Q

What is the eruptive process controlled by?

A

Amount of dissolved gasses in magma
Viscosity

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3
Q

What are some examples of dissolved gasses which can be trapped in magma?

A

Water
CO2
SO2

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4
Q

What is viscosity?

A

fluids resistance to flow & ability to degas

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5
Q

What is viscosity of lava related to?

A

Silica content
Lava temperature

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6
Q

What does a higher silica content mean?

A

higher viscosity

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7
Q

What will the viscosity of a cooler lava be?

A

higher

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8
Q

How do volcanic systems form at convergent boundaries?

A

Partial melting oceanic crust
Melt rises and collect in magma chamber (pressure builds)
Erupts on surface forming volcanic cone
Post eruption- magma solidify and plug vent
Magma continually accumulates and cools

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9
Q

What will eruptions at convergent boundaries/ subduction zones be like?

A

Most violent eruptions

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10
Q

What will magma be like at convergent boundaries?

A

High viscosity of intermediate to felsic magma
high volatile content (water dehydration melting)

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11
Q

What type of colour rock will be produced from felsic magma?

A

lighter coloured rock (rhyolite)

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12
Q

What will felsic magma be enriched by?

A

lighter elements (silicon, oxygen, aluminium, sodium, potassium)

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13
Q

What is the viscosity of felsic magma like?

A

high viscosity- sticky doesn’t flow easy

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14
Q

What does having high viscosity magma mean for the explosivity at convergent coundaries?

A

pressure can build making eruption more explosive

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15
Q

What type of volcanoes is most associated with convergent boundaries?

A

stratovolcanoes

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16
Q

What is an example of a stratovolcanoes?

A

Mt St Helens

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17
Q

What are stratovolcanoes also known as?

A

composite volcanoes

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18
Q

Why are stratovolcanoes known as composite volcanoes?

A

comprised of interbedded (andesitic) lava flows and pyroclastic debris

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19
Q

What do the layers on composite volcanoes reflect?

A

gradual calm and then more violent phases of volcanic development

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20
Q

What leads to a well formed cone shape for composite volcanoes?

A

Moderate to high viscosity
High angle of repose for pyroclastic debris

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21
Q

What happens to the neck and vent of a volcano post eruption?

A

plugged with fragments of pyroclastic debris and lava

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22
Q

What will begin to form over time below the plugged section of the neck and vent?

A

pressure increases and build a lava dome

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23
Q

What happens when magma and expanding gases being to move in the volcano when it is plugged?

A

plug can rupture leading to eruption

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24
Q

What happens to material around the vent when an eruption occurs?

A

pulverised

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25
Q

What is the charging element of pyroclastic debris?

A

gas

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26
Q

What is pyroclastic debris?

A

airborne volcanic fragments

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27
Q

What speed can pyroclastic debris reach?

A

200km/h

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28
Q

What allows pyroclastic debris to reach such high speeds?

A

minimal drag and frictional resistance

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29
Q

What can fast moving pyroclastic debris be called?

A

pyroclastic flow or glowing avalanches

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30
Q

What happens when water mixes with loose ash in the atmosphere?

A

lahars- destructive mudflows are created

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31
Q

How long might it take for fine ash to come down from the atmosphere?

A

depends on ejection height but can range from months to years

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32
Q

When did Mt St Helens erupt?

A

1980

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33
Q

What was different about the pyroclastic flow for the Mt St Helens eruption?

A

occurred on the north flank with a lateral eruption. Created largest mass movement as whole north face collapsed

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34
Q

How could geoscientists tell an eruption was imminent at Mt St Helens?

A

Shallow earthquakes sign of magma moving combined with bulging and deformation of volcano

35
Q

What path will magma take when moving through a volcano?

A

the weakest point in the structure

36
Q

How much of the surrounding area was devastated by the 1980 Mt St Helens eruption?

A

150 miles*2

37
Q

How long was Mt St Helens dormant before the 1980 eruption?

A

120 years

38
Q

What are cinder cone volcanoes like?

A

small volcanic cones generally developed on flank of large volcano (found in clusters)

39
Q

What are cinder cones mainly composed of?

A

pyroclastic cinders (pea sized blobs of lava) cooled mid air

40
Q

What will there rarely be evidence of with with cinder cones?

A

lava flows

41
Q

What are the typical dimensions of cinder cones?

A

rarely over 250m in height or 500m in diameter

42
Q

What happens when a caldera erupts due to be so explosive?

A

the top of he volcano collapses in

43
Q

What are huge amounts of pyroclastic debris and gases released from in calderas?

A

ring fractures

44
Q

What is a caldera?

A

the crater left after eruption when the volcano collapses in because of the mass ejection of magma

45
Q

What is an example of a past caldera?

A

Crater lake (oregon) last Mt Mazama eruption 7000yrs ago

46
Q

What was the scale of crater lake caldera?

A

Relatively small with 10km diameter

47
Q

What is the last eruption of Mt Mazama associated with?

A

ash deposition across western US and Canada

48
Q

How many super volcanoes are there?

A

20

49
Q

When was the last super volcano eruption?

A

27,000 years ago Taupo New Zealand

50
Q

What are tuffs description?

A

fragments 2mm in diameter
look sedimentary but lithified volcanic ash
Welded

51
Q

What does welded mean with volcanic deposits?

A

material was still hot enough that they partially recrystallised when emplaced and consolidated

52
Q

What are Breccias?

A

rock formed from angular gravel and boulder-sized clasts cemented together in a matrix

53
Q

What is the structure of breccias like?

A

rock formed from angular gravel and boulder-sized clasts cemented together in a matrix

54
Q

How does pumice form?

A

when lavas have a lot of water and other volatiles

55
Q

What is pumice like?

A

Glassy texture
Light (mostly air many void spaces)
Frothy appearance

56
Q

What are cinders? (volcanic deposits)

A

pebble sized particles

57
Q

What are volcanic bombs?

A

baseball to car sized chunks (due to cooling in air will have head and tail shape)

58
Q

What are mantle plumes?

A

stationary plumes of heat in the mantle that dome the lithosphere and produce magma by decompression

59
Q

What does the magma plume process lead to on land and in oceans?

A

Rift formation within continental plates
Volcanic island chain on oceanic plates

60
Q

What will usually happen first from a mantle plume before hotspot volcanoes?

A

Flood eruptions

61
Q

What happens with flood eruptions?

A

very fluid basaltic lava creates a extensive lava plateau

62
Q

What are some examples of flood eruptions?

A

Deccan traps- India in cretaceous
Siberian traps- Permian, Triassic boundary

63
Q

What are the general characteristics of mafic magma?

A

Usually darker rock produced
Low silica Fe and Mg rich
Less viscous - less sticky

64
Q

What type of eruption will you get with mafic lava?

A

effusive as lava flows

65
Q

What is created with hotspot activity under continental crust?

A

rift systems

66
Q

What is the characteristic geometry of rift systems?

A

“triple junction”

67
Q

What is triple junction geometry?

A

where three lithospheric plates meet

68
Q

How does continental crust cause assimilation of magma to share similar characteristics with subduction volcanoes?

A

Under continent more rock to get through for magma to get to surface so more time for assimilation of material

69
Q

Where is the greatest volume of volcanic rock produced?

A

along oceanic ridges at divergent boundaries

70
Q

What type of basalt is extruded underwater at divergent plate boundaries?

A

pillow basalt

71
Q

How does pillow basalt shape form?

A

rapid exterior quenching of lava bodies

72
Q

What are eruptions like at mid ocean ridges?

A

gentle due to low viscosity and low volatile content

73
Q

What type of magma will form shield volcanoes?

A

mantle derived mafic lava
hot
non-viscous (runny)
dry (limited volatiles)

74
Q

How do shield volcanoes form?

A

Basaltic lava builds up in flat layers creating a large volcano with gentle slopes

75
Q

What is an example of a shield volcano?

A

Typical of Hawaii (Mauna Kea)

76
Q

What type of lava is mafic lava?

A

Block lava

77
Q

What are the characteristics of block lava?

A

less mobile more viscous
Rough irregular surface

78
Q

What happens when block lava cools?

A

goes to angular chunky texture causes lava to thicken and break apart as flow continues downslope

79
Q

What are vesicles in block lava?

A

few and irregular in shape

80
Q

What is the texture of blocky lava referred to as?

A

a-a (sound you make when walking on a flow)

81
Q

What is another name for ropey lava? (mafic lava)

A

pahoehoe

82
Q

What is ropey lava like?

A

more mobile
less viscous
wrinkled but smooth and shiny

83
Q

What are vesicles like in ropey lava?

A

more abundant
small spherical shape

Atoms to volcanoes - 33739 (16 decks)

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