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Geological natural hazards > Magma properties > Flashcards

Magma properties Flashcards

1
Q

What is a hazard?

A

The process that can cuse harm

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2
Q

What should considerations of natural hazards take into account?

A

Frequency
Future probability
Onset speed
Magnitude
Duration
Extent

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3
Q

What does risk consider?

A

the exposure, sensitivity and resilience of:
- populaiton
- Economy
- Land use
- Infrastructure
- Culture
- natural resources

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4
Q

What is risk?

A

the interaction between hazards and human populations depending on vulnerability/ exposure

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5
Q

What is volcanism?

A

The manifestation at a planetary surface of internal thermal processes through the emission of solid, liquid or gaseous products

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6
Q

WHat are some examples of volcanic hazards that are not direclty related to magmatic eruptions?

A

Gas emissions
Pheartic activity
Volcanic landslides

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7
Q

What does volcanism originate from?

A

Melting of earths mantle

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8
Q

What specific conditions are needed for the melting of earths mantle?

A

Relativley high elevated temperatures and reduced pressure in parts of the mantle
Adding fluids to the mantle to reduce melting temp

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9
Q

What is produced when the conditions needed to melt earths mantle are met?

A

partial melt will occur if this ascends towards and reaches the surface volcanism occurs

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10
Q

WHat are the different settings for volcanism?

A

Subduction zones (volcanic arcs)
Rift
Intraplate

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11
Q

How many of earths volcanoes form as a result of subduction?

A

90%

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12
Q

What are the general differnecs between the volcanis settings?

A

In hazard terms fairly comparable but Arc volcanoes span a wider composition range and stsle and thus hazard

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13
Q

What are some examples of subduction zones/ volcanic arcs?

A

Andean volcanic zones
Japan
Indonesia
Lesser Antilles
Tonga

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14
Q

What type of melting occurs at subduction zones?

A

flux melting (addition of water/ volatiles)

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15
Q

How does crustal evolution affect subduction zones?

A

wide composiitonal range commonly evolved, volatile rich magmas Andesites and rhyolites)

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16
Q

What type of plate boundary is rift setting volcanics?

A

divergent

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17
Q

What is an example of a rift setting volcano?

A

Laki (Iceland)

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18
Q

What type of magma is present at the laki ruption?

A

basaltic

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19
Q

What are the eruptions in iceland augmented by?

A

mantle plume

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20
Q

WHat type of melting occurs at roft settings?

A

Decompression melting

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21
Q

What is decompression melting?

A

weight of mantle above creates high pressure raising meltin point when the rock moves to a lower pressure area the melting point is lowered

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22
Q

What does mantle genreation have its origin from?

A

Partial melting of mantle perioditite

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23
Q

What are the original magma melts generally like compositionally?

A

Basaltic in composition (~50wt%SiO2)

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24
Q

What happens to the original melts/ magmas as they ascend through the lithosphere?

A

evolve and accumulate

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25
Q

What sort of processes occur as the magmas rise through the lithosphere?

A

Mixing
Assimilation
Fractional crystalisation

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26
Q

What catergories do the majority of volcanic rock compositions fall into?

A

Basalts
Rhyolites

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27
Q

WHat are basalts like?

A

Primitive
Mafic (basic)

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28
Q

What are rhyolites like?

A

Evolved
Felsic (acid)

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29
Q

What types of volcanic rock does more than 50% of volcnic rock fall into?

A

Basalt
Basaltic andesite
Andesite
Rhyolite

30
Q

How long are sites of volcanic acitivity acitve for?

A

Hundreds of thousands to millions of years

31
Q

What is polygenetic volcanism?

A

The build up of large volcanic landforms through a very large number of eruptions
Linked to long lived focused crustal magma storage

32
Q

What is monogenetic volcanism?

A

when the vents might have greater ditribution forming volcanic fields with several small volcanic constructions

33
Q

What is the typical condition of most volcanic sites?

A

Not to be in eruption

34
Q

The properties of magma affect how it reaches the surface and the hazards produced. What are these properties?

A

Volatile content
Chemical composition
Viscosity
Temperature
Crystallinity

35
Q

Unlike in other areas of life what typically makes volcanoic impacts greater?

A

Size (it does matter in some cases)

36
Q

What is another name for a polygenetic volcano?

A

Stratovolcanoes

37
Q

Lifetime

Volume
Output
example

What are the characteristics of a typical stratovolcano?

A

Lifetime - over 100 kyr
Volume - over 100 km3
Output flux - 0.1 to 1 km3 magma per year

Example - popocatepetl (mexico)

38
Q

What might be closely similar between monogenetic volcanic fields and polygenetic volcanoes?

A

Overall output rate

39
Q

What are the characteristic of a monogenetic volcanic field? (Chichinautzin volcanic field, mexico city)

A

Active for over 100 kyr
Hundreds of vents active in single eruption and form monogentic cones

40
Q

What controls how the products of an eruption are distributed?

A

eruption style

41
Q

What will deposits be like for a powerful explosive eruption?

A

widely dispersed

42
Q

What will deposits be like for a small explosive eruption?

A

Localised
Layered cones from pulsatory

43
Q

What will lava flows be like from low-viscosity lava?

A

sheet like or linear thin lava flows

44
Q

WHat will lava flows be like for high viscosity lava?

A

Thick piles of lava (lava domes)

45
Q

What does it mean saying many eruptions have changing behaviour?

A

explosive phase follwed by an effusive phase

46
Q

What is an example of a volcanic eruption changing behaviour?

A

Chaiten chile 2008
POwerful explosive in may 08 followed by effusive eruption of lava dome

47
Q

What leads to varying volcanic landforms and sizes?

A

Different eruption styles along with the dominant style at a long lived volcano

48
Q

How is size of volcanic eruption indicated?

A

VEI (Volcanic Explosivity Index)

49
Q

On the global scale what are more common small or large eruptions?

A

Small

50
Q

Why is looking at eruption patterns important?

A

For assesing risk and monitoring and mitigation stratergies

51
Q

Why is understanding the hazrads from a long lived volcano eruption challenging?

A

Due to the many different eruption styles, size, magma composition

52
Q

What is a silicate melt?

A

polymerised mixture of silicon and oxygen atoms with varible proportions

53
Q

What are the varied cations and volatiles in silicate melt?

A

Cations - Ca, Na, K, Fe, Mg, Al etc
Volatiles - water, co2, halogens etc

54
Q

What happens in the magma as it ascends?

A

The magma cools as pressure reduces allowing volatiles to exsolve intto gas bubbles and crystals begin to form

55
Q

What is magma usuaully like in the upper crust?

A

polyphase mixture of liquid, solid and gas

56
Q

What might mean the proportions of gas bubbles in polyphase mixtures in the upper crust varies?

A

The bubbles are only present above volatile saturation conditions (pressure depedent)

57
Q

What causes over pressure to occur in crustal magma bodies?

A

Due to no open link to the surface, continued input of fresh, deeper magma, and exsolution of magma

58
Q

What happens if overpressure exceeds the strength of the surrounding rock?

A

Propagating fracture will form allowing magma to ascend

59
Q
A
60
Q

What is the name for the magma filled fracture produced by over pressure?

A

A dyke

61
Q

What is the silica content difference between basalt and rhyolite?

A

B = 50%
R = 70%

62
Q

What is the liquidus temp difference between basalt and rhyolite?

A

B - 1200c
R - 900c

63
Q

What is the melt density difference between basalt and rhyolite? (Annhydrous)

A

B - 2600kg/m3
R - 2300kg/m3

64
Q

How does increasing the silica content of magma affect viscosity?

A

MOre silica = more viscous magma

65
Q

What does a large increase in vicosity also promote?

A

crystallisation

66
Q

What out of hazards, exposure and vulnerability can be chnaged to reduce risk?

A

exposure and vulnerability

67
Q

What is exposure in relation to hazards?

A

something for the hazard to interact with

68
Q

How many active/ erupting volcanoes are there each year?

A

~50

69
Q

How many volcanoes have erupted in the holocene (10,000yrs)?

A

~1500

70
Q

What does the presence of more volatiles do to explosivity do?

A

increase it

71
Q

What causes fluids to be released from the subducting plate?

A

metamorphism

Geological natural hazards (11 decks)

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