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Geological natural hazards > Effusive eruptions > Flashcards

Effusive eruptions Flashcards

1
Q

What 2 ways can the behaviour of magam be classified when it reaches the surface?

A

Explosive
Effusive

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2
Q

What are key controls of explosive eruptions?

A

viscosity and gas content

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3
Q

What is first needed for an effusive eruption to occur?

A

magma must reach the surface as a liquid (lava)

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4
Q

What is low viscosity magma?

A

this occurs if bubbles can escape and/or if there’s
insufficient gas to drive explosive fragmentation of the magma

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5
Q

What is high viscosity magma?

A

this occurs if there is no gas loss or high gas content
(and slow ascent)

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6
Q

What is lava?

A

magma reaching earths surface in liquid form

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7
Q

What is tephra?

A

the fragmental material generated by the break-up of magma during explosive eruptions

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8
Q

What structures do low and high viscosity lava produce?

A

Low - lava flows
High - lava domes

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9
Q

What are the typical compositions of lava flows and domes?

A

Flows are mafic
Domes are felsic

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10
Q

What composition are the majority of explsoive eruptions?

A

MOre evolved higher viscosity

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11
Q

What is lava viscosity/ behaviour dependant on?

A

Silica content
Other chemical species (volatiles)
Temperature
Crystal content

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12
Q

What does viscosity determine?

A

Lava flow - speed, thickness and surface morphology

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13
Q

What do viscous lavas form?

A

Blocky thick relatively short flows, coulees and domes

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14
Q

What are pahoehoe and A’a lava flows?

A

BAsaltic lava flows
P - ropy
A - rubbly

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15
Q

Which magma type is more polymerised basalt or rhyolite?

A

rhyolite is more polymerised, more silica-rich lower amount of other cations that inhibit polymerisation. The silica content is by far the strongest influence on polymerisation and viscosity.

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16
Q

Which is more dense in general out of basalts and rhyolite lava?

A

The basalt – it is much richer in Mg, Fe, Ca

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17
Q

What is density of lavas dependant on?

A

temperature - higher temp = lower density

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18
Q

what is the relationship between viscosity and temperature?

A

reduced viscosity with increased temp

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19
Q

how does adding dissolved h2o to silicate melt affect viscosity?

A

significant reduction in viscosity

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20
Q

What might the eruotion differeces be between a degassed rhyolite and a water rich rhyolite?

A

Water rich magma, with relatively lower viscosity, is able to flow more easily, potentially ascend
more rapidly and more likely to erupt explosively than a gas-poor rhyolite, which may ascend
slowly and erupt effusively in styles typical of very viscous magma (lava domes)

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21
Q

What area of monserrat was most affected by the 1990s to 2013 eruption?

A

South of the island

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22
Q

What is the contrast of lava flows between Monseratt and Hawaii?

A

Hawaii much more runny can flow on very gentle gradients

23
Q

How are Pahoehoe and A’a linked?

A

Pahoehoe is the thin runny ropy phase but as the crust cools and solidifies it creates a protecive shell insulating the interior allowing continued flow

24
Q

Will lava flows by a one and done hazard or does it occur in pulses?

A

Pulses with layers building up on top of each other

25
Q

What was the feeder structure for the 2018 eruption of Kilnauea east rift zone?

A

A fissure which was predominanlty sub-surface apart from eruption/ surface break area

26
Q

What were the effects of the 2018 Kilnauea east rift eruption?

A

2000 evacuated
Infrastructure damage - 700 homes, roads and electrical plant ($800 million)
Over an area of 35km2

27
Q

What do hawaiian lava flows usually pose a threat to?

A

infrastruce with them being so slow humans arent at too much threat

28
Q

What are the chracteristics of lava flows in HAwaii?

A

Relatively fast moving
Spreading rapidly even on low gradients, forming sheet-like bodies
Fed by fissures where dykes breach the ground surface

29
Q

Why are the Hawaiian flows relatively fluid/ low viscosity?

A

Although they have a low water content, they are high temperature and crystal poor
Their advance is controlled by cooling, formation of a surface crust and crystallisation

30
Q

What re the hazards associated with Hawaiian lava flows?

A

Infrastructure damage, property destruction (resurfacing of the land)
Site/extent of flows can be unpredictable, requiring evacuation

31
Q

What are the lava of Etna generally like?

A

More crystalline
Cooler on eruption
Relatively more viscous
Slower and thicker

32
Q

How would the movement of Etna lava flows be described?

A

Linear usually flowing down steeper slopes/ following valleys

33
Q

What will Etna lava flows look like?

A

Blocky surface with rubbly front

34
Q

What will the length of etna lava flows be?

A

Can exceed 10km

35
Q

What can the flow speed of Hawaiian lava flows be like?

A

10s to 100s of metres an hour

36
Q

In what scenenario will flow velocity of basaltic lava be greater than 1km an hour?

A

In the insulation of a lava tube

37
Q

What is an example of the flow speed of Mt Etna eruptions from 2003?

A

3km in 50 days so 2.5m per hour

38
Q

What does lava flow length reflect?

A

eruption rate (advance) and cooling rate (hinderance)

39
Q

What will happen when one lava flow ceases to advance but the eruption of lava still continues?

A

Lava flows will take other paths and keep advancing downslope by diverting laterally creating lava fields

40
Q

What is lava distribution facilitated by?

A

Channels
Tubes
Breakout lobes

41
Q

How are lava tubes produced?

A

When lava flows down slope it will colelct and build up rubble forming levees as these grow eventually meeting at the top cooling and forming a roof which insulates the alava allowing effecient movement

42
Q

What is lava tube developemnt dependant on?

A

Discharge rate and slope angle

43
Q

What are some examples of where lava flow diversion has been attempted?

A

USA tried bombing hawaii flow to open crust and cool the flow this was unsuccessful
In heimay iceland sucessfully used seawater
In Etna they create embankments to change direction and use explosives to slow flows

44
Q

What was a succesful exampel of lava diversion from etna 1992?

A

diversion using embankments and ditches saving the town of Zafferana

45
Q

What is an example of an unusally fast lava flow that did leave to a loss of life?

A

Nyirangongo 2002 in the DRC
Flow speed over 10m/s

46
Q

What were the effects of the fast Nyirangongo lava flows?

A

72 deaths 10km from vent
half of city Goma destriyed
~150 fatalities
~250,000 displaced

47
Q

What 2 ways can lava domes grown?

A

Endogenous (undreground)
Exogenous (above ground)

48
Q

How do endogenous lava domes form?

A

Magam rising from below forces the ground up

49
Q

What is the main problem associted with lava domes?

A

Their lack of stability shown by blocky scree material at their base if they collpase it can lead to catastrophic flows

50
Q

What leads to the production of domes over flows?

A

Crystallisation and degassing as the magma rises

51
Q

How long can it take for lava domes to form?

A

Growth is slow and make take months of years

52
Q

What are vulcanian exposions?

A

Small short lived eruptions which are caused by pressure build up from the small amount of gas present in lava domes

53
Q

What was the effect of the Monseratt eruption on the island?

A

South was most developed area with airport and deep sea port which were destroyed along with the town of plymouth
2/3 of 13000 population have not returned remaining population limted to north of island

54
Q

What does exogenous growth of lava domes produce?

A

solid lava spines (an example from the Mount Pelee in 1902 was taller than old joe, 300m high lasting for 11 months)

Geological natural hazards (20 decks)

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