Explosive eruptions Flashcards by Andrew Jeavons

What are the relevant parameters that cause magma to erupt in an explosive way?

Volatile content
Magma viscosity (and therefore composition)
Mass eruption rate and ascent rate
Presence of external water (e.g. groundwater, surface water)

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What s tephra?

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

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What can tephra be?

Lithic clasts, pyroclastic (juvenile) material, crystals.
The grain-size of this material includes lapilli (2 – 64 mm) and ash (<2mm)

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What does high fragmentation mean for a volcanic event?

Large production of fine particles (volcanic ash)

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What does high fragmentation correlate with?

Explosive intensity
Column height and dispersal

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When do column height and dispersal increase?

Mass eruption rate

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What is the explosive eruption stye dependant on?

Composition and physical magma properties

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What is fundamental to explosive eruption dynamics?

presence and behaviour of bubbles in magma

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What wil the water content of ocean ridge basalts be like?

Low water content (<0.5wt%)

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What will the water content of Arc (subduction) magmas be?

Higher than ocean basalts >5wt%

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How does high pressure affect the presence of volatiles?

At high pressure, volatiles are completely dissolved (i.e. the magma is under- saturated in these compounds)

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How does lower pressure affect volatiles in magma?

Volatiles have a lower solubility, and they thus become saturated in the melt. At this point they can come out of solution, forming bubbles
In reality, bubble nucleation may be limited by kinetic factors, and may require nucleation sites such as crystals

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What happens to the bulk magma density as bubbles form?

Decreases

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What does decreased magma bulk density result in?

Increasing buoyancy and ascent velocity

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What must be the case with bubbles in magma for an explosive eruption to occur?

Bubble exsolution must be sufficient to lead to fragmentation

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How does low viscosity lava affect bubble behaviour?

bubbles may expand, coalesce, and ascend through the magma (buoyantly), and are more able to escape the magma

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What drives explosive eruptions in low viscosity lava?

Explosive eruption driven by expanding bubbles and deformable magma, in bursts and jets

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What are some examples of low viscosity magma explosive eruptions?

Hawaiian and Strombolian eruption styles

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How do explosive eruptions occur in higher viscosity magmas?

viscosity may hinder bubble buoyancy and expansion.
Bubble overpressure is generated and bubble cannot coalesce: ascent is accompanied by closely packed small bubbles and eventual fragmentation (by
expansion and/or brittle fracture)

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What are an exmaple of explosive eruption types linked ot high viscosity magma?

sub- to ultra- Plinian eruption styles

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What happens if magma reaches the surface without fragmentation?

Effusive eruptions occur

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What is a scoria pyroclast?

A scoria pyroclast – the product of explosive
eruption of mafic magma (e.g. basalt). Scoria clasts
typically reach heights of a few hundred metres, with their ascent driven by their initial momentum from the energy of the explosion. They fall back to the
surface locally, building up around the vent

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What will mean most pyroclasts fall relatively local to the vent?

low ash content (i.e. dominated by relative
coarser clasts) and low height (hundreds of m to
perhaps a few km)

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How are concentric cinder cones formed?

Repeated pulsatrry eruptions of pyroclasts which create a central cone with often well defined internal bedding

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When did the Paricutin eruption occur?

In mexico between 1943-52

What was the lava type at the Paricutin eruption?

Basaltic

What was the behaviour of the Paricutin eruption?

first phase was strombolian explosions followed by lava flows

How were humans affected by the 43-52 Paricutin eruption?

Two towns entirely destroyed 26 km2 lava >400 m cone height No direct deaths

What evidence is there that volcanism in Paracutin is unpreditable in both timing and location?

Very large number of prehistoric cones in the volcanic field

What is fragmentation?

Transition from a bubbly melt to a gas with suspended melt fragments (pyroclasts). Occurs when bubbles become close packed, forming a continuous gas phase with suspended particles of melt

How does fragmentation affect the lava?

Dramatic lowering of bulk density and viscosity, leading to further acceleration towards the surface

To what velocity can the magma mixture be accelerated to?

Gas expansion and reduced viscosity and density speed it to >100m/s through the conduit

What affects the explsoive eruption style?

The interaction of the momentum fuelled jet with the atmosphere

How can the atmopshere affect the style of explosive eruption?

Efficient mixing with the air will lead to convection and buoyant ascent of the plume and tephra fallout Insufficient mixing the plume density remains > then atmospheric density the plume fall back to the surface creating pyroclastic density currents

What is the trend of a normal atmospheric density gradient?

decreases with height

What happens to the atmospheric density gradient when a volcanic jet mixes with air?

Jet mixes with air causing a heating of air leads to expansion and lower density creating buoyancy which drives ascent

What happens when plume density is less than atmospheric density?

means plumes form large volcanic eruptions can ascend well into the stratosphere (25km or more)

What does turbulent entertainment do to the momentum of the jet when its interacts with the atmosphere?

decelerates the mixture

What does initial kinetic energy allow the ascent of the plume to be?

~10*2-3 m

What is critical for higher ascent of gas plumes?

Buoyancy

What are typical heights of gas plume for sub-Plinian/Plinian eruptions?

10-30km

What can mean both plume clouds and pyroclastic density currents?

larger particles may separate, forming PDCs; the remainder forming a buoyant plume

How can Pilnian eruption column heights be affected by release height?

Exceed the tropopause into the stratosphere the plume can then be dispersed rapidly be strong stratospheric winds

What can happen to material dispersal by stratospheric winds in the tropics?

Can spread into both hemispheres

What is ash fall?

Tephra particles that settle out of this cloud, forming tephra fall deposits

What is dispersal of ash fall controlled by?

Wind speed and direction Particle size and density (more small particles, more distant impacts) Column height

How do dispersion paramters affect the depostion of fall deposits?

Predictable thickness chnage (broadly exponential thinning) and grain size (coarse to fine)

What can be used to map and calculate volumes of fall-deposits?

Isopach maps (show lines of constant thickness)

What are typicall fall deposit characteristics?

Blanket topography Well sorted Consistent density/grain-size relationships Predictable thinning and fining relationships

What are clasts often like? (from explosive eruptions)

pumice (low density, very highly vesicular)

What is the most devestating impact of fall deposits?

Burial/ loading especially to vegetation and infrastructure (roof collapse)

What is one of common areas affected by ash plumes and fallout?

Aviation

What is an example of an area where the avaition industry was affected by volcanic ash plumes?

Bariloche airport, Argentina – 200-300 km E of Chilean volcanoes Several months disruption after Chaiten (2008), Puyehue (2011), Calbuco (2015)

How are planes affected by ash plumes?

Engine damage (cases of complete loss) Abrasion

What are the broader impacts of ash fallout?

air quality, agriculture, electrical systems, pumps etc.; and secondary impacts from wind-driven remobilisation

On what scale will the largest of volcanic eruptions be?

Global

What is the most significnt and widely dispersed element of large eruptions?

Sulphur with particular impacts for the climate

How does sulphur dioxide affect the climate?

Acts as a reflectant in the atmosphere leading to a net warming of upper atmopshere but a net cooling of the surface

What does a higher mass eruption mean lead to?

Faster and greater flux volume of magma promoting greater explosivity

What is framentation generally?

how intensive/ efficient the explosive eruption process is

What is lithostatic pressure?

The pressure of overlying rock which increases with deth

How are solubility and pressure linked?

The more soluble a volatile the more of it that can be held at a greater pressure

Is the differnce between obsidian and pumice chemical or physical?

Physical

What is the bubble differnece between high and low viscosity?

LOw viscosuty bubbles can expand and bacome buoyant leading to ascension will there are more restrictive forces in high viscosity creatng small high frequency bubbles

What does fragmentation do to the magma?

Changes it from a bubble liquid to a gas with suspended solids

Why does overshooting of the gas column occur and what stops continued ascension?

Overshoot due to moment this stopeed by area of neutral buoyancy where air cannot rise higher due to reduce pressure difference