GEOS2111 Chap.8 Volcanic eruptions and their hazards
Volcanic eruptions and their hazards
Week 8 is the second of the geological weeks and the unit frames volcanoes as a lithospheric hazard alongside earthquakes. The stated scope is where volcanoes are located, the different types, the scales used to size them and the hazards they set off, with the lectorial turning to monitoring and management through case studies including volcanic hazards in Australia. Location is not random.
The tectonic section used in this unit places volcanoes in three settings: at an oceanic spreading ridge where plates diverge, above a subducting plate producing an island or continental arc aligned parallel to the trench, and over a hot spot independent of any boundary.
Reading set with the unit adds the arc mechanism, in which magma is generated at a depth of around one hundred kilometres before rising to intrude the crust or erupt explosively.
Two edifice types are named and each is a readable record of eruption style: a shield volcano is broad and gently sloping, built by fluid lava that travelled far, while a stratovolcano is steep and layered, built by alternating flows and fragmental material close to the vent.
The productive way to hold the topic is as a causal chain rather than a taxonomy: magma properties decide eruption style, style decides which hazards are produced, and the hazard set decides what management is possible. Fluid magma lets gas escape and produces effusive eruptions; viscous magma traps gas and produces explosive ones.
Once a flow begins, the unit names three controls on how it travels downslope: the viscosity of the magma, the eruption rate and the topography. The hazard vocabulary follows from the flow field: tephra carried on the prevailing wind, lava tubes that insulate a flow so it travels further, breakouts, volcanic smog and the plume formed where lava meets the sea.
Management is unusually tractable because vents are fixed and precursors are real, which is why the chapter closes on a case in which the science worked and the decision chain did not.
What this chapter covers
- 01
Three tectonic addresses: spreading ridge, subduction arc and hot spot
- 02
Shield against stratovolcano, and why the shape is a record of eruption style
- 03
A chain of islands over a fixed plume, and why the spatial sequence is a temporal one
- 04
Magma properties to eruption style: viscosity, dissolved gas, effusive and explosive
- 05
Three controls on a flow once it starts: viscosity, eruption rate and topography
- 06
The flow-field vocabulary: tephra, lava tube, breakout, volcanic smog and the coastal plume
- 07
Volcanically induced earthquakes, and precursors as both hazard and signal
- 08
Why hazards divide by timescale, and why that division drives everything about management
- 09
Recency-based hazard zoning, and why the same logic fails for floods
- 10
Ash management across the four phases, and a monitoring success that was a decision failure
Advising on evacuation below an active vent
- +1Separate the two hazard clocks. Lava advance is slow, and eight kilometres through a flow field is measured in days rather than minutes, so lives are not immediately at risk from the flow. The road is different: it crosses the flow field, so it can be cut long before the settlement is threatened, and once cut, evacuation stops being a choice.
- +1Read the monitoring signals for what they are. Rising shallow volcanic earthquakes indicate magma and fluid moving in the crust, and a new breakout means the tube system is not carrying all of the supply. Together they indicate an increasing supply rate, which is one of the three controls on how far and fast a flow travels.
- +1Act first on the thing that expires first. Establish an alternative route, or stage transport on the settlement side of the flow field, before the road is lost. This is an engineering and logistics control applied to exposure rather than to the hazard.
- +1Stage the evacuation rather than making it binary. Pre-position it, set an explicit trigger tied to a monitored quantity such as flow front position or a stated change in seismicity, and communicate that trigger to residents in advance so the eventual order is expected rather than surprising. Full evacuation now is defensible but expensive and erodes compliance if the flow stalls.
- +1State the residual risk and the assumption. Ash and gas can affect the settlement without any lava arriving, so air quality monitoring and respiratory advice belong in the same plan. And the whole recommendation assumes the eruption stays effusive; if the monitoring agency revises that, every timescale collapses and the staged plan becomes an immediate one.
Key terms
- Shield volcano
- A broad, gently sloping edifice built by fluid lava flows that travelled far from the vent. Its profile is a record of effusive eruption style.
- Stratovolcano
- A steep, layered edifice built by alternating lava flows and fragmental material deposited close to the vent, characteristic of explosive eruption style.
- Mantle plume
- A persistent column of rising hot mantle material that produces volcanism independent of any plate boundary. Because the plume stays put while the plate moves, it builds an age-progressive chain.
- Hot spot
- The surface expression of a mantle plume. The Hawaiian chain is the unit's worked example, with island ages increasing away from the only actively erupting volcanoes.
- Magma viscosity
- The resistance of magma to flow. It determines whether dissolved gas can escape freely, and therefore whether an eruption is effusive or explosive.
- Effusive eruption
- An eruption in which gas escapes readily and molten rock is poured out as flows rather than thrown out. It destroys property, builds broad cones and rarely kills.
- Explosive eruption
- An eruption in which trapped gas pressurises viscous magma until it fragments, producing airborne material that travels far and arrives fast.
- Lava flow
- Molten rock pouring out onto the surface during an eruption, then running downhill at a speed and in a manner set by how viscous the magma is, how fast it is erupting, and the shape of the ground.
- Lava tube
- A roofed channel formed when the surface of a flow crusts over, insulating the molten interior so that it travels much further than an open flow would.
- Breakout
- A point at which lava escapes a tube, which is why a flow front can advance somewhere nobody was watching.
- Tephra
- Fragmental material thrown out by an eruption and carried on the prevailing wind, so its hazard footprint is decided by weather rather than by terrain.
- Volcanic smog
- The persistent air-quality hazard produced by released volcanic gases. It is the hazard that outlasts the eruption and affects people who are never threatened by lava.
Volcanic eruptions and their hazards FAQ
Why do volcanoes occur where they do?
In three settings, and the settings explain most of what a volcano will do. At an oceanic spreading ridge, plates diverge and new crust forms, producing generally fluid eruptions.
Above a subducting plate, magma generated at a depth of around one hundred kilometres rises to intrude the crust or erupt explosively, producing arcs of volcanoes aligned parallel to the trench, and these are the systems that produce the most dangerous eruptions.
Over a hot spot, a persistent mantle plume produces volcanism independent of any boundary, and because the plate moves over the fixed plume the result is an age-progressive chain of islands whose spatial sequence is also a temporal one. Being able to place a volcano in one of the three settings gives you a first estimate of its eruption style before you know anything else about it.
What decides whether an eruption is explosive?
Magma properties, specifically viscosity and dissolved gas content, and the chain runs in one direction only. Where magma is fluid enough for gas to escape as it rises, pressure never builds and the eruption is effusive, producing flows that build broad, gently sloping cones.
Where magma is viscous enough to trap gas, pressure accumulates until the magma fragments, and the eruption is explosive, producing airborne material and steep, layered cones. Once a flow has started, the unit names three separate controls on how it behaves as it travels downslope: the viscosity of the magma, the eruption rate and the topography.
A question about why one flow reached a town and another stalled is answered from those three, and the combination of viscous magma, low eruption rate and shallow gradient is the one that stops.
Why is recency a defensible way to map lava hazard?
Because the two things that decide where a future flow will go are stable. Vents are persistent features that erupt repeatedly, and the downslope path is controlled by topography that changes slowly and largely through the flows themselves. A map of where lava has recently gone is therefore a reasonable map of where it will go next, and recency also indexes how active the producing vent is.
That is why the unit's own practical classifies historical flow polygons by age into five hazard zones, with recent flows implying very high hazard. It is worth noticing where the same reasoning fails: for riverine flooding, channel geometry, land cover and infrastructure change on decadal timescales, so a map of historical flood extents in a developing catchment can understate present hazard rather than represent it.
Why are volcanic hazards divided by timescale?
Because the division drives management completely. Flows and ground-level material advance at a pace people can usually outrun, so they destroy property, displace communities and rarely kill; the management response is evacuation, land use zoning and, occasionally, diversion.
Airborne and rapidly moving material travels far, arrives fast and is what produces mass casualties; the management response has to be in place before the event, because there is no time to improvise it. Two historical eruptions used in the unit's century-scale disaster figure killed on the order of tens of thousands of people each, and neither did so with lava that anyone could have walked away from.
When you write about a volcanic disaster, name which category of hazard produced the deaths you are describing.
What does the risk-assessment failure case teach about volcanoes specifically?
That volcanoes are the best-monitored hazard in this unit and that monitoring alone is not a control. Vents are fixed, precursors are real, and instruments can be concentrated on a known point rather than spread across a landscape, so when a volcanic disaster occurs the science has usually already done its part.
In the case the unit anchors risk assessment to, a national agency had published a quantified short-term probability of eruption about a week beforehand. What failed was that a qualitative risk assessment for the tourism activity had not been commissioned, and that the published risk was not conveyed to the public in a form that changed behaviour. Twenty-two people died and twenty-five were injured.
The generalisable lesson is that a probability which never reaches the person about to be exposed has not reduced anyone's risk.
Exam move
Learn this chapter as a chain and you will not need to memorise a taxonomy. Write the chain across one page: magma properties, then eruption style, then hazard set, then management response, with two or three items under each. Then add the three controls on a flow, which are the most quotable detail in the week and answer any question about why one flow travelled further than another.
Second, draw the hot spot chain once with the island ages marked, because it is the cleanest available demonstration that a spatial pattern can encode time, and because it justifies the recency zoning rule the practical uses. Third, make a two-column list of hazards by timescale, slow and fast, and attach the management response to each; that division is the fastest route into any management question.
Fourth, hold the risk-assessment failure case with its numbers, since it serves three chapters at once: it is a volcanic case, a risk-assessment case and a risk communication case. Finally, note where the unit's own materials will be your source rather than this book.
The Week 8 lecture covers how volcanoes are measured and the lectorial covers precursor signals and Australian volcanic hazards, so take the measurement scale, the alert-level system and the Australian cases from your own slides, because alert ladders differ between countries and the one your unit uses is the one that will be examined.
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