Engineering Geology and Geohazards Lecture 6: Volcanic hazards

Question 1

Define Lava flows

Answer 1

Product of effusive eruptions.
– From effusive volcanoes (Hawaii, Stromboli…)
– From composite/stratovolcanoes (Vesuvius, Teide…)

Question 2

What are Lava flows usually made up of?

Answer 2

Generally (not always)
basaltic composition.

• Basalts- Erupted at
  high temperatures
  ~1200 °C
• Basalts- Some of the
  denser rocks on earth

Question 3

What are the main 3 morphological activity

Answer 3

• Aa,
  – Pahoehoe,
  – Blocky

Question 4

Define and describe Blocky lava flows

Answer 4

Rough surface, similar to aa BUT
fractured in large blocks with smooth
faces.
* Continuous advance
* Extremely slow advance (1- 0.01 km/yr)
* Might generate channels in steep slopes.
Tend to spread.
* Width (< 1000 m) & thickness (> 10 m)
conditioned by rheology and topography.
* Lengths dependent mainly on rheology,
generally <4km

Question 5

Define Pahoehoe lava flows

Answer 5

Smooth and/or ropy surfaces
* Pulsed advance (toes), less often as
sheets.
* Slow advance of fronts (1-0.1 km/day)
* Generate frequently tubes
* Width conditioned by topography
* Thickness depends on growth by
“inflation”
* Length dependent of length (time wise)
of the effusion

Question 6

Describe a Pahoehoe flow field

Answer 6

Created by the continuous expansion of an original pahoehoe flow.

thickness conditioned by “inflation”, caused when the
intrusion of new flows lift the original surface.
* Tubes or channels can get blocked or overwhelm creating new
flows

Question 7

Define and Describe Aa lava flows

Answer 7

Rough, contorted surfaces,
topped with loose rubble
* Continuous advance of
front until solidified
* “Fast” advance of fronts
(1-10 km/h)
* Produce channels and less
frequently tubes

Width of flows conditioned by cooling.
* Thickness conditioned by
viscosity
* Length dependent on effussion
rate
* Main lengthening infirst
48h

Question 8

What are the hazards of lava flows

Answer 8

– Ignition - Houses, structures, plants
* Very important- FOREST FIRES!!

– Destruction- Permanent~ Require removal of material or
reconstruction.
* Buildings- Demolished or surrounded
* Structures & Infrastructures- Demolished, surrounded or invadedroads, burst pipes
* Facilities- Destroyed – sky lifts, harbours, touristic attractions, etc.
* Land- Invasion, changes on properties of soils (permeability)

Question 9

List some mitigation methods for lava flows

Answer 9

-Levee disruption
-Contention barrier
-Diversion barriers
-Solidification by cooling with water
-Tube blockage
-Dam Barrier

Question 10

Describe mitigation of lava flows; before, during and after

Answer 10

BEFORE- Land planning- Building at certain distance of
effusive volcanoes

DURING- Stopping lavas to approach areas of interest
- Lava diversion barriers
- Lava damming barriers
- Levee disruption
- Lava tube/channel blockage
- Cooling lava flows
- Lava disruption with explosives

AFTER- Removal of the lava
once solidified

Question 11

Describe the mitigation method of lava cooling

Answer 11

The aim of this procedure is to stop or slow the movement of a lava front OR stop a marginal area from expanding.n

• The procedure consist of pumping water to a front/margin in order
  to solidify it and stop the advance.
• Can be applied to pahoehoe and aa flows

Question 12

Describe ballistic tephra (Projectiles)

Answer 12

Radial distribution; within 3 - 5km of vent

• Wind has little effect.
• Larger sizes may reach further (less resistance).
• Projectiles > ~ 10cm may have terminal velocities.
• Parabolic trajectories.

Impacts:
– Destruction of property and facilities
– Lethal to people
– General disruption
– Burns

Question 13

How is ballistic tephra mitigated

Answer 13

• Evacuation/restrict access to areas while eruption lasts
• Bomb Shelters
• Helmets/ padding

Question 14

Describe Ashfall (tephra)

Answer 14

Most voluminous product of
explosive eruptions (VEI 2- 8)

• Eruption columns typically up to
  10km (may reach > 50km)
• Strong wind influence
• Downwind transport velocities
  <10 - <100 km/h
• Exponential fall in thickness
  downwind
• Can extent >1000km downwind

Question 15

List impacts of ashfall

Answer 15

Pumice may be hot enough to
ignite fires at 30+km
* Density of compacted wet ash
may be 1.6 tonnes/cubic m
* 30cm may collapse roofs
* Visibility may be a few 10s cm for hours
* Highly abrasive

Surface crusting of fine ash
promotes runoff
* Provide source for lahars

Health issue for humans and livestock.

Damages Infrastructure

Very dangerous for aircraft

Question 16

List ashfall mitigation measures

Answer 16

• Evacuation
• Removal of accumulating tephra from roofs
• Availability of face masks / goggles/ protective gear
• Medical care - respiratory problems
• Plans for infrastructure disruption
• Availability of water and food supplies