Engineering Geology and Geohazards Lecture 5: Volcanology Flashcards
Define a direct impact
A Local impact- Economic / Social
Global impact
Define a secondary impact
A Famine for example
Which is the Main killer in terms of eruptions;
300,000 dead Asama, Japan,1783
90,000 dead Tambora, Indonesia,1815
Define VEI ( Volcanic Explosivity Index )
VEI is a measure of the intensity of a volcanic eruption.
Measures of the size of an eruption through :
-the volume of lava ejected as pumice and ash (tephra) during an explosive phase
-the volume of lava extruded during an effusive phase.
List the VEI
0 – Non explosive
* 1 – Small
* 2 – Moderate
* 3 – Moderate-large
* 4 – Large
* 5 – Very large
* 6 -
* 7 -
* 8 - Cataclysmic
Define and explain a volcano
What is a volcano?
* A volcano is one of the ways in which a planet loses heat
Planets are hot in the centre due to heat - caused by radioactive decays, gravitational forces, solar heating.
- Most heat is lost by simple conduction
- However, intense local heating can cause;
-melting of planetary material
-Hot, molten material escapes to the surface, bringing heat with it - a volcano
Define Terrestrial volcanism
Terrestrial volcanism is controlled by active mantle processes and plate tectonics
It is the eruption of Silicate magmas derived by
partial melting of upper mantle or lower crust
- Magma temperatures on eruption -
700-11500C - Acid (high silica) - Intermediate - Basic
(low silica)
Describe the cycle of Eruptive events
Cycle of Eruptive Events:
* Dormant phase
* Seismic indications
* Gas emission and composition change – meteoric to magmatic
* Magmatic inflation
* Eruptive phase
List the Empirical classification of eruptions (Gradually increasing in viscosity)
- Hawaiian (fissure, Icelandic)
- Strombolian
- Vulcanian
- Plinian (Ultra-Plinian)
- Pelean
- Surtseyan (phreatic)
Describe a Hawaiian eruption
Effusive Eruptions
–> Produce Lava flows
Less viscous (the magma has come up through thinner sections of crust, this magma is less developed/primitive form of magma) , High temperature
Define Viscosity
Viscosity is the thickness and ‘stickyness’ of magma, as well as the amount of gas bubbles present.
The longer magma spends rising through the crust, the more gas bubbles it accumulates and therefore more developed/viscous it becomes.
The more gas bubbles/higher viscosity an eruption the more explosive it is, as the gas bubbles ‘pop’.
Describe a Strombolian eruption
More viscous, thicker, more gas bubbles. Eruptions more explosive than Hawaiian.
Eruption launches lava bombs into air by force of explosion due to volcanic gas bubbles exploding.
Describe a vulcanian eruption
Vulcanian eruptions are even more viscous and explosive than strombolian eruptions.
Often there is a volcanic plug, which ‘bursts’ when the volcano erupts.
Describe a Plinian/Ultraplinian eruption
Even more explosive and viscous than vulcanian.
Releases huge amounts of volcanic gas, these clouds are extremely dense.
Ultraplinian eruptions are the largest and most explosive eruption type.
For example Pinatubo 1991, Eruptive column rose 50km above volcano.
Describe a Pelean eruption
In a Pelean eruption the magma reaches the surface in a very solid state with lots of gasses. The very solid magma compresses the gas bubbles.
The magmas in these eruptions is very viscous.
These eruptions often create pyroclastic flows.
Describe a Surtseyan/(Phreatic) eruption
A Surtseyan eruption is one where water interacts with the eruption and the magma released.
This causes significant evaporation of the water which adds to the gas cloud as well as launching additional ash up into the air.
Logically happen on island volcanoes and near coasts.
