GEOL1002 Lecture Notes - Lecture 12: Lava Dome, Pumice, Tephra
5 Jul 2018
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Volcanology
Products of volcanic eruptions
Products of an eruption occur in three forms:
1. Lava flows
2. Pyroclastic
3. Debris/gas
Lava flows
Not all lava behaves the same way.
The character of lava is a reflection of its viscosity which in turn is dependent on its
chemical composition and its SILICA CONTENT.
The polymerisation of silicate minerals results in molecules being unstable to flow
past each other easily. The higher the silica content the higher the viscosity.
Viscosity is also dependant on temperature. Thermal energy breaks chemical bonds
linking molecules so hotter magmas are less viscous.
Basaltic lava flows
Basaltic lava has very low viscosity – very hot and low in silica.
On steep summits, basaltic flows can reach up to 30kph, however it slows
significantly as it cools.
Most flows are less than 1km in length, but can reach several kms.
As lava flows the outside cools and ‘crusts over’ insulating the hot interior allowing it
to flow until the whole lava flow solidifies.
Lava flow surface textures depend on the timing of freezing relative to its
movement.
Flows with warm, smooth surfaces wrinkle into glassy, ropy ridges (Pahoehoe).
Flows that are too viscous to contort into Pahoehoe ropes tend to break up into
rubbly angular fragments (a’a).
Basaltic flows that erupt underwater cool extremely quickly. Submarine basaltic lava
can travel only a short distance before it freezes, producing a glass-encrusted blob
(pillow). A flow made of these blobs is called pillow lava.
As the last of a lava flow’s heat radiates outward, the rock in the lava flow shrinks
and contracts as a result of losing heat. This shrinkage causes it to crack, forming
natural fractures called joints.
Andesitic lava flows
Intermediate lava is more viscous than basaltic lava so it doesn’t flow as easily.
When erupted, andesitic lava first forms as a mound above the vent of the volcano.
These mounds build up and then eventually advance down the volcano at a slow rate
of 1-5m a day as a lumpy flow.
High viscosity and slow flow means that the surface solidifies quickly forming angular
chunks. The whole flow is called a blocky flow. These flows often crumble and fall as
a rockslide down the volcano.
Rhyolitic lava flows
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Document Summary
Products of an eruption occur in three forms: lava flows, pyroclastic, debris/gas. The character of lava is a reflection of its viscosity which in turn is dependent on its chemical composition and its silica content. The polymerisation of silicate minerals results in molecules being unstable to flow past each other easily. The higher the silica content the higher the viscosity. Thermal energy breaks chemical bonds linking molecules so hotter magmas are less viscous. Basaltic lava has very low viscosity very hot and low in silica. On steep summits, basaltic flows can reach up to 30kph, however it slows significantly as it cools. Most flows are less than 1km in length, but can reach several kms. As lava flows the outside cools and crusts over" insulating the hot interior allowing it to flow until the whole lava flow solidifies. Lava flow surface textures depend on the timing of freezing relative to its movement.
