September 2022
CHAPTER 9
The Wrath of Vulcan: Volcanic Eruptions
9.2 The Products of Volcanic Eruptions
Products of an eruption come in three forms; lava ows, pyroclastic debris and gases.
The character of a lava primarily re ects its viscosity, and not all lavas have the same
viscosity. This is in uenced by the chemical composition, temperature, gas content and
crystal content.
- When the proportion of silica in lava increasers, the lavas ability to ow decreases.
Basaltic (ma c) lava has low viscosity and ows easily when it rst emerges from a volcano.
Due to the high temperature and low concentration of silica.
- Rapid cooling causes the surface of the ow to harden after the ow has moved a short
distance -> new molten lava injects itself between the original ground surface and the
new solid crust -> this addition of lava in ates the ow, jacking up the hardened crust
and making the overall ow thicker -> the lava only moves thought a tunnel-like
passageway or lava-tube.
- Columnar jointing; type of fracturing that yields roughly hexagonal columns of basalt.
- Pahoehoe; a basalt lava ow with a surface texture of smooth, glassy, ropelike ridges.
Andesitic lava ows less easy than basaltic lava due to a higher viscosity. Because it moves
slower the lava has more time to solidify. As it moves the surface breaks up into angular
blocks.
Rhyolitic lava is the most viscous of all lavas because it has the highest silica concentration
and the coolest temperature. In tends to accumulate above the vent in a bulbous mass (lava
dome).
Pyroclastic debris is fragmented igneous material forcefully ejected from a volcano to form
a layer of tephra. This forms a layer of pyroclastic rock when it becomes solid and
coherent.
- Any accumulation of pyroclastic debris or the deposits of slurries and streams is referred
to as volcaniclastic deposit.
Basaltic magma can contain volatiles, these rise faster than the magma itself, and when the
bubbles reach the surface, they burst and ejects clots and drops of molten lava upward to
form dramatic fountains.
- Blocks; large pieces of angular volcanic rock broken up during the eruption
- Bombs; lumps of erupted igneous rock that have streamlined, polished surfaces.
Andesitic and rhyolitic lavas are more viscous than basalt and tend to be more gas-rich.
These eruptions tend to be more explosive. Debris ejected in this include ash, pumice/
accretionary lapilli.
1
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CHAPTER 9
The Wrath of Vulcan: Volcanic Eruptions
9.2 The Products of Volcanic Eruptions
Products of an eruption come in three forms; lava ows, pyroclastic debris and gases.
The character of a lava primarily re ects its viscosity, and not all lavas have the same
viscosity. This is in uenced by the chemical composition, temperature, gas content and
crystal content.
- When the proportion of silica in lava increasers, the lavas ability to ow decreases.
Basaltic (ma c) lava has low viscosity and ows easily when it rst emerges from a volcano.
Due to the high temperature and low concentration of silica.
- Rapid cooling causes the surface of the ow to harden after the ow has moved a short
distance -> new molten lava injects itself between the original ground surface and the
new solid crust -> this addition of lava in ates the ow, jacking up the hardened crust
and making the overall ow thicker -> the lava only moves thought a tunnel-like
passageway or lava-tube.
- Columnar jointing; type of fracturing that yields roughly hexagonal columns of basalt.
- Pahoehoe; a basalt lava ow with a surface texture of smooth, glassy, ropelike ridges.
Andesitic lava ows less easy than basaltic lava due to a higher viscosity. Because it moves
slower the lava has more time to solidify. As it moves the surface breaks up into angular
blocks.
Rhyolitic lava is the most viscous of all lavas because it has the highest silica concentration
and the coolest temperature. In tends to accumulate above the vent in a bulbous mass (lava
dome).
Pyroclastic debris is fragmented igneous material forcefully ejected from a volcano to form
a layer of tephra. This forms a layer of pyroclastic rock when it becomes solid and
coherent.
- Any accumulation of pyroclastic debris or the deposits of slurries and streams is referred
to as volcaniclastic deposit.
Basaltic magma can contain volatiles, these rise faster than the magma itself, and when the
bubbles reach the surface, they burst and ejects clots and drops of molten lava upward to
form dramatic fountains.
- Blocks; large pieces of angular volcanic rock broken up during the eruption
- Bombs; lumps of erupted igneous rock that have streamlined, polished surfaces.
Andesitic and rhyolitic lavas are more viscous than basalt and tend to be more gas-rich.
These eruptions tend to be more explosive. Debris ejected in this include ash, pumice/
accretionary lapilli.
1
fi fl fl fl fl fl fl fl fl fi fl fl
