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Lecture Notes for LEC 172 Lecture 6
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Summary lecture notes for LEC 172 Geology Lecture 6 - Volcanism & Plutonism 1.
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Friday 26th November 2021 LEC 172Volcanism & Plutonism – 1
Material Products of Eruptions:
Igneous Rock rock formed from magma cooling and solidifying.
Magma is molten rock below the Earth’s surface.
When magma reaches the surface and erupts, it either forms lava flows (continuous flow of molten material)
or pyroclastic material (fragmented solid material of various sizes).
Lava of 2010 Eyjafjallajökull volcano, Iceland eruption; Flank eruption on Fimmvörduháls 20 March-12 April:
lava fountaining and lava flows
0.02 km3 of basaltic magma
negligible ash, no flight disruptions
tourist attraction
Pyro clasts form when magma is explosively ejected from a volcano as fragments (can be μm-, mm-, cm-, m-
etc sized) with volcanic gases.
Pyroclastic rocks form when pyroclastic sediments consolidate.
Pyro clasts - classification according to particle diameter:
o Ash - <2 mm diameter
o Lapilli (Latin = little stones) - 2-64 mm diameter
o Volcanic Bombs - >64 mm diameter fragments hurled from the vent which
land close (normally within 100s metres) to the vent; can be house-sized
Components of ash:
• Pumice/Scoria - fragments of vesicular lava
Top: Pumice - spherical
• Lithics - rock fragments vesicles
• Crystals - individual and broken Bottom: Tube Pumice -
elongate vesicles
• Glass Shards - chilled magma, vesicle walls
Components of lapilli:
• Pumice/Scoria - fragments of vesicular lava
• Lithics - rock fragments
* Pumice – Light coloured, vesicular (bubbly) rock, that isn’t very dense. Scoria is very similar but darker.
Pumice - light coloured vesicular igneous rock which forms through rapid solidification of melt. Vesicular
texture is the result of gas trapped in the melt at the time of solidification.
Processes & Deposits of Eruptions:
Pyroclastic Fall – Sedimentation of particles from a volcanic plume, controlled by gravity, density, viscosity of
atmosphere & diameter and density of particles, produce a Pyroclastic Fall Deposit.
Pyroclastic Density Current (AKA Pyroclastic Flow) – Gravity flow of hot, supercharged gases & particles down
the flanks of the volcano (10-300m/s & up to 1000°C), producing a Pyroclastic Density Current Deposit.
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