Silicic Volcanism
Magma composition is high in Silica
EX// Basalt is ~45-50% silica
Andesite: ~60% silica
Dacite: ~65% silica
Rhyolite: ~75% silica
Obsidian: ~75% silica
Magma is low temperature
EX// Basalt ~1200-1400oC
Silica rich magmas ~700-900oC
What does this do to the viscosity of the magma?
Where are Silicic volcanoes found?
Subduction zones
What process is creating magma?
What is being melted?
How are ocean to ocean convergence different than ocean to continental convergence?
Ocean to Ocean convergence
What type of matererial is being melted?
What composition of material is being generated?
Creates
Basaltic andesite to andesite in composition
Eastern Pacific
Explosive due to higher silica
Continental to Ocean convergence
What type of material is being melted?
What type of magma is being created?
Creates continental volcanic arcs
Magma composition ranges from andesite to rhyolite
Very explosive due to high nature of silica
Lava Flows?
Are lava flows found on this type of volcano and if yes what do they look like?
What are some features/products of Strato volcanoes other than lava?
Other Products of Silicic volcanoes
Composite cones
(strato volcanoes)
Ash and lapilli
Volcanic Plumes
Pyroclastic flows
Lahars
Lava domes
Collapsed calderas
Pyroclastic material
Blocks:
>32 mm
What are they?
Lapilli
4-32 mm
Include Pumice
Ash
< 4 mm
Glass
Why does material get thrown out of a volcano?
Ash and the Plume
Ash is created as magma is torn from the volcano
Ash is the primary component of the volcanic column or plume
Will there be a plume on a basaltic volcano?
Ash deposits created ash tuff
Hazards of Ash?
Volcanic Plumes
Fed at base by hot gas and particles that rise at ~100-600 m/s (328-2000 ft/sec)
Three primary phases to a plume
Jet Phase
Convective Phase
Umbrella Region
Jet Phase
Ash is forced upward by pressure
Ejected material has a greater density than surrounding atmosphere
Convective Phase
Atmosphere is drawn into cloud by turbulence
Air heats up and expands
Plume density decreases and continues to rise due to buoyancy
Umbrella Region
Density of cloud matches density of surrounding atmosphere
Cloud spreads out and material begins falling out
Lightning?
Why does lightning occur in Volcanic plumes?
Ash is charged (+ and – side)
Creates a potential and lighting forms when there is enough energy
Electrostatic release
Falling ash and fragments create tuff deposits
What size particles would be found closer to the vent?
What size particles would be found further from the vent?
Isopach and Isopleth maps
Isopach maps
Thickness of various deposits
Where would the volcanic deposits be the thickest?
Isopleth maps
Size of particles in deposits
Where would the largest particles be found?
How could geologist use information like this?
Maps are used to understand past eruptions
Where was the vent?
What direction was the wind blowing?
What was the size of the plume (i.e. how big was the eruption)
Plume size and distribution
The size of the plume is controlled by
Magma composition
Amount of volatiles in melt
Rate of discharge
Geometry of the vent
How much air gets drawn into the vent
If little to no air is drawn into the vent:
Column remains more dense than the surrounding atmosphere
Collapses and creates pyroclastic flow
Pyroclastic flows
Density driven cloud of hot gas, ash and rocks
Speeds from 60-200mph
Due to hot gas, buoyant and travels long distances
Also known as Nuee Ardents
Pyroclastic flow deposits create ignimbrites or welded tuff
How else are pyroclastic flows created?
Lava Domes
Silicic magma oozes up and slowly piles up.
Unstable, often collapses creating pyroclastic flow
Eventually, over time, creates new summit of volcano
Lahars
Volcanic Mudflow
Generated by
Glaciers
Lakes
Rain
VERY DANGEROUS