PowerPoint Presentation - VISCOSITY

• The resistance of material to flow
– the higher the viscosity, the less easily
something (magma/lava) can flow
• Motor oil: Flagstaff vs. Phoenix
– hotter temps = lower viscosity oil
– lower temps = higher viscosity oil
Cinder cones/maars
Short lived (<10 years?)
Cinder/scoria, lava flows (out the
Most any tectonic environment
Tephra, tephra, tephra
~1 km wide, 300 m tall
Shield volcanoes
Very wide, tall
Centuries to a few million years
Often have a caldera at the rim
Very low angle slopes
Highly fluid flows VERY low viscosity
Subduction zones and hotspots (larger
volcanoes = hotspots)
• Cool picture is here:
Relatively small (few hundred m on a side)
(usually) dacite to andesite to rhyolite
Convergent/subduction margins
Pyroclastic flows often due to collapse of the
• Can be several thousand years, but really
variable – a few years to several thousand
• Toothpaste
• http://a.abcnews.com/images/US/volcanoe_1120
Stratovolcanoes (also called
composite volcanoes)
• Erupts everything (pyroclastic, lava, mudflows
• Intermediate (basalt to dacite)
• Hundreds of thousands years lifespan
• Big! 4000’ high from base (so Cascade
volcanoes often 12,000’ or more)
• Subduction zone/convergence
• High viscosity, often explode
• polygenetic
• Largest volcanoes on earth – 15 x 30 km
• Largest eruptions 2-1000 km3
• Magma rising to earth’s surface swells the surface,
erupts pyroclastic material, magma chamber collapses.
Shallow magma chamber.
• RHYOLITE!! (also dacite)
• All types of tectonic settings
• Lifespan to several million years
• Tuff is the most common erupted material, both as flows
and as material falling from the air: pyroclastics rule
• Vents along margins of caldera (lava domes),
earthquakes common
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