Igneous Structures: Volcanic
Viscosity
• Units: pascal-seconds (kg/m-sec)
• How many pascals pressure x how many
seconds to squash fluid flat?
• Viscous: Material flows as long as pressure is
applied, however small
• Pseudoviscous: Material flows after some
threshold stress
Viscosity of Common Materials
Material
Viscosity Pa-Sec
Hydrogen gas, 15 degrees K
0.0000006 (6 x 10-7)
Air, 0 degrees C
0.000017 (1.7 x 10-5)
Liquid Hydrogen
0.00001 (10-5)
Water 0 degrees C
0.0018 (1.8 x 10-3)
Water 100 degrees C
0.0003 (3 x 10-4)
Heavy Machine Oil 15 C
0.66
Glycerin, 20 degrees C
1.5
Honey 20 degrees C
1.6
Geologic Viscosities
Material
Viscosity Pa-Sec
Basalt Lava
1000
Rhyolite Lava
1,000,000
Rock Salt
1014 (Pseudoviscous)
Marble
1016
Granite, Quartzite
1018 - 1020
Asthenosphere
1019 - 1020
Deep Mantle
1021 - 1022
Shallow Mantle
1023 - 1024
What Drives Magma?
• Magma is typically 10% lighter than equivalent
solid rock
– Why pressure inhibits melting
• Buoyancy drives magma upward
– Weight of rock around volcanic conduit > weight
of magma
• Gas pressure powers volcanic eruptions
• Mafic (dense) rocks can sink in crust by
viscous flow
Plate Tectonics and Volcanoes
A Cinder Cone:
Wizard Island, Crater Lake, Oregon
Paricutin,
Mexico
1943-1952
Anatomy of a Cinder Cone, Hawaii
Tuff Ring: Diamond Head, Hawaii
Tuff Ring: Diamond Head, Hawaii
Shield Volcano: Haleakala, Hawaii
Stromboli
Mount Lassen and Mount Tehama
Mount Shasta, California
Shasta and Shastina
Shastina and Landslide Deposit
Mount Shasta and Landslide Deposit
Mount Saint Helens and Spirit Lake
May 18, 1980
The Mountain Collapses
Mount St. Helens
Mt. St. Helens
Bezymianny, Russia
Lava Dome, California
Rhyolite Plugs and Pumice Rings
Rhyolite Plugs and Pumice Ring
Obsidian Dome, California
Obsidian
Rhyolite Plugs and Pumice Rings
Maar, Nevada
Volcanic Neck, New Mexico
Topographic Inversion, Nevada
How Calderas Form
Crater Lake, Oregon
Mount Mazama: After
Mount Mazama: Before
Jemez Caldera, New Mexico
Valles Caldera, New Mexico
Tuff, Valles Caldera, New Mexico
Welded Tuff, California
Santorini
(Thera),
Greece
Santorini (Thera), Greece
Santorini, Greece
Santorini, Greece
Ash Layer, Santorini
Ash Layers, Santorini
What Really Destroyed the Minoan
Civilization
Collapsing Volcanoes - Hawaii
Molokai Scarp
Nuuanu Pali, Oahu
Tambora 1815
1816: “Year Without A Summer”
• 100 cubic km of ash erupted
• Global sunset color effects for months
• New England
– Snow in June and August, Frost in July
– Exodus to Midwest
• Europe: High prices, food riots
Tambora
Long Valley Caldera
Long Valley Caldera
Bishop Tuff
Compaction of Bishop Tuff
Toba, Sumatra
Eyjafjallajokull, Iceland
Flood Basalts, Washington
Lava “Delta,” Washington
Columnar Jointing, Washington