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