Volcanism
The study of volcanoes
and their activities
5/18/80 - Before
“Vancouver, Vancouver, this is it!”
During
May 18, 1980
July 22, 1980
After
The Lahar
Cascade Volcanic Peaks
Adams
Lassen
Hood
Baker
Rainier
ShastaShuksan
St. Helens
A picturesque, cone-shaped structure that periodically erupts violently!
Are all eruptions explosive?
Why the difference?• Temperature – hotter
is less viscous• Composition – the
more silica, the more viscous
• Dissolved gases – gases increase fluidity and decrease viscosity
• Viscosity increases explosiveness
Materials extruded from volcanoes
• Lava – pahoehoe (ropy), aa (blocky) and pillow lava (water cooled rolling formation)
• Gases – 1-6% of the mass (water vapor, carbon dioxide, nitrogen, sulfur, chlorine, hydrogen, argon, and methane)
• Pyroclastics – ejected fragments
Pahoehoe
A’a
Pyroclastic flow
A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and
volcanic gas that rushes down the side of a volcano as fast as 100
km/hour or more. The temperature within a pyroclastic flow may be greater than 500° C, sufficient to
burn and carbonize wood.
Pyroclastics
• Ash and dust
• Cinders
• Lapilli (little stones)
• Pumice (frothy lava)
• Blocks (large rocks)
• Bombs (blobs of lava)
Volcanic Cones
• Successive eruptions from a central vent result in a mountain-like accumulation of material called a volcano (cone)
• Let’s look at shield, cinder, and composite cone types!
This type of volcano can be hundreds of miles across and many tens of thousands of feet high. The individual islands of the state of Hawaii are simply large shield volcanoes.
Mauna Loa, a shield volcano on the "big" island of Hawaii, is the largest single
mountain in the world, rising over 30,000 feet above the ocean floor and reaching
almost 100 miles across at its base. Shield volcanoes have low slopes and consist
almost entirely of frozen lavas. They almost always have large craters at their summits.
Shield cone
Mauna Loa
As you might expect from the name, these volcanoes consist almost entirely of loose,
grainy cinders and almost no lava. They are small volcanoes, usually only about a mile
across and up to about a thousand feet high. They have very steep sides and
usually have a small crater on top.
Cinder Cone
These volcanoes are typically tens of miles across and ten thousand or more feet in height. As illustrated in the figure above, they have moderately steep sides and sometimes have small craters in their
summits. Volcanologists call these "strato-" or composite volcanoes because they
consist of layers of solid lava flows mixed with layers of sand- or gravel-like volcanic
rock called cinders or volcanic ash.
Composite Cone
Volcanic Hazards
• Explosion – Krakatoa
• Burial – Vesuvius (Pompeii)
• Nuee ardente – Pelee (St. Pierre)
• Lahar – Mt. St. Helens & Nevado del Ruiz
• Lava flow - Kilauea
Krakatoa before and after
Vesuvius and Pompeii
St. Pierre?
Poisonous Gases
Kilauea
Other Volcanic Landforms
Caldera
Volcanic dome
• Viscous magma• Usually in
association with existing cones
• Some independent, like Mono Craters CA
Volcanic neck
• Vent materials more resistant
• Flanks wear away• “neck” left behind
Vents and pipes
Fissure flows
Columbia Flood Basalts
Pyroclastic flows
Distribution of Igneous Activity
• Zones of divergence (seafloor spreading)
• Zones of convergence (subduction)
• Therefore, crustal plate margins
• Hot spots (intraplate volcanism)