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)