the good earth/chapter 6: volcanoes and other mountains

The Good Earth/Chapter 6: Volcanoes and Other Mountains

As a scientifically literate citizen, what 3 questions would you ask about this volcano if you moved to the city in the foreground (Tacoma, Washington)?

Volcanoes and Other Mountains

The Volcano Commandos

• 1,500 active volcanoes worldwide

− a third have records of previous eruptions

− 2 or 3 eruptions per decade are major disasters

− 500 million people live near active volcanoes

− Fewer than 200 volcanoes have instruments to assess potential for eruption


What exactly is a Volcano?

Location where molten rock and

other mantle materials, are

released, to the surface.

Parts of the Volcano

• Magma:– Molten rock – Less dense than crust

• Lava:– Magma that has

reached the surface and erupted out of the volcano

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Crater

Lava

Central vent

Pipes

Parts of the Volcano

• Magma Chamber– Underground pools of magma

• Pipes– Pathways for magma

• Vents– Area where magma reaches the surface

• Crater– Depression at the top of a volcanic form after an

eruption

2 Types of volcanoes

1. ShieldMade from fast flowing basaltic lava

(Basaltic: A hard, dense, dark volcanic rock)

Large and flat due to movement of the lava



Hawaiian Islands formed this way



http://www.history.com/shows/how-the-earth-was-made/videos/how-was-hawaii-formed -

how-was-hawaii-formed

QuickTime™ and a decompressor


http://www.history.com/shows/how-the-earth-was-made/videos/how-was-hawaii-formed#how-was-hawaii-formed




Volcano types

2. Strato-volcanoesmajority of the worlds volcanoes are these most explosive and destructive

Associated with subduction zones



Where do they occur?

• All boundaries except transform

Famous Volcanoes / areas of volcanoes

The Mount St. Helens Eruption


• Cascade Mountains – volcanic arc in Pacific Northwest− Major cities within 100 km of

active volcanoes− Mount St. Helens eruption of

May 18, 1980



• Cascade Mountains − Volcanoes formed above

subduction zone where Juan de Fuca plate slides beneath North America

− Mount St. Helens is most active volcano in conterminous US



Prior Activity

• Early (March) unrest featured− Minor eruptions

− Earthquakes

− Release of volcanic gases

• Followed by change in shape of cone (bulge on North flank)

• Increasing frequency of earthquakes



May 18 Eruption

• A moderate earthquake triggered a massive landslide (debris avalanche) on the North side of the volcano

− Debris clogged streams

− Pressure released on near-surface magma

− Lateral blast produces an initial sideways eruption to North

− Later vertical eruption


• Eruption of Mount St. Helens reduced height of volcano by 400 meters

• Features near volcano were blown over or carried away by products of eruption

Geologist David Johnston (right) died at this site (Johnston’s Ridge) located 10 km from the volcano. (this is 6.25 miles or from here to the pro-football hall of fame.

Products of Volcanic Eruptions


Major products of volcanic eruptions:• Airborne – lateral blast, tephra, volcanic gases• Flows on land – lava, pyroclastic flows, lahars



• Rare lateral blasts can destroy objects up to 12 km away and knock down trees more than 25 km distant

− Effect of lateral blast only seen on North flank of Mount St. Helens

Airborne Eruption Products



• Tephra represents particles blasted into air by eruption

− Volcanic bombs and ash are found near and far from eruption source, respectively


Blobs of magma solidify to form lava bombs

Wind can transport fine volcanic ash for hundreds of kilometers downwind



• Volcanic gases (water vapor, sulfur dioxide, carbon dioxide) may affect climate patterns

− Sulfur dioxide may block insolation, temporarily (up to 1 year) reducing global temperatures

− Widespread release of carbon dioxide and higher temperatures due to faster rates of volcanic activity approximately 120-80 million years ago


Trees killed by excessive carbon dioxide released by magma under Mammoth Mountain, California.



Eruption Products on Land

• Low viscosity lava can flow up to 50 km from its source

− Lava transported to front of lava flows in long lava tubes

− Lava flows build up in a series of layers




• Low viscosity lava can flow up to 50 km from its source

− Lava transported to front of lava flows in long lava tubes

− Lava flows build up in a series of layers

Walter’s Kalapana store, Hawaii, was buried in lava within a few weeks in 1990




• Higher viscosity lava remains within volcano crater

− Lava dome formed in crater of Mount St. Helens




• Pyroclastic flow – dense cloud formed from combination of tephra and volcanic gases

− Fast moving, up to 700 C




• Lahars – mudflows formed when volcanic debris mixes with streams or melting ice

− Often confined to stream channels

Lahar along Muddy River reached depths of 20 meters following Mount St. Helens eruption

Ring of fire area surrounding the pacific

plate with VERY high activity of Volcanoes



the good earth/chapter 6: volcanoes and other mountains

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