Volcanoes

Windows Into the Earth

•Volcano –

a mountain formed by lava & pyroclastics

•Crater –

opening at the top of a volcano

•Caldera –

summit depression >1 km-usually caused by collapsed magma chamber

Important Vocabulary

• Active – erupted in recent history (within the last

few 100 years)• Dormant –

no recent eruptions (not within the past several 100 years), not eroded or worn down

• Extinct – no recent eruptions (not within the past

1000 years) and eroded

Where are all the Volcanoes?

• 1,900 volcanoes are active today or are known to have been active during historic times.

• 90% of these volcanoes are on the Ring of Fire– a band of volcanoes circling the Pacific Plate

Practice Questions

• What is the difference between a crater and a caldera?– size

• Is the picture below a crater or a caldera?– caldera

http://earth.jsc.nasa.gov/EarthObservatory/CraterLake,Oregon_files/ISS006E15238_Crater-Lake.jpg

Practice Questions

• What is the difference between a dormant and an extinct volcano?– Time since last eruption & amount of

erosion

• Where are most of the volcanoes in the world located?– Ring of Fire– Convergent and Divergent Plate Boundaries

(not transform)

Magma vs. LavaBoth are molten (liquid)

rock with suspended mineral grains and dissolved gases

• Magma – interior of the earth

• Lava – magma that reaches

the earth’s surface

Magma and Viscosity

•Viscosity can be affected by many variables

Mineral compositionTemperatureDissolved gases

Viscosity is a fluids resistance to flow

Practice Question

• Rank the following substances from high to low viscosity.– Ketchup– Vitamin Water– Magma– Maple syrup

1. Magma

2. Ketchup

3. Maple syrup

4. Vitamin Water

Mineral Composition

• Remember silica?– Silica is a very common mineral composed

of silicon and oxygen (SiO2). Silicates make up about 95% of the earth’s crust

• A magma’s viscosity is directly related to its silica contentHigh silica magmas (granitic) are

highly viscous with short thick lava flows

Low silica magmas (basaltic) are more fluid and may travel up to 90 miles and have low viscosity

Temperature•High temperatures decrease

viscosity and make magmas and lavas more fluid= low viscosity

•As a lava flow cools it begins to thicken and movement decreases= high viscosity

Dissolved Gases• As magmas reach the surface, the pressure is

reduced and gases expand with explosive results

• Gas composition varies, but it usually consists of:– Mostly H2O (water vapor) & some CO2 (carbon

dioxide)– Minor amounts of Sulfur, Chlorine, and Fluorine gases

• The more gas, the more explosive the eruption!• However, gas content affects viscosity as well:

– High Gas = Low Viscosity– Low Gas = High Viscosity

Practice Questions

• What are 3 things that affect the viscosity of magma?– Silica content– Heat– Gas content

• How does each of these affect viscosity?

Viscosity Review

1) Temperature - temp. Viscosity2) Silica Content - silica Viscosity3) Dissolved Gas - gases Viscosity

• Deep below the surface of the earth, the pressure is great.

• Volcanic gases are dissolved in the liquid….

• If the magma has low viscosity, the gas bubbles can easily escape...

• If the magma has high viscosity, the gas bubbles are trapped in the thick, gooey liquid. And gas pressure builds up until….

Kaboom!An explosive eruption!

So low viscosity magmas that allow gas to escape produce non-explosive eruptions.

High viscosity magmas that trap gas produce explosive eruptions.

Types of Eruptions• Non-Explosive

Lava Flows – Low viscosityEx. Hawaii & Mid-Ocean Ridge

• ExplosiveHigh viscosity lava flows & high gas

contentPyroclastics – “fire fragments”Ex. Mount St. Helens, Krakatoa

Soda Bottle Analogy

• Dissolved gases under pressure inside bottle.

• Soda (lava) escapes violently as pressure drops!

Practice Questions• What type of eruption would

happen if the magma has a high silica content and a high gas content?– Explosive

• What type of eruption would happen if the magma has a low silica content and a low gas content? – Non explosive

Lava Flows• Dark colored = mafic lava (rich in

magnesium & iron, low in silica) – low viscosity– Also called basaltic lavas

Pahoehoe – smooth, rope-like lavaaa – clinkery, rough lava

• Light colored = felsic lava (rich in silica)– High viscosity

Pyroclastics“Fire Fragments”

• Ash- very fine glassy fragments• Welded tuff – fused glassy shards • Cinders – pea sized pyroclasts• Lapilli – walnut sized pyroclasts• Lava blocks – large hardened

pyroclasts• Lava bombs – incandescent lava that

cools as it flies through the air, football shaped

• Pumice – frothy material w/ air spaces

Volcanic Locations1) Divergent Plate Boundaries

Where two plates are pulling apart--Mid Ocean Ridge & Continental Rift

2) Subduction Zones Where an oceanic plate converges

with another oceanic plate or a continental plate--Pacific Ring Of Fire

3) Hot Spots Fixed source of magma in the center

of a plate-- Hawaii, Yellowstone

Types of Volcanoes

•Shield Volcano•Cinder cone•Composite (stratovolcano) cone•Fissure eruptions

Shield Volcano• Mafic, basaltic lava• Rapid streams of low viscosity lava

flow easily form gentle slopes.• Very flat and low compared to

diameter• Each flow is only a few meters thick• Mauna Loa

2.5 miles above see level (13,679 ft)

6 miles above sea floor60 miles across at base

Cinder Cone• Small, steep cone shaped volcanoes• Usually very forceful release of gas• Little to no lava flow• Magma and rock are flung from

volcano• Pea size pyroclastics (cinders) pile up

and form distinctive steep sided cone

Composite Cone

• Alternate layers of lava and pyroclastics build up

• Viscous, gas charged andesitic lava

• Violent eruptions• http://www.pbs.org/wnet/savag

eearth/animations/volcanoes/index.html

Fissure Eruptions• A fissure is a

fracture or crack in rock along which there is an obvious separation

• Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

Practice Questions• Arrange the types of volcanoes from least

explosive to most explosive. (in general)Shield, cinder cone, composite/strato

• Which volcanoes have lava as part of the eruption? shield, fissure eruptions & composite/strato

• Which volcanoes have pyroclastics as part of the eruption? cinder cone & composite/strato

Volcanic Hazards• Lava and Pyroclastics•Lahars – volcanic mudflows

created by ash and water•Nuée Ardente – incandescent debris w/ hot gases that moves like an avalanche

Glowing cloud

20000 F

120 mph

•Toxic Gases – CO, CO2, Sulfur, HCl

Lake Nyos - Cameroon

•Steam Explosion

Phreatic eruption

Seawater is heated and explodes

Krakatau100 million tons of TNT

Future Eruptions

• Decade Volcanoes– 16 volcanoes that have been designated as

Decade Volcanoes by the International Association of Volcanology and Chemistry of the Earth's Interior b/c of their eruptive history and proximity to major population centers.

Volcano Pictures

• Mount Etna, Italy

•Pacific Northwest – Rainier and Mt. St. Helens

•Hekla, Iceland

•Olympus Mons, Mars

Eyjafjallajökull, Iceland 2010

The End