Types of VolcanoesTypes of Volcanoes

Chapter 11Chapter 11

Section 2Section 2

Types of Eruptions:Types of Eruptions:

• Quiet

• Explosive

• Determined by:– Amount of water vapor/trapped gases in the

magma.– Amount of silica (silicon and oxygen) in the

magma.

Quiet EruptionsQuiet Eruptions

• Contain basaltic magmas– Low in silica– Flows very easily

• Contain gases that are easily released due to the high viscosity (ability to flow) of the magma.

Quiet EruptionsQuiet Eruptions

• Pahoehoe– Lava cooled in a rope-like structure

• Aa– Forms at lower temperatures than pahoehoe

• Pillow lava– Igneous rock structures shaped like baloons,

tubes, or pillows.

Quiet EruptionsQuiet Eruptions

• Examples:– Hawaii– Rift zones (Iceland)

Explosive EruptionsExplosive Eruptions• Contain granitic magma

– High in silica– Thick – Low in viscosity (ability to flow)– Contains many trapped gases

• May also contain andesitic magma– Intermediate silica content

Explosive EruptionsExplosive Eruptions• Examples

– Convergent boundaries

Forms of VolcanoesForms of Volcanoes

• Dependent upon type of magma and type of eruption.

• 3 basic forms:– Shield volcanoes– Cinder cone volcanoes– Composite volcanoes

Shield VolcanoShield Volcano

• Broad volcano with gently sloping sides.

• Result of basaltic magma and quiet eruptions.

• Example: Hawaii

Flood BasaltsFlood Basalts

• Occur when basaltic magma flows onto the surface of the earth through large cracks called fissures.– When the magma cools, it covers large areas with

thick igneous rock.– Accounts for largest volume of erupted volcainc

material on Earth.

• Examples:– Columbia Plateau– New ocean floor

Cinder Cone VolcanoCinder Cone Volcano

• Form when tephra fall to the ground and form a loosely-packed steep-sided structure.– Tephra consist of bits of rock or solidified lava

drpped from the air.• Includes volcanic ash, cinders, and larger rocks

called bombs and blocks

• Example:– Paricutin – Feb 20th, 1943

Types of TephraTypes of Tephra• Volcanic Ash

– Tephra particles less than 2 mm in diameter.

• Volcanic Dust– Particles less than 0.25 mm in

diameter.

• Lapilli– Larger tephra particles, less than 64

mm in diameter.• Generally fall near the vent.

Types of TephraTypes of Tephra

• Volcanic Bombs– Tephra particles with round or spindle shape.

• Volcanic Blocks– Solid rocks blasted from a fissure.

Composite VolcanoComposite Volcano

• Formed from alternating quiet and explosive eruptions, depending on the amount of trapped gases and silica content at the time of eruption.

• Result is alternating layers of tephra and lava.• Examples:

– Convergent boundaries – Mount St. Helen’s, Mount Raineer

KrakatauKrakatau• Island in the Sunda Straits near

Indonesia.• Erupted in August of 1883, causing the

“island” to collapse into the empty magma chamber (caldera-forming).

• More than 36,000 people were killed (mainly by a tsunami)

• Global temps dropped as much as 1.8 degrees Celsius for five years.

Effects of EruptionsEffects of Eruptions

• Lava flows

• Collapse of buildings from ash fall

• Air pollution (lung disease/stress)

• Acid rain

• Pyroclastic flows– Huge, hot, fast rush of ash and debris

down the mountain (can result in fires)

• Permanent/Temporary Evacuation