Volcano :

An opening or vent in the

Earth’s crust through which

molten rock, gases, ash, and

pyroclastic materials erupt

A landform or mountain built

from the products of eruption

that develops around the

opening

Magma Composition

The amount of Silica present

Controls Volcanoes!

Effects the character of the

eruption (mild – explosive)

Determines the shape of the

volcanic landform produced

Viscosity : resistance to flow

• Influenced by temperature and

composition of magma

• Composition : % of Silica

Why do Volcanoes Erupt??

1. MAGMA IS BUOYANT

• Molten rock is less dense

than solid rock, so it rises

2. MAGMA CONTAINS GASES

• If pressure is lowered quickly,

the gas is released rapidly,

causing violent eruptions

Dissolved Gases in Magma

Water Vapor

Carbon Dioxide

Sulfur Dioxide

Lesser Amounts of

Carbon Monoxide, Hydrogen Gas,

Methane, Hydrogen Sulfide,

Mercury, Helium , and others

Types of Magma

1. Basaltic Magma :

Mafic – low in silica

Low viscosity – thin and runny

High Temperature

Contains least amount of gases

Mild / “gentle” eruptions

Found most often at rifts and

oceanic hotspots

Ex. Kilauea , Hawaii

2. Andesitic Magma :

Intermediate silica content - @ 60%

Intermediate gas content

Intermediate viscosity

Intermediate temperature

Eruptions – sometimes explosive

Found mostly at subduction

boundaries

Ex. Mount St. Helens (Washington),

Mt. Fuji (Japan), Mt. Rainier

(Washington)

3. Rhyolitic Magma

Felsic – high in silica

High viscosity – thick , resists flow

High gas content – trapped,

pressure builds

Violent eruptions

Lowest temperature

Found mostly where hot spots

underlie continental plates

Ex. Yellowstone

Where do Volcanoes Occur??

** Plate Boundaries and Hot Spots

Hot Spots

Areas of volcanic activity that

result from plumes (rising

streams) of hot materials that

originate deep in the mantle

Hawaii

• Pacific Plate moved Northwest

over a Hot Spot

• Magma breaks through the

crust forming a volcano

• Hot Spot remains in the same

spot

Plate Boundaries

1. Convergent – 2 lithospheric

plates move toward each

other

Denser plate is subducted

2 oceanic plates

1 oceanic and 1 continential

2. Divergent – 2 plates move apart

most along ocean floor

Creates deep rift valleys

Ex. Iceland – Mid Atlantic Ridge

rises above sea level