Chapter 2: Volcanoes
Section 2: Volcanic Activity
How magma reaches Earth’s surface
 Liquid magma is less dense than the surrounding solid
material, so it flows upward into any cracks in the rock above
 During a volcanic eruption, the gasses dissolved in magma
rush out, carrying the magma with them
Inside a volcano
Characteristics of magma
 The force of volcanic eruption depends partly on the amount
of gas dissolved in magma
 Other important factors that affect an eruption: How thick
or thin the magma is, the temperature and its silica
content.
 More silica= higher viscosity (thicker)
 Less silica= lower viscosity (thinner)
Types of volcanic eruptions
 The silica content of magma helps to determine
whether the volcanic eruption is quiet or
explosive
 High viscosity (thick) causes an Explosive
eruption
 Low viscosity (thin) causes a Quiet eruption
 A volcano erupts quietly if its lava flows easily
 Two types of lava produced by quiet eruptions
 Pahoehoe -fast moving, hot lava (Low viscosity)
 Aa -slow moving, cooler lava (High viscosity)
Aa
Pahoehoe
 Magma rises through the lithosphere
because it is less dense and it is rising
through the solid rock. As the magma
materials rise, the rock cracks and magma
goes through.
 Both quiet and explosive eruptions can
cause damage far from the crater’s rim.
3 Stages of a Volcano
 Active - it will erupt or is erupting
 Dormant - there is a possibility that it
will erupt (may not be for a long time)
 Extinct - it will not erupt
Other types of volcanic activity
 Hot spring: forms when groundwater heated by
nearby magma, rises to the surface and collects in
a natural pool
 Pressure can build and cause a geyser- a
fountain of water and steam that erupts from
the ground
 Geothermal Energy: a clean, reliable energy
source caused when water is heated by magma
Hot Spring
Geyser