Volcanic Activity
How Magma Reaches the Surface
Magma rises because magma is less dense
than the surrounding solid material.
 It will rise until it reaches the surface or
becomes trapped beneath layer of rock.

A Volcano Erupts
A volcano erupts when an opening
develops in weak rock on the surface.
 During a volcanic eruption, the gases
dissolved in magma rush out, carrying the
magma with them.
Inside a Volcano:
 Magma Chamber – The pocket beneath a
volcano where magma collects

The magma moves through the pipe, a
long tube in the ground that connects the
magma chamber to Earth’s surface.
 Vent - an opening in the volcano where
gases and molten rock leave.
 Lava flow – The area covered by lava as it
pours out of a vent.
 Crater – A bowl-shaped area that may
form at the top of a volcano around the
volcano’s central vent.

Characteristics of Magma
Gas content, how thick or thin the magma is,
temperature and silica contents are important
factors as to the force of a volcanic eruptions.
 The amount of silica in magma helps to
determine how easily the magma flows. Silica is
formed from the elements oxygen and silicon
and is abundant in the crust and mantle. The
more silica content in magma the thicker the
magma.

Types of Volcanic Eruptions
Silica content determines whether the
volcanic eruption is quiet or explosive.
Quiet Eruption:
 A volcano erupts quietly if its magma
flows easily.
 Quiet eruptions produce two types of lava:
Pahoehoe – fast-moving, hot lava
aa – cooler, slower moving lava

Explosive Eruptions:
 If magma is thick and sticky, a volcano will
erupt explosively.
 The explosion will break the lava into
fragments that quickly cool and harden
into pieces of different size called Tephra.
 Smallest pieces are called ash (fine, rocky
particles as small as a grain of sand),
cinders are pebble-sized particles and
bombs are larger pieces that range in size
from a baseball to a car.
Stages of a Volcano
Active – a volcano that is erupting or has
shown signs that it may erupt.
 Dormant – sleeping and may become
active in the future.
 Extinct – a volcano that is unlikely to erupt
again.

Other Types of Volcanic Activity
Hot Spring – forms when groundwater
heated by a nearby body of magma rises
to the surface and collects in a natural
pool.
 Geyser – a fountain of water and steam
that erupts form the ground.
 Geothermal Energy – water heated by
magma that provides a clean, reliable
energy source.

Monitoring Volcanoes

Geologists have been somewhat
successful in predicting eruptions. They
use the same devices that are used to
monitor earthquakes (tiltmeters, laserranging devices, etc.). Geologist will also
monitor the small earthquakes that
normally accompany a volcano.
Volcano Hazards
Although quiet eruptions and explosive
eruptions involve different volcano
hazards, both types of eruptions can
cause damage far from the crater’s rim.
 Volcanic ash can bury entire towns,
damage crops, and car engines. Eruptions
can also cause landslides and avalanches.
