Volcanic Eruptions

ch. 6 section 2

Inside a Volcano

Beneath a volcano, magma collects in a pocket called a magma chamber. The magma moves upward through a pipe, which is a long tube in the ground that connects the magma chamber to the Earth’s surface. Molten rock and gas leave the volcano through an opening called a vent.

A volcanic eruption

Dissolved gases are trapped in magma and

are under great pressure. As magma

reaches the surface, the dissolved gases

begin to expand and exert an enormous

force. When a volcano erupts, the force of

the expanding gases pushes magma from

the magma chamber through the pipe until it

flows or explodes from the vent.

What do you think?

What might happen if

more magma is injected

into the chamber from

below?

Kinds of volcanic eruptions

Geologists classify volcanic eruptions as

quiet or explosive. The properties of

magma determine how the volcano erupts.

The type of eruption depends on the

magma’s silica content. Silica is the

material found in magma that is formed

from the elements oxygen and silicon

Quiet eruptions

A volcano erupts quietly if its magma is

low in silica. Low silica magma is thin,

runny and flows easily. The gases in the

magma bubble out gently.

Quiet eruptions

Quiet eruptions can produce two types of

lava. Pahoehoe is fast moving, hot lava that

is thin and runny. It looks like wrinkles

when it hardens. Aa is cooler and slower

moving. It is also thicker and has a rough

surface when it hardens.

Explosive eruptions

A volcano erupts

explosively if its

magma is high in silica.

High silica magma is

thick and sticky. The

gases cannot escape the

thick magma so they

build up pressure until

they explode!

Explosive eruptions

An explosive volcano breaks lava into

pieces that cool and harden in all different

sizes. A pyroclastic flow is a type of

explosive eruption that hurls out a mixture

of hot gases, ash and cinders.

What do you think

Identify if each of the following is an example of a high silica or low silica lava.

1) A lava river flowing away from a volcano

2) A plug of lava covering a volcano’s vent

3) Small pieces of lava being blasted high into the air.

Volcano hazards.

Both types of eruptions can cause damage

far from the volcano’s location. During a

quiet eruption lava flows from vents setting

fire to, and then burying everything in its

path..

Volcano hazards

During an explosive eruption, a volcano can

release deadly gases, as well as ash and

cinders

Life cycle of a volcano

The activity of a volcano may last from less

than a decade to more than 10 million

years. Geologists use the terms active,

dormant and extinct to describe a

volcano’s stage of activity.

Life cycle of a volcano

An active, or live volcano, is one that is

erupting or showing signs to erupt in the

future. A dormant, or sleeping volcano is

expected to awaken and become active in

the future. An extinct, or dead volcano is

unlikely to erupt again.

Hot Springs and Geysers

Hot Springs and geysers

are found in areas of

present or past volcanic

activity. A hot spring

forms when water deep

underground is heated by

magma or hot rock. The

hot water rises to the

surface and collects in a

pool.

Hot springs and geysers

Sometimes, the rising water and

steam become trapped in a narrow

crack. The pressure builds until it

sprays above the surface as a geyser.

A geyser is a fountain of steam and

water that erupts from the ground.

Monitoring volcanoes

Geologists use tiltmeters and other

instruments to detect changes in or around

a volcano. The changes may give warning

a short time before an eruption. Scientists

cannot predict the type of eruption or how

powerful it may be.