The document discusses different types of volcanoes that form based on the viscosity of the magma and type of eruption. There are three main types of volcanoes - cinder cones form from explosive eruptions of viscous magma and are the smallest volcanoes. Shield volcanoes form from gentle eruptions of thin, non-viscous basaltic magma and are the largest volcanoes. Composite volcanoes form from both gentle and explosive eruptions of variable viscosity andesitic magma, resulting in layers of ash and lava building up into a cone shape.

1. VOLCANOES - TYPES
- Review Questions VOLCANOES 101 Part 1: 1-8
- Review: Formation / Magma / Eruptions
- Types of Volcanoes

2. Magma Types and Eruptions
The type of magma determines the type of eruption.
There are 2 major types of eruptions.
Explosive Eruptions
Gentle Eruptions

3. Explosive Eruptions
1. Explosive eruptions occur because magma is viscous
(thick).
2. The more silica in magma, the more viscous it is.
3. When viscous magma rises, it does not flow up easily.
4. The magma usually gets stuck.
5. Gasses like CO2 and water Vapor also get stuck.
6. The gasses pressurize and eventually blow out
explosively.

4. Gentle Eruptions
1. Gentle eruptions occur because magma is non-viscous
(thin).
2. The less silica in magma, the less viscous it is.
3. When non-viscous magma rises, it flows easily.
4. The magma does not get stuck.
5. Gasses like CO2 and water vapor do NOT get stuck either.
6. Gasses easily released = no violent pressure built
explosions – Gentle oozing out of fissures.

5. WHY IS THE
ERUPTION TYPE
SO IMPORTANT?
The eruption
type determines
the type of
volcano that
forms.

6. Eruptions and Volcanic Formation
Some volcanoes are tall and steep and others are
big and broad…
There are 3 major types of volcanoes:
1. Cinder Cones
2. Shield Volcano
3. Composite

7. Cinder Cones
 Cinder cones are the smallest, but most
violent volcanoes.
 They form from explosive eruptions.
 The granitic magma is very viscous, thus
trapping gasses.
 When they erupt, they explode ash and lava
high into the air.
 Lava and ash quickly cool forming tephra
that falls to the ground.
 The tephra piles up forming the cinder cone.
EXAMPLE: Krakatau
TEPHRA:
Rock and Ash that forms when magma cools quickly in the air.

11. Shield Volcanoes
1. Shield volcanoes are the biggest volcanoes, but most
gentle.
2. They form from quiet eruptions.
3. The basaltic magma is very thin, thus flows very easily.
4. When they erupt, the magma just oozes out.
5. Lava runs down the side of the volcano cooling to form
rock.
6. The volcano is built wider and more broad as lava builds
up.
EXAMPLE: Mauna Loa, Hawaii

15. Composite Volcanoes
1. Composite volcanoes are the “typical-looking” volcano.
2. They form from both quiet and explosive eruptions.
3. The andesitic magma can be both viscous and non-
viscous.
4. Sometimes more silica is in the magma than other times.
 When silica levels are high – Explosive Eruptions
 When silica levels are low – Gentle Eruptions.

16. Composite Volcanoes
1. During Explosive Eruptions – ash, gas, and
tephra erupt.
2. During Gentle Eruptions – andesitic lava flows
out.
3. As a result, layers of lava and ash build up
forming a cone.
4. First ash, than lava. Then ash again. Then
lava…and so on…