Download Getting to Know: Where Volcanoes Form

Getting to Know: Where Volcanoes Form
In 2010, a volcano erupted in Iceland. It spewed a giant ash cloud for weeks and disrupted travel throughout Europe. In January 2011, Krakatau
volcano in Indonesia erupted for several days, displacing thousands of people. What caused
these volcanoes to erupt? Why do volcanoes form in some areas but not in others? In this concept, you’ll learn more about why volcanoes form in certain areas, the causes of eruptions, and the different types of eruptions that can occur. Do volcanoes only form in
certain areas?
Volcanoes tend to form at the boundaries
of tectonic plates.
Volcanoes primarily form at tectonic plate boundaries. Tectonic plates are enormous pieces
of Earth’s crust that are slowly moving. As tectonic plates move, the edges of the plates can
collide, separate, or slide past each other. Tectonic plates collide at convergent boundaries,
and they separate at divergent boundaries. Tectonic plates slide past each other at transform
boundaries. Volcanoes tend to form at convergent and divergent plate boundaries—they are
not usually associated with transform boundaries.
Why do volcanoes tend to form at plate boundaries?
When plates move away from each other at divergent plate boundaries, interesting things
happen. For example, when plates move apart, molten rock from the Earth’s core ascends and
forms new crust. This molten rock can erupt when it reaches Earth’s surface. At convergent
plate boundaries, one plate can be pushed under another plate. When this happens, the plate
can melt and turn into magma. This molten rock can rise up to the surface, forming a volcano,
and causing volcanic eruptions.
Misconception 1: Is it true that volcanoes are only on land?
That is not correct. There are many volcanoes on the ocean floor. When volcanic
eruptions happen on the ocean floor, they can create volcanoes that are completely
underwater. In particular, divergent plate boundaries such as the Mid-Atlantic Ridge are
characterized by numerous volcanoes.
Concept: Where Volcanoes Form
Getting to Know
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Why does the molten rock rise to the surface?
Molten rock rises to the surface because it has a different density than the surrounding rock.
Density is a measure of the amount of mass in a given volume. The more dense something
is, the more mass it has per unit of volume. Molten rock is less dense than solid rock and will
rise above solid rock if there is passage for it to travel through. The molten rock, also called
magma, rises to the surface of the Earth through cracks and fissures in Earth’s crust and can
erupt in dramatic explosions when it reaches the surface.
What’s the difference between magma and lava?
Magma and lava are both forms of molten rock. Magma is molten (or melted) rock that is
beneath Earth’s surface. Lava is the term for magma that reaches Earth’s surface (usually
during a volcanic eruption). Lava and magma can both have the same chemical composition.
Why do some volcanoes have violent eruptions?
The type of magma in a volcano determines the type of volcanic eruption. All types of magma
contain gases. When magma reaches the surface, these gases can expand, causing a violent
eruption. However, magma varies in its ability to hold gases. One type of magma, called mafic
magma, has high iron content and tends to be less viscous. Gases can escape from this type of
lava easily; when the lava reaches the surface, there’s less gas, so the eruption is less violent.
Another type of magma, felsic magma, is thicker
and traps more gases. That means when felsic
magma reaches the surface, there’s a lot of
expanding gas and a more violent eruption.
In addition to plate boundaries, are
there other places where volcanoes
can form?
The Hawaiian Islands are located far from
tectonic plate boundaries.They formed
from a magma chamber called a hot spot.
Volcanoes can also form at places called hot spots.
Hot spots are magma chambers that form at places
other than plate boundaries. They can even form in
the middle of tectonic plates. The islands of Hawaii
were formed from volcanoes that developed at a
hot spot.
Misconception 2: Is it correct that after a volcano erupts it is considered extinct?
It’s not necessarily true that volcanoes are extinct once they have erupted. A volcano is
considered extinct when it no longer has a supply of magma. A volcano is active when it
does have a supply of magma. It can be quite difficult for scientists to determine when
a volcano has lost its magma supply. Some volcanoes can lie dormant for hundreds of
years and suddenly erupt!
You’ll learn more about hot spots and volcanoes as you explore this concept.
Concept: Where Volcanoes Form
Getting to Know
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.