Download Volcanoes – Mountains from Magma

Name: ____________________
Volcanoes – Mountains from Magma
The Earth’s surface is full of volcanoes. Some of them erupt frequently and somewhat quietly,
and you can actually watch the lava flow out of them from a safe distance. Others erupt only
once every few hundred years – but when they do, it is wise to be as far away as possible.
Shield Volcanoes
Shield volcanoes do not occur at
the edges of plates. Instead, they
can be found anywhere on a plate
– even rising out of the ocean
floor! These volcanoes are formed
above hot spots in the mantle,
where the magma collects in
enormous pools. The hot magma
eventually melts the rock above it, and pours out through the hole onto the Earth’s surface as
lava. The lava that comes out of a shield volcano is usually very runny, and hardens into a type
of rock called basalt. This lava is so fluid that it is very rare for a shield volcano to erupt
explosively. The lava simply pours out of the volcano like a river and then hardens. Because of
this, shield volcanoes build up very gradually and have gently sliding slopes.
When a shield volcano forms on the ocean floor, the
lava pouring out hardens more quickly than it would
on land. More lava pours on top, forming a volcanic
cone. The cone may build up until it rises above sea
level, and forms an island. This is how Mauna Loa, a
shield volcano on the island of Hawaii, was formed.
Stratovolcanoes
Stratovolcanoes are the type of volcano that you
usually hear about in the news. These volcanoes
erupt extremely explosively, blowing ash and rock
many kilometers up into the atmosphere, along
with an enormous amount of steam and gas.
These volcanoes form where two plates collide,
with one of the plates sliding on top of the other.
As the light rocks on the subducting plate heat
up and melt, the magma rises up against the plate
above and melts through it. This forms a hole so
the magma can eventually escape.
While shield volcanoes have very liquid
magma, the magma in a stratovolcano is
thick and sticky. Because the magma is
created from melted crust, and the lower
plate often contains water, the lava from
stratovolcanoes also contains steam
underneath of the enormous pressure. It
is this combination of thick magma and
steam that causes stratovolcanoes to
erupt so violently and explosively. As the
lava rises, the high-pressure steam
escapes, carrying lava and ash with it.
Mount St. Helens is one of the most
famous stratovolcanoes.
Eruptions where the Plates Part
As you have already seen, lava also flows up through the crust and onto the Earth’s surface
where plates are moving apart, usually on the ocean floor where the crust is thinnest. Here, the
lava cools quickly and forms basalt (rock that is hardened lava) in a long ridge on each side of
the crack in the ocean floor. Some of these ridges rise high enough to reach the surface of the
ocean, creating islands such as Iceland. These islands are initially caused by eruptions, but there
are sometimes no volcanoes created because the lava simply hardens into rock overtop of its
escape point.
Name: ____________________
Volcanoes – Mountains from Magma
1. Which type of volcano has very liquid, runny magma?
2. Where are stratovolcanoes found? How are they created?
3. Where can eruptions occur without forming volcanoes?