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?