Name: ___________________________
Date: _____________________
Section: ____
Content Explorations
5
Mountains Formed by Magma
By Patti Hutchison
You have probably heard of Mount Saint
Helens. This mountain is part of the Cascade
Range in the northwestern United States. You
know that these mountains were formed by
magma. But did you know that the Adirondack
Mountains in New York were also formed by
magma?
1
Volcanic mountains are formed when
magma erupts onto earth's surface. Many of
these mountains are found along mid-ocean ridges. These are divergent boundaries where
tectonic plates are spreading apart.
2
As the sea floor spreads apart, magma moves upward to fill the space. It cools and
forms mountains. As the eruptions continue, the mountains grow taller. Huge mountain
ranges are formed under the ocean.
3
Single volcanic mountains form over hot spots in the earth. These are areas where the
mantle is extremely hot. As a tectonic plate moves over this hot spot, the molten mantle is
forced through the crust. It forms a volcanic peak. A chain of volcanoes forms as the plate
continues to mover over the hot spot. As they grow taller, they can form mountainous
islands such as the Hawaiian Islands.
4
Some volcanic mountains have formed on land. As lava erupts, along with ash and
cinders, it cools. Over time, and with many eruptions, a mountain is formed. Mount Saint
Helens is an example of this type of mountain.
5
The Adirondacks are not volcanic mountains. How can they have been formed by
magma? You know that magma is less dense than the solid rock that surrounds it. The
molten rock is forced upward through cracks in the layers of rock. This is called intrusion.
6
But magma doesn't always reach the earth's surface. It sometimes gets trapped under the
crust. The magma pushes the overlying rocks upward. There is no folding or faulting. Over
a long period of time, the magma cools and forms hardened rock.
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Mountains formed in this way are called dome mountains. They are shaped like a halfsphere. Over time, the overlying rock is eroded. The hardened magma is exposed, forming
a mountain. In addition to the Adirondacks, the Black Hills of South Dakota are an example
of dome mountains.
8
Mountains made from magma sometimes seem to rise up out of the land or sea around
them. Beautiful lakes can form in the valleys. These mountains are often a place for
recreation such as skiing, hiking, and fishing.
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Copyright © 2008 edHelper
Name: ___________________________
Date: _____________________
Section: ____
Content Explorations
Mountains Formed by Magma
1. Name two types of mountains that
are formed by magma.
2. Most volcanic mountains are found
near:
Mid-ocean ridges
At divergent boundaries
Both a and b
3. Single volcanic mountains are
4. What is intrusion?
formed over:
Mid-ocean ridges
New York
Hot spots
5. Intrusions can form what type of
mountain?
Folded
Dome
Volcanic
6. An example of dome mountains is
the:
Adirondack Mountains
Hawaiian Islands
Cascade Mountains
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Name: ___________________________
Date: _____________________
Section: ____
Content Explorations
5