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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?
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.
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.
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.
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.
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.
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.
Mountains formed in this way are called dome mountains. They are shaped like a half-sphere. 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.
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.
Mountains Formed by Magma
Questions
1. Name two types of mountains that are formed by magma.
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2. Most volcanic mountains are found near:
A. mid-ocean ridges
B. at divergent boundaries
C. both a and b
3. Single volcanic mountains are formed over:
A. New York
B. hot spots
C. mid-ocean ridges
4. What is intrusion?
5. Intrusions can form what type of mountain?
A. folded
B. dome
C. volcanic
6. An example of dome mountains is the:
A. Hawaiian Islands
B. Adirondack Mountains
C. Cascade Mountains
Write a paragraph explaining three places volcanic mountains can form. How are they formed in each place?
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Find out more information about the Black Hills of South Dakota. Write a paragraph about them.