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Volcanic Activity
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Printed: March 10, 2013
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C ONCEPT
Concept 1. Volcanic Activity
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Volcanic Activity
Lesson Objectives
• Explain how volcanoes form.
• Describe places where volcanoes occur.
• Describe what volcanic hot spots are and where they occur.
Vocabulary
• fissure
• hot spot
• mantle plume
Introduction
Volcanoes are fantastic displays of the power of the Earth. What is a volcano? How and where are they formed?
Why do some places have lots of volcanoes?
Where Volcanoes Are Found
Volcanoes rise where magma forms underground. Volcanoes are found at convergent plate boundaries and at
hotspots. Volcanic activity is found at divergent plate boundaries. The map in Figure 1.1 shows where volcanoes
are located.
FIGURE 1.1
This map shows where volcanoes are
located.
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Divergent Plate Boundaries
There is a lot of volcanic activity at divergent plate boundaries in the oceans. As the plates pull away from each
other, they create deep fissures. Molten lava erupts through these cracks. The East Pacific Rise is a divergent plate
boundary in the Pacific Ocean (Figure 1.2). The Mid-Atlantic Ridge is a divergent plate boundary in the Atlantic
Ocean.
Continents can also rift apart. When mantle gets close enough to the surface, volcanoes form. Eventually, a rift
valley will create a new mid-ocean ridge.
Convergent Plate Boundaries
Lots of volcanoes form along subduction plate boundaries. The edges of the Pacific Plate are a long subduction
boundary. Lines of volcanoes can form at subduction zones on oceanic or continental crust. Japan is an example of
a volcanic arc on oceanic crust. The Cascade Range and Andes Mountains are volcanic arcs on continental crust.
FIGURE 1.2
The Pacific Ocean basin is a good place
to look for volcanoes. The light blue wavy
line that goes up the right-center of the
diagram is the East Pacific Rise. Trenches
due to subduction are on the west and
east sides of the plate.
Hawaii trends
southeast-northwest near the center-top
of the image.
Volcanic Hot Spots
Some volcanoes form over active hot spots. Scientists count about 50 hot spots on the Earth. Hot spots may be in
the middle of a tectonic plate. Hot spots lie directly above a column of hot rock called a mantle plume. Mantle
plumes continuously bring magma up from the mantle towards the crust (Figure 1.3).
As the tectonic plates move above a hot spot, they form a chain of volcanoes. The islands of Hawaii formed over a
hot spot in the middle of the Pacific plate. The Hawaii hot spot has been active for tens of millions of years. The
volcanoes of the Hawaiian Islands formed at this hot spot. Older volcanoes that formed at the hot spot have eroded
below sea level. These are called the Emperor Seamounts.
Loihi seamount is currently active beneath the water southeast of the Big Island of Hawaii. One day the volcano will
rise above sea level and join the volcanoes of the island or create a new island (Figure 1.4).
Hot spots may also be active at plate boundaries. This is especially common at mid-ocean ridges. Iceland is formed
by a hot spot along the Mid-Atlantic Ridge.
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Concept 1. Volcanic Activity
FIGURE 1.3
Mantle plumes are found all over the
world, especially in the ocean basins. The
size of the eruptions is different at different plumes.
FIGURE 1.4
A bathymetric map of Loihi seamount.
Loihi will be the next shield volcano in the
Hawaiian-Emperor chain.
Hot spots are found within continents, but not as commonly as within oceans. The Yellowstone hot spot is a famous
example of a continental hot spot.
Lesson Summary
• Volcanoes form when magma reaches the Earth’s surface.
• Volcanoes occur most often along plate boundaries.
• Convergent plate boundaries, where oceanic crust is forced down into the mantle, form many of the volcanoes
found on Earth.
• Divergent plate boundaries produce huge mountain ranges under water in every ocean basin.
• Volcanoes like those that make up the islands of Hawaii form over areas called hot spots.
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Lesson Review Questions
Recall
1. What is a hot spot?
2. How is a hot spot related to a mantle plume?
3. Why do hot spot volcanoes form in lines?
Apply Concepts
4. What plate tectonic setting produces the most volcanoes?
5. What are the ages of hotspot volcanoes relative to each other?
6. What are the ages of volcanic arc volcanoes relative to each other?
Think Critically
7. Volcanoes have been found on Venus, Mars, and even Jupiter’s moon Io. What do you think this indicates to
planetary geologists?
Points to Consider
• When you look at the map of tectonic plates (Figure above), what areas besides the Pacific Ring of Fire would
you expect to have volcanic activity?
• Why do you think some volcanoes are no longer active and probably never will be again?
• Why do you think it’s hard to study hot spots?
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