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Answer Key VOLCANOES 101:
The word volcano comes from Vulcan, the Roman god Vulcan who “lived” under Mt.
Etna. The study of volcanoes is referred to as Volcanology. Volcanoes are
landform that develops where Stratovolcanoes, Cone, and shield volcanoes
erupt. Stratovolcanoes most often erupt with explosions, cone volcanoes
erupt less violently and shield or rift volcanoes just
ooze lava. There are two major ways volcanoes
form, they are the collision of tectonic plates
(subduction) and hotspots where magma plumes
burn through a crustal plate. Most volcanoes form
on the edges of tectonic plates and most volcanoes
are found under the sea. In convergence of plates,
there is sinking of a more dense plate into the
asthenosphere as the plate begins to melt rock, it forms magma. The hot, low density magma rises or floats
toward Earth’s surface. Eventually, magma pushing up forms a plume. Magma turns into the lava when it
spills onto Earth’s surface. After several eruptions, lava cools and builds up and a rocky/ash hill called a
volcano is formed. Volcanoes are important because when they erupt they can change the composition of air,
even change the earth’s weather and disrupt or even nearly end all life as they have done at least 20 times in
earth’s history.
HOT SPOTS
A hotspot is an area in the lower mantle that is melted by a plume. Hot magma rises because it is less dense.
Pressure is put onto the surface of the Earth, forming a bulge. A chain may form
over many eruptions. Examples of a hot spot found in a plate would be
Yellowstone and Hawaii. Hawaii sits on the pacific plate which is moving to the
northwest. There is a hot spot causing magma to rise. The plume is located in the
mantle, and is rising. The plate moves over the hot spot, forming a line of islands.
The northern Hawaiian Islands are the oldest islands as they were formed first.
Magma and Viscosity
While volcanoes form in similar ways, all volcanoes are not alike. Some are tall, steep, violent, gentle… Why
so different? To understand what makes one volcano different from another we need to look into the heart of
the volcano, its viscosity. The type of volcano and how it erupts depends on magma’s viscosity:
The more silicon in the magma, the thicker and viscous it is. The thicker it is the more likely the gasses
trapped in side it will explode when released. Silica tetrahedrons link together in long chains that stop magma
from flowing easily. The more silica chains, the more gasses are trapped.
Magma with Silica content that is:
High- Granite
Medium-Andesite
Low-Basalt
Answer these questions in complete sentences on this paper. Be detailed in your answers.
1. What is a volcano?
An eruption of magma and gasses from the mantle, hardening on the surface of the earth, (lava)
forming a Stratovolcanoes, cone or shield volcano.
2. What are the two major ways in which volcanoes most often form?
Subduction (plate collision) and Hotspot
3. Describe, in several sentences how a hot spot forms a volcano, like in Hawaii?
As the mantle plume rises it burns through the moving tectonic plate above forming a string of
volcanoes.
4. What does viscosity mean? How does silica content affect viscosity?
Viscosity is the thickness of the lava, the more silicon the thicker it is and the more gasses can be
contained.
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5. Why is viscosity so important?
Viscosity is important because the more silicon, the thicker it is and the more gasses can be contained,
that leads to explosive volcanoes.
6. Volcanoes can form on the ocean floor. [True or False.]
7. There is no relationship between plate tectonics and volcanoes.[ True or False.]
8. Cinder cone volcanoes produce moderately quiet eruptions. [True or False.]
9. A Stratovolcano often forms a mountain when layers of lava and volcanic ash erupt and build up.
10. One factor that determines whether a volcanic eruption will be quiet or explosive is viscosity.
11. Magma forms deep inside Earth because of heat form the radioactive core.
12. Basaltic magma produces oozing eruptions such as those at Hawaii.
13. Silica-rich magma produces violent eruptions.
14. Two important factors that control whether an eruption will be explosive or quiet are viscosity and
silica content.
15. How are a soft-drink container and magma alike?
Both contain dissolved gasses, that when the pressure is released, expand violently.
16. Why do volcanoes form at plate boundaries and hot spots?
Hot magma rises from the core and rises through plates and plate edges.
17. What are the three major types of volcanoes?
Stratovolcano, Cone and shield volcanoes
18. How did Hawaii form?
As the Pacific plate moved over a hot mantle plume it burns through and creates a chain of volcanic
mountains.
19. Describe the magma types for each of the three major types of volcanoes.(*generalization- there are exceptions!)
Stratovolcano –Granite
Cone-Andesite
Shield-Basalt
20. Describe and identify some of the major volcanic hazards and effects from volcanic eruptions.
Volcanoes are important because when they erupt they can change the composition of air, even change
the earth’s weather and disrupt or even nearly end all life as they have done at least 20 times in earth’s
history.
21. Using what you know from topographical maps, and the third picture on page one, which of the Hawaiian
islands is the tallest? Hawaii- is the tallest as it has the most contour lines.
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