Volcanoes
and
Plate Tectonics
Section 1- Pages 88-91
1- Where are volcanoes found?
Most volcanoes occur at plate
boundaries.
 Either divergent boundaries like the
mid-ocean ridge,
Or…
 Convergent boundaries where
subduction occurs

2- How do volcanoes form at
divergent plate boundaries?
As the plates move away from
each other, lava pours out the
cracks creating volcanoes.
3- How do volcanoes form at convergent
boundaries?
•When two plates collide, a subduction zone is
created.
•As the older more dense plate sinks below the
less dense plate, the material melts as it heats up.
•As it heats up, it rises because it becomes less
dense.
•The magma finds its way through cracks in the
crust and creates volcanoes.
4- How do hot spot volcanoes form?
Plumes of magma rise from deep in
the mantle and melts through the
crust creating volcanoes in the
middle of plates.

Lo`ihi Seamount is
an active volcano
built on the seafloor
south of Kilauea
about 30 km from
shore. The
seamount rises to
969 m below sea
level and generates
frequent earthquake
swarms, the most
intense of which
occurred in 1996.
Volcanic Activity
Section 2- Pages 93-102
1- Explain how a volcano erupts.
Molten material rises forcing its way
through the crust.
As pressure increases eventually the
magma reaches the surface causing
the eruption of lava and gases.
2- Describe the characteristics of magma.
Magma is hot molten rock and
dissolved gasses.
 Magma could either be thick and flow
slowly or thin and flow quickly.
 Depends on the amount of gasses,
temperature, water and silica in the
magma.

3- Compare/Contrast magma with
HIGH silica content and magma with
LOW silica content.
Magma with HIGH silica content
makes the magma thicker and
clumpy,
But…
Magma with LOW silica content is
thin and runny.
4- Describe a QUIET eruption, and
explain the lava.
A quiet eruption has lava that is thin
and runny and flows out easily.
 It can create very long lava flows.
 Can create Pahoehoe or aa.


http://www.youtube.com/watch?v=488BkTUsMa4
5- Describe an EXPLOSIVE
eruption, and explain the lava.
An explosive eruption is when the very
thick and sticky magma causes the pipe to
clog up and the volcano to explode due to
the huge amount of pressure.
 Ash- fine grain size rock, cinders- smaller
than a baseball, and bombs- larger than a
baseball are thrown from the eruption along
with the release of gases. (pyroclastic flow)



http://www.youtube.com/watch?v=EupnfA-PDaw explosive eruptions
http://www.youtube.com/watch?v=Cvjwt9nnwXY pyroclastic flow
6- How do geologists monitor
volcanoes?
They use similar devices that are
used to monitor faults because small
earthquakes occur before an eruption.
 Geologists also monitor ground water
temperatures. An increase in
temperature indicates magma moving
into the magma chamber.

7- What are some hazards of volcanic
eruptions?





During a quiet eruption, lava can set fires and
bury everything in its path.
During an explosive eruption, hot and
poisonous gas clouds can be emitted. Ash and
cinders can also cover an enormous area.
Eruptions can also cause landslides which can
cause a lot of damage over a very large area.
http://www.youtube.com/watch?v=4aYQixhdWY4
(Top 10 Most Active Volcanoes)