Investigation 5: Earthquakes, Volcanoes, and Mountains
Vocabulary Words: fault
subduction zone
continent-continent collision zone
transform boundary
shield volcano
composite volcano
divergent plate boundary
convergent plate boundary
hot spot
magma
mid-ocean ridges
Ring of Fire
Earthquakes
Figure 1: San Andreas Fault
Rocks are solid material. It takes a large force to break
them. Plate movements cause large forces to build up in the
lithosphere, and when these forces become very large, the rock
breaks. This break in rocks is called a fault. Faults are
fractures in the Earth’s surface where the rocks have broken
and moved due to plate tectonics. The San Andreas Fault
(Figure 1) and Hayward Faults in California are examples of
faults. When these rocks break, they slide past each other until
the energy from the forces is gone. This is felt as an earthquake.
After a while, the fracture “heals”, and it makes the rocks
stronger again. This is why faults will slip and stick again and
again.
Earthquakes and Plate Movements
Many of the largest earthquakes happen at a subduction zone (remember that
subduction happens at a convergent plate boundary, that’s where plates come together).
This can be either an ocean plate being subducted under a continental plate (Figure 2), or
at a continent-continent collision zone (Figure 3).
Figure 2: Oceanic crust and continental crust
convergent plate boundary (subduction zone).
Figure 3:
Continentcontinent
collision
zone.
Notice the
new
mountain
range,
made of
folded
mountains
.
Earthquakes are also very common at a transform boundary.
The plates are “stuck” together until the forces become too great,
and then they “slip” and an earthquake happens.
Figure 4: Transform plate boundary.
Volcanoes
A volcano is a place where molten rock, along with solid rock and gases, erupt from
within the Earth to the Earth’s surface. There are two major types of volcanoes: shield
volcanoes and composite volcanoes. Both volcanoes contain magma, which has
dissolved gases in it, much like a 2-liter bottle of soda.
Shield Volcanoes
Shield volcanoes have magma with low gas levels. This
lava flows out of a volcano peacefully, without powerful
explosions. Hawaii is an example of a shield volcano.
Figure 5: Shield Volcanoes
Composite Volcanoes
Composite volcanoes have magma with high gas levels. This gas bubbles out of the
magma as it reaches the Earth’s surface, and causes
powerful explosions. These explosions blow magma, ash,
and broken rock into the air. This is the type of volcano that
exploded at Pompeii.
Figure 6: A composite volcano.
Volcanoes and Plate Tectonics
Figure 7: Divergent
Plate Boundary
Volcanoes can form in three different places:
along mid-ocean ridges (at divergent plate
boundaries), in subduction zones (at
convergent plate boundaries) and at hot spots.
1. Divergent Plate Boundaries
In investigation 3, you learned how
volcanoes formed along mid-ocean ridges.
These volcanoes are mostly located deep in
the ocean, but sometimes build islands above sea level. Iceland is an example of a midocean ridge volcano that is above the ocean. See Figure 7.
2. Convergent Plate Boundaries
Most large volcanoes occur at
convergent plate boundaries.
Scientists think this is because the
ocean crust that is being subducted
contains water. As the crust goes
deeper into the Earth, the water is
released from the rocks and helps to
make more magma by lowering the
temperature at which rocks melt.
Figure 8: Convergent Plate
Boundary. Notice the
volcanoes being formed.
3. Hot Spots
There are hot spots in the mantle that make magma for a long
period of time. Hot spots stay in one spot and don’t move with
the lithospheric plates. Volcanoes form over this hot spot, and
scientists can tell how plates have moved by the line of
volcanoes formed by the hot spot. See Figure 9.
Figure 9: Hot spots
How are Earthquakes and Volcanoes related?
Along subduction zones, there are a lot of major earthquakes and large volcanoes.
The Pacific Ocean is known as the “Ring of Fire” because of this. There are subduction
zones all around the Pacific Ocean. See Figure 10.
In continent-continent collision zones, there are earthquakes but not volcanoes. An
example of this is the Himalayan Mountains where the Indian Plate is colliding with the
Asian Plate.
Figure 10: The Pacific “Ring of Fire”