1. Volcanoes Chapter 29.1

2. Unit Ten: Earth Science
Chapter 29 Formation of Rocks
 29.1 Volcanoes
 29.2 The Surface of Earth
 29.3 Rocks and Minerals

3. 29.1 Volcanoes
 The eruption of Mount St.
Helens in 1980 reduced the
height of this mountain
from 2,932 meters (9,677
feet) to 2,535 meters (8,364
feet).
 Early in the morning of May
18, 1980, an earthquake
triggered a landslide that
caused the bulge to eject
magma, water, and gases.

4. Mount St. Helens

6. 29.1 Volcanoes
 *Solid rock melts and becomes magma under
certain conditions that lower the melting point of
the material.
 *At subduction zones, water is the key for solid rock
to melt and become magma.

7. 29.1 Volcanoes
 *Most volcanic
activity is found at
the edges of
tectonic
plates, namely at
divergent and
convergent plate
boundaries, but
does not occur at
transform plate
boundaries.

9. Shield Volcanoes:
 *Low
viscosity, fast-
flowing lava is
associated with
shield volcanoes.
 Because this lava
easily flows down
hill, shield
volcanoes are
gently sloped and
flattened.

10. Composite Volcanoes
 *High viscosity
lava is associated
with
stratovolcanoes
(also called
composite
volcanoes).
 These volcanoes
range in height
from 500 to 10,000
meters high.

11. Cinder Cone Volcanoes
 *Cinder cone
volcanoes are steep
stacks of loose
pyroclasts (clumps
and particles of
lava).
 Cinder cones are
rarely higher than
300 meters.

12. Lava Facts:
 *Lava Explosive eruptions occur when the lava has a
lot of water and dissolved gases.
 Gentle eruptions are associated with fast-flowing
lava from oceanic crust. viscosity also determines
how explosive an eruption will be.

13. Where volcanoes form
 Volcanoes also form when an oceanic plate slides under
another oceanic plate.

14. Hydrothermal Vents
 *Hydrothermal vents are deep sea, chimney-like
structures that occur along midocean ridges.

15. Gemstones
 Some gemstones are also associated with volcanic
activity.
 For example, diamonds form at high temperatures deep
underground when carbon crystallizes inside rocks called
kimberlites.

16. Geothermal Energy
 *Geothermal energy is
the useful product of
volcanic activity.
 When steam from
magma collects below
ground, it can be tapped
just like water in a well.
 *The pressurized steam
can be used to generate
electricity.

17. 29.1 Volcanoes
 *Volcanic activity
results in the
formation of two
kinds of igneous
rocks:
 extrusive
 intrusive
 A batholith is a large
underground rock
that formed when a
mass of magma
cooled underground.

18. Key Questions:
1. Why do some volcanoes erupt
explosively?
2. Which volcanoes have a slow
flow of lava down the slopes?
3. Where do volcanoes occur?
4. What ocean does the Ring of
Fire surround?