Chapter 12
Volcanoes
Introduction
 A volcano is the opening in the Earth that
erupts gases, ash and lava.
 There are over 1500 volcanoes in the
world.
 Over 600 are currently active.
 169 volcanoes in the Unites States are
currently active. Being monitored by the
USGS
 Kilauea in Hawaii is the most active (but
not explosive) volcanoes on Earth
 Check out chart on 346 – why not explosive?
Mauna Loa on Hawai'i Big
Island (active) is world’s
biggest volcano.
 Olympus Mons is a shield volcano
that towers to an amazing 26 km.
That makes it 3 times the height of
Mt. Everest. Unlike Everest,
Olympus Mons has a very gentle
slope. It is up to 550 km at its base.
Active – Dormant - Extinct
 Active – is a volcano that has had at least
one eruption during the past 10,000
years. May be dormant or erupting.
 Dormant – have note erupted recently,
but may at any time
 Extinct – have not erupted in over 10,000
years.
 Since 1980, 45 eruptions and 15 cases
of notable volcanic unrest have
occurred at U.S. volcanoes.
t3
 Many volcanoes are found underwater.
 Where are a good deal of these
located?
 In some cases, lava can build up
high enough to form islands (Hawaii,
Iceland)
 Test question - tell what creates the
volcanoes at each of the locations on
the following slide.
Types of magma
 Basaltic
 42-52% silica content
 Dark in color
 Low viscosity
 Non-Explosive
 Low gas content
 Andesitic
 52-66% silica content
 Granitic:
 >66% silica content
 Lighter in color
 High viscosity
 High gas content
 Explosive
Viscosity
 A liquid’s ability to resist flow
 Related to SiO2 - More silica =
more viscosity
 Low viscosity – runny - watery
 High viscosity – thick - milkshake
Magma composition
 Page 350
 Be able to classify the three types of
magma based on their silica content.
 Volcano Apps WK – quiz next week
 Thursday or Friday
Types of volcanoes
 Shield: gentle-sloped, quiet
eruptions, basaltic lava, low sillicacontent, low trapped-gas content.
Kilauea.
 Composite: made of both lava and
tephra. Mount Rainier.
 Cinder cone: steep-sloped, made of
tephra, explosive, granitic lava, high
silica, high trapped gases. Krakatoa.
Hazards from volcanoes
 Lava (enough said)
 Volcanic ash - can cause lung cancer,
can also smother crops and cause
other damage
 Pyroclastic flows
 Lahars
 Poisonous gases - sulfuric gases
(rotten eggs). Can also combine with
water vapor to create sulfuric acid and
cause acid rain
 Can cause tsunamis
Pyroclastic Flows
 Flow of extremely hot material including
ash, poisonous gas and tephra.
 Can reach 700 km/h (450 mph).[2] The
gas can reach temperatures of about
1,000 °C (1,830 °F).
 #1 cause of volcano-related deaths
 (29,000 at Mount Pelee 1902)
Lahars
 A lahar is a type of mudflow or debris flow
composed of a slurry of pyroclastic material,
rocky debris, and water
 Vary in size, speed and deadliness.
 Snow and glaciers can be melted by lava or
pyroclastic flows during an eruption.
 The 1991 eruption of Mount Pinatubo caused
lahar flows: the first eruption itself killed six
people, but the lahar flows killed more than
1500.
Predicting volcanic eruptions
 There can be several signs that sometimes
help predict the eruption of an earthquake
and necessitate evacuation:





Seismic activity
Bulging of volcano’s exterior
Gas Emissions
Thermal monitoring
Mass movements
Plus Sides to volcanoes




New Land
Fertile Soil
Geothermal Energy
Mineral Resources
 aluminum, diamonds, gold, nickel, lead, zinc,
and copper.
 Tourism
 Spas and resorts
 Volcano: an opening in the Earth
where lava, ash and gas escapes
 Tephra: rock thrown into the air
 http://www.webpages.uidaho.edu/~simkat/cors220_files/t
ephra_sizes.jpg
 http://vulcan.wr.usgs.gov/Imgs/Gif/Hazards/Tephra/tephra_plot_dista
nce.gif
 Shield: gentle-sloping volcano,
generally display quiet eruptions
Crater - a circular
depression in the ground
caused by volcanic activity
 Vent: opening of a volcano

 Dike: a vertical igneous rock
formation.
 Magma: hot, melted rock beneath
the surface.
 Caldera: large crater formed when
a volcano collapses
 Cindercone- steep sided volcano
known for explosive eruptions
 Neck: a solid igneous core that
remains after the volcano has been
eroded.
 Batholith: intrusive igneous rock
formed under ground
 Composite- volcano made of both
lava and tephra
 Sill- horizontal igneous rock
formation

 Pluton - any type of igneous rock
intrusion
 Hot spot- place above an
exceptionally hot part of the
mantle. (not at plate boundaries)
Tephra
 First, what type(s) of volcano erupts
tephra as a product?
 Classified by size
 Ash - particles smaller than 2 mm (0.08 inches)
in diameter
 Lapilli or volcanic cinders - between 2 and
64 mm (0.08 and 2.5 inches) in diameter
 Volcanic bombs or volcanic blocks - larger than
64 mm (2.5 inches) in diameter.
Where do volcanoes occur?




See map, page 337
Divergent boundaries
Subduction zones
Hot spots
 Know two realistic examples of each