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Volcanoes
What is a Volcano?
• A volcano is an opening in Earth’s crust
that releases lava, smoke and ash when it
erupts.
• Non active volcanoes are called dormant.
• The hole from which the magma emerges
is called a vent.
•
http://video.nationalgeographic.com/video/player/environment/environmentnatural-disasters/volcanoes/volcano-lava.html
Cinder Cone Volcano
• Is the smallest and most common of the three
types of volcanoes (most less than 300m)
• Has steep sides and a bowl shaped crater at the
top
• Known for violent, explosive eruptions
• Streaming gases carry liquid lava blobs into the
atmosphere that fall back to earth around a
single vent to form the cone. The volcano forms
when ash, cinders and bombs pile up around the
vent to form a circular or oval cone.
Cinder Cone Examples
Krakatau, Indonesia
Paricutin, Mexico
La Poruna, Chile
Cinder Cone Volcano Diagram
Vent
Fine Ash
Magma
Cinder
Shield Volcanoes
• Made of gently sloping layers composed entirely of
cooled lava.
• Low, broad shape formed by hot spots
• Shield volcanoes are the largest of the 3 volcanoes.
• Made of layers upon layers of cooled lava, ash is rare in
shield volcanoes.
• They do not erupt violently
• The Hawaiian Islands are shield volcanoes
• A hot spot is an area of volcanic activity produced my
magma rising from the mantle. The temperature under
the crust at a hot spot is much higher than elsewhere.
The causes the magma to be forced upward through
cracks in the Earth’s crust.
Examples of Shield Volcanoes
Mauna Loa, Hawaii
Kilauea, Hawaii
Ilgachuz Range, BC (Extinct)
Mount Edizza, BC
Shield Volcano Diagram
Composite Volcano
• Is made of alternating layers of lava and
cinder/ash.
• The eruption may be quiet or violent and is
intermittent (hundreds or thousands of years of
inactivity separating a few years of activity.)
• Most Complex
• Formed by melting of crust when colliding with
other rock surfaces (subduction)
• Can be very large because of their long lives
and resistance to erosion.
Examples of Composite Volcanoes
Mount Baker
• Active Volcano
• 3285m
• Last eruption was in the
1800’s, but eruption of hot
fluids and gas in the late
1970’s show the volcano is still
active
• Glaciers cover Mt. Baker – an
eruption would not only
produce large quantities of
ash, but create volcanic debris
flow, floods, and landslides
that would cause extensive
damage in BC.
Examples of Composite Volcanoes
Mount St. Helens
• Active Volcano
• 2950m
• Explosive eruption was on May
18, 1980 – the rock on one
side started to bulge a few
days prior and there were
several earthquakes
• 400 km2 of forest was leveled
and 24 km of valley was
covered by debris flow
• The eruption cloud rose over
20 km and ash deposits were
7- 10 cm deep
• http://www.fs.fed.us/gpnf/
volcanocams/msh/
Examples of Composite Volcanoes
Mount Meager
• Dormant Volcano in BC
• The oldest known eruption was
2.2 million years ago
• The ash plume from the last
eruption travelled into Alberta
• A lava flow dammed the
Lillooet River
• The pumice created in
subsequent eruptions made
Canada’s only pumice mine
possible, on the northeast side
of Mt. Meager
• The explosive nature of Mt.
Meager’s past eruptions has
caused scientists to infer that
the volcano is a threat to
Pemberton and the Lillooet
River area.
RING OF FIRE
• Volcanoes often form along plate
boundaries
• The volcanoes encircling the Pacific
Ocean are called the RING OF FIRE.
• The Ring of Fire is composed over 75% of
the world's active and dormant volcanoes.
Volcanoes are so DANGEROUS,
so why do people live near them?
• People live close to volcanoes because
the soil is very fertile. Volcanic ash is rich
in nitrogen, which makes the soil ideal for
growing plants, fruits and vegetables.
• Sometimes communities are wiped out
due to the volcano. The volcanoes in BC
cause very little personal or economic
damage because they are located in areas
that are not very populated.
Intrusive Volcanic Features
• Most magma never reaches Earth’s
surface.
• Instead magma cools slowly underground
and produces rock bodies that could
become exposed later by erosion.
• Volcanic Neck
– Core of the volcano left behind after the
softer cone has been eroded. Black Tusk,
Garibaldi is an example.
• Sill
– Formed when magma flows parallel to the
layers of rock and then hardens
• Dike
– Formed when magma flows across layers
of rock and hardens
The Relative Age of Rocks
• Geologists can use their knowledge of intrusive volcanic
features and faults to understand the geologic history of
rock layers.
• The Principle of Superposition can be used to determine
the relative age of rocks. In undisturbed layers of rocks,
the oldest rocks are on the bottom and the youngest
rocks are on the top. Sediments are often deposited in
horizontal beds, forming layers.
• Relative age – its age compared to other things – not
exact. If layers of sedimentary rock are moved by a fault,
you know that the layers had to be there first before a
fault could cut through them. Therefore the relative age
of the rocks is older then the relative age of the fault.
• Complete Pardon My Intrusion pg 281
Absolute Age
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Earth is approx. 4.6 billion years old!
Absolute age, is the age of an object.
Geologist determine absolute age by using
properties of some of the atoms that make up
the object.
Geologic Time Scale

Determining the relative age and absolute age
of animals and plants that lived on Earth in the
past has allowed scientists to create a geologic
time scale. The geologic time scale divides the
history of Earth into small Eras. The division
of Eras is based on geologic events and the
fossil record of the appearance and
disappearance of living things.