Volcanoes and
Eruptions
8SCIENCE
Date
Title
16/04/13
Volcanoes and
Eruptions
Page
Review
• Where do volcanoes form?
– Plate boundaries
• How do volcanoes form?
– Heat and pressure from inside the Earth produces
magma and pushes up to the surface
• What is a hotspot?
– Very hot areas in the mantle that produce
volcanoes although they are not found on plate
boundaries
•
• Identify the cause of each volcano below using the words:
subduction, rift, and hot spot.
• a) In 1973, a town in Iceland called Vestmannaeujar was surprised
when lava began erupting from a vent or crack in the Earth’s
surface. The Earth’s crust seemed to split
apart._______________________
•
•
• b) Scientists feel that the Cascade Mountains, extending from
California to British Columbia, are still forming. Two plates are
colliding, forcing magma up from the mantle to form the mountain.
_____________________
•
• c) Mauna Loa in Hawaii is the “growing mountain”. In 1969 and
1974, it erupted and poured out lava through a hole in the Earth.
This lava called and hardened into rock. Below it, molten lava
continues to flow. _____________________
Types of Volcanoes
• Shield
• Cinder cone
• Composite
Introduction to the three types of
volcanoes
• Three types of volcanoes
• As you view the video clip, try to think of
some of the similarities and differences
between the three types of volcanoes.
Shield volcanoes
• Quiet eruption that produces lava and spreads out in flat
layers
• These layers build to form slightly sloping sides
• Ex: Hawaiian islands produces 2 different types of lava:
– Pahoehoe (smooth)
- A’a (rough and chunky)
Pahoehoe and A’a lava
A’a lava
Pahoehoe
Cinder cone volcano
• Explosive eruptions that throw lava and rocks
high into the air
• These bits of rock and hardened lava are
called tephra
– Tephra layers build up to form steep sided
volcanoes
Tephra lava
Tephra lava erupted from
the 1980 Mount St.
Helens Eruption in
Washington State
Composite volcano
• Produces eruptions that are quiet then violent
• Cycles of lava and tephra form different layers
• Usually found where one plate is forced under
another(subduction zone)
Let’s review with animations…
• Types of Volcanoes
What causes an eruption to be quiet
or violent?
• Amount of water vapor and
other gases trapped in magma
– High gas content = more violent
eruption
– Gases are trapped in magma and pressure builds up
– As magma moves closer to the surface, there is less
pressure and the gas can escape
• Amount of silica (silicon and oxygen)
present in magma
– Low in silica = more quiet eruption
Other volcanic structures
• Not all magma flows to the surface
• Some may rise from the volcano and cools
underground to form other rock structures
• Erosion of the rock layers make these visible
to us today
CALDERAS IN
OREGON (US) AND
PHILLIPINES
DEATH VALLEY,
CALIFORNIA
SHIPROCK, NEW
MEXICO
COLOMBIA