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Volcanoes (part 2)
Earth Science - 6th Grade; Week 16; Mrs. Rubright
Review of Volcano Formation
• Heat and pressure cause rocks to melt and form magma.
• Magma needs to get out, too much pressure!!!!!!
• Rise in temperature or drop in pressure causes magma to form
faster.
Where do most volcanoes occur?
• Volcanoes occur at
both divergent and
convergent
boundaries and also
at hot spots.
Why do we have different types of
Volcanoes?
• The process of magma formation is
different at each type of plate boundary.
• Therefore, the composition of magma
differs in each tectonic setting.
• Tectonic settings determine the types of
volcanoes that form and the types of
eruptions that take place.
Types of volcanoes
Types of volcanoes
• Shield volcano: usually form at hot spots, from non
explosive eruptions
• Cinder cone volcano: a small, cone-shaped volcano built
of ash and cinders; form from explosive eruptions, very
steep.
• Composite volcano: a deep-sided volcano of lava, ash,
and cinders; form from both explosive and non explosive
eruptions.
Shield volcano
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•
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Form from many layers of “runny” lava.
Very wide, not to steep.
Biggest type of volcanoes
Tallest mountain in the world is Mauna Kea
(measures from sea floor to top)
• non explosive eruptions
Mauna Kea, Hawaii
Cinder cone volcano
• Smallest type of volcano
• Most common
• Made from pyroclastic material (material
shot out of a volcano)
• Form a large crater
• Explosive!
Paricutin, Mexico
Composite volcano:
• Eruptions alternate between explosive and
non-explosive.
• Sometimes they have runny lava layers,
other times the have pyroclastic materials
form layers.
• Have a wide base and steep sides.
• Have a crater
• Mount Fuji
Mount Fuji
2 Types of eruptions
• Explosive: volcanoes that
build enough pressure to
blow its top, sending
pyroclastic material into
the air.
• Non explosive: Build only
enough pressure to allow
lava to run down its sides.
• Eruption Recipe
Thin magma, little gas → quiet eruption
Thin magma, much gas → big eruption
Thick magma, little gas → quiet eruption
Thick magma, much gas → BOOM!
Non explosive eruption
• Mafic: refers to rocks and magma rich in
iron and magnesium.
• This type of lava that is very runny.
• As magma nears the surface there is little
pressure, causing gasses escape easily.
• Magma low in Silica have quiet eruptions
Explosive eruptions
• Felsic: means magma with high silica and
feldspar content.
• Felsic magma traps water and gas
bubbles, which leads to lots of pressure.
• Silica acts like a cork
• Explosive eruptions are caused by a build
up of high pressure.
• Convergent zones contain lots of water,
therefore have explosive eruptions.
Pyroclastic materials
• Material that is thrown into the air during
an explosion.
• Volcanic bombs: large blobs of magma
that harden in the air.
• Lapilli: pebble size rocks
• Volcanic ash: tiny powder like material
Pyroclastic materials
• Solid volcanic material such as ash and
rocks that is thrown into the air during an
explosion.
• Volcanic bombs: large blobs of magma
that are shot into the air; harden in the air.
• Lapilli: pebble size rocks
• Volcanic ash: tiny powder like material
Four types of lava
• Aa: lava that has a rough surface
• Pahoehoe: lava that has a smooth or
billowy surface; forms thin crust and
wrinkles
• Pillow lava: lava that erupts under water,
has a round shape
• Blocky lava: cooler, lava that does not
travel far from eruption, jagged when it
dries.
Aa
• lava that is thick and sharp
Pahoehoe:
• forms thin crust and
wrinkles
Pillow lava
• lava that erupts under
water, has a round
shape
Stages of a Volcano
• There are 3 stages to a volcano's life
– Active: a volcano that has erupted recently or
will erupt soon
– Dormant: a volcano that has not erupted for a
long time by may erupt again
– Extinct: a volcano that can no longer erupt
Questions
• What is the difference between magma and lava?
• What do you think determines how easily lava flows?
• Relate the shapes of volcanoes to their explosivity.
• Why do you think that many people believe all
volcanoes are dangerous and explosive?