Download Volcanoes

Volcanoes
A place where molten rock, hot gases, and
solid rock erupt through an opening in the
crust. It is also the mountain built up by these
materials.
How Volcanoes Form
• Beneath the volcano is a source of magma.
• Magma rises to the surface through cracks in
the earth.
• In time the magma reaches the surface.
• The magma erupts through the central
opening known as the vent.
• When the magma comes out of the vent it is
called lava.
• Erupted material cools and hardens around
the vent in a mound.
• After many eruptions it can pile up into a big
hill or mountain called a volcano.
Parts of a Volcano
• Vent – central opening
to the volcano
• Crater – cuplike
hollow around the vent.
– Caldera – when
a volcano collapses and
forms a very wide crater
• Lava – magma that
has reached the
surface
Where Volcanoes Form
• Occur along the plate boundaries in belts or
long lines.
• 80% of all Earth’s volcanoes are found within:
– Pacific Ring of Fire
– Mediterranean Sea through Iran
Pacific Ring of Fire
Rift Volcanoes
• Where plates are moving apart volcanoes
form along the gaps at the edges of spreading
plates.
• Most are located deep underwater along midocean ridges.
Along Colliding Plates
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One plate plunges beneath another.
The downward-moving plates melts.
Magma forms.
It moves upward through the rocks and forms
volcanoes.
The Hawaiian Islands
• A chain of volcanoes, many not active
• In the middle of a plate
• Believed to be moving over a hot spot
– Kauai is the oldest island.
– It formed when it was located over a hot spot.
– As the plate moved, Kauai moved away from the
hot spot and was no longer active.
Hot Spot
• A very hot part of the mantle
• As a plate moves over the hot spot, magma
melts up through the crust and forms
volcanoes.
• Other examples:
– The Azores and the Galapagos
What Causes Eruptions
• Magma is less dense than the rock around it.
• The magma rises up toward Earth’s surface.
• Near the surface there is less rock overhead
pressing down on the magma.
• If the pressure decreases enough, it is like
removing the cap of a shaken bottle of soda.
• Gases that were dissolved in the magma came
boiling out.
• The gases shoot lava or partly hardened chunks
of lava out the vent.
Types of Eruptions
• Depends on how much gas is in the magma
and how thick the magma is.
Types of Volcanoes
Cinder-cone Volcano
Has both explosions and flowing eruptions
Cinder-Cone Eruptions
• Magma that is thick and has a lot of gas in it.
• Lumps of magma may get stuck as the magma
rises to the surface.
• In time the magma bursts free.
• Gases explode our of the magma.
• Lava blasts outward and hardens.
• What falls to the ground is a rain of hot rocks
in sizes from tiny droplets to huge boulders.
• Builds into a steep-sided cone.
Shield Volcano
flowing eruptions
http://www.youtube.com/watch?v=G3ZWAgN3Wzc
Shield Volcano Eruptions
• Lava flows out of the vent due to a lot of gases
already having escaped from the magma.
• The lava spreads out and hardens into a wide,
flat mound.
Composite Volcano
Explosive eruptions
http://www.youtube.com/watch?v=j5FBwCZ6xOs
Composite Volcano Eruptions
• The eruptions “take turns”
– An eruption may explode and the next time may
flow out of the volcano.
• The switching of eruptions repeats over and
over.
• Have beautifully symmetrical shapes
– The shape of one side of the cone matches the
opposite side.
Active Volcanoes
• Erupting now or have erupted recently
• Many located in the Pacific Ring of Fire
Japan
• Made up of volcanic island including active
volcanoes
• These islands were built up from the ocean
floor along a convergent boundary.
– One plate is plunging under another.
– Molten rock pushed through the surface and
forma string, or arc, of volcanoes.
– The volcanoes grew in size over time from
undersea volcanoes to islands far above sea level.
Dormant Volcanoes
• When active volcanoes stop erupting
• Has not been active for a long time but has
erupted in recorded history.
Extinct Volcanoes
• Has not erupted in recorded history.