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Volcanoes
Volcanoes and Plate tectonics
 Volcano
is a mountain formed when layers of lava
and volcanic ash erupt and build up
 Most are dormant
 About 600 are active
What cause Volcanoes
 Magma
is less dense then solid rock
 It slowly rises to the earths surface
 Flows out through an opening called a vent
 Lava cools quickly and forms igneous rock
 Opening at the top of the vent is called a crater
 Pyroclastic flow – massive avalanches of hot
glowing rock and gases (complete destruction)
Where do Volcanoes Form
 1.
Divergent Boundaries
 Mid Atlantic
Ridge
 Forms long deep cracks called rifts where magma flows
from
 2.
Convergent boundaries
 Cascade
Mt. Range in Oregon and Washington
 Mt. Saint Helen
 Pacific Ring of Fire
 3.
Hot Spots
 Hawaiian
 Far
 Some
Islands
from the edge of any plate
area of the mantle are hotter then others
 These hot spots melt rock which rise to the surface to
form volcanoes
 Once the volcano rises above the water it becomes an
island
What determines how a volcano
Erupts
 Composition
of the magma plays a big role in
determining the manner in how a volcano erupts
 Lave – more silica (silicon and oxygen) tend to be
thicker and resists flow
 Lava containing more iron and magnesium and
less silica tend to flow easier
 Amount
of water vapor in lava influences how lava
erupts
 More water (steam) the more pressure builds up
and the more violent the eruption is
Forms of Volcanoes
 Three
basic volcanoes
 Shield
volcanoes
 Cinder volcanoes
 Composite volcanoes
1. Shield
 Quiet
eruption
 Broad volcanoes
 Gently sloping sides
 Ex. Hawaiian , rift zones like Iceland
Cinder Cone
 Explosive Volcano
 Throws
lava into the air, lava cools and hardens
 Tephra
 Forms
– small- cinders , large – bombs
steep sides
 Sunset crater
Composite Volcano
 Can
vary between quiet or violent
 Alternating layers of lava and tephra
 Convergent boundaries above subduction zone
 Mt. St. Helens
 Fissure
Eruption
 Magma
that is highly fluid can ooze from cracks or
fissures in earths surface