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 HelenPacific Ring of Fire

3. Hot Spots Hawaiian Islands

Far from the edge of any plate

Some 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 – small- cinders , large – bombs Forms 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