A volcano is an opening, or vent, in theEarth’s crust through which magma andgases are expelled.
![C:\Documents and Settings\wilesr\Local Settings\Temporary Internet Files\Content.IE5\W15XD9BH\MC900282924[1].wmf](data/images/img5.png)
Where do volcanoes form?
A large number of volcanoes lie directly on tectonicplate boundaries where it is possible for magma toform and rise to the surface.
80% of active volcanoes form where plates collide.
15% of active volcanoes form where plates separate.
The remaining volcanoes occur far from any tectonicplate boundary.
The plate boundaries in the Pacific Oceanhave so many volcanoes that it is known asThe Ring of Fire.
Volcanoes can also form at subductionzones where temperature and pressurebuild to create magma.
What are the parts of a volcano?
Volcanic Eruptions
The composition, or make up, of magma candetermine how explosive a volcanic eruption will be.
At this moment eruptions are occurring around theworld and on the ocean floor.
Non-explosive Eruptions
Most common type of eruption
Produce relatively calm flows of lava
Can release huge amounts of lava
Much of the sea floor is made up of lava from non-explosive eruptions
Explosive Eruptions
Occurs much more rarely
Can be incredibly destructive
Expells clouds of hot debris, ash & gas into the air
Instead of lava flows, causes molten rock to be blowninto the air where tiny particles harden and fall
Larger pieces of debris fall closer to the volcano
Can blast millions of tons of lava and rock in a matterof seconds, demolishing entire mountain sides
Effects of Eruptions
Climate change
Other example: In 1815,Mount Tambora blanketedIndonesia in darkness from the volcanic ash thatremained in the atmosphere after it erupted. 12,00 diedin the blast, but 80,00 died from resulting hunger anddisease. Volcanic ash can blocked the sunlight andcaused a drop in temperature, causing crops to fail.
Volcanic Landforms
Volcanic eruptions can cause profound changes inclimate, but they are best known for their volcaniclandforms.
![C:\Documents and Settings\wilesr\Local Settings\Temporary Internet Files\Content.IE5\W15XD9BH\MP900422859[1].jpg](data/images/img15.jpg)
Shield Volcanoes
Formed from layers of lava released from repeatednon-explosive eruptions
Sides are gentle slopes, but can be enormous in size
Hawaii’s Mauna Kea is the tallest mountain on Earth!Measured from its base on the sea floor, it is taller thanMount Everest.
Cinder Cone Volcanoes
Made from pyroclastic materials that where usuallyproduced from moderately explosive eruptions
Usually erode quickly because the material is notcemented together.
Small, but has steep slopes
Often occur in clusters, commonly on the sides ofother volcanoes
Composite Volcanoes(also known as stratovolcanoes)
Most common type of volcano.
Formed from explosive eruptions of pyroclasticmaterials followed by quieter flows of lava.
Have broad base and sides that get steeper toward thepointed peak.
Lava Dome Volcanoes
Formed from thick lava that doesn’t flow far as it poursover and around its vent, or opening.
Craters
A crater is a funnel shaped pit near the top ofthe central vent of a volcano.
Calderas
A large, semicircular depression that forms when themagma chamber below a volcano partially empties andcauses the ground to sink.
Lava Plateaus
A wide flat landform that results from repeated non-explosive eruptions of lava that spread over a large area
