1. Volcanoes and Eruptions
8SCIENCE

2. Review Where do volcanoes form? How do volcanoes form?
Plate boundaries
How do volcanoes form?
Heat and pressure from inside the Earth produces magma and pushes up to the surface
What is a hotspot?
Very hot areas in the mantle that produce volcanoes although they are not found on plate boundaries

3. Types of Volcanoes
Shield
Cinder cone
Composite

4. Shield volcanoes
Quiet eruption that produces lava and spreads out in flat layers
These layers build to form slightly sloping sides
Ex: Hawaiian islands produces 2 different types of lava:
Pahoehoe (smooth)
- A’a (rough and chunky)

5. Pahoehoe and A’a lava
A’a lava
Pahoehoe

6. Cinder cone volcano
Explosive eruptions that throw lava and rocks high into the air
These bits of rock and hardened lava are called tephra
Tephra layers build up to form steep sided volcanoes

7. Tephra lava
Tephra lava erupted from the 1980 Mount St. Helens Eruption in Washington State

8. Composite volcano Produces eruptions that are quiet then violent
Cycles of lava and tephra form different layers
Usually found where one plate is forced under another

9. What causes an eruption to be quiet or violent?
Amount of water vapor and other gases trapped in magma
High gas content = more violent eruption
Gases are trapped in magma and pressure builds up
As magma moves closer to the surface, there is less pressure and the gas can escape
Amount of silica (silicon and oxygen) present in magma
Low in silica = more quiet eruption

10. Other volcanic structures
Not all magma flows to the surface
Some may rise from the volcano and cools underground to form other rock structures
Erosion of the rock layers make these visible to us today

11. CALDERAS IN OREGON (US) AND PHILLIPINES
DEATH VALLEY, CALIFORNIA
SHIPROCK, NEW MEXICO
COLOMBIA