1. Types of Volcanoes

2. Frequency and Quantity
There are ~ 550 historically active volcanoes
About 60 erupt annually
The majority are 1) at the edge of continents and 2) underwater
Dormant volcanic cone in Japan

3. What causes volcanoes?

4. Underwater Volcanoes
Rift valley
plate
plate
magma

5. Mid ocean ridges

6. Mid ocean ridge

7. Mid ocean ridges Are continuous underwater mountain ranges
Are formed by rising magma
Cause sea floor spreading
Form a rift valley between spreading plates
Pillow lava at mid ocean ridge

8. Hot Spots
Hot spots create chains of islands like the Hawaiian islands or the Aleutian chain.

10. Hawaiian Islands – hot spot

12. Land Based Volcanoes

13. A volcano at the edge of a continent
Mid ocean ridge ?
Sinking plate
Melted rock forming magma

14. Three Types of Land Based Volcanoes
Volcanoes take different shapes depending on the amount of SILICA and DISSOLVED GASSES in the magma
High Silica content…
More likely to be explosive!!!
Why?
Silica has a thick, stiff consistency
Flows slowly
Tends to Harden in the volcano’s vent
High Dissolved Gas content…
More gas comes out, an eruption can launch ash and debris great distances
However, if lava is viscous and flows quickly, these eruptions may not be that dramatic

15. What Erupts from a Volcano?
Blocky lava
Lava can be
thick or thin.
Pahoehoe Aa
Pillow lava

16. What Erupts from a Volcano?
Pyroclastic material
Rock fragments created by eruptions
magma explodes from volcano and solidifies in the air
existing rock is shattered by powerful eruptions
EXPLOSIVE
Lapilli
Volcanic bombs
Volcanic blocks
Volcanic ash

17. Let’s see it in action
ment/natural-disasters/forces-of- nature.html?section=t

18. Shield Volcanoes
Volcanoes that build up from many slow, steady, flows of hot lava, are called shield volcanoes. This kind of volcano is low and broad with gently sloping sides. They look like a warriors shield.
They have LOW silica content and LOW dissolved gases.
Unlikely to yield massive explostions

19. Shield Volcanoes

20. Cinder Cone Volcano
Cinder cone volcanoes form when solid rock and ash shoot up into the air and fall back around the volcano opening. The cinder cone volcano has steeply sloped sides.
Cinder cone volcanoes have LOW silica content and HIGH dissolved gas content
Cinder cones can have violent eruptions, but are unlikely to send ash or lava too far

21. Cinder Cone

23. Composite Volcano
Composite volcanoes, erupt with molten lava, solid rock, and ash. The layers pile up much like layers of cake and frosting. The layers form into symmetrical cones, and the slopes are steep.
Composite volcanoes have HIGH silica content and HIGH dissolved gas.
Composite volcanoes erupt violently and can send ash and lava great distances

24. Composite Volcano

26. Composite Volcano
Mt St Helens one day before eruption

27. Mt St Helens 4 months after eruption

28. Another before…

29. …and 2 years later

30. Now, it is your turn. Pick a type of volcano and build it out of clay
Now, it is your turn. Pick a type of volcano and build it out of clay. Then, we are going to make TOPOGRAPHIC MAPS.