1. Volcanoes

2. A Volcano is…
An opening in Earth’s crust through which molten rock, gases, and ash erupt.
Also, the landform that develops around this opening.

3. Why Volcanoes form
Volcanic Eruptions occur when magma rises to the surface. This will happen when the asthenosphere melts enough to flow. There are three things that can cause this:
1- A decrease in pressure (like at a mid-ocean ridge or rift valley)
2- An increase in temperature (like at a hot spot)
3- An increase in the amount of water in the asthenosphere (like at subduction)

4. Where Volcanoes Form Hot Spots Most volcanoes occur at:
DIVERGENT boundaries, and at
CONVERGENT boundaries that have SUBDUCTION.
Hot Spots

5. Volcanoes at Divergent Boundaries
Decrease in pressure as plates pull apart lets magma rise.
These fissure volcanoes are located at the Mid-Atlantic Ridge and the Great African Rift Valley

6. Divergent- Great Rift Valley

7. Volcanoes at Convergent Boundaries (Subduction)
Subduction increases the amount of water in the asthenosphere, which lowers the melting temp.
As the denser oceanic crust is pushed lower, it melts into magma.
When continental and oceanic plates converge, a volcano forms on land.
When 2 oceanic plates converge together, a volcano forms an island.
Mount Hood/ Portland

8. Volcano- Convergent
Sibayak, Indonesia
Mt St. Helens

9. Hot Spots We call these HOT SPOTS.
Sometimes, volcanoes occur at places that aren’t plate boundaries.
We call these HOT SPOTS.
Hot spots are areas where hot magma rises from deep in Earth’s mantle.
Magma escapes where the crust is the thinnest or weakest.
It starts out solid then it melts when it reaches areas of lower pressure.

10. Example of Hot Spot
Volcanoes that make up Hawaii.

11. Shield Volcanoes Cinder Cone Volcanoes Composite Volcanoes
Types of Volcanoes
There are 4 main types of Volcanoes:
Shield Volcanoes
Cinder Cone Volcanoes
Composite Volcanoes
Fissure Volcanoes

12. Shield Volcanoes
Shield Volcanoes form from runny lava (low viscosity) that tends to flow long distances before hardening. They generally have quiet eruptions.
Volcano has a broad base and gently sloping sides.
Volcano has a less explosive eruption than other types because the lava flows more easily
Type of magma- Mafic Basalt: black, runny lava
Made of layers of lava
Example: Mauna Loa, Hawaii

14. Cinder Cones
Form when molten lava is thrown into the air from a vent. As it falls, it breaks into fragments called ash or tephra that harden before hitting the ground.
The ash and tephra make a cone-shaped mound.
They are smaller than other types.
Erupt explosively because magma is thick, which allows pressure to build up.
Magma composition: Felsic Rhyolite (light colored)

15. Krakatau, Indonesia

16. Cinder Cones
New Guinea
Iceland

17. Composite or Stratovolcano
Form from alternating eruptions of quiet lava and explosive ash. The layers build up and make a moderate-sized volcano.
Most common kind of volcano
Made of layers of ash/tephra and lava.
Magma composition- Andesite: medium color

18. Example of Composite/Stratovolocano
Mt. Pinatubo-Philippines

20. Fissure Volcanoes
Form in long cracks where plates are pulled apart and near other volcanoes where the crust is weakened. Cinder Cone or Shield Volcanoes may also be nearby.

21. Yellowstone
Check out how large some of the lava flows were from the Yellowstone Volcano!!!

22. How does a Caldera Form? A) A Volcano’s pressure starts to build up
B) The Volcano releases large amounts of Lava
C) As the Lava is released, the pressure decreases and the volcanic mountain begins to collapse, forming a concave shape in the center of the volcano.
D) The center of the volcanic mountain may begin to fill with water and form a lake.

23. Crater Lake in Oregon State should actually be called Caldera Lake

24. Label the parts of a volcano
Word Bank:
Main Vent/Crater
Secondary Vent
Magma Chamber
Lava
Ash & Dust Cloud
Tephra Bombs
Cone