1. Volcanism
Definition: Any crack in the Earth’s surface where molten rock has moved towards or onto the Earth’s surface.

2. Divergent Boundary Volcanoes
Black Smoker
Occurs underwater at Mid-Ocean Ridges at divergent tectonic plate boundaries.
Can occur on land as a fissure volcano in places such as Iceland.
Mid Ocean Ridge – Rift Volcano

3. Iceland Volcanic Eruption – 1970s Divergent Boundary Fissure Volcano

4. Subduction Zone Volcanoes
Oceanic Crust will melt as it slides underneath another piece of crust.
Forms volcanic arcs (Island chains or continental arcs)

5. Subduction Zone Volcanoes
The most explosive and violent volcanoes occur along subduction zones.
Magma contains a lot of water from oceans pulled into the mantle by a subducting plate.
Mt. St Helens Eruption – 1980s – Washington State:

6. Ring of Fire – Zone of Active volcanism around the Pacific Ocean
Ring of Fire – Zone of Active volcanism around the Pacific Ocean. Several subduction zones can be found here.
Ring of Fire Video:

7. Hot Spots Area where magma continuously rises from the Earth’s core.
Not related to plate tectonics.
Hawaii and Yellowstone Park are examples of Hot Spots in the United States.

8.  Kiluea Volcano, Hawaii
Yellowstone Hot Springs, Wyoming 

9. Types of Volcanoes Type of volcano depends upon two factors:
Viscosity of Magma – How “sticky” magma is. High viscosity magma is very sticky and slow to flow.
Amount of dissolved gas – Lots of gases can increase the strength of a volcanic eruption.

10. Types of Volcanoes: Cinder Cone
Small volcanoes that often form on the side of larger volcanoes.
Eject lots of gas and rock fragments.
Lower viscosity magma
Example: Stromboli in Italy and Pocpocopetal in Mexico.

11. Types of Volcanoes: Shield Volcanoes
Large and relatively flat volcanoes that have gentle eruptions.
Looks like a shield placed on the ground.
Lava has very low viscosity.
Example: Mauna Loa, Hawaii.

12. Hawaiian Shield Volcanoes
Mauna Kea
Mauna Loa

13. A closer look at Mauna Loa

14. Stratovolcano (Composite)
Tall volcano with steep sides
High Viscosity magma
Infrequent eruptions.
Has pyroclastic flows
Example: Mt. St. Helens in Washington State

15. Pyroclastic Flow
Fast moving flows of rock, ash, and debris during eruptions of composite volcanoes.
The flows can move at speeds greater than 50 miles/hour.
The temperatures of the gases in these flows can be up to 1,000 degrees Fahrenheit.
Pyroclastic Flow

16. Mt. St. Helens Erruption
Mt. St. Helens in a composite volcano located in the Cascade Range on the west coast of the United States.
This continental volcanic arc forms as the “Juan de Fuca” plate is subducted underneath the North American Plate.
Mt. St. Helens Erruption

17. Pillow Lava
Contact with water causes and outer shell of rock to cool and solidify quickly.
The rock inside is still molten and pushes its way through the outer shell.
Pillow Lava

18. Hot Spot Volcanism
“Hot Spots” are where volcanism occurs but is NOT related to Plate Tectonics.
It is theorized that giant plumes of magma rise up from the Earth’s core continually for unknown reasons.
Volcanoes on a hot spot become extinct once the tectonic plate they lie on moves.