1. Volcanoes
A volcano is a weak spot in the crust where the molten material, or magma, comes to the surface.

2. Volcanoes and Plate Tectonics
About 600 active volcanoes on land and even more under the sea.
An underwater volcano is called a seamount.
Volcanic belts form along the boundaries of Earth’s plates.
Volcanoes form along mid-ocean ridges as well as subduction plate boundaries.

3. Volcanoes and Plate Tectonics
Volcanoes form along the mid-ocean ridges, which are created by diverging boundaries.
Volcanoes can also form near colliding plates.
Volcanoes formed through this version of subduction can also create island arcs.

4. Volcanoes and Plate Tectonics
Some volcanoes result from “hot spots” in the Earth’s mantle.
A hot spot is an area where material from deep within the mantle rises and then melts, forming magma.
A volcano forms above a hot spot when magma erupts through the crust and reaches the surface.

5. Properties of Magma
Magma is melted earth under the ground. When it breaks the surface, it is called lava.
Magma has physical and chemical properties that help classify the volcano and its surroundings.
Magma!

6. Properties of Magma
A physical property of a substance is any characteristic that can be measured or observed without changing the composition of the substance.
Viscosity is a physical property of magma.
Viscosity is the resistance of a liquid to move or flow.
So – high viscosity means it moves slowly.
Low viscosity means it moves fast.

7. Properties of Magma Viscosity depends on Silica content
Silica is a compound (SiO2)
The amount of silica in magma determines the viscosity
High in silica….. High Viscosity
Low in silica….. Low Viscosity

8. Properties of Magma
Aa lava
Temperature also affects how fast or slow magma travels.
High Temperature…. Low Viscosity
Low Temperature… High Viscosity
Pahoehoe lava

9. Volcanic Eruptions
When a volcano erupts, the force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.

10. Volcanic Eruptions
Geologists classify eruptions as quiet or explosive.
Quiet eruptions will occur when the magma has a low viscosity… The lava just oozes slow and gentle from the volcano
Explosive eruptions will occur when the magma has a high amount of silica and therefore a high viscosity. The magma is sticky and is slow to move. This creates a tremendous build up of pressure causing an explosive eruption.

11. Volcanic Eruptions Pyroclastic flows occur during explosive eruptions.
Ash, bombs, and cinder hurl from the volcano.

12. Volcano Life Cycle
Active – a volcano that is currently erupting or is showing signs of possible activity so that it might erupt in the near future.
Dormant – a volcano that was once active but has not shown signs of activity for x amount of years. “sleeping”. It is possible for a dormant volcano to become active. Many volcanoes in Alaska are considered dormant, but show signs of possible activity.
Extinct – a volcano that no longer shows signs of activity and is unlikely to “wake up” or become active.

13. Volcanic Landforms
Volcanic eruptions create landforms made of lava, ash, and other materials. These landforms include shield volcanoes, cinder cone volcanoes, composite volcanoes, and lava plateaus.

14. Volcanic Landforms
Shield Volcanoes are created from thin layers of lava.
Such lava flows gradually build wide, gentle sloping mountains

15. Volcanic Landforms
Cinder Cone Volcanoes are created from explosive eruptions that contain ash, bombs, and cinder.
These materials build up around the vent in a steep, cone shaped hill or small mountain called a cinder cone.

16. Volcanic Landforms
Composite volcanoes are created from a layering effecting of quiet and explosive eruptions.
Tall, cone shaped mountains formed by alternating layers of lava with layers of ash, bombs, and cinder.

17. Volcanic Landforms
Lava Plateaus – When a volcano erupts, sometimes a mountains doesn’t form…. Rather a high, flat area of land is created.

18. Volcanic Landforms
Calderas – A huge hole left by the collapse of a volcanic mountain.
Crater Lake, Ore.

19. Volcanic Landforms
Volcanic Necks - This feature is created when magma hardens within the pipe of the volcano.
Ship Rock, NM

20. Volcanic Landforms
Dikes and Sills - When magma forces itself through layers of rock, it can form a sill. If the magma creates layers between hardened rock layers, then it’s called a dike.
Volcanic dike
Volcanic sill
In Antarctica

21. Volcanic Landforms
Batholiths - A mass of rock formed when a large body of magma cools inside the crust. See page 104 in textbook for examples of batholith locations.

22. Volcanic Landforms
Dome Mountains – These land features are exposed batholiths that have eroded or worn away as they were pushed through the crust. The Black Hills of South Dakota were formed this way.

23. Geothermal Activity Geysers, Hot Springs, Geothermal Activity
All this activity creates heated water underground. Once again…. Pressure builds and needs to be released through the cracks in the Earth’s surface… even for water and heated minerals and gases!
Some communities use these natural resources as a source of electricity!

24. Review…. Key Terms Key Terms Volcano Magma Lava Island arc Hot spot
Viscosity
Dormant
Key Terms
Types of eruptions
Geothermal Activity
Silica
Ring of Fire
Types of volcanoes
Types of landforms
Extinct

25. The End!!!
KLM ENTERPRISES 2010