Volcanoes

1. Volcanoes

2. Volcanoes • A volcano is a weak spot in the crust where molten rock comes to the surface.

3. Molten Rock • There are two types of molten rock: • Magma – molten rock found below the surface of the earth • Lava – molten rock that has reached the surface of the earth • Once lava or magma cools, it becomes solid rock.

4. Volcanoes • Volcanoes form at: • Divergent plate boundaries • Convergent plate boundaries • Hot spots

5. Divergent Plate Boundaries • Volcanoes form along mid-ocean ridges at divergent boundaries. • Lava pours out of the ridges, creating new oceanic crust. • Volcanoes can also form at rift valleys on land.

6. Convergent Plate Boundaries • Volcanoes will form at convergent plate boundaries where subduction is occurring. • This means at two oceanic plates or at an oceanic and a continental plate.

7. Convergent Plate Boundaries • As the old, dense oceanic plate is moving back into the mantle, it is going to melt some of the rock above it. • This happens because of friction.

8. Convergent Plate Boundaries • Because liquids are less dense than solids, the molten rock is less dense than the rock above it. • This is going to cause the molten rock to start to move toward the surface of the earth.

9. Convergent Plate Boundaries • When the molten rock reaches the crust it will push the rock up forming a mountain and eventually break through the crust, forming a volcano.

10. Hot Spots • A hot spot is an area where material deep within the mantle rises and melts, forming magma.

11. Hot Spots • Just like at convergent plate boundaries, this molten rock will move towards the surface until it breaks through the crust. • This will form a string of volcanic mountains as the plate above it continues to move.

12. Viscosity • Viscosity is the resistance of a liquid to flow. • Some liquids can flow more easily than others because they have different viscosities.

13. Viscosity • Liquids with high viscosity flow slowly. • Honey is a high viscosity liquid. • What else might have a high viscosity?

14. Viscosity • Liquids with a low viscosity flow quickly. • Water is a low viscosity liquid. • What other liquids might have a low viscosity?

15. Lava • Not all types of magma and lava are the same.

16. Lava • One way that lava is different is its viscosity. • The viscosity of lava depends on its silica content and its temperature.

17. Silica Content • A major ingredient in lava and magma is silica. • Silica is made of oxygen and silicon.

18. Silica Content • The silica content in molten rock ranges from 50% to 70%.

19. Silica Content • The more silica molten rock has, the higher its viscosity. • The less silica molten rock has, the lower its viscosity.

20. Silica Content • High silica lava is light in color, sticky, and does not flow very far. • Low silica lava is dark in color and can flow much further than high silica lava.

21. Temperature • Viscosity increases as temperature decreases. • The hotter it is, the lower the viscosity. • The colder it is, the higher the viscosity.

22. Pahoehoe • Pahoehoe is a fast moving, hot lava with low viscosity. • It is dark in color and has a small amount of silica.

23. Aa • Aa is cooler and slower-moving than pahoehoe. • It has a higher viscosity, is lighter in color, and has more silica.