1. Magma
A mixture of rock-forming substances, gases, and water from the mantle. When it reaches the surface it is called lava.

2. Viscosity - the resistance of a liquid to flowing.
The viscosity of magma depends upon its silica content and temperature.

3. The more silica in magma the higher the viscosity
The more silica in magma the higher the viscosity. The less silica magma contains, the lower the viscosity.

5. The higher the temperature of the magma the lower the viscosity.

6. Pahoehoe (pah HOH ee hoh ee )- fast moving, hot lava that has low viscosity.

7. Aa (AH ah) - cool, slow moving lava.

8. Inside a volcano

9. Some examples of intrusive and extrusive igneous rock bodies.

10. Low viscosity causes quiet eruptions
Low viscosity causes quiet eruptions. High viscosity causes explosive eruptions.

11. Volcanic Eruption
The force of the expanding gases pushes magma from the magma chamber through the pipe until it flows or explodes out of the vent.

12. Pyroclastic flow occurs when an explosive eruption hurls out a mixture of hot gases, ash, cinders, and bombs.

13. Life Cycle of a Volcano
Active or live - is erupting or shows signs that it may erupt in the near future.
Dormant - not active but may be in the future.
Extinct - unlikely to erupt again.

14. Shield Volcanoes - a wide, gentle sloping mountain made of layers of lava and formed by quiet eruptions.

15. Hot Spot Volcanoes are volcanoes that form over “hot spots” in the Earth’s mantle. The volcanoes that form the Hawaiian Islands are an example.

16. Mauna Loa

18. Cinder Cone Volcanoes - a steep, cone-shaped hill made of volcanic ash, cinders and bombs piled around a volcano’s opening.

20. Composite or Strato Volcanoes - tall, cone-shaped mountains in which layers of lava alternate with layers of ash and pyroclastic material.

22. Mt. St. Helens before and after.

26. Mt. St. Helens Eruption

28. Lava Plateaus - is made up of many layers of thin, runny lava that erupt from long cracks.

31. Thick sequence of thin basaltic lava flows have built up this flat plateau along the Columbia River between Oregon and Washington states.

32. Calderas - form when a volcano’s magma chamber empties and the roof of the chamber collapses.

34. Was there a volcano here. Yes, and there still is
Was there a volcano here? Yes, and there still is! For approximately 400,000 years, volcanic eruptions here built up a 10,000 – 12,000 foot mountain now called Mt. Mazama. Seven thousand seven hundred years ago, the volcano exploded in a cataclysmic eruption. During this eruption, so much material was evacuated from the internal magma chamber that afterwards, there was not enough left to support the remaining mountain and it collapsed and created the hole – the caldera – that we now see today half filled with water.

37. Volcanic belts form along the boundaries of Earth’s plates.
Ring of Fire- a major volcanic belt formed along the rim of the Pacific Plate.

38. Soils From Lava and Ash
Volcanoes can clearly cause much damage and destruction, but in the long term they also have benefited people. Over thousands to millions of years, the physical breakdown and chemical weathering of volcanic rocks have formed some of the most fertile soils on Earth. In tropical, rainy regions, such as the windward (northeastern) side of the Island of Hawaii, the formation of fertile soil and growth of lush vegetation following an eruption can be as fast as a few hundred years. Some of the earliest civilizations (for example, Greek, Etruscan, and Roman) settled on the rich, fertile volcanic soils in the Mediterranean-Aegean region. Some of the best rice-growing regions of Indonesia are in the shadow of active volcanoes. Similarly, many prime agricultural regions in the western United States have fertile soils wholly or largely of volcanic origin. -- Excerpt from: Kious and Tilling, 1996, This Dynamic Earth: The Story of Plate Tectonics: USGS General Interest Publication