1. 3-2 Notes
Volcanoes

2. eruptions
Volcano- an opening in Earth’s crust through which molten rock, rock fragments, and hot gases erupt.

3. eruptions
A mountain built up from erupted material is also called a volcano.

4. eruptions
The violence of the eruption depends on the type of magma that erupts.

5. magma
Magma contains gases that expand as the magma is heated and rises.

6. magma
All magma contains silica.

7. magma
Magma that is high in silica doesn’t flow easily, so gases that are trying to expand get trapped.

8. magma
High silica magma causes violent explosions.

9. magma

10. magma
Magma that is low in silica flows easily, so gases expand and escape easily.

11. magma
Low silica magma causes oozing, flowing eruptions.

12. magma
Underground, magma rises toward the surface when it is less dense than the rock around it.

13. magma
It collects in a magma chamber.

14. magma
It either cools into rock inside the chamber, or erupts when the chamber overflows.

15. Rocks Volcanoes release large amounts of rock fragments.
These rock fragments form as:

16. Rocks
Molten rock is torn apart as gas bubbles pop.

17. Rocks
Lava cools and hardens into rock as it flies through the air.

18. Rocks
Rocks rip loose from the volcano’s walls during the eruption.

19. Rocks
Tiny rock fragments: volcanic ash

20. Rocks
Medium sized rock fragments: volcanic cinders

21. Rocks
Large rock fragments: volcanic blocks or volcanic bombs

22. gases
“Smoke” rising from a volcano is a mix of ash and gases.

23. gases
The gases are mostly water and carbon dioxide.

24. gases
Pyroclastic flow- a dense, superhot cloud of rock fragments that races downhill.

25. gases
Pyroclastic flows can be as hot as 800 degrees Celsius (1500 degrees F!) and move as fast as 160 km/her (100 mph!)

26. gases
In AD 79, Mt. Vesuvius erupted and buried the town of Pompeii in a deadly pyroclastic flow, killing almost 16,000 people immediately.

27. volcanoes form along plate boundaries
Volcanoes are common along tectonic plate boundaries at subduction zones and divergent boundaries.

28. volcanoes form along plate boundaries
Volcanoes can also form over hot spots.

29. volcanoes form along plate boundaries
There are ~1500 potentially active volcanoes in the world, 500 of which we know have erupted.

30. volcanoes form along plate boundaries
400 of the 500 volcanoes known to have erupted occur in the Ring of Fire.

31. shapes and sizes
Most volcanoes erupt from openings in bowl- shaped pits called craters.

32. shapes and sizes
The type of magma a volcano erupts determines its shape.

33. Shield volcano
Shaped like a broad, flat dome.

34. Shield volcano
Lava is low in silica so it flows easily and spreads out in thin layers.

35. Shield volcano
Example: Mauna Loa in Hawaii, U.S.

36. Cinder cone Steep, cone-shaped hill
Formed by the eruption of cinders and rock fragments.

37. Cinder cone Magma contains large amounts of gas.
Example: Sunset Crater Volcano, Arizona, U.S.

38. Composite volcano
Cone-shaped; built up by alternating layers of lava and rock fragments.

39. Composite volcano Magma is high in silica
Tends to be steep near the top and flattens out toward the bottom.

40. Composite volcano
Example: Mt. Fuji, Japan

41. Composite volcano Composite volcanoes have violent eruptions because:
Gases trapped in the magma cause high pressure explosions.
Cooled, hardened lava plugs up previous openings, causing pressure to build again.

42. shapes and sizes
Both shield and composite volcanoes can form calderas.

43. shapes and sizes
Caldera- a huge crater formed by the collapse of a volcano when magma rapidly erupts from underneath it.

44. Scientists Monitor volcanoes
In 1991, Mt. Pinatubo in the Philippines erupted for the first time in 500 years.

45. Scientists monitor volcanoes
Scientists knew it was becoming active months before and the area was evacuated, saving thousands of lives.

46. Scientists monitor volcanoes
Scientists monitor volcanoes for signs of eruption, such as:
Small earthquakes in the area

47. Scientists monitor volcanoes
Explosions of steam.

48. Scientists monitor volcanoes
Changes in the tilt of the ground.

49. Scientists monitor volcanoes
Scientists also:
monitor the temperature of water near volcanoes.

50. Scientists monitor volcanoes
Study the ages and types of rocks around a volcano to understand its history.

51. Scientists monitor volcanoes
Keep a close watch on volcanoes near cities.
Example: Mt. Rainier (Seattle)

52. review 1. What makes a pyroclastic flow so dangerous?
Its speed and temperature
Its speed and its cinder content
Its temperature and its silica content
Its temperature and its volume

53. review 2. Volcanoes form at
converging boundaries and diverging boundaries only.
hot spots and faults only
diverging boundaries and hot spots only
converging boundaries, diverging boundaries, and hot spots

54. review
3. Which of the following signs might indicate that a volcano might soon erupt?
increased wearing away of volcanic rocks.
falling temperatures in volcanic lakes
rising temperatures in volcanic lakes
decreased wearing away of volcanic rocks