2. Introduction to volcanoes
Volcano:
an opening in the earth’s surface
through which lava, hot gases, and
rock fragments erupt

3. Origin of Volcanoes
1. Magma 50-100 miles below the
earth’s surface slowly begins to rise
to the surface
2. As the magma rises it melts gaps in
the surrounding rock
3. As more magma rises a large
reservoir forms as close as 2 miles
below the surface (magma chamber)

4. Origin of Volcanoes
4. Pressure from the surrounding
rock causes the magma to blast
or melt a conduit (channel) to the
surface where magma erupts onto
the surface through a vent
(opening)

5. Origin of Volcanoes
5. The magma, now called lava, builds
up at the vent forming a volcano

6. Origin of Volcanoes
6. Often the volcano sides will be
higher than the vent forming a
depression called a crater

7. Crater:

8. Caldera:
an unusually large crater or the
remains when the cone collapses
into its own magma chamber

9. Anatomy of a Volcano
Cone:
the above ground structure
built from lava and/or tephra

10. Conduit:
the path that magma takes from
the magma chamber to the vent

11. Magma Chamber:
the reservoir located under the
volcano where magma collects and
becomes the supply of magma/lava
to build the volcano

12. Parasitic Cone:
 a smaller secondary volcano built
on the side of or near the main
volcano, but sharing the same
conduit to the magma chamber

13. Fumarole:
a secondary vent that emits
only gases

14. Fissure:
a long fissure (crack) from
which lava flows

15. Vent: opening of the volcano, through which lava,
ash and gases flow

16. Take a minute to
label the parts on
the diagram (not
all parts are
shown)

17. Crater
Ash Cloud/Gases Vent
Parasitic Cone Lava Flow
conduit
mantle
Magma chamber

18. Lava— There are 3 kinds:

19. Pahoehoe lava:
 Hot, thin, fast
flowing
 harden with a
relatively smooth
surface
 Often has a ropy or
wrinkled appearance

20. Pahoehoe lava:

21. Aa lava:
• Cooler, thicker,
slow moving
• Hardens with a
rough, jagged,
sharp edge
surface

22. Pillow Lava:
 Lava suddenly cooled by
water
 shows sack-like
segments (stuffed
pillows)

23. Can you identify the kinds
of lava from the pictures?
Circle your choice.

28. Tephra (pyroclastic, rock fragments)
Volcanic Dust: Smallest particles
and carried by atmosphere circulation

29. Volcanic Ash:
 0.25-0.5 cm diameter
 Generally settles out within miles
of the cone but can be carried
greater distances by stronger
winds.
 Forms a mudflow when mixed with
water

30. Bomb:
 Smaller bombs (gravel, pea size) are called
cinders.
 Walnut size bombs are called lapilli.
 Larger fragments up to 4+ feet in diameter
are called bombs.

31. Lahar (mudflow):
 mixture of ash, eroded land, and
water flowing down river valleys

32. Lahar (mudflow):

33. Gases:
water vapor, carbon dioxide,
nitrogen, sulfur dioxide,
hydrogen sulfide, chlorine

34. Locations of Volcanoes
Divergent Boundaries:
 As the plate move long cracks
(rifts) form and lava builds up
forming
volcanoes.

35.  If the boundary is on the ocean
floor, volcanoes can grow tall
enough to break the surface of
the ocean and become islands
(Iceland)

36. Convergent Boundaries:
 Places where plates are moving
toward each other forming a
subduction zone.
 One plate melts under the other
and the magma moves upward to
form volcanoes.

37.  Example: Pacific Ring of Fire

38.  Example: Cascade Volcanoes

39. Hot Spots
 Magma that may originate in the
mantle or outer core will move
upward, breaking the surface and
forming a volcano, they are
independent of plate boundaries and
a chain of volcanoes may form as the
plate moves across a hot spot.

40. Hot Spots
 (Examples: Hawaiian Islands and
Yellowstone National Park)

41. Types of Volcanic Eruptions
Two factors determine the type of
eruption:
 Amount of water vapor &
other gases in the magma
 The chemical composition of
the magma

42. Explosive Eruptions
 Trapped gases under high pressure will
violently explode when the magma reaches
the lower pressure of the surface.
 Has granitic magma is very thick and plugs
the vent causing the pressure to build until
it blows violently out the vent
 The high water content of the magma
produces more water vapor which when
mixed in granitic magma produces
explosive eruptions

43. Explosive Eruptions
Mt. Pinatubo
Mont serrat
Mt. St. Helens

44. Quiet Eruptions
 Low pressure gas
 Has basaltic magma (is more
fluid and will flow instead of
explode)
 And has low water content

45. Types of Volcano Mountains
Cinder Cones:
 Small base, steep-sided, loosely
consolidated
 Up to 1000 feet tall
 Life span of a few years
 Commonly built from gravel size lava
rock fragments call cinders
 Has violent eruptions, dangerous when
close.

47. Types of Volcano Mountains
Cinder Cones:

48.  High pressure gas bubbles cause thick lava
to explode into the air; lava begins to cool as
it rises and falls, becoming very sticky
 When lava hits the ground it sticks rather
than flows
 This builds a steep cone with a small base

49. Sketch a Cinder Cone Volcano:

50. Types of Volcano Mountains
Shield Volcanoes:
 Large base, gentle slope, lava rock
layers
 A few miles wide
 Life span of a million years or more
 The lava is hot, thin, very fluid,
often basaltic.
 Example: Hawaiian Islands

51. Take a look at these examples:
http://www.volcano.si.edu/world/tpgallery.cfm?category=Shield%20Volcanoes
The Mauna Loa volcano in Hawaii—
the largest volcano on Earth—has
the broad expanse characteristic of
shield volcanoes. It spreads across
half the island of Hawaii.
Shield volcano on Mars;
Taken from space

52. Shield Volcanoes
Mauna Kea

53. Sketch a sheild Volcano:

54. Types of Volcano Mountains
Composite (strato) Volcanoes:
 Large mountain volcano often snow capped, a
few miles high
 Life span of million years or more
 Have alternating eruptions of tephra (air-
borne) and lava. The tephra adds height to
the volcano and the lava cements the tephra
together and adds to the base.
 Found mostly in subduction zones and have
violent eruptions.
Examples: Mt Rainier, Mt Fuji, Mt Kilimanjaro

55. Composite (strato) Volcanoes:
(Make a Flip-Book)
Mt. Fuji
Mt. Rainier
Mt. Kilimanjaro

56. Sketch a composite Volcano:

57. Volcano Activity Levels (Stages)
Active (awake):
 Has erupted within recent time and
can erupt again at any time.
Pre-eruption activities:
 Increase in earthquake activity
under the cone
 increase in temperature of cone,
 melting of ice/snow in the crater
 swelling of the cone
 steam eruptions
 minor ash eruptions

58. Mt St. Helens

59. Dormant (sleeping):
 No eruption within recent times,
but there is record of past
eruptions
 Can become active and erupt again
after a “wake up” period
 Example: Mt. Rainier

60. Extinct:
 No eruption within recorded
history
 Not expected to ever erupt again
Example: Mount Mazama (Crater Lake)

61. Crater Lake

62. Mount Rainier
• The most dangerous volcano in the US
• The danger is mostly from lahars traveling down
river valleys at a speed of 25mph and destroying
everything in its path
• 100,000 people live on the solidified mudflows of
previous eruptions

63. Mount Rainier
• The mountain is dangerously unstable, a tall,
steep heap of loose rock held together by the
force of gravity and a cubic mile of glacier ice
that could be melted or shaken loose
• Lahar flows average every 500 years and have
gone as far as the Puget Sound lowlands (1 in 7
chance of it happening during your lifetime)
• Mount Rainier has erupted 4 times in the last
4000 years with the last eruption 200 years ago

64. VolcanoVideos
• http://video.nationalgeographic.com/video/player/
environment/environment-natural-
disasters/volcanoes/volcano-eruptions.html
• http://video.nationalgeographic.com/video/player/
environment/environment-natural-
disasters/volcanoes/volcano-lava.html
• http://video.nationalgeographic.com/video/player/
environment/environment-natural-
disasters/volcanoes/volcano-eruptions.html