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

3.  Magma 50-100 miles below the
earth’s surface slowly begins to rise
to the surface
 As the magma rises it melts gaps in
the surrounding rock
 As more magma rises a large
reservoir forms as close as 2 miles
below the surface (magma chamber)
How do they form?

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)
How do they form?

5.  The magma, now called lava, builds
up at the vent forming a volcano
How do they form?

6.  Often the volcano sides will be
higher than the vent forming a
depression called a crater
How do they Form?

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. Lava:
molten, liquidliquid rock on the surface
of the earth

13. 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

14. Fumarole:
a secondary vent that emits
only gases

15. Fissure:
a long fissure (crackcrack) from
which lava flows

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

17. What comes out of volcanoes?What comes out of volcanoes?
•Lava
•Tephra
•Gases

18. Lava—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. Tephra
 Basically, rockrock fragments
• Also known as pyroclasticpyroclastic rock
fragments.
• There are many different possible
sizes, from very small (volcanic ashash
or dustdust to much larger rocks (called
volcanic bombsbombs)

24. Ash & dust

25. Volcanic Bombs
This dacite breadcrust
bomb (about 15 cm in
diameter) was erupted
from the lava dome at
Mount St. Helens,
Washington.

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

27. Lahar (mudflow):

28. GASES
water vapor, carbon dioxide,
nitrogen, sulfur dioxide,
hydrogen sulfide, chlorine

29. Where do Volcanoes Occur?

30. Where do Volcanoes Occur?
location Examples
Divergent
Boundaries:
where plates move
APARTAPART
IcelandIceland

31. Where do Volcanoes Occur?
location Examples
convergent
Boundaries:
where plates comecome
togethertogether
Cascade rangeCascade range
volcanoes, Andesvolcanoes, Andes

32.  Cascade Volcanoes

33. Where do Volcanoes Occur?
location Examples
Hot Spots:
plates riding over anplates riding over an
especially hot placeespecially hot place
in the mantlein the mantle
Hawaii, Yellowstone,Hawaii, Yellowstone,
IcelandIceland

34. Hot Spots

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

36.  Trapped gases
under high
pressure will
violently
explode when
the magma
reaches the
lower pressure
of the surface.
Explosive EruptionsExplosive Eruptions

37.  Has granitic
magma is very
thick and plugs
the vent
causing the
pressure to
build until it
blows violently
out the vent
Explosive EruptionsExplosive Eruptions
Mt. St. Helens

38.  The high water
content of the
magma produces
more water
vapor which
when mixed in
granitic magma
produces
explosive
eruptions
Explosive EruptionsExplosive Eruptions
Mont serrat

39.  Low
pressure
gas
Quiet EruptionsQuiet Eruptions

40.  Has
basaltic
magma (is
more fluid
and will flow
instead of
explode)
Quiet EruptionsQuiet Eruptions

41.  …and has low
water content
 Examples: Hawaii
Quiet EruptionsQuiet Eruptions

42. Cinder conesCinder cones
oSmall base, steep-sided, loosely consolidated
oUp to 1000 feet tall
oLife span of a few years
oCommonly built from gravel size lava rock fragments
called cinders
oViolent eruptions, dangerous when close---High pressure
gas bubbles causes thick lava to explode into the air, lava
begins to cool as it rises and falls becoming very sticky
oWhen lava hits the ground it sticks rather than flows
oThis builds a steep cone with a small base

44. Cinder Cones:
Cinder Cones

45. Shield volcanoesShield volcanoes
oLarge base, gentle slope, lava rock layers
oA few miles high
oLife span of a million years or more
oThe lava is hot, thin, very fluid often basaltic
oExample: Hawaiian Islands, Iceland

46. Shield volcano on Mars;
Taken from space
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.

47. Shield Volcanoes
Mauna Kea
Shield volcanoes

48. Composite or stratoComposite or strato
Large mountain volcano often snow capped, a
few miles high
Life span of million years or more

49. Composite or stratoComposite or strato
oHave 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.
oFound mostly in subduction zones and have
violent eruptions.

50. STrato/CompositeSTrato/Composite
aima Volcano, Chile.

51. Composite (strato)Composite (strato)
Volcanoes examples:Volcanoes examples:
Mt. Rainier
Mt. Fuji
Mt. Kilimanjaro

52. Composite/strato volcanoes

53. 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
Volcano Activity Levels (Stages)

54. Mt St. Helens

55. 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
 ExampleExample: Mt. Rainier

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

57. Crater Lake

58. Mount RainierMount Rainier
• The most dangerousdangerous volcano in the US
• The danger is mostly from laharslahars 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

59. Mount Rainier
• The mountain is dangerously unstableunstable, 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 years500 years and have
gone as far as the Puget Sound lowlands
• Mount Rainier has erupted 4 times4 times in the last
4000 years with the last eruption 200 years ago