1. Edward J. Tarbuck & Frederick K. Lutgens
Earth Science, 10e
Edward J. Tarbuck & Frederick K. Lutgens

2. Volcanoes and Other Igneous Activity Chapter 8
Earth Science, 10e
Stan Hatfield and Ken Pinzke
Southwestern Illinois College

3. Volcanic eruptions Factors that determine the violence of an eruption
Composition of the magma (% silica content)
Temperature of the magma (Very hot and cool)
Dissolved gases in the magma (Water vapor etc…)
Viscosity of magma (function of silica content)
Viscosity is a measure of a material's resistance to flow

4. Let’s Review
What types of Magma are there?...
How does temperature relate to a type of magma?
How Does viscosity relate to a type of Magma?

5. Volcanic eruptions Viscosity of magma Factors affecting viscosity
Temperature (hotter magmas are less viscous)
Composition (silica content)
High silica – high viscosity (e.g., rhyolitic lava)
Low silica – more fluid (e.g., basaltic lava)
Dissolved gases (volatiles)
Mainly water vapor and carbon dioxide
Gases expand near the surface
Hotter magmas have less gases
Cooler magmas trap more gases

6. Volcanic eruptions Viscosity of magma (continued…)
Factors affecting viscosity
Dissolved gases (volatiles) thicken the magma
Provide the force to extrude lava
Violence of an eruption is related to how easily gases escape from magma
Easy escape from fluid magma
Viscous magma produces a more violent eruption

7. Fluid Lava: The Columbia River basalts

8. Let’s summarize
Viscous magma: Felsic/Andesitic or Mafic?
Cooler magma Felsic/Andesitic or Mafic?
Violent eruptions: Felsic/Andesitic or Mafic?

9. Materials associated with volcanic eruptions
Types of Basaltic Lava flows
Basaltic lavas are more fluid
Types of basaltic lava
Pahoehoe lava (resembles braids in ropes)
Aa lava (rough, jagged blocks)
Gases
Represent one to 5 percent of magma by weight
Mainly water vapor and carbon dioxide

10. A Pahoehoe lava flow

11. A typical aa flow

12. What is a Hot Spot?
An intermittent stationary source of magma that originates deep inside the core or the mantle
Plates riding over a hot spot form a chain of volcanoes. i.e. Hawaiian chain
The chain changes direction as the plate changes its direction over the hot spot
Only the volcano over the hot spot is active

13. A size comparison of the three types of volcanoes

14. Types of Volcanoes
There are three types of volcanoes defined according to their shape and size:
Shield volcano
Broad, slightly domed
Primarily made of basaltic (fluid) lava
Generally large size
e.g., Mauna Loa in Hawaii
Mostly found on the Ocean floor and/or over
hot spots

15. A shield volcano

16. Types of Volcanoes Types of volcanoes (continued…) Cinder cone
Built from ejected lava fragments
Steep slope angle
Rather small size
Frequently occur in groups

17. Sunset Crater – a cinder cone near Flagstaff, Arizona

18. Types of Volcanoes Types of volcanoes (continued…)
Composite cone (or stratovolcano)
Are made of Interbedded lavas and pyroclastics
Most are adjacent to the Pacific Ocean also called Ring of Fire (e.g., Mt. St. Helens, Mt. Pinatubo,…)
They are of large size
They produce the most violent eruptions (i.e. Mt. St Helens May 18, 1980 eruption)

19. Volcanoes Types of volcanoes (continued…)
Composite cone (or stratovolcano)
Often produce nuée ardente (Pyroclastic flow)
Fiery pyroclastic flow made of hot gases infused with ash
Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour
May produce a Lahar – deadly volcanic mudflow

20. A composite volcano (stratovolcano)

21. Mt. St. Helens – a typical composite volcano

22. Pyroclastic flow on Mt. St. Helens

23. What’s on a Volcano’s arsenal?
Pyroclastic flow (very hot ash)
Lahar (deadly mud flow)
Crater Lake’s deadly gases

24. A lahar along the Toutle River near Mt. St. Helens

25. Other volcanic landforms
Calderas
Steep walled depression at the summit
Formed by collapse of the crater
Nearly circular
Size exceeds one kilometer in diameter
When filled with water calderas form
crater lakes

26. Crater Lake, Oregon is a good example of a caldera

27. Crater Lake in Oregon

28. Materials associated with volcanic eruptions
Pyroclastic materials
"Fire fragments"
Types of pyroclastic material
Ash and dust – fine, glassy fragments
Pumice – from "frothy" lava
Lapilli – "walnut" size
Cinders – "pea-sized"
Particles larger than lapilli
Blocks – hardened lava
Bombs – ejected as hot lava

29. A volcanic bomb
Bomb is approximately 10 cm long

30. Volcanoes General features
Conduit, or pipe caries gas-rich magma to the surface
Vent, the surface opening (connected to the magma chamber via a pipe)
Crater
Steep-walled depression at the summit
Caldera (a summit depression greater than 1 km diameter)

31. Mt. St. Helens following the 1980 eruption

32. Other volcanic landforms
Volcanic pipes and necks
Pipes are short conduits that connect a magma chamber to the surface
Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone

33. Origin of magma
Magma originates when essentially solid rock, located in the crust and upper mantle, melts
Factors that influence the generation of magma from solid rock
Role of heat
Earth’s natural temperature increases with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle

34. Origin of magma
Factors that influence the generation of magma from solid rock
Role of heat
Additional heat is generated by
Friction in subduction zones
Crustal rocks heated during subduction
Rising, hot mantle rocks

35. Origin of magma
Factors that influence the generation of magma from solid rock
Role of pressure
Increase in confining pressure causes an increase in melting temperature
Drop in confining pressure can cause decompression melting
Lowers the melting temperature
Occurs when rock ascends

36. Origin of magma
Factors that influence the generation of magma from solid rock
Role of volatiles
Primarily water
Cause rock to melt at a lower temperature
Play an important role in subducting ocean plates

37. Plate tectonics and igneous activity
Global distribution of igneous activity is not random
Most volcanoes are located on the margins of the ocean basins (intermediate, andesitic composition) called subduction zone
Second group is confined to the deep ocean basins (basaltic lavas) called Mid Ocean ridges
Third group includes those found in the interiors of continents at Continental rift valley

38. Locations of some of Earth’s major volcanoes

39. Plate tectonics and igneous activity
Plate motions provide the mechanism by which mantle rocks melt to form magma
Convergent plate boundaries
Deep-ocean trenches are generated
Descending plate partially melts
Magma slowly rises upward
Rising felsic to andesitic magma can form
Volcanic island arcs in an ocean (Aleutian Islands)
Continental volcanic arcs (Andes Mountains)

40. Plate tectonics and igneous activity
Plate motions provide the mechanism by which mantle rocks melt to form magma
Divergent plate boundaries
The greatest volume of volcanic rock is produced along the oceanic ridge system
Lithosphere pulls apart
Less pressure on underlying rocks
Partial melting occurs
Large quantities of fluid basaltic magma are produced

41. Plate tectonics and igneous activity
Plate motions provide the mechanism by which mantle rocks melt to form magma
Intraplate igneous activity
Activity within a rigid plate
Plumes of hot basaltic magma rise
Form localized volcanic regions called hot spots
Examples include the Hawaiian Islands and the Columbia River Plateau in the northwestern United States

42. Tectonic Settings and volcanic activity
Divergent margin: Has volcanic activity
Convergent margin: Has volcanic activity
Transform fault: Is a margin but has no volcanic activity
Hot spot: Not a margin but has volcanic activity

43. Formation of a volcanic neck

44. Intrusive igneous activity
Two types of igneous intrusive features
Sills, Laccoliths, and dikes
Lens shaped masses
Batholiths and Stocks
Largest intrusive bodies
Often occur in groups Surface exposure 100+ square kilometers (smaller bodies are termed stocks)
Frequently form the cores of mountains

45. Intrusive igneous structures exposed by erosion

46. A batholith exposed by erosion

47. Intrusive igneous activity
Most magma is emplaced at depth
An underground igneous body is called a pluton
Plutons are classified according to
Shape
Tabular (sheetlike)
Massive

48. Intrusive igneous activity
Plutons are classified according to
Orientation with respect to the host (surrounding) rock
Discordant – cuts across existing structures
Concordant – parallel to features such as sedimentary strata

49. Intrusive igneous activity
Types of igneous intrusive features
Dike, a tabular, discordant pluton
Sill, a tabular, concordant pluton
e.g., Palisades Sill, NY
Resemble buried lava flows
May exhibit columnar joints
Laccolith
Similar to a sill

50. A sill in the Salt River Canyon, Arizona

51. End of Chapter 8

52. 1-Which Magma is more viscous?
Basaltic
Felsic/Andesitic
Ultramafic
None of these

53. 2-which magma traps more gases?
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure

54. 3-Which Magma is Hotter? Mafic/Ultramafic Felsic /Andesitic
None of these
I am not sure

55. 4-Which magma produces more violent eruptions?
Mafic/Ultramafic
Felsic/Andesitic
None of these
I am not sure

56. 5-Magma viscosity is a function of…
Temperature
Silica content
Gases content
All of these
b only

57. 6-Which volcano is over a hot spot?
Mt St Helens (Washington State)
Mt Rainier (Washington State)
Mauna Loa (Hawaii)
All of these
a and c only

58. 7-Which one is a Shield Volcano?
Mt St Helens (Washington State)
Mt Rainier (Washington State)
Mauna Loa (Hawaii)
All of these
a and c only

59. 8-Which one is a Composite cone?
Mt St Helens (Washington State)
Sunset Crater (Arizona)
Mauna Loa (Hawaii)
All of these
a and c only

60. 9-Which volcano type produces lava + pyroclastic flows
Mt St Helens (Washington State)
Kula (Hawaii)
Mauna Loa (Hawaii)
All of these
a and c only

61. 10-Volcanoes with steep cones have
Fluid lava
Viscous lava
No relation whatsoever
I am not sure

62. 11-Volcanoes at divergent boundary erupt mainly…lava
Felsic to Andesitic
Mafic
None of these
I am not sure

63. 12-Volcanoes at convergent margins produce mainly…lava
Felsic to Andesitic
Mafic
None of these
No relation whatsoever

64. 13-Which volcanoes are more explosive?
Shield volcanoes
Composite cones
Cinder cones
None of these
I am not sure

65. 14-Volcano explosiveness is influenced by …
Viscosity
Temperature
Amount of gases
All of the above
A and c only

66. 15-A Pahoehoe lava is Gases-free with jagged surface lava
Wrinkled, ropy, and gaseous lava
Explosive ash and fragments flow
None of these
I am not sure

67. 16 - Quartzite is what type of metamorphic rock?
Regional metamorphism
Contact metamorphism
Not a metamorphic rock

68. 17 – In what type of metamorphism do pressure and temperature work together?
Contact metamorphism
Regional metamorphism
All of the above
None of the above

69. 18 - What is the parent rock of limestone?
Quartzite
Granite
Marble
Basalt
This does not make sense

70. 19 – Which one of these is a foliated metamorphic rock?
Marble
Granite
Mica-schist
Basalt
Shale

71. 20 - Which one is the metamorphic rock?
Do not take
SANDSTONE for
GRANITE (granted)!
Have a
c. GNEISS (nice) day!

72. THE END
THAT’S ALL FOLKS!
THANK YOU!!!