1. Volcanoes and Other Igneous Activity

2. Volcanoes: Content Standards Addressed
6th 1
Plate tectonics accounts for important features of Earth's surface and major geologic events. As a basis for understanding this concept: 1a
Students know evidence of plate tectonics is derived from the fit of the continents; the location of earthquakes, volcanoes, and mid-ocean ridges; and the distribution of fossils, rock types, and ancient climatic zones 1d
Students know that earthquakes are sudden motions along breaks in the crust called faults and that volcanoes and fissures are locations where magma reaches the surface. 1e
Students know major geologic events, such as earthquakes, volcanic eruptions, and mountain building, result from plate motions 1f
Students know how to explain major features of California geology (including mountains, faults, volcanoes) in terms of plate tectonics.

3. Origin of magma
Magma originates when essentially solid rock, located in the crust and upper mantle, melts

4. How Magma Rises
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5. Where do volcanoes form?
Volcanoes form at:
Hot Spots (10% of all volcanic activity)

6. Where do volcanoes form?
Volcanoes form at:
Hot Spots (10%)
Spreading Centers (80% of all volcanic activity)

7. Where do volcanoes form?
Volcanoes form at:
Hot Spots (10%)
Spreading Centers (80%)
Convergent Plate Boundaries
(10% of all volcanic
activity)
Ocean–Continental
Ocean – Ocean

8. Plate tectonics and igneous activity
Intraplate igneous activity
Activity within a rigid plate
Plumes of hot mantle material rise
Form localized volcanic regions called hot spots
the Hawaiian Islands & the Columbia River Plateau

9. Plate tectonics and igneous activity
Divergent plate boundaries
Produces the greatest volume of volcanic rock
Lithosphere pulls apart
Less pressure on underlying rocks
Partial melting occurs
Large quantities of fluid basaltic magma are produced

10. Plate tectonics and igneous activity
Convergent plate boundaries
Descending plate partially melts & magma slowly rises upward
Rising magma can form
Volcanic island arcs in an ocean (Aleutian Islands)
Continental volcanic arcs (Andes Mountains)

11. Tectonic Settings and Volcanic Activity
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12. Plate Boundary Features
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13. Volcanic Eruptions Factors that determine the violence of an eruption
Temperature of the magma
Composition of the magma
Dissolved gases in the magma
The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption

14. Viscosity, Temperature and Water Content of Magmas
Rock Type
Basalt
Andesite
Rhyolite
SiO2 content
45-55%
55-65%
65-75%
Magma temperature
1,000 – 1,250ºC
800 – 1,000 ºC ºC
Viscosity
Low
High
Gas escape from magma
Easy
Difficult
Eruptive style
Peaceful
Explosive
increasing
increasing
increasing

15. Plate-Tectonic Setting of Volcanoes Revisited
Why more volcanic activity at spreading centers?
Low SiO2 content
High temperature
Low pressure as plates pull apart
Fluid basaltic lavas generally produce quiet eruptions
Why less volcanic activity at subduction zones?
High SiO2 content
Lower temperatures
Higher pressures
Highly viscous lavas produce more explosive eruptions

16. Volcanic Explosivity Index
VEI
Description
Plume Height
Volume
Classification
How often
Example
non-explosive
< 100 m
1000s m3
Hawaiian
daily
Kilauea 1
gentle m
10,000s m3
Haw/Strombolian
Stromboli 2
explosive
1-5 km
1,000,000s m3
Strom/Vulcanian
weekly
Galeras, 1992 3
severe
3-15 km
10,000,000s m3
Vulcanian
yearly
Ruiz, 1985 4
cataclysmic
10-25 km
100,000,000s m3
Vulc/Plinian
10's of years
Galunggung, 1982 5
paroxysmal
>25 km
1 km3
Plinian
100's of years
St. Helens, 1981 6
colossal
10s km3
Plin/Ultra-Plinian
Krakatau, 1883 7
super-colossal
100s km3
Ultra-Plinian
1000's of years
Tambora, 1815 8
mega-colossal
1,000s km3
10,000's of years
Yellowstone, 2 Ma
Source:

17. Refers to both the VENT and the CONE
Volcano: Any landform that releases lava, gas, or ashes or has done so in the past
Refers to both the VENT and the CONE
Many shapes and sizes due to magma chemistry and origin

18. Volcano Types
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19. Volcanoes can be classified into 6 major types
Based on their size, shape, and origin

20. Features Of The Two General Categories Of Volcanoes
Central Vent Volcanoes
Large-scale Volcanic Terrains
central vent
summit crater
flank eruptions
fissure eruptions
no central vent
network of source material
generally massive
Suggested tips to give students: Remember that “A volcano is any landform from which lava, gas, or ashes escape from underground or have
done so in the past” – not always the “classic” type shown on the first page of this chapter.

21. Comparison Of The Six Types Of Volcanoes

22. Shield volcano
Low silica, low gas magma originates in the mantle = fluid, basaltic lava
Broad, gentle slopes.
Phreatomagmatic eruptions occur when lava contacts water.

23. SHIELD VOLCANOES ARE A TYPE OF CENTRAL VENT VOLCANO.
Mauna Loa

24. Cinder Cones high lava fountains on the vents of shield volcanoes.
Suggested tips to give students: Cinder cones are often classified as a separate type of volcano.
Composed of pyroclastic debris.
Cinders formed by high gas content.

25. Sunset Crater, Flagstaff, Arizona

26. Mt. Fuji
Aniakchak Caldera
Stratovolcanoes And Rhyolite Caldera Complexes Are Central Vent Volcanoes.

27. Composite cone or stratovolcano
Alternating andesitic lava flows and layers of explosively ejected pyroclastics.
Intermediate magma = viscous lava that is difficult to erupt.
Explosive eruptions due to buildup of gases.
Often produce nuée ardente
May produce a lahar - volcanic mudflow

28. Mt. St. Helens

30. Mt. St. Helens New Dome Vent 1980-1986 Dome Rock Glacier

31. Mt. St. Helens, October 1, 2004

32. Eruptions in the Cascades Ranges

33. Calderas High-silica, high-gas magmas.
Massive explosions (most explosive of all types).
Collapse, producing an “inverse volcano”, or Caldera (Spanish for cauldron).

34. Calderas Famous (or infamous) collapsed calderas:
Long Valley, California (Mammoth)
Crater Lake (Mount Mazama), Oregon
Yellowstone, Wyoming
Krakatau, Indonesia, 1883
Santorini and the Lost Continent of Atlantis

35. How Calderas Form
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36. Large-scale volcanic terrains lack a central vent

37. San Francisco Volcanic Field
Monogenetic Fields
Poorly understood.
Multiple maar vents and cinder cones.
Erupt at different times.
Grow laterally.
Usually a single magma source
San Francisco Volcanic Field
Suggested tips to give students: A maar is a broad, low-relief volcanic crater caused by phreatomagmatic eruption. Monogenetic = “”single source (mono=single , genetic=think “Genesis”) or remember that Monogenetic fields have a “monopoly” on their source magma.

38. Monogenetic Fields Large Igneous Provinces
Fed by massive mantle plumes
Caused by flood basalts
Discharge over time through long fissures (cracks).
Create large plateaus.
Columbia River Basalts

39. The Columbia River basalts

40. Monogenetic Fields Mid-ocean Ridges develop at Spreading Centers
Basaltic flow creates global network of interconnected ridges
submarine volcanism
Suggested tips to give students: “pillow basalts” are interesting to talk about at this point because they do look like big, fluffy pillows.

41. Other volcanic landforms
Lava Domes
Bulbous mass of congealed lava
Most are associated with explosive eruptions of gas-rich magma
One is currently developing in Mt. St. Helens

42. 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

43. Formation of a volcanic neck

44. Volcanic Hazards Threaten Human Communities
Damage is caused by:
Volcanic Hazards Threaten Human Communities
Lava flows
Pyroclastic flows.
Lahars (mudflows).
Ash falls.
Volcanic bombs.
Massive lethal, violent explosions.
Earthquakes.
Landslides.
Open cracks and chasms.
Phreatomagmatic eruptions.
Climate change.
Suggested tips to give students: “lahar” is an Indonesian word for any type of flow – a “lahar” obliterated Pompeii and Herculaneum in 79 AD.
Maurice and Katia Krafft were French volcanologists who died on 3 June 1991 when they were hit by a pyroclastic flow at Unzen volcano in Japan. American volcanologist Harry Glicken died during the same event.

45. Lava flows
Molten rock that has flowed out onto the Earth’s surface

46. Pahoehoe lava (braids or ropes)

47. Aa lava (rough, jagged)

48. Pillow basalts

49. Gases One to 5 percent of magma by weight
Mainly water vapor and carbon dioxide

50. Pyroclastic materials
Ash and dust – fine, glassy fragments
Pumice – from "frothy" lava
Cinders – "pea-sized"
Lapilli – "walnut" size
Particles larger than lapilli
Blocks
Bombs
weblink

51. TYPES OF PYROCLASTIC DEBRIS
ASH
PUMICE
volcanic bombs
Suggested tips to give students: Pumice is useful for calluses on your feet, bombs are aerodynamically shaped, welded” tuff” looks soft like a sand hill but is “tough”.
LAPILLI
VOLCANIC BLOCK
WELDED TUFF

52. Nuée Ardentes
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 & 800º C
Examples:
Mt. Vesuvius
Mt. Shasta
Mt. St. Helens
Mayon Volcano, Philippines, in 1984
Source

53. Lahars (mudflows) How to make a lahar:
Mount Unzen, Japan. Note the broad paths of the pyroclastic flows across flat cultivated land. source
Lahars (mudflows)
How to make a lahar:
Add water + volcanic material = lahar
Water sources:
Melt glaciers
Displace lakes
Rain

54. Intrusive igneous structures

55. Intrusive Igneous Features
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56. ~ End ~