1. Plate Tectonics and Magmas
Chapter 6
Volcanic Eruptions:
Plate Tectonics and Magmas
Lecture PowerPoint
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2. Vesuvius, 79 C.E.
Cities of Pompeii and Herculaneum buried by massive eruption which blew out about half of Mt. Vesuvius
Similar to 1991 eruption of Mt. Pinatubo in Philippines
Clouds of hot gas (850oC), ash and pumice enveloped city
Many tried to escape near sea, but were buried by pyroclastic flows
Figure 6.1
Figure 6.3

3. Vesuvius, 79 C.E.
Vesuvius was inactive for 700 years before 79 CE eruption
People lost fear and moved in closer to volcano
After 79 CE, eruptions in 203, 472, 512, 685, 993, 1036, 1049,
500 years of quiet, then 1631 eruption killed 4,000 people
18 cycles of activity between 1631 and 1944, nothing since then
3 million people live within danger of Vesuvius today; 1 million people on slopes of volcano

4. The Hazards of Studying Volcanoes
Eruptive phases are often separated by centuries of inactivity, luring people to live in vicinity (rich volcanic soil)
400,000 people live on flanks of Galeras Volcano in Colombia
Many people killed each year by volcanoes, sometimes including volcanologists
Volcanoes may be active over millions of years, with centuries of inactivity

5. How We Understand Volcanic Eruptions
Understand volcanoes in context of plate tectonics
Variations in magma’s chemical composition, ability to flow, gas content and volume determines whether eruptions are peaceful or explosive

6. Plate-Tectonic Setting of Volcanoes
90% of volcanism is associated with plate boundaries
80% at spreading centers
About 10% at subduction zones
Remaining 10% of volcanism occurs above hot spots
Figure 6.6

7. Plate-Tectonic Setting of Volcanoes
Subduction carries oceanic plate (with water-rich sediments) into hotter mantle, where water lowers melting temperature of rock
Rising magma melts continental crust it passes through, changing composition of magma
Figure 6.7

8. Plate-Tectonic Setting of Volcanoes
No volcanism associated with transform faults or continent-continent collisions
Oceanic volcanoes are peaceful
Subduction-zone volcanoes are explosive and dangerous
Subduction zones last tens of millions of years
Volcanoes may be active any time, with centuries of quiet
Figure 6.7

9. Chemical Composition of Magmas
Of 92 naturally occurring elements:
Eight make up more than 98% of Earth’s crust
Twelve make up 99.23% of Earth’s crust
Oxygen and silicon are by far most abundant
Typically join up as SiO4 tetrahedron, that ties up with positively charge atoms to form minerals
Figure 6.8

10. Chemical Composition of Magmas
Mineral formation in magma: crystallization
Order of crystallization of different minerals in magma can be determined:
Iron and magnesium link with aluminum and SiO4 to form olivine, pyroxene, amphibole and biotite families
Calcium combines with aluminum and SiO4 until calcium replaced by sodium, to form plagioclase feldspar family; calcium and sodium are later replaced by potassium, to form potassium feldspar and muscovite families; finally only Si and O remain, forming quartz

11. Chemical Composition of Magmas
Figure 6.9

12. Chemical Composition of Magmas
Elements combine to form minerals
Minerals combine to form rocks
Different compositions of magma result in different igneous rocks
If magma cools slowly and solidifies beneath surface  plutonic rocks
If magma erupts and cools quickly at surface  volcanic rocks

13. Viscosity, Temperature, and Water Content of Magmas
Viscosity: internal resistance to flow
Lower viscosity  more fluid behavior
Water, melted ice-cream
Higher viscosity  thicker
Honey, toothpaste
Viscosity determined by:
Higher temperature  lower viscosity
More silicon and oxygen tetrahedra  higher viscosity
More mineral crystals  higher viscosity
Magma contains dissolved gases: volatiles
Solubility increases as pressure increases and temperature decreases

14. Viscosity, Temperature, and Water Content of Magmas
Consider three types of magma: basaltic, andesitic and rhyolitic
Basaltic magma has highest temperatures and lowest SiO2 content, so lowest viscosity (fluid flow)
Rhyolitic has lowest temperatures and highest SiO2 content, so highest viscosity (does not flow)
Basaltic makes up 80% of magma that reaches Earth’s surface, at spreading centers, because it forms from melting of mantle
Melted mantle at subduction zones rises through continental crust before reaching the surface, incorporating continental high SiO2 rock as it rises, to become andesitic or rhyolitic in composition before it erupts

15. Viscosity, Temperature, and Water Content of Magmas
Insert Table 6.5

16. Viscosity, Temperature, and Water Content of Magmas
Water is most abundant dissolved gas in magmas
As magma rises, pressure decreases, water becomes steam bubbles
Basaltic magma has lower water content  peaceful, safe eruptions
Rhyolitic magma has higher water content and high viscosity  many steam bubbles form and can not escape through thick magma, so explode out  violent, dangerous eruptions
Figure 6.11
Figure 6.12

17. Plate-Tectonic Setting of Volcanoes Revisited
Spreading centers have abundant volcanism because:
Sit above hot asthenosphere
Asthenosphere has low SiO2
Plates pull apart so asthenosphere rises and melts under low pressure, changing to high-temperature, low SiO2, low volatile, low viscosity basaltic magma that allows easy escape of gases  peaceful eruptions

18. Plate-Tectonic Setting of Volcanoes Revisited
Subduction zones have violent eruptions because:
Magma is generated by partial melting of the subducting plate with water in it
Melts overlying crust to produce magmas of variable composition
Magma temperature
decreases while SiO2,
water content and
viscosity increase 
violent eruptions
Figure 6.13

19. How a Volcano Erupts Begins with heat at depth
Rock that is superheated (heated to above its melting temperature) will rise
As it rises, it is under less and less pressure so some of it melts (becomes magma)
Volume expansion leads eventually to eruption
Three things will cause rock to melt:
Lowering pressure
Raising temperature
Increasing water content
Lowering pressure is most common way to melt rock  decompression melting

20. How a Volcano Erupts
Magma at depth is under too much pressure for gas bubbles to form (gases stay dissolved in magma)
As magma rises toward surface, pressure decreases and gas bubbles form and expand, propelling the magma farther up
Eventually gas bubble volume may overwhelm magma, fragmenting it into pieces that explode out as a gas jet
Figure 6.14

21. How a Volcano Erupts Eruption Styles and the Role of Water Content
Concentration of water in magma largely determines peaceful or explosive eruption
Basaltic magma can erupt violently with enough water
Rhyolitic magma usually erupts violently because of high water content, high viscosity (secondary role)
Styles of volcanic eruptions
Nonexplosive Icelandic and Hawaiian
Somewhat explosive Strombolian
Explosive Vulcanian and Plinian
Figure 6.16

22. How a Volcano Erupts Some Volcanic Materials
Low-water content, low-viscosity magma  lava flows
High-water content, high-viscosity magma  pyroclastic debris
Insert Table 6.6

23. How a Volcano Erupts Nonexplosive eruptions
Pahoehoe: smooth ropy rock from highly liquid lava
Aa: rough blocky rock from more viscous lava
Figure 6.17
Figure 6.18

24. How a Volcano Erupts Explosive eruptions
Pyroclastic debris: broken up fragments of magma and rock from violent gaseous explosions, classified by size
May be deposited as:
Air-fall layers (settled from ash cloud)
High-speed, gas-charged pyroclastic flow
Figure 6.20
Figure 6.19a

25. How a Volcano Erupts Explosive eruptions Very quick cooling:
Obsidian: volcanic glass forms when magma cools very fast
Pumice: porous rock from cooled froth of magma and bubbles
Figure 6.21

26. Side Note: How a Geyser Erupts
Geyser: eruption of water superheated by magma
Can only exist in areas of high heat flow underground
Water boils (becomes gas) at 100oC unless it is under pressure – no room for expansion to gas state
Water can be heated to higher than boiling temperature  superheated
When superheated water reaches point of lower pressure, it flashes to steam violently, and erupts out of the ground
Figure 6.22

27. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Viscosity may be low or high
Controls whether magma flows easily or piles up
Volatile abundance may be low, medium or high
May ooze out harmlessly or explode
Volume may be small, medium or large
Greater volume  more intense eruption

28. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
By mixing different values for the three V’s, can forecast different eruptive styles for volcanoes
Insert Table 6.7

29. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
By mixing different values for the three V’s, can define different volcanic landforms
Insert Table 6.8

30. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Shield Volcanoes: Low Viscosity, Low Volatiles, Large Volume
Basaltic lava with low viscosity and low volatiles flows to form gently dipping, thin layers
Thousands of layers on top of each other form very broad, gently sloping volcano like Mauna Loa in Hawaii
Great width compared to height
Figure 6.24

31. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Hawaiian-type Eruptions
“Curtain of fire”: lines of lava fountains up to 300 m high
Low cone with high fountains of magma
Floods of lava spill out and flow in rivers down slope
Eruptions last days or years, usually not life-threatening but destroy buildings and roads
Figure 6.26

32. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Hawaiian-type Eruptions
Hawaiian volcanoes include Haleakala on Maui, five volcanoes of island of Hawaii and subsea Loihi (969 m below sea level)
Figure 6.25

33. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Killer Event of 1790
Rare Hawaiian killer pyroclastic events
King Keoua’s army passing through Kilauea area was stopped by eruptions and split into three groups to escape area
Base surge overtook middle group, killing all 80
Explosion column burst upward as dense basal cloud swept downhill
Cloud of hot water and gases sometimes with magma fragments

34. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Icelandic-type Eruptions
Most peaceful type of eruption
Fissure eruptions:
Lava pours out of linear vents or long fractures up to 25 km long
“Curtain of fire” effect
Low-viscosity, low-volatile lava flows almost like water
Build up volcanic plateaus (even flatter than shield volcanoes) of nearly horizontal basalt layers

35. In Greater Depth: Volcanic Explosivity Index
Provides a means of evaluating eruptions according to volume of material erupted, height of eruption column and duration of major eruptive blast  scale from 0 to 8
Insert Table 6.9

36. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Flood Basalts: Low Viscosity, Low Volatiles, Very Large Volume
Largest volcanic events known on Earth
Immense amounts of basalt erupted
Geologically short time (1 to 3 million years)
Different from hot spots that last hundreds of millions of years
Can have global effects as huge amounts of gases (including CO2 and SO2) are released into atmosphere
Some flood basalts coincide with mass extinctions:
Siberia (250 million years ago): 3 million km3 of basalt
India (65 million years ago): 1.5 million km3 of basalt

37. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Scoria Cones: Medium Viscosity, Medium Volatiles, Small Volume
Low conical hills (also known as cinder cones) of basaltic to andesitic pyroclastic debris built up at volcanic vent
Can have summit crater with lava lake during eruption
Form during single eruption lasting hours to several years
Figure 6.27

38. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Strombolian-type Eruptions
Scoria cones usually built by Strombolian eruptions
Named for Stromboli volcano in Italy, erupting almost daily for millennia (tourist attraction)
Central lava lake with thin crust that breaks easily to allow occasional frequent eruptive blasts of lava and pyroclastic debris
Michoacan, Mexico
New scoria cone born in farm field and built up by nine years of eruptions, burying area and destroying two towns

39. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Stratovolcanoes: High Viscosity, High Volatiles, Large Volume
Steep-sided, symmetrical volcanic peaks
Composed of alternating layers of pyroclastic debris and andesitic to rhyolitic lava flows
Eruptive styles from Vulcanian to Plinian
Figure 6.28

40. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Vulcanian-type Eruptions
Alternate between highly viscous lava flows and pyroclastic eruptions
Common in early phase of eruptive sequence before larger eruptions (‘clearing throat’)

41. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Plinian-type Eruptions
Named for Pliny the Younger (descriptions of 79 C.E. eruption of Mt. Vesuvius)
Occur after ‘throat is clear’, commonly final eruptive phase
Gas-powered vertical columns of pyroclastic debris up to 50 km into the atmosphere

42. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Vesuvius, 79 CE
Caused by subduction of Mediterranean seafloor beneath Europe, by northward movement of Africa
Most of 4,000 people who remained in Pompeii killed by thick layers of hot pumice or pyroclastic flows from Vulcanian-type eruption, followed by Plinian-type eruption
Seismic waves define 400 km2 magma body 8 km under Vesuvius today
Millions of people live around Bay of Naples area

43. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Vesuvius, 79 CE
Figure 6.29

44. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Vesuvius, 79 CE
Plinian-type eruptions can create ‘volcano weather’, when steam in eruption column cools and condenses to fall as rain, mixing with ash on volcano’s slopes and creating mudflows (lahars) that can be devastating
Lahars buried Herculaneum
Figure 6.30

45. Side Note: British Airways Flight 9
1982 flight from Kuala Lumpur, Malaysia to Perth, Australia lost all four engines at 37,000 feet
Plane descended to 12,000 feet before engines started again
Emergency landing in Jakarta
Plane had flown through eruption cloud of hot volcanic ash and pyroclastic debris from Mount Galunggung

46. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Lava Domes: High Viscosity, Low Volatiles, Small Volume
Form when high-viscosity magma at vent of volcano cools quickly into hardened plug
Gases accumulated at top of magma chamber power Vulcanian and Plinian blasts until most volatiles have escaped
Remaining magma is low-volatile, high-viscosity paste
Oozes to vent and cools quickly in place, forming plug
Figure 6.32

47. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
A Typical Eruption Sequence
Gas-rich materials shoot out first as Vulcanian blast, followed by longer Plinian eruption
After gas depleted, high-viscosity magma builds lava dome over long period
Vulcanian precursor  Plinian main event  lava dome conclusion

48. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Calderas: High Viscosity, High Volatiles, Very Large Volume
Calderas: large volcanic depressions (larger than crater) formed by inward roof collapse into partially emptied magma reservoirs
Form at different settings:
Summit of shield volcanoes, such as Mauna Loa or Kilauea
Summit of stratovolcanoes, such as Crater Lake or Krakatau
Giant continental caldera, such as Yellowstone or Long Valley
Ultraplinian eruptions at Toba on Sumatra (74,000 years ago) formed 30 x 100 km caldera with central raised area – resurgent caldera

49. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Calderas: High Viscosity, High Volatiles, Very Large Volume
Figure 6.34
Figure 6.33

50. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Crater Lake (Mount Mazama), Oregon
Formed about 7,600 years ago from stratovolcano Mt. Mazama
Major eruptive sequence of pyroclastic flows and Plinian columns emitted ash layer recognizable across North America
Large enough volume of magma erupted to leave void beneath surface  mountain collapsed into void leaving caldera crater at surface that filled with water to form Crater Lake
1,000 year old successor volcanic cone Wizard Island
Figure 6.35

51. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Crater Lake (Mount Mazama), Oregon
Figure 6.36

52. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Krakatau, Indonesia, 1883
Part of volcanic arc above subduction zone between Sumatra and Java
After earlier collapse, Krakatau built up during 17th c.
Quiet for two centuries then resumed activity in 1883
Moderate Vulcanian eruptions from dozen vents
Led up to enormous Plinian blasts and eruptions 80 km high and audible 5,000 km away
Blew out 450 m high islands into 275 m deep hole
Triggered tsunami 35 m high killing 36,000 people
Has been building new cone Anak Krakatau since 1927

53. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Santorini and the Lost Continent of Atlantis
Mediterranean plate subducting beneath Europe  many volcanoes including stratovolcano Santorini
Series of eruptions around 1628 B.C.E.:
6 m thick layer of air-settled pumice
Several meter thick ash deposits from when seawater reached magma chamber  steam blasts
56 m thick ash, pumice, rock fragments from collapse of cones
Layers of ash and rock fragments from magma body degassing
Figure 6.37

54. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Santorini and the Lost Continent of Atlantis
Effects on local Minoan culture:
Akrotiri had three-story houses, sewers, ceramics and jewelry, trade with surrounding cultures
Destruction of part of Minoan civilization made great impact  story of disappearance of island empire of Atlantis made be rooted in this event
Figure 6.38

55. In Greater Depth: Hot Spots
Shallow hot rock masses/magmas or plumes of slowly rising mantle rock operating for about 100 million years
Used as reference points for plate movement because almost stationary, while plates move above them
122 active in last 10 million years, largest number under Africa (stationary plate concentrates mantle heat)
Oceanic hot spots:
Peaceful eruptions build shield volcanoes (Hawaii)
Spreading center hot spots:
Much greater volume of basaltic magma, peaceful (Iceland)
Continental hot spots:
Incredibly explosive eruptions as rising magma absorbs continental rock, form calderas (Yellowstone)

56. In Greater Depth: Hot Spots
Figure 6.39

57. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Three calderas in U.S. known to have erupted in last million years:
Valles caldera in New Mexico, about 1 million years ago, in Rio Grande rift
Long Valley, California, about 760,000 years ago, edge of Basin and Range
Yellowstone, Wyoming, about 600,000 years ago, above a hot spot
Occur where large volumes of basaltic magma intrude to shallow depths and melt surrounding continental rock, to form high-viscosity, high-volatiles magma

58. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Yellowstone National Park
Resurgent caldera above hot spot below North America, body of rhyolitic magma 5 to 10 km deep
North American plate movement (southwestward 2-4 cm/yr) is recorded by trail of volcanism to the southwest
Figure 6.40

59. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Yellowstone National Park
Three recent catastrophic (ultra-Plinian) eruptions:
2 million years ago, 2,500 km3
1.3 million years ago, 280 km3
0.6 million years ago, 1,000 km3, created caldera 75 km by 45 km, covering surrounding 30,000 km2 with ash
Figure 6.41

60. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Eruptive Sequence of a Resurgent Caldera
Very large volume of rhyolitic magma bows ground upward
Accumulates cap rich in volatiles and low-density material
Circular fractures form around edges  Plinian eruptions, then pyroclastic flows as more magma is released than can vent upwards
Figure 6.42

61. The Three V’s of Volcanology: Viscosity, Volatiles, Volume
Eruptive Sequence of a Resurgent Caldera
As magma body shrinks, land surface sinks into void
New mass of magma creates resurgent dome  next eruption
Figure 6.42

62. End of Chapter 6