Volcanoes

1
Volcanism
2
Volcanism

• The process whereby magma and associated gas
  rises through the crust and are extruded onto the
  surface or into the atmosphere.
• Major node in the rock cycle.
• Constructive geologic process.
• Dynamic recycling process.
• Destructive geologic hazard.

3
Fig. 1-12, p. 20
4
Volcanism

• About 550 volcanoes are active (erupted during
  historic time). A larger number are dormant (not
  active recently, but may erupt again) or are
  extinct (permanently inactive).
• The mid-ocean ridge system is a vast chain of
  underwater volcanic centers that are nearly
  constantly active
• Other bodies in the solar system are volcanically
  active (Triton and Io)

5
(No Transcript)
6
(No Transcript)
7
Products of Volcanism

• Volcanic Gas
• Lava flows
• Pahoehoe
• Aa
• Lava lakes fountains
• Lava channels tubes
• Columnar joints
• Pillows

• Pyroclastics
• Ash
• Lapilli
• Volcanic bombs blocks
• Lahars, mudflows, etc.
• Nuee ardente
• Tuff
• Constructional features (volcanoes)

8
Volcanic Gas

• Magma (and lava) has dissolved gas, like a soda.
  And like a soda, if you decrease the pressure (by
  opening it) the gas wants to escape
• From last time, recall that hot, mafic lavas (low
  viscosity) release gas easily. Cooler, felsic
  lavas (higher viscosity) do not allow gas to
  escape easily. Danger of explosive release!!

9
Fig. 5-2, p. 116
10
Fig. 4-8h, p. 96
11
Volcanic Gas

• Small amount of gas (few wt. ) dissolved.
• 50-80 of the total gas is water vapor (H2O).
• Other important gases
• Carbon dioxide (CO2)
• Nitrogen (N2)
• Sulfur (SO2, H2S)
• Carbon monoxide (CO)
• Hydrogen (H2)
• Chlorine (Cl2)

12
Volcanic Gas

• Important effects of gas content
• Explosive hazard of eruptions (viscosity of
  lava)
• Toxicity (SO2, CO2)
• Global climate (volcanic winter, volcanic
  greenhouse, mass extinctions)

13
Lava Flows

• Least dangerous extrusive phenomenon.
• Relatively slow moving.
• Predictable flow.

14
Figure 1b, p. 118
15
Pahoehoe Lava
16
Fig. 5-4a, p. 119
17
(No Transcript)
18
(No Transcript)
19
Aa Lava
20
(No Transcript)
21
(No Transcript)
22
Scoria surface on an aa flow
23
Lava Flows

• What determines if a mafic lava erupts as aa or
  pahoehoe?
• Viscosity
• Strain rate (how fast force is applied)
• Temperature
• Gas content
• If lava slows, cools, and stops as viscosity
  increases ? stays pahoehoe.
• If lava forced to keep flowing when viscosity
  increases ? transition to aa.

24
Fig. 5-4b, p. 119
25
(No Transcript)
26
Lava Channels

• Narrow, curved or straight open pathways through
  which lava moves on the surface of a volcano.
• Some of the lava congeals and cools along the
  banks to form natural levees that may eventually
  enable the lava channel to build a few meters
  above the surrounding ground.

27
Lava Tubes

• Underground lava conduits
• Form by the crusting over of lava channels and
  flows. If supply of lava stops, lava in the tube
  system drains downslope leaving empty tubes.
• Commonly exhibit "high-lava" marks, flat floors,
  and lava stalactites that hang from the roof.

28
Fig. 5-3a, p. 117
29
Fig. 5-3b, p. 117
30
(No Transcript)
31
Pressure Ridges

• As surface of a flow solidifies, it protects
  the lava in the channel, allowing it to continue
  to flow.
• The crust becomes buckled and cracked due to the
  flow, and develops an up-warped and fractured
  ridge

32
(No Transcript)
33
Fig. 5-5a, p. 119
34
Fig. 5-5b, p. 119
35
Spatter Cones

• Very fluid fragments of molten lava ejected from
  a vent that flatten and congeal on the ground.
• Spatter will build walls of solidified lava
  around a single vent to form a circular-shaped
  spatter cone or along both sides of a fissure to
  build a spatter rampart.

36
Spatter Cones
37
Spatter Cone line
38
Lava Fountain

• Jet of lava sprayed 10 to 500 m into the air by
  the rapid formation and expansion of gas bubbles.

39
Lava Lakes

• Large volumes of molten lava, usually basaltic,
  contained in a vent, crater, or broad depression.
  
• Active lava lakes have a partially solidified
  shiny gray crust (5-30 cm thick a few minutes or
  hours old) formed by lava constantly cooled by
  the atmosphere.
• Crust continually circulates, breaks, and sinks
  into the moving molten lava below. The pattern of
  movement looks like plate tectonics!

40
(No Transcript)
41
(No Transcript)
42
Pillow Lava

• Mounds of elongate lava "pillows" formed by
  repeated oozing and quenching of the hot basalt.
• First, a flexible glassy crust forms around the
  newly extruded lava, forming an expanded pillow.
  Next, pressure builds until the crust breaks and
  new basalt extrudes like toothpaste, forming
  another pillow.

43
Fig. 5-7a, p. 120
44
Fig. 5-7b, p. 120
45
Columnar Joints

• Most materials contract as they cool. Lava does
  this.
• As lava coolss and contracts, forces cause it to
  fracture, forming joints.
• Cracks form a hexagonal pattern (due to geometry
  of most efficient cooling.
• Cracks extend down through the flow forming
  columns

46
(No Transcript)
47
Fig. 5-6a, p. 120
48
Fig. 5-6b, p. 120
49
Tephra

• General term for fragments of volcanic rock and
  lav, regardless of size, that are blasted into
  the air by eruptions.
• Fallen tephra size gets smaller with distance
  from volcano. thickness of the resulting deposit
  also becomes thinner with distance from source.
• Small tephra stays aloft in the eruption cloud
  for longer periods of time, which allows wind to
  blow tiny particles farther from an erupting
  volcano.

50
Fig. 5-8, p. 121
51
Ash

• Rock, mineral, and volcanic glass fragments
  smaller than 2 mm (0.1 inch) in diameter.
• Ash is extremely abrasive, similar to finely
  crushed window glass, mildly corrosive, and
  electrically conductive, especially when wet.
• Volcanic ash is created during explosive
  eruptions by the shattering of solid rocks and
  violent separation of magma (molten rock) into
  tiny pieces.

52
Lapilli

• Rock fragments between 2 and 64 mm in diameter
  (little stones) that were ejected from a
  volcano during an explosive eruption.

53
Volcanic Bombs Blocks

• Bombs are lava fragments (gt64 mm diameter) that
  were ejected while viscous (partially molten).
  Many acquire rounded aerodynamic shapes during
  their travel through the air.
• Blocks are solid rock fragments (gt 64 mm in
  diameter) that were ejected from a volcano during
  an explosive eruption. Blocks commonly consist of
  solidified pieces of old lava flows that were
  part of a volcano's cone.

54
Scoria

• Vesicular (bubbly) glassy lava rock of basaltic
  to andesitic composition.
• The bubbly nature of scoria is due to the escape
  of volcanic gases during eruption.
• Like pumice, but denser.

55
Fig. 4-16d, p. 101
56
Pumice

• Light, porous volcanic rock that forms during
  explosive eruptions.
• Resembles a sponge because it consists of a
  network of gas bubbles frozen amidst fragile
  volcanic glass. All types of magma (basalt,
  andesite, dacite, and rhyolite) will form pumice.
  
• During an explosive eruption, volcanic gases
  dissolved in the liquid portion of magma expand
  rapidly to create a foam or froth (glass).

57
Fig. 4-16c, p. 101
58
Fig. 4-16b, p. 101
59
Reticulite

• Basaltic pumice in which nearly all cell walls of
  gas bubbles have burst, leaving a honeycomb-like
  structure.
• Does not float in water because of the open
  network of bubbles.

60
Peles Tears and Peles Hair

• Pele's tears small bits of molten lava formed
  into spheres or tear drops in lava fountains.
• Peles hair thin strands of volcanic glass
  drawn out from molten lava in lava fountains. A
  single strand, with a diameter of less than 0.5
  mm, may be as long as 2 m. Wind can blow the
  glass threads several tens of kilometers from a
  vent.

61
Types of Volcanic Deposits

• Airfall tephra can be distributed by fall
  through the atmosphere. Usually finer particles
  like ash. Airfall tephra of basaltic eruptions
  are much less voluminous than those of
  intermediate to rhyolitic eruptions due to the
  less explosive style of basaltic volcanic
  activity. Deposits typically well sorted.
• Pyroclastic flow and surge movement of large
  volumes of material with the general behavior of
  lava flows. They act as heavy fluids controlled
  in their movement by gravity and the topography
  of the underlying land surface. Deposits can be
  well sorted or unsorted.

62
Nuee Ardente

• Fiery Cloud. Hot ash flow. Pyroclastic flow.
• Mobile, dense cloud of hot (1000 C) pyroclastic
  material and gases. Travel very rapidly (100
  km/hr) and go long distances.
• Gas expands as lava rises.
• Lava breaks up into fragments supported by
  escaping gas.
• Cloud flows downhill.
• Very destructive volcanic hazard!

63
(No Transcript)
64
Fig. 5-12b, p. 127
65
Fig. 5-12a, p. 127
66
Fig. 5-1a, p. 114
67
Fig. 5-CO, p. 112
68
Fig. 5-1b, p. 114
69
Fig. 5-1c, p. 114
70
Tuff

• A volcanic (extrusive) igneous rock that forms
  when ash deposits are consolidated. Can be
  air-fall or flow deposit.

71
Fig. 4-16a, p. 101
72
Lahars and Mudflows

• Indonesian word for a rapidly flowing mix of rock
  debris water that originates on the slopes of a
  volcano.
• Mudflows (lots of water) or debris flows (less
  water).
• Hot or cold.
• Form by the rapid melting of snow and ice by
  pyroclastic flows, intense rainfall on loose
  volcanic rock deposits, breakout of a lake dammed
  by volcanic deposits, and as a consequence of
  debris avalanches.
• Very destructive volcanic hazard!

73
Fig. 5-11a, p. 126
74
Fig. 5-11b, p. 126
75
Volcanic Breccia

• Volcaniclastic (extrusive igneous/sedimentary)
  rocks composed predominantly of angular volcanic
  particles greater than 2 mm in size. Usually
  debris flow/mudflow/lahar deposit or coarse
  (near) part of tuff airfall/flow deposit or aa.

76
Volcanoes

• Conical mountains formed around a vent where
  lava, gas, and pyroclastics are erupted to the
  surface.
• Main crater (circular depression) at the summit
  fed by a volcanic pipe. Subordinate vents on the
  volcano flanks may also extrude material
  (parasitic cones).

77
(No Transcript)
78
Kinds of Volcanoes

• Shield Volcano
• Cinder Cone
• Composite Volcano
• Lava Dome

79
Shield Volcanoes

• Resemble an upside-down shield with low, rounded
  profiles and gentle slopes (2-10 degrees max).
• Built of thin, low-viscosity mafic flows (99).
• Massive constructional features. Example Mauna
  Loa (Hawaii) 100 km across 9.5 km tall 50,000
  km3
• Hawaiian-style eruptions.

80
(No Transcript)
81
(No Transcript)
82
Olympus Mons, Mars
83
Cinder Cones

• Volcanic peak comosed of pyroclastic materials
  resembling cinders (scoria, pumice, lapilli, and
  ash).
• Form when pyroclastic material is ejected and
  falls close to the vent, piling up into a
  steep-sided cone.
• Up to 400 m high, often with a prominent crater.
• Often form in the caldera of a larger volcano.

84
(No Transcript)
85
(No Transcript)
86
Fig. 5-10, p. 125
87
Figure 1, p. 126
88
(No Transcript)
89
Composite Volcanoes

• Stratovolcanoes. Built of both lava flows and
  pyroclastics in layers. Usually andesitic
  composition material. Large, high mountains with
  concave profiles (steep summit and shallow
  flanks).
• Erupt explosively (especially in Plinean and
  Pelean events). Lahar and nuee ardente
  pyroclastic flows are real hazards. Airfall
  deposits often voluminous.
• Typical of subduction-zone volcanic arcs.

90
(No Transcript)
91
(No Transcript)
92
Mount Pinatubo, Phillipines
93
Figure 1a, p. 118
94
Mount Saint Helens, Washington State
95
Lava Dome

• Characteristic of viscous (e.g. cool felsic)
  magmas.
• Bulbous, steep-sided constructs, often in the
  craters of larger composite volcanoes.
• Grow slowly, often in pulses. Tend to collapse
  to create ash flows, nuee ardents, and other
  pyroclastic debris flows.

96
(No Transcript)
97
(No Transcript)
98
Calderas

• Characterize large volcanic centers like those
  responsible for ignimbrites and supervolcano
  eruptions. Exceed 1 km in diameter with steep
  sides.
• Examples Yellowstone Long Valley, CA Mount
  Mazama, Oregon

99
Fig. 5-9, p. 124
100
(No Transcript)
101
(No Transcript)
102
(No Transcript)
103
(No Transcript)
104
Table 5-1, p. 115
105
(No Transcript)
106
Ignimbrite Eruptions (Supervolcanoes)

• Exceedingly large, caldera forming eruptions,
  typically associated with andesitic to rhyolitic
  magmas.
• Caldera forming (no volcanic mountain) events
  where very large volumes (1000 km3 ) of material
  is ejected into the atmosphere. Airfall on a
  continental scale as well as significant local
  pyroclastic (tuff, welded tuff) flows.
• Explosive force of thousands of nuclear weapons
  (Yellowstone was about 2500 times more powerful
  than Mt. St. Helens).
• Examples Yellowstone Long Valley, CA
• Planetary-scale geologic hazard

107
Eruptions every 600,000 years since 2.1 Ma
Last one 640,000 years ago!
Produced 85 x 45 km caldera. 3000 square miles
of pyroclastic flows. Sent airfall across most
of the continent.
Whens the next one?
108
(No Transcript)
109
Fig. 4-16a, p. 101
110
Welded Tuff

• A volcanic (extrusive) igneous rock that forms
  when still-warm tephra (tuff) accumulates.
  Particles are hot and soft, and weld together
  under the weight of overlying deposits, forming a
  hard rock.

111
Fissure Eruptions (Flood Basalts)

• Basaltic magma in huge volumes erupted along
  fissures. Dozens to hundreds of lava flows pile
  on top of one another.
• Fluid magma ? no volcanic mountain little
  pyroclastics.
• Largest volcanic eruptions on Earth 2000 km3
  of material in single flow (few days?). Compare
  with Kilauea (Hawaii) with 1.5 km3 in 16 years!
  Also release tremendous volumes of gas ? may play
  role in mass extinctions (the timing may be
  right)?
• Thought to arise due to mantle plumes impinging
  on the lithosphere
• Planetary-scale geologic hazard

112
(No Transcript)
113
Flood Basalt Provinces

• Columbia River basalt plateau erupted 17 Ma to 5
  Ma.
• 164,000 km2 area to 1 km thickness.
• Deccan traps and Siberian traps are order of
  magnitude larger

114
Flood Basalt Provinces

• FLOOD BASALT PROVINCE SIZES
  Province                 
  Age (Ma)                  Area (km2 est.)
  -------------------------------------------------
  ---------------------------- Siberian             
         2503                          2,000,000
  Paraña                      1305                
            2,000,000 Deccan                     
  662                            1,500,000
  Columbia River        171                       
           200,000 Laki, Iceland             1783
  A.D.                               565 Northern
  CAMP        2011                         
  4,200,000 Southern CAMP       1993              
              5,900,000
• Total CAMP              1993                     
     10,100,000 -----------------------------------
  -------------------------------------------
  Sources Hooper, 1988, The Columbia River
  Basalt, in Macdougall, ed.,    Continental Flood
  Basalts Kluwer, p. 1-33. Rampino Stothers,
  1988, Flood basalt volcanism during    the past
  250 million years Science, v. 241, p. 663-668.
  Thorarinsson, S., 1969, The Lakagigar eruption
  of 1783    Bull. Volcanology, v. 33, p.
  910-929. McHone, J.G., Puffer, J.H., 1999 (?).
  Flood basalt provinces    of the Pangaean
  Atlantic rift Regional extent and environmental
     significance. In Olsen, P.E., LeTourneau,
  P.M., (Editors),    Aspects of Triassic-Jurassic
  Rift Basin Geoscience Columbia Univ.  Press,
  New York, in press

115
Fig. 5-13a, p. 129
116
Fig. 5-13c, p. 129
117
Fig. 5-15, p. 131
118
Mid-ocean Ridge Volcanism
119
Hawaiian/Icelandic Eruptions

• Magma fluid, basaltic.
• Explosive activity Weak ejection of very fluid
  blebs long-lived lava fountaining and lakes.
• Effusive activity Thin, often extensive
  long-lived flows of fluid lava.
• Dominant ejecta Cow-dung bombs and spatter very
  little ash (pyroclastics).
• Vent structures Spatter cones and ramparts very
  broad flat lava cones and shields.
• Examples Hawaiian islands Iceland.

120
Strombilian Eruptions

• Magma moderately fluid, basaltic.
• Explosive activity Weak to violent fountaining
  and explosion of pasty blebs during intermittent
  (rhythmic or irregular with a period of minutes
  or hours) release of volcanic gases.
• Effusive activity Occasionaly, thick, not
  extensive flows of moderately-fluid lava. Long
  lived flows (months to years).
• Dominant ejecta Elliptical bombs cinder small
  to large amounts of glassy ash.
• Vent structures Cinder cones
• Examples Stromboli and Etna, Sicily.

121
Vulcanian Eruptions

• Magma viscous and gas-rich basaltic to
  rhyolitic.
• Explosive activity moderate to violent ejection
  of solid or very viscous hot fragments of gas-
  and water-rich lava.
• Effusive activity Flows commonly absent, but
  when present they are thick, and stubby ash
  flows, etc. are rare. Well-bedded, widely
  distributed airfall is the common pyroclastic
  material.
• Dominant ejecta Large, dark mushroom cloud of
  glassy ash glassy to lithic blocks pumice
  breadcrust bombs.
• Vent structures Ash cones block cones block
  and ash cones.
• Examples Aleutian Islands, Alaska.

122
Pelean Eruptions

• Magma viscous, andesitic to rhyolitic.
• Explosive activity moderate to violent ejection
  of solid or very viscous hot fragments of
  effervescing lava common glowing avalanches (hot
  ash flows and nuee ardents).
• Effusive activity domes and/or very short, thick
  flows may occur.
• Dominant ejecta Glassy to lithic blocks, pumic,
  and ash. Not much airfall.
• Vent structures Ash and pumice cones (unstable)
  steep domes and spines.
• Examples Mt. Pelee, Martinique

123
Plinian Eruptions

• Mamga viscous, felsic but becoming more mafic
  with time.
• Explosive activity voluminous, gas-rich
  eruptions with catastrophic ejection of large
  volumes of ash and accompanying caldera collapse.
  Mega-tonnes of energy.
• Effusive activity small to very voluminous (up
  to 1,000 km3) sheets of widely-dispersed air-fall
  pyroclastics very high (11 km) eruption
  columns no lava, but pyroclastic flows are
  common.
• Dominant ejecta glassy ash and pumice
  well-sorted and no welding.
• Vent structures Widespread beds of pumice,
  lapilli, and ash generally no cone building.
• Examples Vesuvius, Italy Mt. Saint Helens, WA
  Pinatubo, Phillipines

124
(No Transcript)
125
What controls explosiveness?

• Gas content (H2O, CO2, N2, SO2, H2S)
• Viscosity
• Temperature
• SiO2 content

126
(No Transcript)
127
(No Transcript)
128
St Helens Before
129
(No Transcript)
130
(No Transcript)
131
(No Transcript)
132
Surtseyan (Phreatic) Eruptions

• Magma viscous, basaltic (rarely andesitic).
• Explosive activity violent, continuous or
  rhythmic explosions due to magma contacting
  ground- or shallow surface-water ejection of
  solid, warm, highly-fragmented (thermally
  shocked) piecies of magma.
• Effusive activity short, locally pillowed, rare
  lava flows.
• Dominant ejecta lithic blocks and ash forming
  pyroclastic cones (tuff rings) of quenched magma
  no spatter, fusiform bombs or lapilli.
• Vent structures tuff rings.
• ExamplesSurtsey, Iceland.

133
(No Transcript)
134
Table 5-1, p. 115
135
Fig. 5-14, p. 130
136
Fig. 5-16a, p. 132
137
Fig. 5-16b, p. 132
138
Fig. 5-16c, p. 132
139
Igneous Activity and Plate Tectonics

• Two questions
• 1. What are zones of igneous activity on earth
  concentrated in belts.
• 2. Why are magmas in ocean basins different than
  those in the continents?

140
Igneous Activity and Plate Tectonics

• Most volcanic activity occurs where plates
  diverge and converge

141
Igneous Activity and Plate Tectonics

• Divergent Margin Volcanism
• Convergent Margin Volcanism
• Hot Spot Volcanism

142
(No Transcript)
143
Igneous Activity and Plate Tectonics

• Gabbro basalt at divergent margins e.g. oceanic
  spreading ridges and rifts. Produced by
  upwelling, hot ultramafic mantle. It melts as it
  is decompressed. 1st partial melt fraction
  richer in Si ? basalt.

144
(No Transcript)
145
Igneous Activity and Plate Tectonics

• Granite rhyolite in continental interiors
  continent-continent collision zones.
• Diorite andesite in island arcs subduction
  zones
• Partial melt from subducted slabs and the bottom
  of the crust that is then changed

146
Fig. 5-17, p. 133
147
Igneous Activity and Plate Tectonics

• But there are hot spots within plates, too!

148
(No Transcript)
149
(No Transcript)