Transcript Chapter 6 Part II

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 8.23
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
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
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
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
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 8.25
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
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 8.26
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’)
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
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
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
Vesuvius, 79 CE
Figure 8.27
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 8.28
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
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 lowvolatile, high-viscosity paste
– Oozes to vent and cools
quickly in place, forming plug
Figure 8.30
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
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
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
Calderas: High Viscosity, High Volatiles, Very Large
Volume
Figure 8.32
Figure 8.31
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 8.33
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
Crater Lake (Mount Mazama), Oregon
Figure 8.34
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
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
Santorini and the Lost Continent
of Atlantis
Figure 8.35
• 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
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 8.36
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)
In Greater Depth: Hot Spots
Figure 8.37
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
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 8.38
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 8.39
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 8.40
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 8.40
End of Chapter 8