Chapter 6
Volcanic Eruptions:
Plate Tectonics and Magmas
Lecture PowerPoint
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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
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, 1138-1139
• 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
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
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
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
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
Plate-Tectonic Setting of Volcanoes
• No volcanism associated with transform faults or continentcontinent 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
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
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
Chemical Composition of Magmas
Figure 6.9
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
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
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
Viscosity, Temperature, and Water Content
of Magmas
Insert Table 6.5
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
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
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
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
How a Volcano Erupts
• Magma at depth is under too much pressure for gas
bubbles to form (gases stay dissolved in magma)
Figure 6.14
• 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
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 Figure 6.16
– Nonexplosive Icelandic and
Hawaiian
– Somewhat explosive Strombolian
– Explosive Vulcanian and Plinian
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
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
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 6.19a
Figure 6.20
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
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
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
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
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
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
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
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
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
Insert Table 6.9
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 6.27
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 6.28
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 6.29
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
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 6.32
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 6.34
Figure 6.33
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
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
Crater Lake (Mount Mazama), Oregon
Figure 6.36
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 6.37
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 6.38
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 6.39
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 6.40
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
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
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
End of Chapter 6