Transcript Lecture Outlines Natural Disasters, 6th edition

Lecture Outlines
Natural Disasters, 7th edition
Volcanic Eruptions: Plate Tectonics
and Magma
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 8.1
Figure 8.3
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 8.5
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 8.6
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 8.6
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 8.7
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 8.8
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
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 8.9
Figure 8.10
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 8.11
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 8.12
• 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)
Figure 8.14
• Styles of volcanic eruptions
– 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
How a Volcano Erupts
Nonexplosive eruptions
• Pahoehoe: smooth ropy rock from highly liquid lava
• Aa: rough blocky rock from more viscous lava
Figure 8.15
Figure 8.16
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 8.17a
Figure 8.18
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 8.19
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 8.20
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
The Three V’s of Volcanology:
Viscosity, Volatiles, Volume
• By mixing different values for the three V’s, can define
different volcanic landforms
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 8.22
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 8.24