1 chapter 6 volcanic eruptions: plate tectonics and magmas lecture powerpoint copyright © the...

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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

• 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

How We Understand Volcanic Eruptions

• 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

Plate-Tectonic Setting of Volcanoes

Figure 6.6

• 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

• 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

• 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

Chemical Composition of Magmas

Figure 6.8

• 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



Figure 6.9

• 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: 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



Insert Table 6.5


Figure 6.12Figure 6.11

• 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

• 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

Plate-Tectonic Setting of Volcanoes Revisited

Figure 6.13

• 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)


• 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

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

Some Volcanic Materials• Low-water content,

low-viscosity magma lava flows

• High-water content, high-viscosity magma pyroclastic debris


Insert Table 6.6

Nonexplosive eruptions• Pahoehoe: smooth ropy rock from highly liquid lava• Aa: rough blocky rock from more viscous lava


Figure 6.17 Figure 6.18

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 6.19aFigure 6.20

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

• 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

Side Note: How a Geyser Erupts

– When superheated water reaches point of lower pressure, it flashes to steam violently, and erupts out of the ground

Figure 6.22

• 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

• By mixing different values for the three V’s, can define different volcanic landforms


Insert Table 6.8

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

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

Hawaiian-type EruptionsHawaiian volcanoes include

Haleakala on Maui, five volcanoes of island of Hawaii and subsea Loihi (969 m below sea level)


Figure 6.25

Killer Event of 1790Rare 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 80Explosion column burst upward as dense basal cloud swept

downhillCloud of hot water and gases sometimes with magma

fragments


Icelandic-type EruptionsMost peaceful type of eruptionFissure 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 waterBuild up volcanic plateaus (even flatter than shield

volcanoes) of nearly horizontal basalt layers


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

In Greater Depth: Volcanic Explosivity Index

Insert Table 6.9

Flood Basalts: Low Viscosity, Low Volatiles, Very Large Volume

Largest volcanic events known on EarthImmense amounts of basalt eruptedGeologically short time (1 to 3 million years)

– Different from hot spots that last hundreds of millions of yearsCan have global effects as huge amounts of gases (including

CO2 and SO2) are released into atmosphereSome 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


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

Strombolian-type EruptionsScoria cones usually built by Strombolian eruptionsNamed 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 debrisMichoacan, Mexico

– New scoria cone born in farm field and built up by nine years of eruptions, burying area and destroying two towns


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

Vulcanian-type EruptionsAlternate between highly viscous lava flows and

pyroclastic eruptionsCommon in early phase of eruptive sequence before larger

eruptions (‘clearing throat’)


Plinian-type EruptionsNamed for Pliny the Younger (descriptions of 79 C.E. eruption of Mt.

Vesuvius)Occur after ‘throat is clear’, commonly final eruptive phaseGas-powered vertical columns of pyroclastic debris up to 50 km into

the atmosphere


Vesuvius, 79 CECaused by subduction of Mediterranean seafloor beneath Europe, by

northward movement of AfricaMost 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


Vesuvius, 79 CE


Figure 6.29

Vesuvius, 79 CEPlinian-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 againEmergency landing in JakartaPlane had flown through eruption cloud of hot volcanic ash and

pyroclastic debris from Mount Galunggung

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

A Typical Eruption SequenceGas-rich materials shoot out first as Vulcanian blast,

followed by longer Plinian eruptionAfter gas depleted, high-viscosity magma builds lava dome

over long periodVulcanian precursor Plinian main event

lava dome conclusion


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


Calderas: High Viscosity, High Volatiles, Very Large Volume


Figure 6.33

Figure 6.34

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

Crater Lake (Mount Mazama), Oregon Formed about 7,600 years ago from stratovolcano Mt. MazamaMajor eruptive sequence of pyroclastic flows and Plinian columns

emitted ash layer recognizable across North America


Figure 6.35

Crater Lake (Mount Mazama), Oregon


Figure 6.36

Krakatau, Indonesia, 1883Part of volcanic arc above subduction zone between

Sumatra and JavaAfter 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


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


Figure 6.37

– 56 m thick ash, pumice, rock fragments from collapse of cones

– Layers of ash and rock fragments from magma body degassing

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 SpotsShallow hot rock masses/magmas or plumes of slowly rising mantle

rock operating for about 100 million yearsUsed as reference points for plate movement because almost

stationary, while plates move above them122 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

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

spotOccur where large volumes of basaltic magma intrude to

shallow depths and melt surrounding continental rock, to form high-viscosity, high-volatiles magma


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

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

Eruptive Sequence of a Resurgent CalderaVery large volume of rhyolitic magma bows ground upwardAccumulates cap rich in volatiles and low-density materialCircular fractures form around edges Plinian eruptions, then

pyroclastic flows as more magma is released than can vent upwards


Figure 6.42

Eruptive Sequence of a Resurgent CalderaAs magma body shrinks, land surface sinks into voidNew mass of magma creates resurgent dome next eruption


Figure 6.42

End of Chapter 6

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