hawaiian-type eruptions hawaiian volcanoes include haleakala on maui, five volcanoes of island of...

Hawaiian-type Eruptions• Hawaiian volcanoes include

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

The Three V’s of Volcanology: Viscosity, Volatiles, Volume

Figure 8.23

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


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


• 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

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


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

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


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

Vulcanian-type Eruptions• Alternate between highly viscous lava flows and

pyroclastic eruptions• Common in early phase of eruptive sequence before

larger eruptions (‘clearing throat’)


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


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


Vesuvius, 79 CE


Figure 8.27

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

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 8.30

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


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 8.31

Figure 8.32

• 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. Mazama

• Major eruptive sequence of pyroclastic flows and Plinian columns emitted ash layer recognizable across North America


Figure 8.33

Crater Lake (Mount Mazama), Oregon


Figure 8.34

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


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 8.35

– 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 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

• 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


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

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

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

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

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