Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Spring 2011
Volcanism
Volcanism is a Planetary Thermal regulatory Mechanism
• Interior temperature is much higher than T at surface
– Heat left over from planetary accretion
– Radioactive decay of long lived isotopes (40K, 235U, 238U, 232Th) gives off heat
• Earth and all planets lose heat to their external environment
• Upward movement of magmas is a very effective mechanism for moving heat
toward the surface
Distribution of Active Volcanoes on Earth
see Simkin, et al., 2006, This Dynamic Planet, USGS, Smithsonian
Institution (interactive map)
http://baird.si.edu/minsci/tdpmap/viewer.htm
Oceanic Volcanism
• Mid-ocean ridge basalt (MORB) volcanism
– More or less continuous volcanism along the mid-ocean ridges
– Mainly quiescent because of low viscosity and low gas content of low-K
tholeiite
– Occasionally above sea level, as in Iceland
• Island arc volcanism
– Caused by ocean-ocean subduction
– Some high-Al basalts, but
– Mostly andesites
– Also some dacites and rhyolites
– Higher in SiO2 and viscosity, and more explosive than MORBs
– Common in the West Pacific
• Oceanic island volcanism
– Related to hot-spots or hot localized plumes of rising mantle
– Hawaii and other oceanic islands
Pacific Ocean Floor Topography
Linear Age/Distance Relation indicates ~constant rate of motion of
Pacific Plate over the Hawaiian Hot Spot
The Hawaiian Islands
Five Volcanoes on the ‘Big Island’ of Hawaii show earliest stages
Classic Shield Volcanoes, 5-10o slope
Stages in the History of a Typical Hawaiian Volcano
1.
2.
3.
4.
5.
6.
7.
Mauna Loa and Kilauea show shield building stage,
subaerial substage (2c)
Kilauea Caldera from the air
Recent eruptions of Kilauea
A’a lava flows
(A’a is blocky, rough, jagged, with a spiny surface)
Pahoehoe Flows
(smooth, billowy, ropy surface)
Maps and Photos of Recent Kilauea Eruptions
1983 – present Pu’u O’o eruptions are most voluminous from the East Rift
of Kilauea in 500 years
Mauna Kea represents the Capping Stage (3)
Review Volcanic History
The Hawaiian Islands
Haleakala Volcano – East Maui
renewed volcanism stage (5) after erosional stage (4)
Rocks at Haleakala Summit (renewed volcanism - Stage 5)
Continental Volcanism
• Continental Arc Volcanism
– Continental Margin Volcanism
– When oceanic crust is subducted beneath continental crust
Continental Margin Volcanism
Stratovolcanoes or Composite Volcanoes
Interlayered Pyroclastics & Lavas
Typical Platy Andesite
Continental Volcanism
• Continental Flood Basalts
– Tholeiites
– Columbia River Basalts (CRBs) - Miocene
– Deccan Flood Basalts (NW India, Bombay) – Cretaceous-Eocene
• Continental Rift Basalts (East African Rift)
– Alkali Basalts
Columbia River Basalts
Distribution of Active Volcanoes on Earth
Eruptive Styles: Plinian
• Large explosive events that form enormous dark columns of tephra and gas high
into the stratosphere, described by Pliny the younger in relation to the disastrous
79 A.D. eruption of Vesuvius
Eruptive Styles: Phreatic Eruptions
• Phreatic eruptions are steam-driven explosions that occur when water beneath
the ground or on the surface is heated by magma, causing water to boil and flash
to steam, thereby generating an explosion of steam, water, ash, blocks and
bombs
Nuée Ardente, ash flow or pyroclastic flow, Peléean eruption
• A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic
gas that rushes down the side of a volcano as fast as 100 km/hr; temperature
within the flow may be greater than 500C
Eruptive Styles: Strombolian
• Intermittent explosion or fountaining of basaltic lava from a single vent or crater,
produces cinder cones like Stromboli in the Mediterranean (lower photo)
Controls of Eruption
• Composition and viscosity
– SiO2 and to a lesser extent Al2O3, control viscosity of magmas, because they
form polymers in magma
– basalt viscosity at eruption T – 102-104 pascal-sec.
– andesite viscosity at eruption T – 104-106 pa-s
– rhyolite viscosity at eruption T ~ 1010 pa-s (up to one-hundred million x
higher than basalt)
• Dissolved magmatic gases
– H2O>>>CO2>>F2,Cl2, N2, SO2 (or H2S)
– Solubility of gases decreases with decreasing Pressure
– Boiling (formation of bubbles) caused by magma rise and crystallization
Boiling of Magma
• Boiling is separation of gas from a liquid
• Evolution of gas produces bubbles which rapidly expand and rise to the top of
the chamber
• Volume expansion of bubbles fragments the surrounding liquid into tiny
fragments and clasts, and provides the driving force for eruption
• Rapid and continuous magma ascent – Plinian eruption
• Continuing gas evolution and magma fragmentation at low to moderate ascent
rates – Cinder cones
• Stagnation and gradual outgassing of magma below a vent plug – Strombolian
eruption