Volcanoes

Volanic rocks make up about 80% of the Earth's surface (if we include the oceans along with the continents). When magma erupts at the Earth's surface can be an awesome spectacle. Products of volcanic eruptions can even change the Earth's climate.

But volcanoes not only contain magma, but also gas (H2O, CO2, SO2). Eruptions of gas very important. Annually the mass of gas erupted is more than magma. Volcanic gas vents are calles fumaroles.

Present distribution of volcanoes

[Map, also don't forget mid ocean ridges]

Physical properties of magma

Viscosity of magma. Gas content of magma

basalt: andesite rhyolite

low viscosity high viscosity

low gas content high gas content

Volcanic deposits
(flows, pyroclastic deposits)

Flows: pahoehoe(fluid basaltic lava), aa (viscous magmas).

Pyroclastic deposits: ash --> tuff;

also: lapilli, bombs, blocks --> volcanic breccia

pyroclastic flows (vaporized magma +gas at high T) --> welded tuff

Lahars: mudflows of ash that occur by mixing of ash with rain, or melting snow

Debris avalanche deposits - occur when a volcano collapses under its own weight - driven by gravity

Types of volcanic edifice and associated features

central volcanoes, flood basalts

Central volcanoes

Eruption through a central vent or crater on top of volcano

Shield volcano: broad, wide cone. Asssociated with fluid flows Hawaii (big island) example of a very large shield volcano

Volcanic domes (=lava dome). Associated with very viscous flows- v. little lateral spreading. often plug the vent inside the crater (Mt. St Helens, Soufriere Hills in Montserrat), but may form large isolated stuctures (Sierra de la Primavera, nr Guadalajara, Mexico).

Composite (strato-) volcano associated with more viscous and gas rich magmas.

pyroclastic cones (a.k.a. cinder cones)- mound of ash around central vent. eg. Cerro Negro, Nicaragua

Minor features: Hot springs, geysers and fumaroles .

Flood basalts

Central volcanoes not only type of edifice that can form. Often occur through fissures - fissure eruptions

Vast outpourings of fluid magma covering hundreds or thousands of km2!! Deccan Traps, India Columbia river plateau, WA, OR (50,000 sq miles 100 cu miles).

Submarine plateaus

[Related to when aesthenospheric hot spot first reaches the lithosphere]

Eruptions and eruption types
[not in textbook]

Eruptions caused by

1) movement of magma to the surface

2) for gas rich magma, gas un-dissolves and bubbles form which cause rapid ejection of gas rich lava - pyroclastic flows, "soda pop" mechanism.

3) when groundwater in volcanic edifice is heated by hot rock. Water expands in an exposive eruption. Phreatic eruption- explosive erution. Does not necessarily involve new magma rising in volcano.

4) when gas charges magmas rapidly inject into lithosphere and crust to form diatremes - produce cinder cones etc Ship Rock, New Mexico.]

Eruption types - in order of increasing energy/violence

Hawaiian - Effusive erutions of low gas magma. Rapidly moving lava flows. Lava lakes, lava fountains.

Strombolian - small eruptions, but often at frequent, regular intervals examples: Stromboli, Italy; Yasur, Tanna island, Vanauatu

Vulcanian - moderately explosive eruption after period of inactivity. Examples:
o Paracutín, Mexico
o Mt Pelee, Martinique, 1902. Pyroclastic flow eruption killed 30,000 people. Not super powerful , but attests to the danger of pyroclastic flows

Plinean - very violent eruption after long period of being dormant. Often associate by pyroclastic (ash) flows. Examples:

o Vesuvius, Italy AD 79 nr Pompeii & Herulaneum
o 1980 Mt St Helen's (123 years or dormancy)
o Mt Minatubo, Phillipines, 199?

Ultra-Plinean - Same style as above, but most violent eruption (previously known as Krakatoan). Examples:

o Santorini, Agean Sea ~4000 BC; deleted Minoan civilization; memory preserved in the Mosaic legend?
o Tambora, 1815 - the year without a summer.
o Kratatau, Indonesia, 1883; sound of exposion heard 2000 miles away in Australia, 30,000 killed by tidal waves

Caldera collapse variation of many Plinean and ultra-Plinean eruptions. Magma chamber may empty so fast that the top of the volcano drops into the magma chamber like putting a cork in a bottle.

o Accompanied by pyroclastic flows
o Later phases of the Santorini eruption seemed Agean sea of this kind
o Later Caldera may be flooded ("Crater Lake" in Oregon) or covered in sediments (Serengeti Plain)

Volcanic Hazards

explosions

ash fall

suffocating gases ( Cameroons , 1980's)

mud flows (Nevado del Ruiz, Colombia - town of Almero wiped out)

tidal waves (Krakatoa)

But benefits: rich soil, geothermal energy (Japan, Nicaragua, Mexico, Italy, Iceland, New Zealand and . . . . California )

Prediction of eruptions

Examining geology can show history of volcano, types of eruptions etc i.e. its "personality" can be determined.

Examination of ash erupted early in eruption cycle: old (re-worked) or new (juvenile)

Seismometers

Tiltmeters (GPS and geodetic satellite variations)

Fumarole temperatures

Thermal imaging satellites

Volcanism on other worlds
if time

Slicate/gas volcanoes on Venus, Moon (flood basalts or Mare), Mars. Sulfur volcanoes on Io [slide].