Our Hazardous Environment GEOG 1110 Dr. Thieme Lecture 8: Volcanoes and Volcanic Hazards

Volcanoes volcano - a conical or dome-shaped landform created when lava and/or tephra accumulates on the earth’s surface lava - molten material on the surface magma - molten material below the surface tephra - igneous material, ranging in size from dust to boulders, that is explosively ejected from a volcano

Distribution of Volcanoes Most frequently associated with subduction zones Also common along mid-ocean ridges Some at hot spots, but much more rare

magma chamber - an accumulation of molten rock beneath the Earth’s surface vent - the opening magma uses to move from the magma chamber to the Earth’s surface crater - the opening through which lava and tephra issues caldera - a very large crater created by explosion or collapse cone - a build-up of lava and/or tephra around a vent, creating a hill or mountain

Types of Volcanoes There are 3 basic types of volcanoes 1) Shield volcanoes - largest 2) Stratovolcanoes (also called composite cones) 3) Cinder cones - smallest

Shield Volcanoes Created by mafic magma Thin and flow easily in comparison to felsic magmas (less “viscous”) Usually hold little gas in comparison to felsic magmas Eruptions of volcanoes consisting of mafic magmas are relatively quiet because of the thin, fluid nature and the absence of gas under pressure Mafic lava flows tend to travel long distances across the surface, and form broad-based volcanoes with gently sloping sides Fissure eruptions on the sides of the volcanoes are common, called flank eruptions Calderas often develop

Shield Volcanoes Composed of multiple stacked lava flows Famous examples include Hawaii, Iceland, and the Azores. Hawaii is formed over a hot spot, Iceland and the Azores over the Mid-Atlantic Ridge hot spot - an area where a plume of magma is welling up from deep within the asthenosphere

’a ’a Pahoehoe

Stratovolcanoes Created by intermediate to felsic magmas thick and gummy magma/lava magma/lava frequently high in gas content eruptions can be very explosive cones are composed of interlayered lava and tephra tall, steep-sided cones

Stratovolcanoes also called composite volcanoes or composite cones stratovolcanoes are known for their scenic beauty famous examples include: Mount Rainier and Mount St. Helens in Washington Mount Shasta in California Mount Fujiyama in Japan volcanoes that most concern scientists explosive eruptions glowing avalanches, scorching hot clouds of ash and debris glowing avalanche from Mount Pelée on the Caribbean island of Martinique killed all but 2 of the 30,000 inhabitants of the city of St. Pierre in 1902

Composite Cone, Mount Shasta, northern California

Mount Lassen east of Redding, CA erupted in June, 1914

Formation of Crater Lake

Figure 4.13

Long Valley caldera eruption 700,000 years ago tephra used to date other rocks and sediments ("tephro-chronology")

“Peléean” eruptions Silica-rich (“felsic”) magma form domes glowing avalanches ash flows

Cinder Cones made up entirely of tephra form when frothy mafic magma is ejected from a vent under high pressure rarely get more than a few hundred meters high, and they are the most easily eroded volcanic cone

Volcanic Hazards gases CO2 SO2 H2S F2 lava flows bombs, ash pyroclastic flows landslides mudflows (“lahars”)

Molten Sulfur and Sulfur Gases in an Indonesian volcano FIGURE 5.30c

Lake Nyos, Cameroon, West Africa

Carbon Monoxide (CO) is poisonous