Volcanoes and Volcanic Deposits 2. IN THIS LECTURE Shield Volcanoes Stratovolcanoes Other Types of Volcanic Centres Flood Basalt Provinces Maar and Tuff Rings Intermediate-silicic centres Rhyolitic volcanoes Submarine spreading ridges and seamounts Intra- or subglacial volcanoes.
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IN THIS LECTURE
Snow-covered Moku`aweoweo Caldera atop Mauna Loa shield volcano (Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep, and is estimated to have collapsed between 600-750 years ago. Several pit craters along the upper southwest rift zone of Mauna Loa (lower right) also formed by collapse of the ground.
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Three-deminsional Space Shuttle Image of the Alcedo Shield Volcano, Galapagos -- The near circular caldera of the Alcedo shield volcano on the big island of Isabela is a feature common to many of the Galapagos shield volcanoes. The image, taken by the Space Shuttle Endeavor, covers an area of about 75 km by 60 km. The oblique view was constructed by overlaying a Spaceborne Radar Image on a digital elevation map. The vertical scale is exaggerated by a factor of 1.87.
Mount Mageik volcano viewed from the Valley of Ten Thousand Smokes, Katmai National Park and Preserve, Alaska. Mageik's broad summit consists of at least four separate structures built above different vents.
Mount St. Helens is the youngest stratovolcano in the Cascades and the most active. Geologists have identified at least 35 layers of tephra erupted by the volcano in the past 3,500 years. This picture is prior to the 1980 eruption
Distinguishing characteristics of maar-type volcanoes
Eruption column generated by phreatic and magmatic explosions rises from the larger east maar.
Aerial view toward N of Ukinrek Maars, Alaska; Lake Becharof at top of photo. Water partially fills the eastern maar and completely covers a lava dome that was erupted in the 100-m deep crater during a 10-day eruption in 1977. Maar is about 300 m in diameter.
Felsic magmas will either (1) erupt explosively to produce extensive deposits of tephra, or (2) nonexplosively to produce degassed, viscous lava (domes, coulees, or obsidian flows) which advance only short distances from their vents. There has been a significant amount of controversy, therefore, over rare rhyolite lavas that appear to occur as large-volume flows (10-100 cubic kilometers).
Most such flows occur near continental hotspots. The best known examples are those associated with (1) the Yellowstone hotspot track near the Idaho-Oregon border, and (2) the Ethiopian hotspot in northeastern Africa. These large-volume felsic volcanic rocks have outcrop, hand specimen, and thin section characteristics typical of lava flows. However, many volcanologists suspect that they are not lava flows at all, but rather rheomorphic ignimbrites. These are densely welded pyroclastic flows of pumice and ash, which were thick and hot enough to flow downslope and obliterate primary pyroclastic structures. They suggest that the original pumice and ash fragments have been streaked out like toffee strands so that the pyroclastic nature of the flow becomes unrecognizable.
Is this the case with the large rhyolite lavas of the Lebombo Monocline?