2. Volcanoes · • volcanoes under water. Composite = stratovolcano, stratocone • well-defined...
Transcript of 2. Volcanoes · • volcanoes under water. Composite = stratovolcano, stratocone • well-defined...
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2. Volcanoes
Stromboli, 1969
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Volcano….. structure where magma & gases are erupted;….. one or more vents;….. pipe or fissure vents;….. includes the erupted deposits around the vent(s).
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Volcano• Active: in eruption, or has erupted recently• Dormant: no recent eruption, but expected to erupt again
in the future• Extinct: no further eruptions expected
Volcanoes may be active for periods ranging from days todecades.
Dormant periods may last for decades, centuries orthousands of years
Repose Period….…….time gap between eruptions
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Monogenetic• conduit used once• continuous eruption• usually small size and simple
Polygenetic• conduit(s) repeatedly used, sealed, used again• multiple vents• usually large and complex
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Volcano Types
• composite volcano (andesite)• shield volcano (basalt)• scoria cone (basalt)• tuff ring and maar (basalt)• lava dome (andesite, dacite, rhyolite)• caldera (andesite, dacite, rhyolite)• volcanoes under water
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Composite= stratovolcano, stratocone
• well-defined cone or overlapping “composite” cones• central vent ± satellite vents• cone made of lavas and breccia• ring plain made of clastic deposits• steep slopes; sector collapse and volcanic landslidese.g. Parinacota, Chile; Ruapehu, NZ; Merapi, Indonesia
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Shield
• gently sloping “shield” profile• radial rift zones, summit crater or caldera• summit vent + rift zone vents• shield made of extensive, thin lavase.g. Kohala & Mauna Kea, Hawaii, USA
20 - 30 km
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Scoria Cone= cinder cone
• usually small; single vent• monogenetic• cone made of outward-dipping beds of loose scoria• mainly pyroclastic fall depositse.g. Mt Capulin, New Mexico,USA
Mt Elephant, Victoria
2.5 km
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Tuff Ring & Maar
• small; usually single vent• monogenetic• vent surrounded by low ring of ash beds• mainly pyroclastic surge depositse.g. Ubehebe, Death Valley, California, USA
< 5 km
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Lava Dome
• usually small• single vent, monogenetic• made of lava & autobreccia• steep margins surrounded by talus, flat tope.g. Novarupta, Alaska, USA; Chao, Chile; Tarawera, NZ
< 5 km
≤ 300 m
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Caldera
• big, multiple-vent volcanic centres• very large volumes erupted explosively
∅ widespread ignimbrite & fall deposits• low topographic relief; capture drainage
∅ lakes• collapse of centre along ring faults• develop over 103 to 105 yearse.g. Rotorua caldera, NZ; Valles caldera, New Mexico, USA
10-20km
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Seamounts
• commonly located on faults;may be elongate in plan
• build up from sea floor• mainly lava and breccia
• may emerge……... island volcano
e.g. Maui, Hawaii, USAWhite Island, NZ
Plan view
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Eruptions in Watere.g. volcanoes on the sea floor, volcanoes in lakes
Lavas– quenched to glass– broken; “hyaloclastite”
– pillow lava
Explosive eruptions– OK in shallow water– deep water: gas cannot expand (P too high)