volcanoes and igneous features. volcanic eruptions factors that determine the violence of an...
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Volcanoes and Igneous Features
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Volcanic eruptions
Factors that determine the violence of an eruption • Composition of the magma • Temperature of the magma• Gases in the magma
Viscosity of magma • Measure of material’s resistance to flow
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Viscosity of magma
• Factors affecting viscosity– Dissolved gases (volatiles)
• Provide the force to extrude lava
–Viscous magma produces a more violent eruption
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Lava Types
• Types of lava – Pahoehoe lava (resembles braids in ropes) – Aa lava (rough, jagged blocks)
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A pahoehoe lava flow
Figure 9.7 B
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A typical aa flow
Figure 9.7 A
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Pyroclastic materials (Fire fragments)
• Ash and dust• Pumice – from “frothy” lava • Cinders (scoria) • Bombs – larger ejected as hot lava
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Volcanic bombs on Kilauea volcano in Hawaii
Figure 9.9
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Volcanoes
Conduit, or pipe, carries gas-rich magma to the surface – Vent, the surface opening (connected to the
magma chamber via a pipe)
• Crater– Steep-walled depression at the summit – Caldera (a summit depression greater than 1 km
diameter)
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Shield volcano• Broad, slightly domed • Primarily made of basaltic (fluid) lava • Generally large size
– e.g., Mauna Loa in Hawaii
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Shield volcano
Figure 9.12
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Cinder cone
• Built from ejected lava fragments • Steep slope angle • Rather small size
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A cinder cone near Flagstaff, Arizona
Figure 9.15
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•Composite cone (or stratovolcano)
• Most are adjacent to the Pacific Ocean (e.g., Mt. Rainier)
• Large size • Interbedded lavas and pyroclastics • Most violent type of activity
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Composite volcano
Figure 9.10
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Mount St. Helens – a typical composite volcano
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Mount St. Helens following the 1980 eruption
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A size comparison of the three types of volcanoes
Figure 9.13
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Other volcanic landforms
Calderas • Steep-walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds 1 kilometer in diameter
Fissure eruptions and lava plateaus • Fluid basaltic lava extruded from crustal
fractures called fissures • e.g., Columbia Plateau
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Crater Lake, Oregon, is a
good example of a caldera
Figure 9.21
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The Columbia River basalts
Figure 9.22
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Volcanic Necks
• Volcanic necks (e.g., Ship Rock, New Mexico) are resistant pipes/vents left standing after erosion has removed the volcanic cone
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Formation of a volcanic neck
Figure 9.24
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Intrusive igneous structures exposed by erosion
Figure 9.25 B
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A sill in the Salt River Canyon, Arizona
Figure 9.27
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A batholith exposed by erosion
Figure 9.25 C
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Locations of some of Earth’s major volcanoes
Figure 9.33