GEOL105 Chapter Notes - Chapter 5: Volcanic Crater, Scoria, Lake Nyos
31 May 2018
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CH 5: Volcanoes
5.1 Introduction to Volcanism
● Volcanism is typically related to plate tectonics
● Most volcanoes are located near active plate boundaries
● Approx ⅔ of all active volcanoes above sea level are located within the Pacific “Ring of
Fire”
● Spreading or sinking plates at plate boundaries interact with other earth materials to
produce molten rock
○ Magma: within earth
○ Lava: when it erupts onto earth’s surface
● Volcanic vent: circular or elongated opening through which lava and pyroclastic debris
are erupted
○ Fissures: elongated cracks
How and Where Magma Forms
● Most magma comes from the asthenosphere
● Three principle processes that generate magma
○ Decompression melting: process by which solid rock melts due to a decrease
in pressure that is caused by upwelling of hot rocks within the interior of the
planet
○ Addition of volatiles
■ Volatiles: chemical compounds that evaporate easily and exist in a
gaseous state at the earth’s surface
○ Addition of heat to rocks
Magma Properties
● Magma is composed of melted silicate rocks and dissolved gases
○ Silica is the primary constituent of magma
● Three major types of magma: basaltic (low, mafic), andesitic (intermediate), and rhyolitic
(felsic)
○ Vary in silica content
● Magma chambers: a large pool of molten rock that accumulates in the subsurface and
is the primary source of lava, and volcanic gases and debris that are extruded to the
surface during an eruption
● Magma evolution: refers to the increase in silica content of a magma as it ascends
toward the surface. The change results from crustal assimilation and chemical
processes within the magma as it rises.
● Viscosity: resistance to flow in fluids
○ Silica rich felsic magma-high velocity, do not flow easily
○ Low silica mafic magma-low velocity, flow readily
○ Strongly influences the mobility of the magma under the surface and the velocity
and form it takes if it reaches the surface as a lava flow
○ Rhyolitic lava flows have high viscosity and move slowly
○ Basaltic lava flows can move rapidly
○ The viscosity is often correlated to the volatile content of the magma
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● Volatile content
○ Content increases with increasing silica content
○ Volcanoes with andesitic-to-rhyolitic magma are more prone to explosive
eruptions than volcanoes with basaltic lava flows
○ Effusive eruptions: eruption of volatile-poor basaltic magma, typified by passive
outpouring of low-viscosity lava onto earth’s surface
○ The rapid formation and expansion of bubbles within volatile rich, high viscosity
magma causes explosive eruptions
○ Pyroclastic debris: volcanic material that are explosively ejected from a volcano
5.2 Volcano Types, Formation, and Eruptive Behavior
● Volcanic explosivity index (VEI): a relative scale by which different volcanic eruptions
can be compared based on quantitative and qualitative measurement of explosivity,
including the plume height, volume of ejected material, and frequency of eruption
○ Scale is logarithmic with a tenfold increase in the volume of the ejected material
● Table 5.1 and 5.2
● Stratovolcanoes: steep-sided volcanic cone produced by alternating layers of
pyroclastic debris and lava flows (also known as a composite cone)
○ Composition: andesite
○ Volatile content: high
○ Shape: cone shaped with steep sides; built of lava flows and pyroclastic deposits
○ Eruption type: combo of lava flows and pyroclastic activity
○ Examples: Mount St. Helens, Washington
○ Plinian-type eruptions: explosive eruptions of stratovolcanoes
● Lava Dome: relatively small volcano type composed of highly viscous magma, usually
rhyolitic in composition, that is often restricted to the vent region. Often found plugging
the vent within the volcanic crater of a recently erupted stratovolcano
○ Composition: andesite to rhyolite
○ Volatile content: low to moderate
○ Shape: dome shaped; steep sided
○ Eruption type: mostly effusive with lavas piling up near the vent, but can be
explosive
○ Examples: Mount Lassen, CA
○ Pelean-type eruptions, VEI 3-4
● Shield Volcano
○ Composition: basalt
○ Volatile content: low
○ Shape: gentle arch or shield shape, with shallow slopes; built up of many lava
flows
○ Eruption type: far-traveling lava flows
○ Examples: Mauna Loa, Hawaii
○ Caldera: formed by the inward collapse of the volcano top due to removal of
magma from the subsurface
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Volcanism is typically related to plate tectonics. Most volcanoes are located near active plate boundaries. Approx of all active volcanoes above sea level are located within the pacific ring of. Spreading or sinking plates at plate boundaries interact with other earth materials to produce molten rock. Lava: when it erupts onto earth"s surface. Volcanic vent: circular or elongated opening through which lava and pyroclastic debris are erupted. Decompression melting: process by which solid rock melts due to a decrease in pressure that is caused by upwelling of hot rocks within the interior of the planet. Volatiles: chemical compounds that evaporate easily and exist in a gaseous state at the earth"s surface. Magma is composed of melted silicate rocks and dissolved gases. Silica is the primary constituent of magma. Three major types of magma: basaltic (low, mafic), andesitic (intermediate), and rhyolitic (felsic)
