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1

magma

molten rock that usually contains some crystals and varying amounts of dissolved gas

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lava

erupted magma

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viscosity

mobility
The more viscous = greater resistance to flow
influenced by: temperature, composition, and dissolved gases

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Temperature

As lava cools it congeals and the viscosity increases

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Composition

chemical composition of the magma

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Mafic

Basaltic magma that contains about 50 percent silica

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intermediate

Andesitic magma contains about 60 percent silica and form rocks such as andesite and diorite

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Felsic

Rhyolitic magma contain more than 70 percent silica and produce rocks such as granite and rhyolite

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The more silica in the magma....

the greater the viscosity

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Dissolved gases

mainly dissolved water which increases the fluidity
gases also give magma their explosive characteristic

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Quiescent Eruptions

On the ocean floor high temperature basaltic magmas reach Earth's surface, where they produce highly fluid lavas.
In continental settings, the density of the crustal rock is less than that of the ascending material causing the magma to pond at the crust-mantle boundary

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Explosive Eruptions

all magmas contain some water vapor and other gasses that are kept in solution by the immense pressure of the over lying rock. As the magma rises, a reduction in pressure occurs and the dissolved gases begin to separate from the melt forming tiny bubbles
ex: opening a soda can

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eruption columns

highly viscous magmas expel particles of fragmented lave and gases at nearly supersonic speeds that evolve into bouyant plumes

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lava flows

the vast majority of lava on Earth (more than 90%) is basaltic (mafic) andesitic (intermediate) lava (less than 10%) rhyolitic (felsic) less than 1%

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hot basaltic lavas

usually very fluid, flow in thin broad sheets or stream-like ribbons and move very fast

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Aa and Panhoehoe flows

aa: rough jagged surfaces of rough jagged blocks with dangerously sharp edges and spiny projections
pahoehoe: smooth surfaces that sometimes resemble twisted braids of ropes, hotter and more fluid and can change into aa flows in lava tubes

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lava tubes

cave-like tubes that were previously conduits for carrying lava from an active vent to the flow's leading edge

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pillow lava

occurs over and over as molten basalt is extruded like toothpaste from a tightly squeezed tube. stacked one atop the other

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pyroclastic materials

pulverized rock and lava fragments from the vent also known as tephra

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pumice

low density vesicular rock that forms during explosive eruptions of viscous magma having an andesitic to rhyolitic composition

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Anatomy of a volcano

conduit:localized path of gas-rich magma that moves up through a fissure into circular chamber
vent: surface opening
volcanic cone: cone-shaped wall structure created by successive eruptions of lava and/or pyrocastic material
crater: located at the summit of most volcanic cones is a somewhat funnel-shaped depression
parasitic cones: flank eruption secondary vent

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shield volcanoes

broad slightly dome-shaped
covers large areas
produced by mild eruptions of large volumes of basaltic lava
Most begin on the seafloor as seamounts; only a few grow large enough to form a volcanic island
Ex: Hawaiian Islands, the Canary Islands, the Galapagos, and Easter Island
Mauna Loa is the largest shield volcano on Earth

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Cinder Cone volcanoes

built from ejected lava fragments that begin to harden in flight to produce the vesicular rock scoria
frequently occur in groups
Lava fields
Paricutin

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Composite volcanoes

most are located near the Pacific Ocean (ring of fire) contain alternating layers of explosively erupted cinders and ash imbedded with lava flows.
ex: mt. st Helens and mt. Edna

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pyroclastic flows

hot gases infused with incandescent ash and larger lava fragments speeds can exceed 100 km per hour
two parts: a low density cloud of hot expanding gases and a ground-hugging portion that is often composed of pumice and other vesicular pyroclastic material
Driven by gravity

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lahar

mudflows on active and inactive cones
occurs when volcanic debris becomes saturated with water amd rapidly moves down steep volcanic slopes

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Blocks from pyroclastic materials

hardened or cooled lava

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bombs pyroclastic materils

ejected as hot lava

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pumice

light gray or pink rock from frothy andesitic and rhyolitic lava

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Scoria

reddish-brown porous rock from frothy basaltic and andesitic lava