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The word volcano, from the Roman god of fire, has
two meanings:
1) The opening in the ground out of which hot molten
rock (lava or magma) and/or volcanic gas issues from
the earth's interior onto the surface.
2) The hill or mountain built up around such an
opening by the accumulation of cooled magma.

Volcanic openings may be:
1) Circular- Lava that is poured or thrown out
of circular pipes (vents) forms tall,
steep-sided mountains (like Mt. Agua).
These are common above subduction zones.
Circular openings vary in size from manhole
covers to those that are larger than the
LBS Ground of IIT Roorkee.

? 2) Long, skinny, snakelike cracks called rifts or fissures.
Lava that is poured or thrown out of long,
skinny cracks (rifts) forms plateus
(like the Columbia or Snake River) or low
angled hills or mountains (like Mona Loa).

? These are common along mid-ocean ridges and hot spots.
? Crack-like openings range in size from pencil width to those that
are far larger than the width of a football field.

Overall there are three major kinds of lava:
? Mafic- called basalt. Composed of lots of iron, magnesium, and calcium
along with aluminum, silica and oxygen to make minerals like olivine,
pyroxene, and calcic plagioclase).
? Felsic- called dacite or rhyolite. Composed of lots of potassium, sodium,
aluminum, silica, and oxygen to form such minerals as quartz, potassium
feldspar, sodic-rich plagioclase.
? Mixed or intermediate- called andesite. This lava is in between the other
two, containing some iron and magnesium-rich minerals as well as
sodium and potassium-rich minerals.

BASALTIC LAVA
? Basaltic lava flows erupt primarily from shield volcanoes, fissure
systems, scoria cones, and spatter cones. These fluid lava flows
can be subdivided into two end-member structural types, based
primarily on the nature of lava flow surfaces:

? Pahoehoe Lava -- Surfaces are smooth, billowy, or ropy. It is
typically the first to erupt from a vent.

? As the smooth lava surface cools to turns to a dark gray color and
becomes less fluid and more viscous, behaving more like a plastic
substance than a truly liquid substance.
? As lava continues to flow underneath this plastic skin, the surface
can bunch up or wrinkle into a form that resembles coiled rope.
Such a surface is called ropy pahoehoe.
? Similarly solidified basalt flows can also display shelly surfaces
called Shelly pahoehoe.

A'a lava -- Surfaces are fragmented, rough,
and spiny, with a "cindery" appearance

? The a'a flows are advancing over older pahoehoe surfaces.
? These are more viscous, and typically thicker, than pahoehoe
lavas.
? Pahoehoe is often converted to a'a as lava advances downslope,
away from the volcano.
? caused by either an increase in flow viscosity, or an increase in the
rate of shear.

Andestic Lava
? andesite generally produces blocky lava
? the surface contains smooth-sided, angular fragments (blocks)

FELSIC LAVA
DACITIC TO RHYOLITIC LAVA
? With increasing silica (SiO2) content and polymerization, the viscosities
of these felsic lavas increases.

Unusual Lava Type
? STRONGLY ALKALINE LAVAS-alkali content,
reflected in their weight percent of
Na2O + K2O

They are typically found in
(1) continental or oceanic intraplate settings, where there is often a lack
of significant tectonic control,
(2) continental rift zones, and
(3) the back-arc setting of subduction zones.

? CARBONATITES - They are defined, when crystalline, by having more
than 50% carbonate (CO3-bearing) minerals, and typically they are
composed of less than 10% SiO2.

? KOMATIITES - komatiites are ultramafic volcanic rocks, having very
low silica contents (~40-45%) and very high MgO contents (~18%).
? A spectauclar identifying
trait of komatiites is their
spinifex texture

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