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Chapter 4
Volcanism and Extrusive Rocks
Index
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Picture on pg. 77
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Definitions
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Magma – Molten rock, usually mostly silica.
Lava – Magma on the earth’s surface.
Pyroclasts – Fragment of rock formed by volcanic
explosion.
Extrusive Rock – Any igneous rock that forms at the
earth’s
surface, whether it solidifies
directly from a
lava flow or is pyroclastic.
Volcanism – Volcanic activity, including the eruption of
lava,
rock fragments, and gas explosions.
Volcano – A hill or mountain constructed by the extrusion
of
lava or rock fragments from a vent.
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Effects on Humans
The Growth of Islands
Geothermal Energy
In some other areas of geologically recent volcanic activity, underground
heat generated by igneous activity is harnessed for human needs.
Effect on Climate
Occasionally, a volcano will spew large amounts of fine, volcanic dust and
gas into the high atmosphere. Winds can keep fine particles suspended
over the earth for years.
Volcanic Catastrophes
Usually hot ashes, tsunamis, lava flow, flying debris, and other mass from
the volcano makes a mess for us humans. (Fig. 4.2)
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Physical Characteristics of
Lava
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What determines the degree of violence associated with volcanic
activity? Whether eruptions are very explosive or relatively “quiet” is
largely determined by two factors: (1) the amount of gas in the lava or
magma and (2) the ease or difficulty with which the gas can escape to
the atmosphere.
 The viscosity, or resistance to flow, of a lava determines how easily
the gas escapes.
 The two most important factors that influence viscosity are (1) the
temperature of the lava relative to the cooler temperature at which it
solidifies and (2) the silica (SiO2) content of the lava. (A third factor is
gas dissolved in magma – the greater the dissolved gas content, the
more fluid.)
Gas
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From active volcanoes we have learned that most of the
gas released during eruptions is water vapor, which
condenses as steam.
 A pyroclastic flow is a mixture of gas and pyroclastic
debris that is so dense that it hugs the ground as it flows
rapidly into low areas. (Fig. 4.6)
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Extrusive Rocks
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Most extrusive rocks are named and identified on
the basis of their composition and texture.
However, some are named exclusively on texture
(e.g., pumice).
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Composition
 Textures
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Composition
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The amount of silica in a lava largely controls not only the
viscosity of lava and the violence of eruptions but also which
particular rock is formed.
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Rhyolite, a silicic rock, is usually cream-colored, tan, or pink;
it is made up mostly of feldspar but always includes some
quartz.
A basalt has a relatively low amount (~50% by weight) of
SiO2. (Fig. 4.10)
Andesite, which crystallizes from an intermediate lava, can
be recognized by its moderately gray or green color. (Fig.
4.9)
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Textures
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Texture refers to a rock’s appearance with respect to the
size, shape, and arrangement of its grains or other
constituents.
Two critical factors determine grain size during the
solidification of igneous rocks: rate of cooling and viscosity. If
lava cools rapidly, the atoms have time to move only a short
distance; they bond with nearby atoms, forming only small
crystals. With extremely rapid or almost instantaneous
cooling, individual atoms in the lava are ‘frozen’ in place,
forming glass rather than crystals. (Fig. 4.8)
Porphyritic Textures
Fragmental Textures
Textures Due to Trapped Gas
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Porphyritic Textures
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Porphyritic rock – An igneous rock in which large crystals
are
enclosed in a matrix (or ground mass)
of
much finer-grained minerals or obsidian.
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Phenocrysts – Any of the large crystals in porphyritic
igneous
rock.
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Textures Due to Trapped Gas
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A magma deep underground is under high pressure,
generally high enough to keep all its gases in a dissolved
state. On eruption, the pressure is suddenly released and
the gases come out of solution.
 When a lava solidifies while gas is bubbling through it,
holes are trapped in the rock, creating a distinctive
vesicular texture. Vesicles are cavities in extrusive rock
resulting from gas bubbles that were in lava.
 In more viscous lavas, where the gas cannot escape as
easily, the lava is churned into a froth. When cooled
quickly, it forms pumice (Fig. 4.11), a frothy glass with so
much void space that it floats in water.
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Fragmental Textures
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Pyroclasts, the fragments formed by volcanic explosion, can
be almost any size. Dust and ash are the finest particles;
cinders are about the size of sand grains; bombs and blocks
are large pyroclasts. When solid rock has been blasted apart
by a volcanic explosion, the pyroclastic fragments are
angular, with no rounded edges or corners and are called
blocks. If lava is ejected into the air, a molten blob becomes
streamlined during flight, solidifies, and falls to the ground as
a bomb, a spindle or lens-shaped pyroclast. (Fig. 4.12)
A tuff is a rock composed of fine-grained pyroclastic
particles.
Fig. 4.13
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Volcano
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Vent – The opening in the earth’s surface through which a
volcanic eruption takes place.
Crater – A basin-like depression over a vent at the summit
of a
volcanic cone.
Flank Eruption – An eruption in which lava erupts out of a
vent
on the side of a volcano.
Caldera – A volcanic depression much larger than the
original
crater.
Fig. 4.14
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Types of Volcanoes
Shield Volcanoes
Cinder Cones
Composite Volcanoes
Volcanic Domes
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Shield Volcanoes
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Shield volcanoes are broad, gently sloping cones
constructed of solidified lava flows. During eruptions, the
lava spreads widely and thinly due to its low viscosity.
Because the lava flows from a central vent, without
building up much near the vent, the slopes are usually
between 2o and 10o from the horizontal, producing a
volcano in the shape of a flattened dome or ‘shield’. (Fig.
4.15)
 A minor feature called a spatter cone, (Fig. 4.18) a small
steep-sided cone built from lava sputtering out of a vent
will occasionally develop a solidifying lava flow.
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Cinder Cones
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A cinder cone is a volcano constructed of pyroclasts
ejected from a central vent (Fig. 4.19). In contrast to the
gentle slopes of shield volcanoes, cinder cones commonly
have slopes of about 30o.
 Cinder cones are a product of concentration of gas in
magma rather than of a particular chemical composition of
magma. Most cinder cones are associated with mafic or
inter-mediate lava. Silicic cinder cones, which are made of
fragments of pumice, are also known as pumice cones.
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Composite Volcanoes
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A composite volcano is one constructed of
alternating layers of pyroclastics and rock solidified
from lava flows. (Fig. 4.20)
 Composite volcanoes are built over long spans of
time. Eruption is intermittent, with hundreds or
thousands of years of inactivity separating a few
years of intense activity.
 Nearly all the better known volcanoes of the world
are composite volcanoes.
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Volcanic Domes
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Volcanic domes are steep-sided, dome- or
spine-shaped masses of volcanic rock formed
from viscous lava that solidifies in or
immediately above a volcano vent. (Fig. 4.25)
 Most of the viscous lavas that form volcanic
domes are high in silica.
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Lava Floods
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Not all extrusive rocks are associated with
volcanoes. Lava that is very non-viscous and
flows almost as easily as water, does not
build a cone around its vents. Such lava is, of
course, mafic (low in silica).
 Plateau basalts – Layers of basalt flows that
have
built up to great
thicknesses.
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Beginning
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Fig. 4.2
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Pg. 81
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Fig. 4.6
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Pg. 84
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Fig. 4.10
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Pg. 86
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Fig. 4.9
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Pg. 86
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Fig. 4.8
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Fig. 4.11
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Fig. 4.12
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Pg. 87
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Fig. 4.13
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Pg. 87
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Fig. 4.14
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Pg. 88
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Fig. 4.15
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Fig. 4.18
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Pg. 89
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Fig. 4.19
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Fig. 4.20
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Fig. 4.25
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Pg. 95
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Fig. 4.28
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Pg. 97
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Fig. 4.29
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Pg. 97
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