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Volcanoes and Plutons
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Mount Etna, an active volcano in Sicily, erupts at night
while mounds of molten lava cool in the foreground.
Fig. 8-CO, p.169
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Geologists retreat from a slowly advancing lava flow on the
island of Hawaii.
Fig. 8-1, p.170
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How does magma form?
The asthenosphere (from 100-300 km deep in upper mantle) is a layer
where temperature is high enough and pressure is low enough that 12% is molten.
Three processes melt the asthenosphere to form magma:
-Increasing temperature (least important cause in asthenosphere)
-Decreasing pressure
-Addition of water
Fig. 8-2, p.171
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Pressure-release melting occurs from decreased pressure. Magma
occupies about 10% more volume than the solid rock it melted from.
High pressure results in the dense, orderly arrangement of solid
mineral while low pressure favors the random, less dense arrangement
of molecules in a liquid magma. Therefore, rock can melt easier near
the Earth’s surface (given the right temp) than when it is under great
pressure.
Fig. 8-3, p.171
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Pressurerelease melting
produces
magma beneath
a spreading
center, where
hot
asthenosphere
rises to fill the
gap left by the
two separating
tectonic plates.
Fig. 8-4, p.172
also,
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A wet rock generally melts at a lower
temperature than an identical dry rock. Certain
tectonic processes add water to the hot rock of
the asthenosphere to form magma.
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NOW, on to Environments of Magma
Formation…
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Spreading Centers: hot, plastic asthenosphere oozes upward
when plates separate; pressure drops as it rises, and melting
forms basaltic magma. The magma is of lower density than
surrounding rock, so it rises. Nearly all of Earth’s oceanic
crust is formed at the Mid-Oceanic Ridge system.

Iceland is one
of the few
places on Earth
where
volcanoes of
the MidOceanic Ridge
system rise
above the sea.
Shown is an
eruption of
Mount Surtsey
of Iceland.
Fig. 8-5, p.173
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Mantle Plume: a rising column of hot, plastic rock from the
mantle. The plume rises because it is hotter than the
surrounding mantle, so it is less dense and more buoyant.
As it rises, pressure decreases and just beneath the
lithosphere is melts to form magma, and continues to rise.
Can occur, for example, within a tectonic plate
(Yellowstone) or beneath the sea (Hawaiian Islands).Fig. 8-6, p.173
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Subduction Zone (SZ): huge quantities of magma form here
from decreasing pressure, addition of water and heating.
The asthenosphere is stirred up and rises. Volcanoes and
plutonic rocks are common features at SZ.
Fig. 8-7, p.173

75% of Earth’s active volcanoes lie in the “ring of fire” (a
chain of subduction zones at convergent plate boundaries
that encircles tha Pacific Ocean.
Fig. 8-8, p.174
Basalt and Granite
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Are the most common igneous rocks. Basalt is the main
rock of oceanic crust; granite is the main rock of continental
crust.
Because rock is a mixture of several minerals with different
melting temperatures it does not melt to form magma at the
same time (unlike a pure substance such as ice). This is
called partial melting. Minerals with the highest silica
content melt at the lowest temperatures (so magma is
always richer in silica than the rock that produced it).
Granite contains more silica than basalt and melts at a lower
temp. So, basaltic magma can melt granitic continental
crust. In this way, magmas with varying compositions are
produced.
Origin of the Continents
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The Earth is thought to have melted shortly after its
formation 4.6 billion years ago. Geologists surmise
the first crust was lava with a composition similar to
today’s mantle (peridotite). When and how did the
granitic continents form?
The oldest rocks known have been dated to 3.96
billion years old (metamorphosed granite), and the
oldest mineral known has been dated to 4.2 billion
years old (the mineral zircon that commonly forms
in granite).
Possibly from partial melting during horizontal and
vertical tectonics of the Precambrian crust.
Fig. 8-9, p.176
Fig. 8-9a, p.176
Fig. 8-9b, p.176
Fig. 8-10, p.177
Magma Behavior
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When magma rises cooling tends to solidify magma while
decreasing pressure tends to keep it liquid. What really
happens depends on the type of magma (its composition).
Granitic magma contains about 70% silica and 10% water;
basaltic magma contains about 50% silica and 1-2% water.
Higher silica makes the magma viscous (stiff). So granitic
magma solidifies in the crust easily, while basaltic magma
can reach the surface.
Water escapes from granitic magma when pressure
decreases as it rises. Its solidification temperature rises,
causing it to crystallize within the crust. Basaltic magma’s
water content is relatively unimportant.
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Plutons: granitic magma solidifies in the Earth’s crust to
form a large mass of granite called a pluton. Tectonic forces
may push it upward, and erosion may expose parts of it at
the Earth’s surface.
Fig. 8-11, p.178
Fig. 8-11a, p.178
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A batholith is a
pluton exposed over
more than 100 sq.
km of Earth’s
surface; a stock is
similar, but exposed
over less than 100
sq. km.
Fig. 8-11b, p.178
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Many mountain ranges,
such as California’s Sierra
Nevada range, contain
large batholiths.
Fig. 8-12, p.178
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Granite plutons make up most of California’s Sierra Nevada
in Yosemite National Park.
Fig. 8-13, p.179
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Smaller magma intrusions may flow into a fracture or
between layers in country rock.
Dike: tabular intrusive rock that cuts across country rock.
Sill: magma oozes between layers of country rock and
forms parallel to layering.
Fig. 8-14, p.179
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Dikes intruding older country rock.
Fig. 8-15, p.179
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A large dike near Shiprock,
New Mexico, has been left
standing after softer
sediment (country rock)
eroded away.
Fig. 8-16, p.180
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A black basalt sill has been injected between layers of
sandstone. Grand Canyon, Arizona.
Fig. 8-17, p.180
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Volcanoes:
Lava is fluid
magma that flows
onto the Earth’s
surface and
solidifies. To right
is pahoehoe lava,
low viscosity lava
that flows while it
cools, forming
smooth, glassysurfaced, wrinkled,
or “ropy” ridges.
Fig. 8-18, p.181
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If viscosity is higher, its
surface may partially
solidigy as it flows. The
solid crust breaks up as the
deeper, molten lava flows,
forming aa lava, with a
jagged, rubbled, broken
surface.
If a volcano erupts
explosively, however, it may
eject liquid magma and solid
rock fragments. A rock
from this material is called a
pyroclastic rock; the
smallest particles are ash,
while cinders are 2-64 mm.
Fig. 8-19, p.181
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Hot lava shrinks as
it cools and
solidifies, pulling
the rock apart and
forming cracks that
can be 5 or 6 sided
and grow
downward through
the solidifying
lava. This is called
columnar jointing.
Fig. 8-20a, p.181
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Columnar joints
viewed from the
top.
Fig. 8-20b, p.181
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Fissure eruptions and lava plateaus: gentle type of volcanic
eruption occurs when magma oozes from cracks (fissures)
in the land surface (or flanks of a volcano) and flows over
the land like water. Common in Hawaii and Icelandic
volcanoes.
Some create flood basalts and lava plateaus.
Fig. 8-21, p.182
Fig. 8-21a, p.182
Fig. 8-21b, p.182
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Types of volcanoes
Table 8-1, p.183
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Lava and rocks commonly erupt from a vent located in the
crater (bowl-shaped depression at the summit).
This photo shows steam rising from vents in the crater of
Marum volcano, Vanuatu.
Fig. 8-22, p.183
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Mount Skjoldbreidier in Iceland shows the typical lowangle slopes of a shield volcano.
Fig. 8-23, p.183
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Cinder cones in southern Bolivia
Fig. 8-24a, p.184
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Pyroclastic fragment
of cinder cones.
Fig. 8-24b, p.184
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A composite cone
consists of
alternating layers
of lava and
pyroclastic
material; Mount
Rainier (below) is
a composite
volcano rising
behind Seattle’s
skyline.
Fig. 8-25, p.185
Fig. 8-25a, p.185
Fig. 8-25b, p.185
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Caldera eruption
Fig. 8-26, p.186
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Rising granitic
magma stretches
and fractures
overlying crust.
Gas separates from
the magma and
rises to the upper
part of the magma
body.
Fig. 8-26a, p.186
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The gas-rich magma
explodes through
fractures, rising as a
vertical column of hot
ash, rock fragments
and gas.
Fig. 8-26b, p.186
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When the gas is used
up, the column
collapses and
spreads outward as a
high speed ash flow.
Fig. 8-26c, p.186
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Because so much
material has
erupted from the
top of the magma
chamber, the roof
collapses to form a
caldera.
Fig. 8-26d, p.186
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Ash-flow tuff forms
when an ash flow
comes to a stop.
Fragments of rock
are carried along
with volcanic ash
and gas. Ash-flows
can travel more than
100 km in distance at
200 km/hr.
Fig. 8-27, p.186
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View of the caldera that
forms Crater Lake, Oregon.
Fig. 8-28, p.187
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Calderas (red dots) and ashflow tuffs (orange areas) are
abundant in wester North
America.
Fig. 8-29, p.187
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Archaeologists uncover
molds of people killed in
the A.D. 79 eruption of
Mount Vesuvius near the
Roman city of Pompeii.
During the eruption an
ash flow streamed down
the volcano and buried
cities and towns under 58 meters of hot ash.
Mount Vesuvius is a
stratovolcano. Recent
seismic studies show that
seismic waves suddenly
slow from 6 to 2.7 km/sec
at a depth of 10 km.
What does this mean?
Fig. 8-30, p.188
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Approximately 1300 active volcanoes are recognized
globally, and 5564 eruptions have occurred in the past
10,000 years. Volcanic eruptions are one of the greatest
geologic hazards.
Table 8-2, p.188
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May 18, 1980
eruption of
Mount St.
Helens.
Fig. 8-31, p.190
Fig. 8-32a, p.190
Fig. 8-32b, p.190
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Plot of temperatures in the Northern Hemisphere shows that
atmospheric cooling follows major volcanic eruptions.
Mount Pinatubo in 1991 produced the greatest ash and sulfur clouds
toward the end of the 20th century, decreasing solar radiation that
reached the Earth’s surface by 2-4% and cooling the globe by a few
tenths of a degree during 1992-93.
Fig. 8-33, p.191
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Do volcanic eruptions always
create global cooling?
End of Permian (248 million
years ago); greatest extinction
known, linked to flood basalts
in Siberia
120 million years ago, vast
submarine lava plateau formed
beneath the Pacific Ocean off
the west coast of SA. Large
quantities of C02 released.
Dinosaurs flourished in
swamps, coal deposits
formed…global climate may
have warmed by 12-15 degrees
C.
p.193