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108 Essentials of Geology
> Figure 4. 17 Why
> the mantle is mainly
> solid This diagram
> shows the geothermal
> gradient (the increase in
> temperature with depth)
> for the crust and upper
> mantle.
Key
Temperature (C)
> Depth in kilometers
> Pressure in kilobars
300
400
500
500 1000 1500 2000 2500
100
200
75
100
150
125
25
50
0
0 0
Solid rock Partial melting Complete melting
The geothermal gradient
(red curve) shows how the
temperature rises with
increasing depth.
The fact that the geothermal
gradient lies completely in
the green area means that
mantle rock is typically solid.
At the depth of the
upper asthenosphere,
the red curve is close
to the orange zone
so that a slight
change in conditions
can cause mantle
rock to partially
melt.
Based on evidence from the study of earthquake waves,
we know that Earths crust and mantle are composed pri -
marily of solid, not molten, rock. Although the outer core
is fluid, this iron-rich material is very dense and remains
deep within Earth. So where does magma come from?
Most magma originates in Earths uppermost man -
tle. The greatest quantities are produced at divergent
plate boundaries, in association with seafloor spreading,
with lesser amounts forming at subduction zones, where
oceanic lithosphere descends into the mantle. Magma
also can be generated when crustal rocks are heated suf-
ficiently to melt.
# Generating Magma from Solid Rock
Workers in underground mines know that temperatures
increase as they descend deeper below Earths surface.
Although the rate of temperature change varies con -
siderably from place to place, it averages about 25C
(75F) per kilometer in the upper crust. This increase
in temperature with depth is known as the geothermal
gradient . As shown in Figure 4.17 , when a typical geo -
thermal gradient is compared to the melting point curve
for the mantle rock peridotite, the temperature at which
peridotite melts is higher than the geothermal gradi -
ent. Thus, under normal conditions, the mantle is solid.
However, tectonic processes trigger melting though vari -
ous means, including reducing the mantle rocks melting
point (the temperature at which a material changes from
solid to liquid).
Decrease in Pressure: Decompression Melting If tem -
perature were the only factor that determined whether
rock melts, our planet would be a molten ball covered
with a thin, solid outer shell. This is not the case because
pressure, which also increases with depth, influences the
melting temperatures of rocks.
Melting, which is accompanied by an increase in
volume, occurs at progressively higher temperatures with
increased depth. This is the result of the steady increase in
confining pressure exerted by the weight of overlying rocks.
Conversely, reducing confining pressure lowers a rocks
melting temperature . When confining pressure drops suf -
ficiently, decompression melting is triggered. Decom -
pression melting occurs wherever hot, solid mantle rock
ascends, thereby moving into regions of lower pressure.
Recall from Chapter 2 that tensional forces along
spreading centers promote upwelling where plates
diverge. This process is responsible for generating magma
along oceanic ridges (divergent plate boundaries) where
plates are rifting apart ( Figure 4.18 ). Below the ridge
crest, hot mantle rock rises and melts, generating a
magma that replaces the material that shifted horizon -
tally away from the ridge axis.
Decompression melting also occurs when ascending
mantle plumes reach the uppermost mantle. If this rising
magma reaches the surface, it triggers an episode of hot-
spot volcanism.
Addition of Water Along with pressure, an important fac-
tor affecting the melting temperature of rock is its water
content. Water and other volatiles, such as carbon diox-
ide, act in a similar way to salt melting ice. That is, water
causes rock to melt at lower temperatures, just as putting
rock salt on an icy sidewalk induces melting.
The introduction of water to generate magma occurs
mainly at convergent plate boundaries, where cool
slabs of oceanic lithosphere descend into the mantle
(Figure 4.19 ). As an oceanic plate sinks, heat and pres-
sure drive water from the subducting oceanic crust and
overlying sediments. These fluids migrate into the wedge
of hot mantle that lies directly above. At a depth of about
100 kilometers (60 miles), the wedge of mantle rock is
sufficiently hot that the addition of water leads to some
melting. Partial melting of the mantle rock peridotite
generates hot basaltic magma whose temperatures may
exceed 1250C (nearly 2300F).
# 4. 5 Origin of Magma
Summarize the major processes that generate magma from solid rock.
Did You Know?
> Since the 1930s carpen-
> ters, mechanics, and
> other people working
> with their hands have
> used Lava Soap , which
> contains powdered
> pumice. Because of the
> abrasiveness of pumice,
> Lava soap is particu-
> larly good at removing
> grease, dirt, paints, and
> adhesives.
> M04_TARB6622_13_SE_C04.indd 108 11/11/16 12:58 PM
CHAPT ER 4 Igneous Rocks & Intrusive Activity 109
CONCEPT CHECKS 4. 5
1. What is the geothermal gradient? Describe how
the geothermal gradient compares with the
melting temperatures of the mantle rock peridotite
at various depths.
2. Explain the process of decompression melting.
3. What roles do water and other volatiles play in
the formation of magma?
4. Name two plate tectonic settings in which you
would expect magma to be generated.
Temperature Increase: Melting Crustal Rocks Mantle-
derived basaltic (mafic) magma tends to be less dense
than the surrounding rocks, which causes the magma to
buoyantly rise toward the surface. In oceanic settings,
these basaltic magmas often erupt on the ocean floor,
generating seamounts, which may grow to form volcanic
islands, as exemplified by the Hawaiian Islands. However,
in continental settings, basaltic magma often ponds
beneath low-density crustal rocks. Because the overlying
crustal rocks have lower melting temperatures than basal -
tic magmas, the hot basaltic magma may heat them suf -
ficiently to generate a secondary melt of silica-rich felsic
magma. If these low-density, felsic magmas reach the sur -
face, they tend to produce explosive eruptions; such erup -
tions occur most often at convergent plate boundaries.
Crustal rocks can also melt during continental colli -
sions that result in the formation of a large mountain belt
(discussed in detail in Chapter 11 ). During these events,
the crust is greatly thickened, and some crustal rocks
are carried to depths where the temperatures are high
enough to cause partial melting. The felsic magmas pro -
duced in this manner usually solidify before reaching the
surface, so volcanism is not typically associated with these
collision-type mountain belts.
In summary, magma can be generated by (1) decom -
pression melting , caused by a decrease in pressure
as magma rises; (2) the introduction of water , which
lowers the melting temperature of hot mantle rock;
and (3) heating of crustal rocks above their melting
temperature.
> Figure 4. 18 Decompression melting As hot mantle rock ascends,
> it experiences continuously decreasing pressure. This drop in confining
> pressure usually initiates decompression melting in the upper mantle.
Asthenosphere
Upwelling
mantle rocks
Rising
mantle rock
Partial
melting begins
Basaltic magma
rises to generate
new oceanic
crust
Continental
lithosphere
Oceanic
crust
Oceanic
lithosphere
Magma
chamber
Solid rock
(peridotite)
Mid-ocean
ridge Water driven
from oceanic crust
causes partial melting
of mantle rock to generate
basaltic magma
Basaltic
magma
rises toward
the surface
Mantle
Solidified
magma
(plutons)
rock melts
Oceanic crust
Trench
Continental
volcanic arc
Continental crust
Asthenosphere
Continental
lithosphere
Subd
u
c
t
i
n
g
o
c
e
a
niclithosphere