CHAPTER 3 Igneous Rocks and Intrusive Activity
80
Recall from Bowen’ s reaction series that
rocks with a granitic composition are composed of minerals with the lowest melting
(crystallization) temperatures—namely,
quartz and potassium feldspar (see Figure
3.20). Also note that as we move up
Bowen’ s reaction series, the minerals have
progressively higher melting temperatures
and that olivine, which is found at the top,
has the highest melting point. When a rock
undergoes partial melting it will form a
melt that is enriched in ions from minerals
with the lowest melting temperatures. The
unmelted crystals are those of minerals
with higher melting temperatures. Separation of these two fractions would yield a
melt with a chemical composition that is
richer in silica and nearer to the granitic
(felsic) end of the spectrum than the rock
from which it was derived (FIGURE 3.23).
In summary, Bowen’ s reaction series is a
simplified guide to understanding how
partial melting produces rocks of various
compositions. As rock begins to melt, those
minerals with the lowest melting temperatures are the first to melt. As a result, partial melting produces magmas enriched
in low-melting temperature components
and richer in silica than the parent rock.
Partial melting of the ultramafic rock peridotite produces a magma of basaltic (mafic)
composition, while partial melting of the
mafic rock basalt generates magma of intermediate (andesitic) composition. Granitic
(felsic) magmas evolve from basaltic magmas and/or the melting and assimilation of
silica-rich crustal rocks.
C O N C E P T C H E C K 3 . 7
What is magmatic differentiation? How
might this process lead to the formation of
several different igneous rocks from a
single magma?
Relate the classification of igneous rocks to
Bowen’s reaction series.
What is partial melting?
How does the composition of a melt
produced by partial melting compare with
the composition of the parent rock?
Intrusive Igneous
Activity
Volcanoes
Intrusive Igneous Activity
Although volcanic eruptions can be violent
and spectacular events, most magma is
emplaced and crystallizes at depth, without
fanfare. Therefore, understanding the
igneous processes that occur deep underground is as important to geologists as the
study of volcanic events.
When magma rises through the crust,
it forcefully displaces preexisting crustal
rocks referred to as host or country rock.
GEODe
ESSENTIALS
OF GEOLOGY
4
3
2
1
FIGURE 3.23 1996 eruption of Mount
Ruapehu, Tongariro National Park, New
Zealand. Volcanoes that border the Pacific
Ocean are fed largely by magmas that have
intermediate or felsic compositions. These
silica-rich magmas often erupt explosively,
generating large plumes of volcanic dust
and ash. (Photo by ImageState/Alamy)
In summary, Bowen successfully
demonstrated that through magmatic
differentiation, a single parent magma can
generate several mineralogically different
igneous rocks. This process, in concert with
magma mixing and contamination by
crustal rocks, accounts in part for the great
diversity of magmas and igneous rocks. We
will next look at another important process,
partial melting, which is also responsible
for generating magmas having varying
compositions.
Partial Melting
and Magma Composition
Recall that the crystallization of basaltic
magma occurs over a temperature range of
at least 200 °C. As you might expect, melting, the reverse process, spans a similar
temperature range. As rock begins to melt,
those minerals with the lowest melting
temperatures are the first to melt. Should
melting continue, minerals with higher
melting points begin to melt and the composition of the magma steadily approaches
the overall composition of the rock from
which it was derived. Most often, however,
melting is not complete. The incomplete
melting of rocks is known as partial melting, a process that produces most magma.
80
Recall from Bowen’ s reaction series that
rocks with a granitic composition are composed of minerals with the lowest melting
(crystallization) temperatures—namely,
quartz and potassium feldspar (see Figure
3.20). Also note that as we move up
Bowen’ s reaction series, the minerals have
progressively higher melting temperatures
and that olivine, which is found at the top,
has the highest melting point. When a rock
undergoes partial melting it will form a
melt that is enriched in ions from minerals
with the lowest melting temperatures. The
unmelted crystals are those of minerals
with higher melting temperatures. Separation of these two fractions would yield a
melt with a chemical composition that is
richer in silica and nearer to the granitic
(felsic) end of the spectrum than the rock
from which it was derived (FIGURE 3.23).
In summary, Bowen’ s reaction series is a
simplified guide to understanding how
partial melting produces rocks of various
compositions. As rock begins to melt, those
minerals with the lowest melting temperatures are the first to melt. As a result, partial melting produces magmas enriched
in low-melting temperature components
and richer in silica than the parent rock.
Partial melting of the ultramafic rock peridotite produces a magma of basaltic (mafic)
composition, while partial melting of the
mafic rock basalt generates magma of intermediate (andesitic) composition. Granitic
(felsic) magmas evolve from basaltic magmas and/or the melting and assimilation of
silica-rich crustal rocks.
C O N C E P T C H E C K 3 . 7
What is magmatic differentiation? How
might this process lead to the formation of
several different igneous rocks from a
single magma?
Relate the classification of igneous rocks to
Bowen’s reaction series.
What is partial melting?
How does the composition of a melt
produced by partial melting compare with
the composition of the parent rock?
Intrusive Igneous
Activity
Volcanoes
Intrusive Igneous Activity
Although volcanic eruptions can be violent
and spectacular events, most magma is
emplaced and crystallizes at depth, without
fanfare. Therefore, understanding the
igneous processes that occur deep underground is as important to geologists as the
study of volcanic events.
When magma rises through the crust,
it forcefully displaces preexisting crustal
rocks referred to as host or country rock.
GEODe
ESSENTIALS
OF GEOLOGY
4
3
2
1
FIGURE 3.23 1996 eruption of Mount
Ruapehu, Tongariro National Park, New
Zealand. Volcanoes that border the Pacific
Ocean are fed largely by magmas that have
intermediate or felsic compositions. These
silica-rich magmas often erupt explosively,
generating large plumes of volcanic dust
and ash. (Photo by ImageState/Alamy)
In summary, Bowen successfully
demonstrated that through magmatic
differentiation, a single parent magma can
generate several mineralogically different
igneous rocks. This process, in concert with
magma mixing and contamination by
crustal rocks, accounts in part for the great
diversity of magmas and igneous rocks. We
will next look at another important process,
partial melting, which is also responsible
for generating magmas having varying
compositions.
Partial Melting
and Magma Composition
Recall that the crystallization of basaltic
magma occurs over a temperature range of
at least 200 °C. As you might expect, melting, the reverse process, spans a similar
temperature range. As rock begins to melt,
those minerals with the lowest melting
temperatures are the first to melt. Should
melting continue, minerals with higher
melting points begin to melt and the composition of the magma steadily approaches
the overall composition of the rock from
which it was derived. Most often, however,
melting is not complete. The incomplete
melting of rocks is known as partial melting, a process that produces most magma.