across existing structures, such as sedimentary strata; whereas others
form when magma is injected between sedimentary layers. Because of
these differences, intrusive igneous bodies are generally classified
according to their shape as either tabular
or massive
and by their orientation with respect to the host rock. Igneous bodies
are said to be discordant
if they cut across
existing structures and concordant
if they form
parallel to features such as sedimentary strata.
Tablular Intrusive Bodies: Dikes and Sills
Tabular intrusive bodies are produced when magma is forcibly injected
into a fracture or zone of weakness, such as a bedding surface (see
Figure 3.24). Dikes are discordant bodies that cut across bedding
surfaces or other structures in the host rock. By contrast, sills are nearly
horizontal, concordant bodies that form when magma exploits weaknesses between sedimentary beds (FIGURE 3.25). In general, dikes serve
as tabular conduits that transport magma, whereas sills store magma.
Dikes and sills are typically shallow features, occurring where the host
rocks are sufficiently brittle to fracture. Although they can range in
thickness from less than a millimeter to over a kilometer, most are in
the 1- to 20-meter range.
Dikes and sills can occur as solitary bodies, but
dikes in particular tend to form in roughly parallel
groups called dike swarms. These multiple structures
reflect the tendency for fractures to form in sets when
tensional forces stretch brittle country rock. Dikes can
also occur radiating from an eroded volcanic neck, like
spokes on a wheel. In these situations the active ascent
of magma generated fissures in the volcanic cone out of
which lava flowed.
Dikes frequently weather more slowly than the
surrounding rock. Consequently, when exposed by
erosion, dikes tend to have a wall-like appearance, as
shown in FIGURE 3.26. Because dikes and sills are relatively uniform in thickness and can extend for many
kilometers they are assumed to be the product of very
fluid, and therefore, mobile magmas. One of the largest
and most studied of all sills in the United States is the
Palisades Sill. Exposed for 80 kilometers along the west
bank of the Hudson River in southeastern New York and
northeastern New Jersey, this sill is about 300 meters
thick. Because it is resistant to erosion, the Palisades Sill
forms an imposing cliff that can be easily seen from the
opposite side of the Hudson.
In many respects, sills closely resemble buried lava
flows. Both are tabular and can have a wide aerial
extent, and both may exhibit columnar jointing
(FIGURE 3.27). Columnar joints form as igneous rocks
cool and develop shrinkage fractures that produce
elongated, pillar-like columns. Further, because sills
generally form in near-surface environments and may be
only a few meters thick, the emplaced magma often
cools quickly enough to generate a fine-grained texture.
(Recall that most intrusive igneous bodies have a coarsegrained texture.)
(concordare = to agree)
(discordare = to disagree)
(tabula = table)
CHAPTER 3 Igneous Rocks and Intrusive Activity
82
Sill
FIGURE 3.25 Salt River Canyon, Arizona. The dark, essentially horizontal
band is a sill of basaltic composition that intruded horizontal layers of
sedimentary rock. (Photo by E. J. Tarbuck)
FIGURE 3.26 The vertical structure
that looks like a stone wall is a dike,
which is more resistant to weathering
than the surrounding rock. This dike
is located west of Granby, Colorado,
near Arapaho National Forest.
(Photo by R. Jay Fleisher)
form when magma is injected between sedimentary layers. Because of
these differences, intrusive igneous bodies are generally classified
according to their shape as either tabular
or massive
and by their orientation with respect to the host rock. Igneous bodies
are said to be discordant
if they cut across
existing structures and concordant
if they form
parallel to features such as sedimentary strata.
Tablular Intrusive Bodies: Dikes and Sills
Tabular intrusive bodies are produced when magma is forcibly injected
into a fracture or zone of weakness, such as a bedding surface (see
Figure 3.24). Dikes are discordant bodies that cut across bedding
surfaces or other structures in the host rock. By contrast, sills are nearly
horizontal, concordant bodies that form when magma exploits weaknesses between sedimentary beds (FIGURE 3.25). In general, dikes serve
as tabular conduits that transport magma, whereas sills store magma.
Dikes and sills are typically shallow features, occurring where the host
rocks are sufficiently brittle to fracture. Although they can range in
thickness from less than a millimeter to over a kilometer, most are in
the 1- to 20-meter range.
Dikes and sills can occur as solitary bodies, but
dikes in particular tend to form in roughly parallel
groups called dike swarms. These multiple structures
reflect the tendency for fractures to form in sets when
tensional forces stretch brittle country rock. Dikes can
also occur radiating from an eroded volcanic neck, like
spokes on a wheel. In these situations the active ascent
of magma generated fissures in the volcanic cone out of
which lava flowed.
Dikes frequently weather more slowly than the
surrounding rock. Consequently, when exposed by
erosion, dikes tend to have a wall-like appearance, as
shown in FIGURE 3.26. Because dikes and sills are relatively uniform in thickness and can extend for many
kilometers they are assumed to be the product of very
fluid, and therefore, mobile magmas. One of the largest
and most studied of all sills in the United States is the
Palisades Sill. Exposed for 80 kilometers along the west
bank of the Hudson River in southeastern New York and
northeastern New Jersey, this sill is about 300 meters
thick. Because it is resistant to erosion, the Palisades Sill
forms an imposing cliff that can be easily seen from the
opposite side of the Hudson.
In many respects, sills closely resemble buried lava
flows. Both are tabular and can have a wide aerial
extent, and both may exhibit columnar jointing
(FIGURE 3.27). Columnar joints form as igneous rocks
cool and develop shrinkage fractures that produce
elongated, pillar-like columns. Further, because sills
generally form in near-surface environments and may be
only a few meters thick, the emplaced magma often
cools quickly enough to generate a fine-grained texture.
(Recall that most intrusive igneous bodies have a coarsegrained texture.)
(concordare = to agree)
(discordare = to disagree)
(tabula = table)
CHAPTER 3 Igneous Rocks and Intrusive Activity
82
Sill
FIGURE 3.25 Salt River Canyon, Arizona. The dark, essentially horizontal
band is a sill of basaltic composition that intruded horizontal layers of
sedimentary rock. (Photo by E. J. Tarbuck)
FIGURE 3.26 The vertical structure
that looks like a stone wall is a dike,
which is more resistant to weathering
than the surrounding rock. This dike
is located west of Granby, Colorado,
near Arapaho National Forest.
(Photo by R. Jay Fleisher)