lava
69
Igneous Textures: What Can They Tell Us?
the aid of a polarizing microscope or other sophisticated techniques (FIGURE 3.5).
Therefore, we commonly characterize fine-grained rocks as being light, intermediate,
or dark in color. Using this system of grouping, light-colored aphanitic rocks are those
containing primarily light-colored nonferromagnesian silicate minerals and so forth.
Common features of many extrusive rocks are the voids left by gas bubbles that
escape as lava solidifies. These nearly spherical openings are called vesicles, and the
rocks that contain them are said to have a vesicular texture. Rocks that exhibit a
vesicular texture usually form in the upper zone of a lava flow, where cooling
occurs rapidly enough to preserve the openings produced by the expanding gas bubbles (FIGURE 3.6).
PHANERITIC (COARSE-GRAINED) TEXTURE. When large masses
of magma slowly crystallize at great depth, they form igneous rocks
that exhibit a coarse-grained texture also referred to as a phaneritic
texture
. Coarse-grained rocks consist of a mass of
intergrown crystals that are roughly equal in size and large enough so
that the individual minerals can be identified without the aid of a
microscope (Figure 3.5B). Geologists often use a small magnifying lens
to aid in identifying minerals in a phaneritic rock.
PORPHYRITIC TEXTURE. A large mass of magma may require tens to hundreds of
thousands of years to solidify. Because different minerals crystallize under different environmental conditions (temperatures and pressure), it is possible for crystals of one mineral
to become quite large before others even begin to form. Should molten rock containing
some large crystals move to a different environment—for example, by erupting at the
surface—the remaining liquid portion of the lava would cool more quickly. The resulting
rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a
porphyritic texture (see Figure 3.4D). The large crystals in such a rock are referred to as
phenocrysts (
,
), whereas the matrix of smaller crystals is
called groundmass. A rock with a porphyritic texture is termed a porphyry.
GLASSY TEXTURE. During some volcanic eruptions, molten rock is ejected into the
atmosphere, where it is quenched quickly. Rapid cooling of this type may generate rocks
having a glassy texture. Glass results when unordered ions are “frozen in place” before
they are able to unite into an orderly crystalline structure.
Obsidian, a common type of natural glass, is similar in appearance to a
dark chunk of manufactured glass (FIGURE 3.7). Because of its excellent
conchodial fracture and ability to hold a sharp, hard edge, obsidian was a
prized material from which Native Americans chipped arrowheads and
cutting tools (see Figure 3.7 inset).
cryst = crystal
pheno = show
visible)
(phaner =
FIGURE 3.6 The larger image shows a lava flow
erupting from Hawaii’s Kilauea volcano. The rock
sample photo is a close-up showing the vesicular
texture of the rock. Vesicles are small holes left by
escaping gas bubbles. (Photo by J. D. Griggs, U.S.
Geological Survey/rock sample photo courtesy of
E. J. Tarbuck)
FIGURE 3.7 Obsidian, a natural glass, was
used by Native Americans for making
arrowheads and cutting tools. (Photo by
E. J. Tarbuck; Inset photo by Jeffrey Scovil)
As this lava cools, it is transformed into a solid
mass of interlocking minerals we call an igneous
rock. (Photo by Griggs/USGS)
69
Igneous Textures: What Can They Tell Us?
the aid of a polarizing microscope or other sophisticated techniques (FIGURE 3.5).
Therefore, we commonly characterize fine-grained rocks as being light, intermediate,
or dark in color. Using this system of grouping, light-colored aphanitic rocks are those
containing primarily light-colored nonferromagnesian silicate minerals and so forth.
Common features of many extrusive rocks are the voids left by gas bubbles that
escape as lava solidifies. These nearly spherical openings are called vesicles, and the
rocks that contain them are said to have a vesicular texture. Rocks that exhibit a
vesicular texture usually form in the upper zone of a lava flow, where cooling
occurs rapidly enough to preserve the openings produced by the expanding gas bubbles (FIGURE 3.6).
PHANERITIC (COARSE-GRAINED) TEXTURE. When large masses
of magma slowly crystallize at great depth, they form igneous rocks
that exhibit a coarse-grained texture also referred to as a phaneritic
texture
. Coarse-grained rocks consist of a mass of
intergrown crystals that are roughly equal in size and large enough so
that the individual minerals can be identified without the aid of a
microscope (Figure 3.5B). Geologists often use a small magnifying lens
to aid in identifying minerals in a phaneritic rock.
PORPHYRITIC TEXTURE. A large mass of magma may require tens to hundreds of
thousands of years to solidify. Because different minerals crystallize under different environmental conditions (temperatures and pressure), it is possible for crystals of one mineral
to become quite large before others even begin to form. Should molten rock containing
some large crystals move to a different environment—for example, by erupting at the
surface—the remaining liquid portion of the lava would cool more quickly. The resulting
rock, which has large crystals embedded in a matrix of smaller crystals, is said to have a
porphyritic texture (see Figure 3.4D). The large crystals in such a rock are referred to as
phenocrysts (
,
), whereas the matrix of smaller crystals is
called groundmass. A rock with a porphyritic texture is termed a porphyry.
GLASSY TEXTURE. During some volcanic eruptions, molten rock is ejected into the
atmosphere, where it is quenched quickly. Rapid cooling of this type may generate rocks
having a glassy texture. Glass results when unordered ions are “frozen in place” before
they are able to unite into an orderly crystalline structure.
Obsidian, a common type of natural glass, is similar in appearance to a
dark chunk of manufactured glass (FIGURE 3.7). Because of its excellent
conchodial fracture and ability to hold a sharp, hard edge, obsidian was a
prized material from which Native Americans chipped arrowheads and
cutting tools (see Figure 3.7 inset).
cryst = crystal
pheno = show
visible)
(phaner =
FIGURE 3.6 The larger image shows a lava flow
erupting from Hawaii’s Kilauea volcano. The rock
sample photo is a close-up showing the vesicular
texture of the rock. Vesicles are small holes left by
escaping gas bubbles. (Photo by J. D. Griggs, U.S.
Geological Survey/rock sample photo courtesy of
E. J. Tarbuck)
FIGURE 3.7 Obsidian, a natural glass, was
used by Native Americans for making
arrowheads and cutting tools. (Photo by
E. J. Tarbuck; Inset photo by Jeffrey Scovil)
As this lava cools, it is transformed into a solid
mass of interlocking minerals we call an igneous
rock. (Photo by Griggs/USGS)