Page 153 - Laboratory Manual in Physical Geology

ACTIVITY centimeters in length. However, if a magma is very vis-
cous (thick and resistant to flow), then atoms cannot eas-
5.5 Crystalline Textures of ily move to nucleation sites. Crystals may not form even
Igneous Rocks by slow cooling. Rapid cooling of very viscous magma
(with poor nucleation) can produce igneous rocks with
THINK | What are igneous rock textures? How a glassy texture (see FIGURE 5.4 ), which indicates an
extrusive (volcanic) origin.
About It is texture used to classify and interpret
igneous rocks? Textures Based on Crystal Size
Several common terms are used to describe igneous rock
OBJECTIVE Review a crystallization experiment,

texture on the basis of crystal size ( FIGURE 5.4 ). Igneous

infer how rate of cooling affects crystal size, and
rocks made of crystals that are too small to identify with
then apply your knowledge to interpret a rock with
the naked eye or a hand lens (generally <1 mm) have a
porphyritic texture.
very fine-grained aphanitic texture (from the Greek word
PROCEDURES for invisible ). Those made of visible crystals that can be
1. Before you begin , read about Textures of Igneous identified with a hand lens or unaided eye are said to have
Rocks (p. 133). Also, this is what you will need : a phaneritic texture (coarse-grained; crystals 1–10 mm) or
pegmatitic texture (very coarse-grained; >1 cm).
____ Activity 5.5 Worksheet (p. 147 ) and pencil
Some igneous rocks have two distinct sizes of crystals.
2. Then follow your instructor’s directions for This is called porphyritic texture (see FIGURE 5.4 ).
completing the worksheet. The large crystals are called phenocrysts, and the smaller,
more numerous crystals that surround them form the
groundmass, or matrix ( FIGURE 5.4 ). Porphyritic textures
may generally indicate that a body of magma cooled slowly
Igneous rocks are also classified into two textural
at first (to form the large crystals) and more rapidly later
groups : intrusive (plutonic) versus extrusive (volcanic).
(to form the small crystals). However, recall from above
Intrusive (plutonic) rocks form deep underground,
that crystal size can also be influenced by changes in
where they are well insulated (take a long time to cool) and
magma composition or viscosity.
pressurized. The pressure prevents gases from expanding, just
Combinations of igneous-rock textures also occur.
like carbonation in a sealed soft drink. The cap seals in the
For example, a porphyritic-aphanitic texture signifies
pressure—an intrusive process. If you remove the cap, then
that phenocrysts occur within an aphanitic matrix.
the carbon dioxide inside the bottle expands and bubbles—
A porphyritic-phaneritic texture signifies that phenocrysts
an extrusive process. Therefore, extrusive (volcanic) rocks
occur within a phaneritic matrix.
form near and on Earth’s surface, where the confining pres-
sure is low and gases begin to bubble out of the magma. This
can help cause explosive eruptions and textures related to Vesicular and Pyroclastic Textures
fragmenting of rocks. Cooler surface temperatures also rob When gas bubbles get trapped in cooling lava they are
thermal energy from magma, so it cools quickly. called vesicles, and the rock is said to have a vesicular tex-
The size of mineral crystals in an igneous rock ture . Scoria is a textural name for a rock having so many
generally indicates the rate at which the lava or magma vesicles that it resembles a sponge. Pumice has a glassy
cooled to form a rock and the availability of the chemicals texture and so many tiny vesicles (like frothy meringue on
required to form the crystals. Large crystals require a long a pie) that it floats in water.
time to grow, so their presence generally means that a Recall that pyroclasts (from Greek meaning fire
body of molten rock cooled slowly (an intrusive process) broken ) are rocky materials that have been fragmented
and contained ample atoms of the chemicals required and/or ejected by explosive volcanic eruptions
to form the crystals. Tiny crystals generally indicate that ( FIGURE 5.3 ). They include volcanic ash fragments
the magma cooled more rapidly (an extrusive process). (pyroclasts < 2 mm), lapilli or cinders (pyroclasts

Volcanic glass (no crystals) can indicate that a magma was 2–64 mm), and volcanic bombs or blocks (pyroclasts >
quenched (cooled immediately), but most volcanic glass is 64 mm). Igneous rocks composed mostly of pyroclasts

the result of poor nucleation as described below. have a pyroclastic texture (see FIGURE 5.4 ).
Nucleation and Rock Texture How to Identify Igneous Rocks
The crystallization process depends on the ability of The identification and interpretation of an igneous rock is
atoms in lava or magma to nucleate . Nucleation is the based on its composition and texture ( FIGURES 5.4 and 5.5 ).
initial formation of a microscopic crystal, to which Follow these steps to classify and identify an igneous rock:
other atoms progressively bond. This is how a crystal
grows. Atoms are mobile in a fluid magma, so they are Steps 1 and 2: Identify the rock’s mafic color index (MCI).
free to nucleate. If such a fluid magma cools slowly, Then, if possible, identify the minerals that make up the
then crystals have time to grow—sometimes to many rock and estimate the percentage of each.

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