Page 154 - Laboratory Manual in Physical Geology

ACTIVITY ACTIVITY

5.6 Rock Analysis, 5.7 Thin Section Analysis and
Classification, and Origin Bowen’s Reaction Series

THINK | How are rock composition and texture THINK | How are rock composition and texture
About It used to classify, name, and interpret About It used to classify, name, and interpret
igneous rocks? igneous rocks?
OBJECTIVE Analyze the composition and textures OBJECTIVE Analyze composition and texture of two

of rocks, identify them, and infer how they formed. thin sections of igneous rock, and then interpret their
origin relative to Bowen’s Reaction Series.
PROCEDURES
1. Before you begin , read about How to Identify PROCEDURES
Igneous Rocks (p. 134). Also, this is what you will 1. Before you begin , read about Bowen’s Reaction
need : Series and Plate Tectonics and Igneous Rocks
___ Activity 5.6 Worksheet (p. 148 ) and pencil below. Also, this is what you will need :
___ Activity 5.7 Worksheet (p. 149 ) and pencil
2. Then follow your instructor’s directions for
completing the worksheet. 2. Then follow your instructor’s directions for
completing the worksheet.

■ If the rock is very fine-grained (aphanitic or batholith, they often find that it contains more than just
porphyritic-aphanitic), then you must estimate one kind of igneous rock. Apparently, more than one kind
mineralogy based on the rock’s mafic color index. Felsic of igneous rock can form from a single homogeneous body
fine-grained rocks tend to be pink, white, or pale gray/ of magma as it cools. American geologist, Norman L.
brown. Intermediate fine-grained rocks tend to be Bowen made such observations in the early 1900s. He then
greenish gray to medium gray. Mafic and ultramafic devised and carried out laboratory experiments to study
fine-grained rocks tend to be green, dark gray, or black. how magmas might evolve in ways that could explain how
more than one kind of igneous rock could form from a
■ If the rock is coarse-grained (phaneritic or pegmatitic),

single body of magma. His work is commonly summarized
then estimate the mafic color index (MCI) and percent-
in a diagram ( FIGURE 5.6 ) called Bowen’s Reaction Series ,
age abundance of each of the specific felsic and mafic
which shows how different kinds of igneous rocks can
minerals. With this information, you can also character-
evolve from a single body of magma as it cools.
ize the rock as felsic, intermediate, mafic, or ultramafic.
Step 3: Identify the rock’s texture(s) using FIGURE 5.4 . Bowen’s Experiment
Step 4: Determine the name of the rock using the flowchart in Other geologic investigations had already suggested that the

FIGURE 5.4 or the expanded classification chart in FIGURE 5.5 . top of Earth’s mantle is made of peridotite. So Bowen placed

pieces of peridotite into bombs , strong pressurized ovens

■ Use textural terms, such as porphyritic or vesicular, used to melt the rocks at high temperatures (1200–1400°C).
as adjectives. For example, you might identify a pink, Once the rocks had melted to form peridotite magma,
aphanitic (fine-grained), igneous rock as a rhyolite. Bowen would allow the magma to cool to a given tempera-
If it contains scattered phenocrysts, then you would ture and remain at that temperature for a while in hopes
call it a porphyritic rhyolite . Similarly, you should call a of having it begin to crystallize. The rock was then quickly
basalt with vesicles a vesicular basalt. removed from the bomb and quenched (cooled by dunking

■ The textural information can also be used to infer the it in water) to make any remaining molten rock form glass.
origin of a rock. For example, vesicles (vesicular textures) Bowen then identified the mineral crystals that had formed
imply that the rock formed by cooling of a gas-rich lava at each temperature. His experiments showed that as magma
(vesicular and aphanitic). Pyroclastic texture implies cools in an otherwise unchanging environment, two series of
violent volcanic eruption. Aphanitic texture implies silicate minerals crystallize in a predictable order.
more rapid cooling than phaneritic texture.

Discontinuous Crystallization of Mafic Minerals (Left
Branch). The left branch of Bowen’s Reaction Series

( FIGURE 5.6 ) shows the predictable series of mafic minerals

Bowen’s Reaction Series
that crystallize from a peridotite magma that is allowed to cool
When magma intrudes Earth’s crust, it cools into a mass of slowly. This series is discontinuous because one mafic mineral
mineral crystals and/or glass. Yet when geologists observe replaces another as the magma cools. For example, olivine is
and analyze the igneous rocks in a single dike, sill, or first to crystallize at very high temperature. But if the magma
Igneous Rocks and Processes ■ 135