Page 160 - Laboratory Manual in Physical Geology

Intermediate Rocks at Convergent Plate Felsic Rocks of the Continents
Boundaries Felsic rocks like granite ( FIGURE 5.13 ) and rhyolite

Factors that contribute to the formation of intermediate ( FIGURE 5.14 ) form within the crust of the continents. Felsic
igneous rocks are plate subduction ( FIGURE 5.7 ), water, and magmas are generated where there is extreme differentiation
magmatic differentiation. Subduction carries water-rich sea- of mafic magmas or partial melting of intermediate rocks
floor rocks (basalt) back down into the mantle. Water, which associated with continental magmatic arcs ( FIGURE 5.7 ).
can lower the melting point of rocks, causes partial melting Felsic magmas are also normally generated above continen-
of mantle peridotite above the subducting plate and develop- tal hot spots, like the Yellowstone Park hot spot.
ment of mafic magma. Although some of this mafic magma
may erupt to the surface as basaltic volcanoes, the mafic
magma tends to differentiate by one process or many and ACTIVITY
take on an intermediate composition. When it cools, it forms
diorite ( FIGURE 5.11 ) and andesite ( FIGURE 5.12 ). Thus, arcs 5.8 Analysis and Interpretation

of volcanoes, called magmatic arcs , develop above subduction of Igneous Rocks
zones and are often of intermediate composition.
THINK How are rock composition and texture
Quartz Plagioclase Biotite K-feldspar About It | used to classify, name, and interpret
igneous rocks?

OBJECTIVE Analyze composition and texture of

igneous rock samples, and then infer how they formed.
PROCEDURES
1. Before you begin , read about Analysis and
Interpretation of Igneous Rock Samples below.
Also, this is what you will need :
___ Activity 5.8 Worksheets (pp. 150 – 151 ) and
pencil
___ set of igneous rocks (obtained as directed by
your instructor)
2. Then follow your instructor’s directions for
completing the worksheets.
Hand sample x2
FIGURE 5.13 Granite (felsic, intrusive, phaneritic).
Granite—an intrusive, phaneritic igneous rock that has a low
MCI (light color) and is made up chiefly of quartz and feldspar Analysis and Interpretation of
mineral crystals. Mafic (ferromagnesian) mineral crystals in granites Igneous Rock Samples
generally include biotite and amphibole (hornblende). Granites rich
in pink potassium feldspar appear pink like this one, whereas those Before you begin this activity, compare the named
with white K-spar appear gray or white. Felsic rocks that resemble rock types in FIGURES 5.8 – 5.14 with the igneous rock

granite, but contain no quartz, are called syenites. classification charts in FIGURES 5.4 and 5.5 . Also consider
the origin of each rock type relative to Bowen’s Reaction
Series ( FIGURE 5.6 ) and plate tectonic setting ( FIGURE 5.7 ).

Read about Bowen’s Reaction Series and Plate Tectonics
and Igneous Rocks (starting on page 134 ) if you have not
already done so.

Intrusion, Eruption, and Volcanic
Landforms
Magma is under great pressure (like a bottled soft drink
that has been shaken) and is less dense than the rocks that
confine it. Like the blobs of heated “lava” in a lava lamp,
Hand sample x2 the magma tends to rise and squeeze into Earth’s cooler
crust along any fractures or zones of weakness that it
FIGURE 5.14 Rhyolite (felsic, extrusive, aphanitic). encounters. A body of magma that pushes its way through
Rhyolite—a felsic, aphanitic igneous rock that is the extrusive Earth’s crust is called an intrusion , and it will eventually
equivalent of a granite. It is usually light gray or pink.
cool to form a body of igneous rock.

Igneous Rocks and Processes ■ 141