Page 138 - Laboratory Manual in Physical Geology

can barely see and feel but are too small to identify dark, light, or made of mineral particles. Therefore, it is
with a hand lens or your unaided eye. Clayey rocks essential to classify a rock based on more than one of its
are mostly made of clay, which has grains too small properties.
to feel or see (even with a hand lens).
Igneous Composition and Texture
■ Layered texture. Some rocks have grains arranged in
layers that can be observed at more than one scale: Recall that igneous rocks form when molten rock (rock
over a region, in an outcrop, or in a hand sample. liquefied by heat and pressure in the mantle) cools to a

Sedimentary rocks generally have flat layers made solid form ( FIGURE 4.1 ). Molten rock exists both below
of either clastic grains (gravel, sand, silt, clay, shells, Earth’s surface (where it is called magma ) and at Earth’s
plant fragments) or crystals of gypsum, halite, or surface (where it is called lava ). Igneous rocks can have
calcite. Metamorphic rock layers are generally not flat- various textures, including crystalline (heterogranular),
lying and foliated (a metamorphic texure described glassy, or vesicular (bubbly). They commonly contain
above in which mineral grains have been aligned or mineral crystals of olivine, pyroxene, or feldspars. Igneous
layered, causing the rock to break or reflect light in rocks from cooled lava flows may have ropy, streamlined
a specific direction like the layered scales on a fish). shapes or layers (from repeated flows of lava). Igneous
Metamorphic rock layers may also be folded , like rocks usually lack fossils and organic grains.
you would fold a napkin. If the folds are smooth and
unbroken, then the rock must have been soft and Sedimentary Composition and Texture
ductile (due to high thermal energy) when it was Recall that sedimentary rocks form in two ways
folded. The foliation is due to directed pressure and ( FIGURE 4.1 ). Lithification is the hardening of
shearing during metamorphism. Brittle rocks do not sediment—masses of loose Earth materials such as clasts
fold easily. They tend to fracture (break, form a clastic (rock fragments, detrital mineral grains, pebbles, gravel,
texture) and move apart along faults. sand, silt, mud, shells, plant fragments) and products
of chemical decay (clay, rust). Precipitation produces
mineral crystals that collect as in situ aggregates, such as
the rock salt that remains when ocean water evaporates.
ACTIVITY The lithification process occurs as layers of sediments
are compacted (pressure-hardened) or cemented (glued
4.5 Rocks and the Rock Cycle together by tiny crystals precipitated from fluids in the
Model pores of sediment).
Thus, most sedimentary rocks are layered and
THINK How are rocks formed, deformed, have a clastic texture (i.e., are made of grains called
About It transformed, melted, and reformed as clasts —fragments of rocks, mineral crystals, shells, and
| a result of environmental factors and plants—usually rounded into pebbles, gravel, sand, and
natural processes of the rock cycle? mud). The sedimentary grains are arranged in layers due
to sorting by wind or water. Sedimentary rocks may also
include fossils —bones, impressions, tracks, or other
OBJECTIVE Analyze and classify rocks, infer how evidence of ancient life.

they formed, and predict how they may change The crystalline sedimentary rocks are layered
according to the rock cycle.
aggregates of crystals precipitated from water. This includes
PROCEDURES the icicle-shaped stalactites that hang from the roofs of

1. Before you begin, read about The Rock Cycle caves. Common minerals of these precipitated sedimentary

(page 114 and FIGURE 4.2 ). Also, this is what you rocks include calcite, dolomite, gypsum, or halite.
will need :
Metamorphic Composition and Textures
___ pencil with eraser
___ Activity 4.5 Worksheets (pp. 127 – 128 ) Recall that metamorphic rocks are rocks that have been
deformed and transformed by intense heat, intense
2. Then follow your instructor’s directions for
pressure, or the chemical action of hot fluids ( FIGURE 4.1 ).
completing the worksheets using Figure 4.5 .
Therefore, metamorphic rocks have textures indicating
significant deformation (folds, extensive fractures, faults,
and foliation). Fossils, if present, also are deformed
(stretched or compressed). Metamorphic rocks often
Rock Classification contain garnet, tourmaline, or foliated layers of mica.
All rocks are classified as igneous, sedimentary, or Serpentine, epidote, graphite, galena, and sphalerite occur
metamorphic, based on their properties of composition only in metamorphic rocks. Metamorphism can occur over
and texture and how they formed. Some properties are large regions, or in thin “contact” zones (like burnt crust
characteristic of more than one rock type. For example, on a loaf of bread) where the rock was in contact with
igneous, sedimentary, and metamorphic rocks all can be magma or other hot fluids.

Rock-Forming Processes and the Rock Cycle ■ 119