Page 181 - Laboratory Manual in Physical Geology

ACTIVITY A. Start with a handful
of mud.
6.6 Hand Sample Analysis and
Interpretation

|
THINK How do geologists describe, classify,
About It and identify sedimentary rocks?

OBJECTIVE Be able to describe, classify, and identify
hand samples of sedimentary rocks.
PROCEDURES
1. Before you begin , read about the Formation B. Compact the mud by
of Sedimentary Rocks, Classifying Sedimentary squeezing it in your
Rocks, and Hand Sample Analysis and fist.
Interpretation below. Also, this is what you
will need :
___ Activity 6.6 Worksheet (p. 179 ) and pencil
2. Then follow your instructor’s directions for
completing the worksheets.

Crystalline and Microcrystalline Textures. Sedimentary C. Release your grip to
rocks that form when crystals precipitate from aqueous observe a piece of
solutions have a crystalline texture (clearly visible mudstone.
crystals; see FIGURE 6.2 ) or microcrystalline texture
(crystals too small to identify; see FIGURE 6.2 ). As the FIGURE 6.4 Compaction of mud to form mudstone. The
crystals grow, they interfere with each other and form an more the mud (silt and clay sized grains of detrital sediment) is
intergrown and interlocking texture that also holds the compacted, the harder (more lithified) it will become. Deeply buried
rock together. mud is also lithified by heat as it is compacted, like baking clay pots
in a kiln.

Formation of Sedimentary Rocks
Lithification is the process of changing loose particles variety of minerals precipitate in a particular sequence.
of sediment (unconsolidated sediment) to solid rock The first mineral to form in this sequence is aragonite
(consolidated sediment). Sediment is loose particles such (calcium carbonate). Gypsum forms when about
as pebbles, gravel, sand, silt, mud, shells, plant fragments, 50–75% of the ocean water has evaporated, and halite
and mineral crystals. Sediment is lithified when it is (table salt) forms when 90% has evaporated. Ancient
compacted (pressure-hardened, squeezed: FIGURE 6.4 ) rock salt units buried under modern Lake Erie probably
or cemented together (glued together by tiny crystals or formed from evaporation of an ancient ocean. The
chemical residues, FIGURES 6.5 , 6.6 ). However, it is also salt units were then buried under layers of mud and

possible to form a dense hard mass of intergrown crystals sand, long before Lake Erie formed on top of them (see
that lock together directly, as they precipitate from water FIGURE 6.7 ).
( FIGURES 6.7 and 6.8 ).

Sand (a sediment) can be compacted until it is
pressure-hardened into sandstone (a sedimentary rock). Classifying Sedimentary Rocks
Alternatively, sandstone can form when sand grains are Geologists classify sedimentary rocks into three main
cemented together by chemical residues or the growth of groups: biochemical, chemical (inorganic), and detrital
interlocking microscopic crystals in pore spaces of the (siliciclastic). Refer to FIGURES 6.2 , 6.9 . and 6.10 .

rock (void spaces among the grains). Rock salt and rock
gypsum are examples of sedimentary rocks that form Biochemical Rocks
in situ by the precipitation of aggregates of intergrown The main kinds of biochemical (bioclastic) sedimentary
and interlocking crystals during the evaporation of salt rocks are limestone, peat, lignite, and coal. Biochemical
water or brine. limestone is made of broken and whole animal skeletons
Ocean water is the most common aqueous solution (usually seashells, coral, or microscopic shells), as in
and variety of salt water on Earth. As it evaporates, a FIGURE 6.6 . Differences in the density and size of the

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