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Sedimentary Rocks
The most vddeIy used classification system for
sedimentary rocks places them into three main categories according to origin of the materials from
which they are made. These three categories are:
clasdc rocks, organic rocks, and chemical rocks.
Focus on [n/s oues~/on as you
> What are me differen[ caregories ot sedimentary
rOCKS~
Rocks
Sedimentary rocks that are made of the fragments
of previously existing rocks are known as clastic
rocks. Clastic rocks are further classified according to
the s~ze and shape of the fragments in them.
Some clastic rocks are made of rounded pebbles
cemented together by clay, mud, and sand. If over a
third of the rock is made of pebbles, the rock ~s
called a conglomerate kahn-~tXHM-er-iht~. The pebbles in conglomerates are smooth and rounded because they have been worn down by the action of
water. Conglomerates are not as common as rocks
made of smaller p~eces because moving water tends
to break large pieces into smalle~ pieces. Because
they resemble an old-fashioned pudding filled with
nuts and chopped fruit, conglomerates are
sometimes called puddingstones
Fi~tur~ 13-28 Clastic rocks are classified according to the size of
[ne rock fragments mey contain. Puddingstone and breccia have
[he largest fragments. How do the fragments tn these two rocks
differ? Sandstone is maoe uo of sand-sized fragments. Shale is
COmposed of dust-sized fragmen[s.
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Clasnc rocks made of small, sand-sized grains are
called sandstones. At least half the particles in a c]as.
tic rock must be sand-sized in order for it to be con.
sidered a sandstone. Sandstones are very common
rocks. They are formed from the sand on beaches
~n riverbeds, and in sand dunes, In sandstones, the
sand grains are cemented together bv minerals. T1qe
minerals harden in the small spaces, or pores, between the grains.
Many geologists use the mrm shale to describe all
the cIastic rocks that are made of particles smaller
than sand. Shales form from small particles of mud
and clay that settle to the bottom of quiet bodies of
water such as swamps. Most shales can be split into
fiat pieces.
Organic: Rocks
Organic rocks come from orgamsms: they are
formed either directly or indirectly from materia~
that was once living. Limestones, which are made
primarily of the mineral calcite, are often (but not
always) organic rocks. Deposits of limestone may be
formed from the limestone shells of creatures such
as clams and certain microorganisms. X/ghen these
organisms die, their shells collect on the oceal~ floor.
Eventually, the shells are compacted into rock.
Living organisms may create limestone directly.
Sometimes many animals with limestone shells live
Figure 13-29 Cooulna is composed primarily of fossil clam
shells The wh~re cliffe of Dover are maoe of chal< To whar grouo
of sedimentary rocks do cocluina and Chalk oelong?
together. They cemen~ their shells together and over
time form large structures called reefs. Corals build
hraestone reefs off the coast of Florida and arouud
many of the Caribbean and Pacific islands. Oysters
build limestone reefs along the Texas Gulf coast.
Have you ever written or drawn with sticks of
cllalk? If so. you have first-hand experience with one
kind ot limestone. Chalk is a type of fine-grained
limestone composed of microscopic shells, small
fragments of shells, and calcite crvstals. Because the
parucles in chalk are tiny and relatively loosely
packed, chalk ~s much softer than other limestones.
Coal is another rock that is formed from the remains of living things. It ts made from plants that
[Ned millions of years ago.
Coral Convers ons
The largest cora~ reef s
~ne Grea~ Barrier Reef ~vn~cn
cara els ~ne normeasterR
coas~ of Australia for a dis~ance of aDou~ 2000 kilomelets, -lOW marb mel:ers ong s
the Coral Reef? t~ow man~
ce~me~ersl Comuare
distance [o iRe as[aqce
acr3ss [ne ~eo States
wRicn s 4517 Km from eas~ ~o
Chernka Rocks
Some sedimentary rocks are formed when a sea
or a lake dries up, l~avil~g large amounts of mineralsF~gure 13-~0 Chemtca~ rocks form
that were dissolved in the water. As you can see in tn many different places. As [ne
sun bears down, evaooration forms
Figure 13-30. the deposited minerals may" create
strange rowers of salt an~ calcium
spectacular formations. Examples of chemical rocks carbonate at Mono LaKe,
formed in this way include rock salt and gypsum.
California. Spectacular formations
Some limestone rocks are formed by inorganic are stowly built underground as
dnPs into a cave and
processes rather than by organisms. The strange and water
~eDosits minerals,
beautiful limestone formations found in many caves
are formed by mineral-rich water drippmg into the
Figgre 13-3~ Interesting
sedfmentary rock structures
include geodes (top left), ripple
marks (center left), concretions
(bottom left), fossils (top right),
and mud cracks (bottom right).
cave. ~hen the water evaporates, a thin deposit of
limestone is left behind. Over a long period oi" time,
the deposits are built up into pillars, spikes, and other struculres. Limestone may also be produced
through chemical changes in ocean water that cause
grains of calcite to form. The small grains get larger
as additional thin layers are deposited from the
ocean water. So these limestones are chemical rocks
rather than organic rocks.
1. How are sedimentary rocks classified? Give an
example of each major group.
2. ~Hat are clastic rocks? How are clastic rocks
classified?
3. How are organic and chemical rocks similar?
How are they different?
Critical Thinldng--Relating Concepts
4. Explain how the fossil of a fish formed and
ended up on the side of a mountain.
P t m ll hic R ¢ks
When already existing rocks are buried deep within the Earth, tremendous heat. great pressure, and
chem’~cal reac~ons may cause them to change hato
different rocks vfith ddfferent text~xes and structures.
The changing of one type of rock into another as
a result of heat. pressure, and or chemical reactions
is called metamorphism (meht-ah-MoR-fihz-uhm).
Focus on this question as you
read.
HoW are existing rocks
changed into metamorphic
rocks?
Figure 13-82 Metamorohism may cause me minerals in a rook to
separate into bands (right). It may a/so cause impurities in a rock
to form minerals SUCh as garners, that are not found in other
types of rocks. Can you explain why schist (SHIEST), the mos[
common metamorphic rock. may oe dotted with garnets (left)?
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