The Rock on the Bottom
By Trista L. Pollard
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As I laid on a rock,
I received such a shock,
Another rock upon me lay,
Dried and hardened throughout the day,
Before I knew it, many more had lain upon me,
Many strata as far as the eye can see...
If sedimentary rocks could talk, would they complain about carrying
the weight of other rocks on their backs? Or would they speak about the
many years of history their layers have witnessed? Since geologists do not
have the privilege of studying "talking" rocks, they must rely on their
observations of the layers and their formation to tell the true story.
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A sedimentary rock's life begins with the sediment. Our planet's surface
is constantly worn down by physical weathering and erosion. The wind and
rain chip away at the rocks on the surface, breaking them into smaller
fragments. These smaller fragments of rocks, minerals, and organic material
are carried by running water, waves, and glacial ice to other areas. As the
sediment is moved, it continues to be broken down physically or chemically
altered. The composition of sediment is based on the source of its materials.
Over time, the sediment is deposited in different areas. It's the build-up of
this sediment that creates the sedimentary rock. The solid materials settle out
of the fluid or water. In fact, the word "sedimentary" comes from the Latin
word sedimentum, which means "settling." Debris from weathered bedrock,
particles of sand from dunes, mud from swamp floors, and surprisingly,
household dust all become part of the sediment train. Now that the sediment
has been deposited, it goes through the processes of compaction and
cementation.
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As new sediment piles on top of old sediment, the layers on the bottom
are compacted. The weight of the overlying layers causes the lower layers to
be squeezed and their volume to decrease. As more layers and time passes,
the layers become cemented together by mineral deposits. These mineral
deposits precipitate from the water and settle into the pore spaces between
the sediment grains. These minerals provide the glue that holds the solid
sedimentary rock together. This process takes place over many years. So it's
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no wonder that geologists study sedimentary rocks for clues about our
planet's past.
Sedimentary rocks are classified into three main groups based on how
they form and their composition. These groups are chemical sedimentary
rocks, organic sedimentary rocks, and clastic sedimentary rocks. When
scientists study sedimentary rocks, they find that accumulating sediment has
two main sources: solid particles from weathered rocks and solid material
produced from chemical weathering. Chemical sedimentary rocks form
when dissolved minerals precipitate out of water. This occurs because the
chemical concentration of the water and minerals changes as the water
flows. Minerals also precipitate when evaporation occurs. Once the water
evaporates, dissolved materials remain. These remaining minerals form
rocks or evaporites. Common evaporates are gypsum and halite.
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Organic sedimentary rocks form when the remains of plants and animals
are buried before they decay. When plant remains are buried in places where
there is very little oxygen in the water, such as swamps, the plants will not
decay as quickly. Over many years the accumulating plant remains are
compacted and cemented into carbonated matter. This is the process that
forms coal. Rocks made from the remains of animals fall into the category of
limestone. Chemical limestones form when chemicals precipitate out of the
water. Organic limestones are made from marine animals like coral, clams,
oysters, and plankton. These shelled organisms use minerals, calcite and
aragonite from the sea water to build their shells. After the organisms die,
their shells settle to the ocean floor. Over time, these shells accumulate and
their layers become compacted and cemented. Chalk, another type of
limestone, is also formed from the shells of marine animals.
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Clastic sedimentary rocks are formed from the fragments of other rocks.
These fragments are transported from their source by wind, water, and ice.
Eventually, these fragments are deposited, compacted, and cemented into
new sedimentary rocks. As with other types of sedimentary rocks, this
process takes a long time. Geologists classify clastic sedimentary rocks
according to their sediment or particle sizes. Conglomerate rocks have
fragments that can be big as boulders or as tiny as mud particles. In fact,
what gives conglomerate rocks their distinctive appearance is the
combination of large round pebbles that are cemented together with finer
sediments or minerals. Breccia rocks differ from conglomerate rocks
because they have angular fragments and sharp corners. Like conglomerates,
breccias have larger fragments that are cemented together by finer
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fragments. In both types of rocks you can clearly see the pieces of sediment.
On the opposite end of the spectrum, there are clastic rocks that are made
of sand-sized grains. Sandstone, which also contains the mineral quartz, is
formed when sand-sized grains are cemented together. Within the
sandstones there are pores between the sand grains. These pores allow
groundwater, natural gas, and other fluids to move through the rock. Shale
has flaky clay-sized particles that have been compacted and cemented. The
flat layers found in shale can be easily broken apart. Sedimentary rocks like
sandstone, limestone, shale, and breccias have provided a lot of information
about the Earth's surface. As geologists have learned, it's the rock on the
bottom that holds the clues to our planet's history.
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Name
_____________________________
Date
___________________
The Rock on the Bottom
1. Breccia is a form of ______.
Clastic sedimentary rock
Chemical sedimentary rock
None of the below
Organic sedimentary rock
2. How do chemical limestone
sedimentary rocks differ from
organic limestone sedimentary
rocks?
3. Based on context clues, what does 4. What are evaporites and how are
the word debris mean?
5. Compare compaction and
cementation.
they formed?
6. How do geologists classify
sedimentary rocks?
Name
_____________________________
Date
___________________
The Rock on the Bottom
7. True/False: Sandstone rarely
contains the mineral quartz as part
of its composition.
8. Conglomerate rocks have a
combination of ______.
Angular fragments and finer
grains of sediment
Large round pebbles
cemented together with finer
grains of sediment
Flaky clay-sized particles
and finer sediment
Smaller round pebbles
cemented together with finer
grains of sediment