Chapter 4: Rocks
Section 4: Sedimentary Rocks
Study Guide
Sedimentary Rocks
• Sediments are loose materials like rock
fragments, mineral grains, and bits of
shell.
• Sediments come from already-existing
rocks that are weathered and eroded.
• Sedimentary rocks form when sediments
are pressed and cemented together, or
when minerals form from solutions.
• Sedimentary rocks form as layers. The
oldest layers are at the bottom because
they were deposited first.
– Sometimes the forces within the Earth
overturn layers of rock, and the oldest are no
longer on the bottom.
Classifying Sedimentary Rocks
• Sedimentary rocks are classified by their
composition and the manner in which
they are formed.
• Sedimentary rocks are classified as
detrital (clastic), chemical, or organic
(nonclastic).
Detrital Sedimentary Rocks
• Detrital, or clastic, sedimentary rocks are
made from the broken fragments of other
rocks. These loose sediments are
compacted and cemented together to
form solid rock.
• The process that breaks rocks into smaller
pieces is called weathering.
– Rocks are weathered by things such as air,
water, and ice.
– Sediments are classified by size. Clay is the
smallest sediment, followed by silt, then
sand, and then gravel, the largest sediment.
• Erosion moves sediments to a new
location where they are deposited.
Pressure from the upper layers pushes
down on the lower layers of sediment.
• If the sediments are small, they can stick
together to form solid rock. This process
is called compaction.
• If sediments are large, pressure can’t
make them stick together. Cementation
occurs when minerals like quartz and
calcite, are deposited by water between
pieces of sediment. The minerals act like
glue and form a detrital sedimentary rock.
• Conglomerate, sandstone, siltstone, and
shale are all detrital sedimentary rocks
named for the shapes and sizes of the
sediments that formed them.
Chemical Sedimentary Rocks
• Chemical sedimentary rocks form when
dissolved minerals come out of solution.
• Minerals collect when seas or lakes
evaporate. The deposits of minerals that
come out of solution form sediments and
rocks.
• Gypsum and rock salt form when water that is
rich in dissolved salt evaporates. Halite forms
rock salt.
• Calcium carbonate is carried in solution in
ocean water. When calcium carbonate comes
out of solution, it forms calcite. Limestone is a
chemical sedimentary rock that is formed as
crystals of calcite grow together.
Chemical Sedimentary Rocks
continued
• Large areas of the United States have
limestone bedrock because seas covered
much of the country throughout geologic
history.
Organic Sedimentary Rocks
• Organic sedimentary rocks are made of
the remains of once living things.
• Fossil-rich limestone is made from the
remains of once-living marine organisms.
– If a rock is made completely of shell
fragments that you can see, it is called
coquina.
– Chalk is another organic sedimentary rock
made of microscopic shells.
• Coal is an organic sedimentary rock that is
formed when pieces of dead plants are
buried under other sediments in swamps.
• The sediments are compacted over
millions of years to form coal.
Organic Sedimentary Rocks
continued..
• Chemical and organic sedimentary rocks
are also called nonclastic sedimentary
rocks, because they are not made up of
broken fragments of other rocks.