Unit 9-2: The Rock
Cycle: Sedimentary
Rocks
Well, I’ve broken out of
wrestling, and now have to
get the the set of another
Scorpion King movie. I’ve
asked a buddy to help out
with the next section.
Well,
there
Well, lets take
a look
at are some nice sediments.
There are three types of sediments:
what makes sedimentary
rock different from igneous -Clastic
-Chemical
rock. Lets start with
-Organic
sediments.
Clastic Sediments:
-Grains of
rock and mineral
Chemical
Sediments:
-Thesegrains
brokethat
off of
other rocks
-Mineral
formed
erosion or
weathering.
duedue
to atochemical
process.
-Can be
a variety
of
sizes.
-Examples
include
acid
rain,
Organic Sediments:
oxidation
(rust),
oceanofsalt
-Made from
pieces
once
wearing living
away material.
the material.
-Examples: plant materials
such as wood.
Sediments have a range of sizes, from largest
The key to
to smallest:
learning how clastic
However, there are
different
sizes
rocks
form-Pebbles,
is by of
looking at what
sediments. I shouldmakes
probably
explain
them.
These rocks are
-Gravels,
that part!
formed by-Sand,
sediments that are
cemented together
-Silt, by time and
pressure.
-Clay.
We sort the sediments by
size using a sieve.
-Sediments are placed into
the top sieve,
-The lower sieves have
progressively smaller
screens to screen out the
sediments.
-Once the system is turned
on, the sieve vibrates.
-After several minutes, we
can see the different
sediments, sorted by size.
Nature also sorts sediments.
-In rivers, fast moving waters carry all
sediments.
-As the rivers slow down, or approach
the edge of the riverbed, sediments
are dropped off.
-The largest and heaviest (pebbles)
get dropped off first.
-The smallest (clay) stay in the river
the longest.
-Dissolved minerals in the ocean
or river water act as cement that
holds the sediments together.
-Silica, lime, and iron can act as
cement.
-The cement works its way into the
pores and irregularities in the
sediments.
-The cement binds to the rock
sediments and holds them
together.
-Clays and silts can bind together when pressure is high enough,
but heavier materials need cement to hold them together.
-Pebbles and gravels form a conglomerate.
-Sand forms sandstone.
-Silt and clay forms shale.
-Conglomerate is the coarsest
of the clastic sedimentary rocks.
-The pebbles and gravels that
make up the conglomerate are
easily visible to the naked eye.
-Conglomerates can be made out of any type of pebble or gravel.
-Quartz is very commonly found in conglomerates because of its
durability.
-The cement that holds the pebbles and gravels together is usually
very fine grained.
-Sandstone is made of sand
sized sediments.
-Generally, all the sand
sediments are quartz grains.
-Cement holds the sand grains
together.
-Because the cement never fills all the spaces between the sand
grains, there can be up to 30% of air space between the grains.
-Sandstone is rough, gritty and durable.
-In some areas of the world, sandstone was commonly used as a
building material and as a sculpting material.
-Shale is made of clay and
silts.
-Clay is composed mostly of
kaolin, so shale is mostly
kaolin.
-The grains in shale are very
tightly packed together.
-Shale is a very soft and smooth stone.
-It breaks very easily.
-Fossils are found in shales, since they form in rivers.
Some sedimentary rocks form
through chemical processes.
-When minerals are dissolved in
water,
react with
other
-The they
mostcan
common
chemical
minerals
or otherrocks
chemicals.
sedimentary
are:
-This produces
a new material.
-Limestone
-Rock salt
-Rock gypsum
-Limestone forms from tiny grains of of calcite.
-Calcium can also come from shells or bones.
-The tiny grains of calcite are usually deposited from sea or lake
water.
-Limestones are usually light
gray or white in color.
-Due to the heavy nature of the
atoms that make the minerals,
limestone is very dense.
-Limestone will feel smooth to
the touch.
-Rock salt is the natural form of
table salt.
-Rock salt is made almost
entirely out of halite.
-Rock gypsum is very much like
rock salt, but made out of
gypsum.
-Both form through the
evaporation of salt lakes or
ocean bays cut off by sandbars.
-Limestone can also form through
organic means.
Now, there is one last type of
-Shells of sea creatures, plankton,
sedimentary rock we should look
bones, etc. pile up on the ocean
at, and that is sedimentary rocks
floor.
that form through organic means.
-As time progresses, these calcium
deposits compact into limestone.
-On land, a common sedimentary
rock is formed from plant remains:
coal.
-Sedimentary rocks show
special features that help to
identify them.
-One method we use is
stratification.
-Stratification is the
arrangement in visible layers.
-How does stratification
develop?
-When a change of sediments occurs, a new layer develops.
-Different size grains, or new sediments appear, a new layer of
sedimentary rock forms.
-This is how sedimentary rocks become stratified.
-Aha! New sediments result in
new layers.
-That’s why we see sandstone
on top of shale, on top of
sandstone.
-Now, there are a couple other
features we can look at:
bedding planes and cross
bedding.
-Bedding plane: horizontal
layers where stratification
begins.
-Cross bedding: A angled
section or layer in stratification.
-It can be due to wind, erosion,
or tectonic forces.
-Many sandstones show ripple marks on the surface of the
bedding plane.
-These formed by the winds and water that formed and deposited
the sediments.
-Mud cracks develop when deposits of wet clay dry out and
contract.
-In a bedding plane, these cracks are filled with other materials.
-Limestones can sometimes
contain hollow spheres of
silica.
-These hollow spheres
contain crystals.
-This is a geode.
-It appears that geodes
formed by water wearing
away a hallow in the stone.
-Over time, this hallow gets
quartz deposited in it, which
grows into crystals.
-Also in limestones, there
can be lumps of chert or flint.
It’s sedimentary, my dear
Watson!
How do you know so much
about these rocks, Mr.
Holmes?