Download Sedimentary Rocks…..Rock?

Sedimentary Rocks…..Rock?
All over the United States we have very beautiful and strange
scenery. South Dakota has the Badlands National Park with its jagged
peaks, steep cliffs, and deep canyons sculpted in colorful rock that is
layered like a birthday cake. The layers of rock are red, orange, pink,
yellow, or tan. These rocks, like many of our other breathtaking rock
formations developed over millions of years as tiny pieces of mud,
sand, and volcanic ash were deposited in thick layers. The mud and
sand slowly changed into sedimentary rock. These rock formations were exposed when the harsh
sun, wind, and water wore away the surrounding material. What were left were the layers of history
in the form of sedimentary rocks.
If you put some muddy water in a jar and let it set a few days, what would you see? The water
would become clear. There would be layers of dirt and stones at the bottom. This is called sediment.
Sediment is small, solid pieces of material that come from rocks or living things. The rocks and living
things have been broken apart by weathering. Wind, water, and ice break down rocks and minerals
into smaller particles. In addition to particles of rock, sediment may be pieces of bone, shells, leaves,
plants, bark, and other parts of living things. These pieces of sediment are what sedimentary rocks
are made from.
Sedimentary rocks only make up about 10 percent of the Earth’s rocks we see. Even though
these rocks are only a small amount of all the rocks on Earth, they help us to know what happened in
Earth’s history. The sediments form layers with the newest layers on top of the old ones. This process
is called Stratification. As we look at the layers we can see what happened in history. Geologists can
study fossils that were cemented into the layers of the rock. These fossils tell us about what type of
living creatures were alive during different time periods. Sedimentary rocks are the only type of rock
that has fossils in them.
Most sedimentary rocks are formed through a series of process: erosion, deposition,
compaction, and cementation. Running water, such as streams and rivers, helps form sedimentary
rocks. The water carries the broken pieces of rock, this is called erosion. Wind and glaciers can also
cause rocks and sediments to break down. The sediments are then dropped in layers wherever the
current, wind flow slows down, or as glaciers melt. This is called deposition. After the layers are
deposited, sedimentary rocks begin to form. This can happen in one of three ways. A process called
compaction can form them. Compaction happens when layers of sediment are squeezed together.
The pressure of the rocks on
top causes this to happen.
Sedimentary rocks can also
be formed by cementation.
Cementation happens when
water evaporates out of the
layers of sediment and the
minerals are left behind.
They "glue" the pieces of rock together. Cementation is kind of like building a brick wall. The
bricklayer puts a mixture of sand and water between the bricks. This layer hardens as it dries, and the
bricks stick together. Sedimentary rocks are formed almost the same way. The minerals between the
rocks dry and stick the pieces together.
Sedimentary rocks are classified based on the types of sediments that make them up. Clastic
rocks are made from pieces of other rocks. They are formed by compaction and cementation. Clastic
rocks can range in size from tiny particles of soil to huge boulders. Conglomerates and breccias are
types of clastic rocks that have larger pieces of rock inside them. The only difference between the
two is that conglomerates have rounded stones in them and breccias have jagged stones. Shale and
sandstone are other types of clastic rocks, which are made from tinier bits of rocks like clay and sand.
Conglomerate
Breccia
Sandstone
Shale
The third kind of sedimentary rock is called organic. Organic rocks form from decayed plants
and animals and not necessarily from other rock fragments. Remember we discussed organic to mean
once living things. They are usually formed in oceans or swamps. The decayed material is compacted
into rock. Two common organic rocks are limestone and coal. Limestone if formed because animals
have calcium in their shells and bones. After the animal dies the calcium combines with other
elements and hardens into a rock. Because limestone is directly made from animals it is common to
find fossils in it. Coal is formed from water animals and plants that died millions of years ago and
were squashed into the ground. Ohio has a lot of coal rocks.
The last type of rock is a chemical sedimentary rock. Chemical sedimentary rocks are formed
when water evaporates and leaves behind minerals that harden into rock. A great example of this
kind of rock is salt. So remember when you're eating salt, you're eating a rock. It's a rock called halite.
Sedimentary rocks give us important resources. Most ground water is found in sedimentary
rocks. Coal, oil, and natural gas are found in these rocks as well. Sedimentary rocks also give us
building materials. Iron is used for making steel. Limestone is made into cement. It is also cut to make
blocks for large buildings. Where would we be without sedimentary rocks?