Download Sedimentary Rocks Notes Teacher

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Sedimentary Rocks Notes
Objectives: Describe the formation of sedimentary rocks; describe the different types
of sedimentary rocks; classify different types of sedimentary rocks.
Most sedimentary rocks form from rock particles.
Most sedimentary rocks are made from other rock particles called sediments.
Sediments are any material that settles out of water or air to form a sedimentary
rock. Examples of sediments include rocks, minerals, and the remains of plants
and animals. Sedimentary rock forms through a five-step process that most often
occurs in water but can occur on land.
Most sedimentary rocks are formed in a five-step process.
There is a process that forms sedimentary rocks. This process is called
lithification. (Technically, this refers to steps 4 and 5, but in general we can use it to
describe the entire process.) There are five steps:
1. Weathering. Weathering forms the sediments that make a sedimentary rock.
2. Erosion. Erosion is the process that transports the sediments formed during
weathering. Two primary causes of erosions are water and wind. These carry
the particles. During this step, many sediments become rounded. This is why
we find many, but not all, sedimentary rocks with rounded grain.
3. Deposition. Deposition is the process by which sediments settle out of water or
wind that is carrying it. The particles are deposited into layers. When
deposition occurs, more dense particles settle before less dense particles. This
is because the dense particles require more energy to keep them moving in
the wind or water.
4. Compaction. Compaction is the process that presses these layers together.
The weight of the layers helps compact everything together, closing the gaps
between the sediments. This creates a very tightly packed, but loosely held
together rock.
5. Cementation. Cementation is the process in which dissolved minerals
crystallize between the sediments. The result is the sediments become
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“glued” together to form the sedimentary rock. If this step doesn’t occur, the
result is a tightly packed group of sediments that can easily break apart.
Not all sedimentary rocks complete all five steps. Silt is a sedimentary rock that is
formed by the first 4 steps. It does not need to be cemented together. Other
sedimentary rocks can form without the 5 steps.
Sedimentary rocks are classified by their sediments
Scientists classify sedimentary rocks into groups based on what sediments make
up the rock. There are three main groups of sedimentary rock: clastic, organic, and
chemical.
Clastic, also known as detrital, rock is the most common sedimentary rock. It
is made of broken pieces of rocks. Common examples include shale made of clay,
sandstone made of sand, conglomerate made of rounded grains, and breccia
made of jagged grains.
Organic rock is made of once-living organisms. The two most common
examples are coal and limestone. Coal is an organic rock made from remains of
plants: wood, bark, leaves, and stems included. Coal forms through the 5-step
processes. Coal that we use today was made from the remains of organisms from
millions of year ago. Limestone is a rock made of carbonate minerals that are from
the shells and skeletons of animals. A very common use of limestone is chalk.
Limestone can form in two ways. Because this limestone is made from once-living
things we call it fossilitic limestone.
Chemical rock forms when minerals dissolved in a solution crystallize.
When water washes over rocks, or if it becomes acidic, it can dissolve certain
minerals. (This is how caves form!) As the water evaporates, the minerals can begin to
recyrstallize. Many rocks form through this process including limestone, gypsum,
and rock salt. These are considered rocks because they are not pure enough to be
minerals. Limestone formed through this process can be called oolitic or tufa.
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