3.4 Notes
How are sedimentary rocks formed?
Objective: Describe two ways that sedimentary rocks are formed
There are three types of rocks: igneous, sedimentary, and metamorphic. Today we will be
discussing how sedimentary rocks are formed.
Sediments are rock particles that are carried and deposited by wind, water, or ice. Two types
of sediments are mud and sand. If you were to
place these two sediments into a jar of water, the
sand would settle to the bottom first because it is
heavier. The mud would settle on top of the sand.
The water in the jar is what deposits the
sediments.
What are sediments?
Many sedimentary rocks form like concrete.
Concrete is made up of sand, gravel, and cement.
A chemical reaction occurs that binds these parts
together with water to from concrete. As the water evaporates, the concrete hardens.
Like concrete, sedimentary rocks are formed in water. The rocks form from sediments at the
bottom of lakes, rivers, or oceans. Over millions of years, the sediments pile up in layers. Some
sedimentary rocks form from sediments deposited by wind or glaciers.
Layers of sediment may be hundreds of meters thick. Lower
layers become tightly packed under the pressure and weight
of sedimentary rock. Older sediments become solid rock
when water and air are squeeze out from between the
layers. Sediments can become solid rock when dissolved
minerals in the water cement the sediments together.
When does sediment become solid rock?
Sedimentary rocks form from the remains of living things. Shells or skeletons of sea animals
are on the bottom of the ocean. These shells and skeletons contain calcium carbonate. When
calcium carbonate dissolves the minerals cement together. This forms solid rock.
Coquina limestone is a sedimentary rock formed from shells.
Chalk limestone also forms from shells.
You cannot see the shells because they were formed from microscopic organisms.
How can shells of marine animals form sedimentary rock?