Sedimentary Rocks for Middle School Science

Feb 20

3 min read

Sedimentary rocks are made when small pieces of rock, sand, and other materials build up in layers over time. Water, wind, or ice carry these pieces to new places, where they settle and get pressed together. Over millions of years, they harden into rock. There are different types of sedimentary rocks, like sandstone, which comes from sand, and limestone, which forms from shells and sea creatures. These rocks are important because they can hold fossils, telling us about Earth's past. They are also used for building and can store water and oil underground.

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Sedimentary Rocks

The secrets of Earth's past hide in sedimentary rock. Sedimentary rock is the only type of rock that contains fossils. Igneous rock forms from cooling magma that is too hot to hold a fossil, and the intense heat and pressure that forms metamorphic rocks destroy any fossils within the rock.

Sedimentary rocks can hold fossils because the hard minerals that make up fossils can withstand the compaction and cementation that turn sediments into sedimentary rock.

Sediments are small pieces of rocks, minerals, plants, or animals. For example, sand is a sediment. Rock sediments form from weathering when wind, water, plants, and animals break larger rocks into smaller pieces. Small pieces of animal bones and shells are also types of sediments.

Wind and water carry sediments from one place to another in a process called erosion. As wind and water slow, they deposit sediments creating beaches, deltas, and small islands.

Over time, sediments build up into many layers. The top layers put immense pressure on the bottom layers. Compaction and cementation turn the sediments into sedimentary rock. Compaction is the process of individual sediments being squeezed together, and cementation is the process of dissolved minerals gluing the sediments into rock as water moves through the layers.

There are three types of sedimentary rock. Clastic sedimentary rock is formed from mostly rock sediments. For example, sandstone is a type of clastic sedimentary rock. Organic sedimentary rock is formed from mostly living things, including shells, bones, and plants. Coal is an example of organic sedimentary rock. Finally, chemical sedimentary rock is formed when the water surrounding dissolved compounds evaporates, leaving the compounds behind. The minerals halite and gypsum are common in chemical sedimentary rocks. Halite is made up of sodium chloride, and gypsum is made up of calcium sulfate. Both of these compounds dissolve quickly in water and precipitate back into a compound when water evaporates.

Limestone forms when water containing dissolved calcium carbonate evaporates. In caves, water dripping from the ceiling leaves behind limestone stalactites and stalagmites. Limestone can be either a chemical sedimentary rock or an organic sedimentary rock. The oceans are full of dissolved calcium carbonate. Animals in the water use the calcium carbonate to make their shells. When these animals die, parts of their shells become sediments at the bottom of the ocean. Over millions of years, compaction and cementation turn the sediments into organic sedimentary rock.

Over time, sedimentary rocks can be weathered back into sediments or pushed deep into the Earth's crust. The high temperature and pressure deep underground turn the sedimentary rock into metamorphic rock. Even deeper in the crust, the heat from the mantle will melt the sedimentary rock. Eventually, the liquid rock, called magma, will cool and become igneous rock. The rock cycle describes how rocks change form as they move through the environment.

Sedimentary Rocks Videos

I like to use videos to teach new concepts because students can use visual cues to support their learning. Here are some of my favorite videos on sedimentary rocks.