How do Sedimentary Rocks Form?

There are three main types of rocks: sedimentary, metamorphic and igneous. Each are formed in a different ways. Sedimentary rocks contain rounded grains in layers. Chalk, shale and limestone are examples of sedimentary rocks.

This activity is a very simple demonstration of how how sedimentary rocks are formed. Sedimentary rocks are formed when layers of sediments build up over time and harden to form a rock with many layers. They are usually soft and relatively easy to break unlike the harder metamorphic (formed by heat pressure under the Earth’s crust ) or igneous rocks ( formed from lava ).

What you need

Sandwich ingredients

Bread

Hummous

Peanut butter

Sandwich meat

Cucumber

Tomatoes

How to make a sedimentary sandwich

Create a delicious sandwich using your ingredients.

Imagine a river flowing over stones breaking them down, carrying them along until they settle. Picture a layer of sand building up over the stones, followed by layers of mud, more rocks or gravel and sand. Build up the sandwich layer by layer, add some sliced tomatoes ( or even raisins and chocolate chips ) to resemble larger rocks.

Push down on the sandwich imagining the layers compacting to form a sedimentary rock.

Sedimentary sandwich made to demonstrate how sedimentary rocks form

The sedimentary sandwich is a very basic demonstration of how sedimentary rocks form but remember the real process can take millions of years!

Layers of broken rock transported from rivers build up at the bottom of the ocean and build up in layers called sediments. This is known as sedimentation. As more and more layers pile up they start to push down on the lower layers. This is called compaction. Over a long period of time, water is squeezed out of the layers of rock and salt crystals form. The salt compounds stick the pieces of rock together. This is called cementation. Eventually sedimentary rocks form.

transport – deposition – sedimentation – compaction – cementation

Extra challenge

Can you create something that looks like an animal fossil to add to your sandwich?

Be a Geologist! Pop the sandwich in the freezer for 20 minutes and then dig through the layers with an apple corer. Geologists drill through the Earth to see the layers below.