Why are the sedimentary rocks called the layers rocks?

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.

How do 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.

Construction for the White House began in 1792, and most of the work was carried out by people who had only recently come to the newly formed country of America. Its outside walls are made of a type of sedimentary rock called sandstone. The sandstone that was used to construct the White House is very porous, which means that rainwater can easily penetrate the sandstone. This made the White House susceptible to water damage in its early days of construction. To stop the water damage, workers had to cover the sandstone in a mixture of salt, rice, and glue, giving the White House its distinct white color.

Figure 4.10 The White House of the United States of America is made of a sedimentary rock called sandstone.

Lesson Objectives

• Describe how sedimentary rocks are formed.
• Describe the properties of some common sedimentary rocks.
• Relate some common uses of sedimentary rocks.

Sediments

In this lesson, you will learn about sedimentary rocks like sandstone, how they form, how they are classified, and how people often use sedimentary rocks.

Sedimentary rocks are formed by the compaction of sediments. Sediments may include:

• fragments of other rocks that have been worn down into small pieces, like sand,
• organic materials, or in other words, the remains of once-living organisms,
• or chemical precipitates, which are materials that get left behind after the water evaporates from a solution.

Most sediments settle out of water (Figure 4.11). For example, running water in rivers carries huge amounts of sediments. The river dumps these sediments along its banks and at the end of its course. When sediments settle out of water, they form horizontal layers. One layer at a time is put down. Each new layer forms on top of the layers that were already there. Thus, each layer in a sedimentary rock is younger than the layer under it and older than the layer over it. When the sediments harden, the layers are preserved. In large outcrops of sedimentary rocks, you can often see layers that show the position and order in which the original sediment layers were deposited. Scientists can figure out the relative ages of layers by knowing that older ones are on the bottom and younger ones are on top.

Figure 4.11: Most sediments settle out of running water, such as in this river.

There are many different types of environments where sedimentary rocks form. Some places where you can see large deposits of sediments today include a beach and a desert. Sediments are also continuously depositing at the bottom of the ocean and in lakes, ponds, rivers, marshes and swamps. Avalanches produce large unsorted piles of sediment. The environment where the sediments are deposited determines the type of sedimentary rock that will form there.

Sedimentary Rock Formation

Sediments accumulate and over time may be hardened into rock. Lithification is the hardening of layers of loose sediment into rock (Figure 4.12). Lithification is made up of two processes: cementation and compaction. Cementation occurs when substances crystallize or fill in the spaces between the loose particles of sediment. These cementing substances come from the water that moves through the sediments. Sediments may also be hardened into rocks through compaction. This occurs when sediments are squeezed together by the weight of layers on top of them. Sedimentary rocks made of cemented, non-organic sediments are called clastic rocks. Those that form from organic remains are called bioclastic rocks, and sedimentary rocks formed by the hardening of chemical precipitates are called chemical sedimentary rocks. Table 4.2 shows some common types of sedimentary rocks and the types of sediments that make them up.

Figure 4.12: This cliff is made of a sedimentary rock called sandstone. The bands of white and red represent different layers of sediment. The layers of sediments were preserved during lithification.

Note from the pictures in the table that clastic sedimentary rocks vary in the size of their sediments. Both conglomerate and breccia are made of individual stones that have been cemented together. In conglomerate, the stones are rounded; in breccia, the stones are angular around the edges. Sandstone is made of smaller, mostly sand-sized particles cemented together. Siltstone is made mostly of silt, particles that are smaller than sand but larger than clay. Shales have the smallest grain size, being made mostly of clay-sized particles and hardened mud.

What do you call the layers in sedimentary rock?

Why are sedimentary layers?

Why are rocks in layers?

Is sedimentary rock layered rock?