Sedimentary Rocks

Sediments - Weathering and Erosion

  • Sedimentary rocks are formed from sediments.

  • Sediments are formed when rocks are exposed to the weather.

  • Over time, ice, rain and wind break down exposed rocks into smaller fragments.

  • Water acts as a 'universal solvent', chemically dissolving minerals and other materials from the rocks and soil.

  • Over time, wind, water, ice and gravity transport sediments in a process called erosion.


Sediments are classified by size. If you add a soil sample to water and stir or shake, you will notice that the sediments will settle in horizontal layers sorted by size.

Clay grains are so small and light they are easily suspended in water, and will only settle as mud where the water is calm.

Watch the video from Open University about how sand is formed from granite.

Note how the sediments are sorted and broken down during chemical and physical weathering, and transport by erosion.

Note which minerals are dissolved, and which mineral remains as the main component of sand.

Eventually, sediments settle in a horizontal layer. Other layers will settle over top of the first layer, and the oldest layer is on the bottom and the youngest on top. These are two very important concepts in geology.

Layers:

4 Youngest

3

2

1 Oldest

TASK 28: Watch the sand video. Collect sand samples from different sources, examine them under a microscope and try to determine how much weathering and erosion they've been subjected to. Look at grain size and shape. Are the grains different sizes, or all much the same? Are the grains rounded or angular? Look for the presence of minerals. What is the percentage of quartz? Are other minerals such as feldspar or mica present? Look at grain texture. Have the grains been transported by water or wind?

Estimate if the samples have been in the environment for a long or short time, and if they come from a marine or desert environment. Like to know more about sand? Note the 'pitting' on the desert sand grains in this image.

Sedimentary Rocks

Over time, layers of sediments may become rocks through compaction and cementation. This process is known as lithification.

Sedimentary rocks can be hard to identify because the same rock types can look completely different. These three rocks are all sandstone.

Sedimentary rocks are classified as:


  • Clastic sedimentary rocks are formed from particles of pre-existing rocks.

  • Chemical sedimentary rocks are formed by the evaporation and precipitation of minerals, which over time may become rocks through compaction and cementation.

  • Biological sedimentary rocks are formed from the skeletons and shells of marine organisms (limestone), or the carbon in plants (coal).

Apart from the evaporite minerals (like salt) and coal, there are three main types of sedimentary rocks; mudstone, sandstone and limestone. The size of the original grains determines if a sediment will form mudstone and sandstone. Limestones incude all carbonate rocks.

Shale is lithified mud

Sandstone is lithified sand

Limestone is rock mostly composed of calcium carbonate

Chert is a hard sedimentary rock made from small quartz crystals

Conglomerate is lithified gravel

Breccia is shattered rock held together by mineral cement

Fossils of once living plants and animals are found in sedimentary rocks.

Dating Rocks

While the actual age of some rocks can be dated using radiometric dating (radioactive decay), geologist can also work out the age of rock layers compared to each other.

  • The oldest are on the bottom and the youngest on top. When they were first laid down the layers were horizontal.

  • Tectonic forces can tilt, break and even fold sedimentary layers, but that must have happened afterwards.

  • Similarly, if magma cuts through layers it must have happened afterwards. This is called relative dating - the order in which events happened, rather than providing an actual date. The study of rock layers is called stratigraphy.

Stratigraphy

TASK 29: Examine the animal tracks and determine the order in which they were made.

TASK 30: Examine the rock strata and determine the order in which the layers were laid down. Assume P is an igneous intrusion (magma).

Using fossils to date rocks

In the 18th and 19th centuries scientists noticed that rocks of the same age contained the same fossil species - even when the rock layers extended over a large area.

Today we know that life on Earth has changed through time. Fossils of shelled animals can be found in all time periods, while humans can only be found in the youngest layers.

We can also see evidence of fossil succession in a single species - trilobites. Trilobites were hard-shelled segmented creatures that lived in Earth's ancient oceans. They are found in the rocks of all continents.Trilobite fossils first appeared in the Cambrian period 521 million years ago, and completely disappeared from the fossil record 300 million years later during the Great Permian Extinction, when 90% of all species on Earth died out. If you are interested you can learn more here.

Artists impression of trilobites

Trilobite fossils

Trilobite

Trilobite

Next we'll look at fossils.