The Science of Chalk

The Science of Chalk

What is Chalk and how is it formed? 

Chalk is a white, grey or yellow limestone formed mainly out of calcium carbonate, CaCo3 in the form of the mineral Calcite. The chalk is formed under water, on the sea bed when there are the correct conditions for the process to take place. It begins as being lime mud which undergoes a series of geological processes which involve heat and pressure removing the water and compacting the mud turning it from a sediment into rock. The process of compression is known as diagenesis. The lime mud itself is formed from the skeletons of plankton that fall to the bottom of the sea bed. 

Most chalks come from the Cretaceous period ( 145.5 Million to 65.5 Million years ago), this is because during this time there were high global temperatures and sea levels due to the break up of the super continent Pangea. The break up of the continent led to magma rising from beneath creating a new sea bed and mid ocean ridges swelled and water was displaced onto the continents by the magma spilling out onto the rest of the sea floor. Then in the sea ways of the flooded continents the chalk was made. 

The Cretaceous chalk is white, why is this? 

The chalk gets its white colour from the skeletons that it is formed from, because they are mainly colourless as is the case with many limestones. The Cretaceous chalk is very white because when it was formed the sea levels were so high that this prevented the chalk from gaining impurities and as the sea floor was very active any organics would have been broken down very quickly.  

Where can the chalk be found?

There are many places you will find chalk. Some of the oldest chalk deposits, formed in the Cretaceous period, are situated in Sweden and England, such as the chalk cliffs of Dover. Some other large deposits are in the USA going from south Dakota, southwards to Texas and eastwards to Alabama. 

In England the chalk runs between Devon and Yorkshire and many of the counties on the South East. There is also chalk underneath London, the North Sea and the Channel. There are three parts to the chalk formation in England, The Upper Chalk, The Middle Chalk and the Lower Chalk. 

What can be found in the chalk?

There are many fossils to be found within the chalk, especially as the chalk itself is made up of plankton skeletons. Commonly found fossils include; bi-valves, echinoids, ammonites, bryozoans and sponges and most of them have been preserved very well by the chalk. It has also been known to very occasionally find preserved fish, starfish and crustaceans, these have been proven to show occasions where the chalk has been transported down a slope and to bury the creatures which were on the sea floor alive. As well as fossils there are other materials that can be found within chalk an example of this is Flint. 

Uses of Chalk

When it comes to uses of chalk many people would immediately think of the stick of chalk used to write on blackboards, however these days that chalk is made from synthetic materials. There are still however many uses for chalk. 

• Chalk is used to raise the ph levels in soil with high levels of acidity in agriculture. 
• To enable toothpaste to have a slightly abrasive effect chalk can be added.
• Chalk with very carefully measured grain size is often used for polishing. 
• Builders putty contains chalk as a filler. 
• Chalk is used in antacids, which when small doses are taken orally help to neutralise stomach acidity. 
• It can be used to enable athletes, such as gymnasts, to have more grip.
• When heated the chalk will turn into lime which can be used for many things such as the production of steel and glass. 
• Chalk is added to make cement mix.
• Chalk is also used as a whitening agent in paint and cosmetics. 

The Channel Tunnel

The channel tunnel goes from Kent all the way under the channel to France. It goes through one particular layer of chalk which is called Chalk Marl. Here is a diagram to show this;

Image from: http://www.tunneltalk.com/Channel-Tunnel-Dec10-20-years-on.php