Chalk

Chalk is a soft, white, and highly porous sedimentary carbonate rock. It is a specific form of limestone composed primarily of the mineral calcite, which is calcium carbonate (CaCO3). This rock is unique because of its high porosity, which refers to the many tiny holes or spaces within its structure. Typical chalk has a porosity ranging from 35 to 47 percent. Because it is so porous and soft, it is easily crumbled and used in many different ways.

The formation of chalk is a biological and geological process that began millions of years ago. Most chalk is created by the accumulation of hard parts from microscopic organisms called plankton. These organisms, such as coccolithophores and foraminifera, lived in the ocean. When they died, their tiny calcite shells or skeletons settled onto the sea floor. These fragments are often extremely small, with many being calcite plates only 0.5 to 4 microns in size. About 10 to 25 percent of the chalk consists of larger fragments between 10 and 100 microns. These larger pieces include the skeletons of bigger organisms like molluscs, echinoderms, or bryozoans.

This process occurred under specific environmental conditions. In Western Europe, much of the chalk formed during the Late Cretaceous Epoch and early Palaeocene Epoch. This was between 100 and 61 million years ago. The organisms settled on continental shelves at depths between 100 and 600 metres. During this time, the climate was likely arid and nonseasonal. This lack of seasonal change reduced erosion from nearby rocks, which kept the chalk very pure. Because the plankton formed stable, low-magnesium calcite skeletons, the sediment did not cement together quickly. This lack of early cementation is why chalk remains so porous compared to other limestones.

Chalk is often found alongside other geological features like flint. Flint is a type of chert that appears as bands or nodules within the chalk. It likely comes from the siliceous remains of organisms like sponges. As the chalk undergoes compaction, water is expelled upwards, often depositing flint in these patterns. In some cases, flint can even replace larger fossils, such as echinoids, molecule by molecule through a process called silicification.

The geographic distribution of chalk is widespread. The Cretaceous Period was actually named after these massive deposits, using the Latin word "creta," which means chalk. In Europe, the Chalk Group forms famous landmarks like the White Cliffs of Dover in England and the Cap Blanc-Nez in France. High cliffs are also found in Germany at Jasmund National Park and in Denmark at Møns Klint. Beyond Europe, chalk deposits exist in North America, Egypt, Australia, and even under the Pacific Ocean in the Solomon Islands.

Humans use chalk for a massive variety of industrial and everyday tasks. In schools, mineral chalk is used for writing on blackboards because it crumbles easily. This allows writing to be erased quickly. Some modern blackboard chalk is made from gypsum, which is calcium sulfate. While gypsum is cheaper, carbonate-based chalk is often sold as "dustless" because it produces larger particles. In sports, athletes use magnesium carbonate chalk to dry their hands. This provides a better grip for gymnasts, rock climbers, and weightlifters.

Other uses are more specialized and involve the chemical properties of the rock. Chalk is used in agriculture to raise the pH in acidic soils. It is also a source of quicklime through thermal decomposition. In the medical world, small doses can act as an antacid. Because it is a mild abrasive, tiny particles of chalk are often found in toothpaste for cleaning teeth. Even the landscape is shaped by chalk; in England, people create "hill figures" by scouring away grass to reveal the white rock beneath. An example is the Litlington White Horse in East Sussex.