How is limestone formed?

So where do the calcite, dolomite and aragonite come from?

Limestone is a sedimentary rock. It forms predominantly on the sea floor where material rich in calcium carbonate ('calcareous' material) accumulates. This calcareous material may be organic, chemical or detrital in origin.

Organic

The sediment that goes to make up limestone may have been derived from the dead remains of marine organisms such as:

  • bryozoa
  • corals
  • crinoids
  • microscopic algae
  • shells

These organisms remove calcium carbonate (CaCO3) from the sea water to construct shells or skeletons.

CaCo3 cement, which may be fine grained (called micrite) or coarse grained (called sparite), holds the fossils (such as coccoliths and foraminifera) together to form a limestone.

Chemical limestones

Ooidal thin section

Chemical precipitation is an important method by which limestones form.

The warm, shallow waters of the Bahamas and the Red Sea, for example, are saturated with CaCO3. Gentle water currents constantly wash grains of sand and shell fragments backwards and forwards. As the grains roll around, they act as nuclei for the precipitation of aragonite, which builds up in concentric layers to form small spheres (called ooids).

The Jurassic limestones of Central England (e.g. the Lincolnshire Limestone) are formed of ooids just like those of the Bahamas and Red Sea, although in this case the aragonite has changed to calcite.

Detrital limestones

Standish Quarry, near Randwick, Gloucestershire

Some limestones are made of, or incorporate fragments of, pre-existing limestones (detritus).

In shallow waters, fringing reefs, barrier reefs, atolls and limestone shorelines are eroded by the sea. The resultant sediment is transported by currents and deposited on the flanks of the reefs and in sheltered lagoons.

Where currents are very gentle, lime muds settle around larger fragments and eventually cement them together.

Where currents are stronger, the lime mud may be washed away to leave only the larger grains, pieces of reef and, perhaps, banks of shells. Later, diagenesis takes place to form the rock we know as limestone.

From sediment to rock

Diagenesis is the process by which soft sediments turn to rock.

Limestones originate as soft lime muds, shells and other fragments of the organisms living in the water. When these sediments are buried under others, they are compacted, the water is squeezed out of pore spaces and, in some cases, mineral grains are physically re-organised or chemically changed by the pressure.

Later, mineralised waters percolate through and minerals precipitate into the pore spaces, cementing everything together to form a rock.

Bedding

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Sediments are deposited in horizontal layers. When these layers undergo diagenesis they form 'beds' of sedimentary rocks. These beds may be thin (as in Jurassic and Cretaceous limestones) or massive (as in the case of Carboniferous limestones). The different beds are separated from each other by 'bedding planes', which represent a change in conditions, or an interruption in sedimentation, when the sediments were accumulating in the sea.

These bedding planes are more or less horizontal, and in massive limestones they are important in allowing the movement of water through the rock. They are, however, lines of weakness along which weathering and erosion can take place.

Joints

Joints

The beds of limestones are divided up by a series of vertical fissures, usually arranged at right angles to each other. These fissures divide the limestone into a series of blocks and are called 'joints'. Joints form when the limestone is placed under tension or pressure, but there is no rock movement associated with them like there is along a fault.

The joints in a massive limestone area are important because they form zones of weakness in the rock, which are attacked by acidic rainwater and chemically weathered.

Coral limestones

Perhaps the type of limestone that many people think of first is the coral reef, which can be seen growing in the world's warmer, shallow ocean margins. Corals, which are marine animals, gradually add skeletal CaCO3 to existing reefs and build structures that can be many kilometres long (e.g. the Great Barrier Reef off Australia).

In fact, reefs are rarely composed of just coral; other organisms contribute to their construction including bryozoa, bivalves, crinoids and the calcareous skeletons of microfossils. Together they build up calcareous structures such as fringing reefs, barrier reefs and atolls.

Fringing reef
Volcanic island
Sunken volcanic island

Freshwater limestones

A freshwater limestone (Bembridge Limestone) from the Isle of Wight.

So far we have considered only limestones that formed in the sea, but we can see limestone in the making in fresh water. Like all sedimentary rocks, this limestone is made up of materials from older rocks. Some freshwater limestones are crowded with the fossil shells of bivalves and 'water fleas' that once lived in the lakes and rivers.

Many freshwater limestones are formed by precipitation. Rivers flowing through limestones carry dissolved calcite in the water. When the water begins to evaporate, it releases the calcite, which builds up to form chemically precipitated limestones called tufa and travertine.

We can also see limestone being created in the caves of Yorkshire where they form stalactites and stalagmites.

Mother Shipton's Cave and petrifying wells

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Petrifying well at Mother Shipton's Cave

In several places within England, 'petrifying wells' have been visited by tourists for hundreds of years, as you can tell from these photographs taken over 100 years ago. The petrifying well at Mother Shipton's Cave at Knaresborough, Yorkshire, is probably the most famous.

Water flows underground through Permian limestone, before pouring over the waterfall at Mother Shipton's Cave to join the River Nidd. As the river flows through the limestone it picks up dissolved CaCO3, so that the water has a high mineral content. The water is especially rich in calcium, sodium and magnesium, but there are also very small traces of other minerals too.

So how does the limestone form?

When the water at the petrifying well emerges from its subterranean course, it begins to evaporate, releasing the calcite which, over time, builds up to form new limestone.

There are two types of limestone forming at the petrifying well at Mother Shipton's Cave:

  • tufa — a fine-grained limestone that was deposited quickly
  • travertine — a limestone with larger crystals of calcite that accumulates more slowly

You can tell how fast this type of limestone is deposited at the petrifying well because toys, hats, umbrellas, etc. are hung under the waterfall and become encrusted with tufa within a few months.

Remember that the hanging objects are just being coated with limestone; the toys are not literally turned to stone as tourist guides might suggest.

Geological map around Mother Shipton's Cave


Mother Shipton's Cave gallery