Sedimentary Rocks

Sedimentary rocks are the most common types on the Earth's surface. In general, they were all formed in a similar way - by the deposition, com­pression and cementing together of numerous small particles of mineral, animal or plant origin. The details of these processes are best exemplified by clastic sedimentary rocks, which consist of mineral fragments derived from pre-existing rocks.As soon as rocks are exposed on the Earth's surface they begin to be broken down by the forces of erosion. The rock fragments, and the minerals washed out of them, are carried by the wind, by streams or by the sea and finally come to rest as a sediment. Eventually it becomes covered with more sediment, and the underlying layers are compressed and cemented together to form sedi­mentary rock - a process called lithification. After millions of years, this rock may be uplifted by Earth movements - thereby again exposing it to the forces of erosion - and the entire process is repeated. This cycle of erosion, transportation, deposition, lithification and uplift is known as the sedimentary cycle.

By studying the various features of a sedimentary rock, geologists can deduce a great deal about the conditions prevalent at the time of its formation.

Sedimentary rocks typically occur as separate horizontal layers called beds, each formed as a result of fairly frequent changes in the sedimenta­tion conditions. When sedimentation stops, the sediments settle; when it resumes, a new layer begins to form on top of the previous one. Unlayered sedimentary rocks - described as mas­sive - therefore reflect long periods of unchang­ing conditions.

Analysis of the grains that make up the rocks may reveal the composition of those from which the fragments originated. In some, the minerals are the same as those in the original rock, but more commonly they have been altered by reactions with water and chemicals in the atmos­phere.

The sizes and shapes of the constituent particles reflect the distance they have travelled and the current conditions they encountered. The faster the current of water, the larger are the rock frag­ments that can be carried by it. Thus large-grained sedimentary rocks were originally formed from large pebbles and boulders deposited by fast-flowing rivers or by the sea. Such rocks are called con­glomerates if their fragments are rounded,-or breccias if they are jagged and angular. Sandstones consist of finer sediments that were laid down by weaker currents.

Extremely small particles can be carried long distances by even very slow-moving water. The sediments that result are silts and muds, which occur in slow-flowing rivers or on the sea floor far away from a turbulent shoreline. When lithified, these very fine sediments form siltstones, mudstones or shales.

A mixture of different sized grains in the same rock may indicate that the current stopped abruptly, thereby suddenly depositing all of the various sized particles it was carrying; such a sedimentary bed is termed poorly sorted. Well-sorted beds, in which all the particles are of approxi­mately the same size, result from stable current conditions.

The shape of the particles in a sedimentary rock indicates the distance the eroded fragments travelled before being deposited and lithified. The farther the fragments travelled, the rounder they are because of the greater amount of abrasion from rubbing against other particles.

Sediment Becoming Rock

It takes millions of years for a sediment to become rock. After deposition, the sediments are com­pressed beneath further layers that accumulate on top of them. The weight of the upper layers forces the underlying particles closer together, causing them to interlock and form a solid mass, but the mass is not yet rock at this stage, because the par­ticles - although tightly packed together - are still separate.

In the next phase - cementation - the particles are bonded together to form rock. Groundwater percolating through rock and sediment often has calcite dissolved in it, leached out of lime-rich rocks by the weak carbonic acid formed when car­bon dioxide in the air reacts with water in rain. The dissolved calcite then precipitates in the minute spaces between grains, thereby cementing them together, the resulting compressed and cemented mass is the sedimentary rock.

Sedimentary Rock Types

In addition to clastic rocks, there are two other principal types of sedimentary rocks: chemical and organic (or biogenic).

Chemical sedimentary rocks are formed when dissolved material precipitates out of water; for example, a bed of salt may be formed when part of the sea becomes cut off from the main body of water and eventually evaporates, leaving a deposit of salt, which may later be over­lain and compressed.

Organic sedimentary rocks are formed from the remains of animals or plants. One of the most common is limestone, which consists of the remains of small marine shellfish. When these creatures die, they sink to the sea bed, where their shells are broken up and then compressed and cemented together in the same way as clastic rocks.