school energy&environment

Sedimentary rocks

The action of water and air tends to transform and demolish the minerals that are contained in the rocks, by causing their disintegration and forming fragments of different size that are called debris. Debris are transported by rivers, sea water, wind and glaciers and then accumulate on the Earth’s surface depressions to form melted deposits. With time passing, transported sediments accumulate and compress the already-existing sediments underneath. Compressed debris are subject to a pressure that provokes the loss of water contained in the fragments. As a consequence, the material progressively becomes more compact. Some mineral substances melt in the water and deposit in the space between the debris, creating a “cement” that keeps them together. It takes millions of years to form a hard and compact rock through the so-called compaction and cementification of melted sediments.  Physical and chemical processes are called diagenesis, which is more active in some specific periods. This leads to the creation of deposits made up of layers that can be easily seen on canyon walls or on the Dolomites.
Where are they located?
Stones of different dimensions are located at the bottom of the mountains. The sand, clay, gravel and round stones that are located along the rivers are clastic sedimentary rocks. In particular, when gravels are kept together by fine sand, they is called conglomerates.
Pyroclastic rocks are sedimentary rocks formed of lava fragments produced by explosive volcanic eruptions and then deposited on layers.
Organogenic rocks are very important and originate from the shells of sea molluscs. Ocean floors are covered by mud formed of planktonic organism shells such as foraminifers, radiolarians and diatoms. Other types of organogenic rocks are created by the activity of sea organisms: polyps of madreporaria build their calcareous skeletons to form coral reefs. The Dolomites are a spectacular example of coral reefs, once submerged into the ocean and now huge mountains. Some blue algae, instead, are able to extract calcium carbonate from the water and deposit it, building dome-shaped structures made up of many layers: stromatolites. Remember that the accumulation of plant material forms fossil coal.
Inside karstic caves, close to water springs or waterfalls, calcium carbonate deposits and forms stalactites, stalagmites, travertine, alabaster, while deposits of water-insoluble minerals like iron and aluminium form laterites and bauxites.
These are chemical sedimentary rocks that derive from a depositing process originated by chemical reactions. Also minerals like calcite, rock salt and chalk deposit in the sea or in a lake after water evaporation, as it happens in the Red Sea, in the eastern part of the Mediterranean and in the Dead Sea, forming evaporites. Evaporite rocks can be found in Emilia Romagna, Sicily and Marche region. They were formed 6-7 million years ago, when Europe got closer to Africa and the Strait of Gibraltar (that guaranteed water exchange between the seas) closed. The Mediterranean water evaporated, new evaporite deposits were created and new lands emerged.
Flint and its varieties like opal (valuable decorative stone) are siliceous decorative stones, made of microscopic quartz crystals. They originate from the accumulation of both marine and fresh water organism-shells. Those shells, like radiolarians and diatoms, are made of silicon.