Rocks

They are generated from magmas that slowly cool down in masses of remarkable extensions in the lithosphere at depths varying from 1 to over 10 km under the surface of the Earth. 

The slow cooling process allows the complete crystallisation of the mass. Crystals of various mineral components reach sizes clearly visible to the naked eye (from 1 mm to 3 cm and more).

Generally, hypabyssal rocks are formed at a shallow depth of the Earth’s crust, reaching limited sizes that cool down faster than intrusive rocks. Usually, the crystallisation process is complete. Granulometry is minute and oftennot visible to the naked eye or with lenses of moderate magnification.

Volcanic rocks come to be when magma (lava in this case) rises to the Earth’s surface causing a volcanic eruption and one or more flows.

The resulting rapid cooling generates either a total crystallisation of the melt, with a very thin granulometry (less than 0,1 mm), or the formation of rocks with a glassy finish or rocks that are part crystalline and part glass. Furthermore, all volcanic rocks may contain large crystals formed within the magma before the eruptive phase.

They consists of fragments of pre-existing rocks (clasts) that have been mechanically disintegrated and deposited after undergoing some transport. The classification of this group is essentially based on the size of clasts without considering neither the sedimentation environment, nor the grain morphology.

An example of a sedimentary rock is sandstone, used as a building material.

These rocks derive from the precipitation of substances in aqueous solutions due to changes in the chemical environment, from the supersaturation due to evaporation and, in some cases, from the action of biological agents.

The subdivision of this group is based on the chemical compound of precipitants: there are rocks with a chemism which is essentially carbonatic (dolomite, limestone, travertine, alabaster, etc.), siliceous (jasper, sylvite, etc.), manganeseferous, ferriferous, sulphatic (gypsum, anhydrite), and aloid (halite, sylvite, etc).

Biogenic rocks are the result of the accumulation of mainly organic, plant and/or animal substances. The classification system employed for these rocks divides them in biogenic rocks and organic-rich sedimentary rocks. Biogenic rocks consist of an accumulation of shells or skeletal structures produced by living organisms. Organic-rich sedimentary rocks consist of products of the transformation of strictly organic biological components.

They include rocks derived from the accumulation of products of volcanic eruptions (whether mixed or not with other debris). That is why pyroclastic rocks are considered a halfway point between volcanic rocks and sedimentary rocks. 

These are the superficial levels of the Earth’s crust deriving from the weathering of the bedrock caused by physical, chemical, biological factors and by human intervention.

These rocks derive from the thermal transformation contact (metamorphism) of pre-existing rocks that came in proximity with a high-temperature magmatic element. The surrounding area, where re-crystallisation processes occur, is called metamorphic aureole.

Tectonites are the rocks produced by dislocation metamorphism, concentrated along faults or thrust tectonics.

Schists originate from regional metamorphism, which is characterised by intense pressures and temperature variations. This type of metamorphism is the most widespread in nature and it affects various areas of the Earth’s crust.

Impactites are extremely rare rocks characterised by very high pressures and temperatures, deriving from the impact of meteorites and terrestrial rocks.

Migmatites are generated by the partial melting of a metamorphic rock due to high temperatures. are generated by the partial melting of a metamorphic rock due to high temperatures.