The rocks
Rock aggregate of minerals on the basis of variations in size, shape and proportion of the presence of the constituent minerals, rocks recognize different origin and composition. The structure of a rock, that is, the physical relationship between the individual minerals, allows to obtain information on the physical conditions of the environment of formation.

The rocks are classified according to their origin and subdivided into:
- Igneous (or igneous or magmatic or endogenous)
- Sedimentary
- Metamorphic

Igneous rocks


Magmatic process
 Is realized through the cooling of magma (a mixture of molten rock mixed with gaseous substances).
Depending on the depth in which the magma originates, of its initial temperature and processes chimicofisici
that it will experience during the ascent you will get extrusive and intrusive igneous rocks.
During the cooling of magma crystallizes the minerals, assuming variable size and shape, giving the structure magmatic features. The minerals present in this type of rocks are predominantly silicates.

Definition of Magma
• The magma is a mixture at high temperature (from 700 to 1500 ° C).
It is a question of a chemical-physical system to many components consisting of a liquid phase (molten) and
a number of solid phases (crystals) in suspension; may also be present a gaseous phase.
• It 's more or less viscous and capable of movement; There are several theories about the origin of the magma:
1. from primordial terrestrial material in the molten state;
2. fusion, total or partial, of pre-existing rocks;
3. by modifications of the original magma contamination.

• The main components of magma are:
silica (40-75%, values ​​expressed as percentages by weight),
alumina (10-20%),
iron oxides (2-12%),
calcium (1-12%),
magnesium (trace-12%), sodium (1-8%) and potassium (traces-7%).
In the gas phase is present mainly water; dioxide
carbon, hydrogen chloride, sulfur dioxide, etc.. are
present in minor amounts.

Depending on the amount of silica present, a magma is defined:
Acid (persilicico,% silica
greater than 65%) or
Basico (iposilicico,% silica
less than 52%)
Neutral (intermediate or mesosilicico,
% Of silica between 52% and 60%).
The viscosity of a magma is related to the temperature and pressure to which it is subjected and to its chemical composition: A magma poor silica, is less viscous (or more fluid) of a magma rich in silica.




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Series of Bowen
The minerals that crystallize first (olivine, plagioclase calcium) are immersed
in a liquid that allows to develop crystals with a regular shape and well-defined
(Mineral idiomorfi).
The minerals that crystallize last (quartz) should be adapted to space debris and
not have well-defined shape (mineral allotriomorfi). For this reason, in the rocks
magmatic quartz has never its typical form of hexagonal prism.

Types of igneous rocks
• If the magma is formed by the merger of materialidella the uppermost mantle, will have a basic nature, is characterized by a high percentage of iron and magnesium, but poor
silica. This magma reaches high temperatures (1200-1500 ° C) and is very fluid. Femiche rocks.
• If the concentration relates rocks of the continental crust drag in depth to tectonic phenomena, originates a magma of acidic nature, rich in silica and very viscous, as there are abundant components which, although
being immersed in a molten mass, are still in the solid state. The temperature is between 700 and 900 ° C. Sialic rocks.
• The merger of the crust and oceanic sediments derived magmas with intermediate composition compared to acidic and basic magmas.

Structure magmatic rocks
• The term structure indicates the shape of individual mineral components of a rock,
their size, how to join and their interdependencies.
The structure mainly depends on the cooling rate of the magma.
• With the term weaving it indicates the arrangement of the components on a large scale in
space and define those aspects determined by the orientations of the crystals
(All the characteristics of a rock in a geological scale).
Granular or holocrystalline faneritica: all fragments minerals are visible to the naked eye and
are essentially the same size. (Intrusive rocks)
Afanitica: typical of extrusive rocks, is characterized by the fact that all fragments are minerals
invisible to the naked eye.
Glass: mineral fragments are so small that the rock has the appearance of a glass. The rocks
vitreous are effusive.
Porphyric: in the rock fragments are present minerals of various sizes due to different
cooling rate. The minerals that constitute the largest fragments are called
phenocrysts and others constitute the matrix.
Pyroclastic: present in many rock fragments produced by explosive volcanic activity
Pegmatitica: characterized by the presence of very large fragments minerals. The pegmatitic rocks are intrusive.
Vesicular: characterized by the presence of many cavities (vesicles) formed by the escape of gas
which give the rock a spongy appearance. The rocks are vesicular effusive.
Depending on their chemical composition, characterized by the percentage of silica, igneous rocks are distinguished:
- Acidic or persiliciche (more than 65% silica)
- Neutral and mesosiliciche (from 52% to 65% silica)
- Basic or iposiliciche (from 45% to 52% silica)
If the chemical composition is taken into account in the percentage of oxides of alkali metals and alkaline earth metals, igneous rocks are divided into two series:
- Alkaline
- Alkali-calcic
Based on their mineralogical constitution characterized by essential minerals, which are the main components in addition to accessory minerals that are not important for the purposes of classification, igneous rocks are distinguoni in:
- Rocks Containing leucocrate in as much mineral sialic white or clear, however,
- Rocks Containing melanocrate in as much mineral Phoenicians dark, brown, green or black
The subdivision of igneous rocks in families is based on the different composition chemical and mineralogical composition which are linked with each other to which the rocks are acidic and basic leucocrate are melanocrate.


Sedimentary rocks
Are in most of the aggregates of mineral granules more or less rounded, whose interstices are filled with finer material. The dimensions of the granules are an index of mechanical energy environment of deposition.




FORMATION OF A SEDIMENTARY ROCK
The formation of a sedimentary rock can be divided into four phases, which represent the "cycle
sedimentary. "
- Phase I: alteration of pre-existing rocks on the Earth's surface with formation of solid deposits and
substances in solution.
- Phase II: transport of detrital material and the current solution by rivers, wind, glaciers, etc..
- Phase III: deposition (sedimentation) of the material in different environments (continental, marine, etc.).. The
sedimentation occurs for successive layers.
- Phase IV: formation of the rock (lithification of sediments) due to pressure from other
sediments which accumulate gradually over them. The processes together are called diagenesis
(Diagenetic processes).


ALTERATION
When any rock comes into contact with the atmosphere begin the alteration processes. These processes can be physical, chemical and biological. Physical processes cause the disintegration of the rock
without modifying the chemical and mineralogical composition (eg, temperature, glacier erosion, abrasion, wind). The chemical processes lead to changes in the composition of the rock and its properties with loss of original characters (eg karst, acid rain). Biological processes have a significant influence on altering facilitate both the physical phenomena that chemical phenomena (eg, lichens, mosses, algae).

Physical weathering processes
• thermoclastism: expansion-contraction-crushing
1) temperature
2) color of the rocks
breakdown can be made:
- Block or flakes (homogeneous rocks such as limestone and
clay)
- Granular (due to expansion and contraction of individual
minerals)
• Crioclastismo or gelivazione: change in volume
water-porosity-fracturing
• Aloclastismo: erosion by crystallization of
salts.
• Humidification and drying clays expand
absorbing water and contract selling it.

TRANSPORT

The transport of detrital material is due to:
1) gravity (landslides, castings, etc..) 2) continental waters (rivers)
3) ocean currents 4) glaciers 5) wind
The transport of the material in solution takes place by means of
waters.
The transport action produces a arrontondamento of the edges
in the elements detrital (clasts), a classazione of the material
(Classification according to the homogeneity of the scale), a
preferential orientation (in the presence of clasts in the form
stretched).
When the alteration is in place, that is without transport of
materials, there is the formation of a soil.

DEPOSIT

Sedimentation may be mechanical, chemical, biochemical.
The sedimentation mechanics regards the debris and differs depending on the environment in which it occurs (marine, fluvial, glacial, etc.).
The sedimentation chemical regards the material transported in solution to changes in the medium (temperature rise, the absence of motion, etc..).
The sedimentation biochemistry still regards the material transported in solution (eg calcium carbonate) which can be fixed by aquatic organisms (molluscs, brachiopods,
corals, foraminifera) for the formation of the shell. The shells, after the death of the animal, are deposited and accumulate in sedimentary basins.
Diagenesis

Set of chemical and physical processes leading to the formation of real rock (lithification).
The temperature which can be reached during diagenesis is less than 200 ° C (at higher temperatures is already talk of metam orphism). The total duration of the processes is equal to some tens of millions of years.
There are different processes in the course of diagenesis. Compaction is caused by the weight of overlying sediments, causes the release of pore water and the approach of the individual grains. Recrystallization involves some unstable minerals present in the sediment. The dissolution and replacement affecting some minerals which can dissolve or be replaced by other minerals, this is a very important process in the formation of rocks of chemical precipitation (transformation
calcite in dolomite - dolomitization). The new precipitation of minerals in the space between the grains of the sediment is called autigenesi; whether precipitation is abundant you get the cementation of
sediment itself.


Chemical or evaporitic rocks
Are formed within a sedimentary basin from chemical components dissolved in seawater. The latter, by coming in contact with rocks, dissolve the salts present in them and carries them with it. If water collects in a closed basin, the concentration of the salts will gradually increasing until it reaches the saturation point: when this point is exceeded (for addition of other salts, or by evaporation of the water), the salts begin to precipitate and settle to the bottom, giving rise, in more or less
long to real rocks.

The evaporite rocks are formed as a result of chemical precipitation of calcium sulphate, sodium chloride and other salts of lesser importance in lagoon basins with hot and arid climates.
Organogenic rocks

• They are formed as a result of the activity of living organisms. Many marine organisms (some molluscs,
some unicellular algae, corals, corals) subtract its salts and water with them
build shells and 'skeletons': When these organisms die, their parts and sidepositano minerals accumulate on the seabed. These activities, among other things, give rise to the atolls and coral reefs. In the course of Earth's history, many coral deposits were raised by endogenous forces and went on to form rock formations surface. Examples are the Dolomites (dolomite) and the Gran Sasso, formed by limestone organogenic origin.

• With regard to the classification of limestones when considering the origin of the materials can be distinguished: native limestone, material generated from local limestone allochthonous, containing material from other areas. The c. natives are generated from marine organisms therefore have names like: c. reef (corals and coral) c. conchigliari (shells of molluscs) c. pelagic (plankton) c. bituminous (layers of limestone
alternating with layers of bitumen); c. asphalt (impregnated oxidized hydrocarbons) c. ammonitici (consisting of ammonites, extinct mollusks with typical spiral shell) c. nummulitici (with nummulites foraminifera
with shell-shaped coin).
Organogenic rocks: silica

The silica gives origin to rocks by precipitation mediated by microorganisms. Bodies deposited silica in amorphous form (opal) and diagenesis of quartz microcrystals are formed through intermediate steps
(Chalcedony).
Geyserite is a rock abiancastra silica that is deposited at the mouth of the geyser and is the only example of direct precipitation of the silica without the intervention of organisms.
Diatomaceous earth (diatomite) is formed in the marine environment by siliceous shells of diatoms mixed with foraminifera. Are deposited in a lacustrine diatoms only giving rise to diatomaceous earth. If these rocks on the compaction mechanism is pushed they become harsh and generate the jasper.
Metamorphic rocks

The metamorphism is the recrystallization in the solid state of a pre-existing rock as a result of important
variations of temperature and pressure. Result of this process precisely the metamorphic rocks are formed from pre-existing rocks that have undergone changes in the mineralogical composition, or at least in structure and texture, due to the change of temperature and pressure to the baseline condition in which they were formed. It 'well known that, up to a certain depth, there is a
increase of temperature varying between 10 ° C and 30 ° C for each kilometer and this is called the geothermal gradient. Parallel to the temperature also increases the value of the pressure.

In the interior of the Earth may also act, in some cases the pressures oriented according to particular
also called stress directions which give rise to various structures visible on the rock as the foliations
(Separation of the mineralogical components into layers of alternating light and dark color due to the different
concentration), lineations (orientation of the mineralogical components along lines parallel) and schistosity '(orientation of the mineralogical components second parallel surfaces).
Contact metamorphic rocks

Rocks are formed when any are in contact with magmatic masses in lifts. The heat
which is liberated during the cooling of magmatic mass causes a recrystallization of
minerals, more or less thrust depending on the temperature, without reaching the melting
of the rocks.

Metamorphic rocks cataclastiche
• They are rocks that are formed as a result of the pressures associated with movements oriented fracture of the earth's crust. During an earthquake along the fault plane along which rivers materials, the crystals undergo transformations due precisely to the pressure in the slip. The mylonites are rocks characteristics of this group.


Regional metamorphic rocks
The regional metamorphism, product contemporary and gradual rising of temperature and pressure in
result of the movement of subsidence and dislocation of the rocks of a wide area subject to subsidence, where the rocks are folded and uplifted to form mountain ranges. The roots of the great mountain ranges are mainly formed by rocks that have had this origin. The most characteristic feature of the rocks is the regional schistosity.