The three main ways rocks are formed:

• Sedimentary rock
  Sedimentary rock
  Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
  
  s are formed through the gradual accumulation of sediment
  Sediment
  Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
  
  : for example, sand on a beach or mud on a river bed. As the sediment is buried it is compacted as more and more material is deposited on top. Eventually the sediment will become so dense that it is essentially rock
  Rock (geology)
  In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
  
  . This process is known as lithification.
• Igneous rock
  Igneous rock
  Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
  
  s are rocks which have crystal
  Crystal
  A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
  
  lized from a melt
  Melt
  Melt can refer to:* Melting, in physics, the process of heating a solid substance to a liquid*Melt , the semi-liquid material used in steelmaking and glassblowing*Melt inclusions, a feature of igneous rock...
  
   or magma
  Magma
  Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
  
  . The melt is made up of various components of pre-existing rocks which have been subjected to melting either at subduction zones or within the Earth
  Earth
  Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
  
  's mantle
  Mantle (geology)
  The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
  
  . The melt is hot and so passes upward through cooler country rock
  Country rock (geology)
  Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
  
  . As it moves it cools and various rock types will form through a process known as fractional crystallization
  Fractional crystallization (geology)
  Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the...
  
  . Igneous rocks can be seen at mid ocean ridges, areas of island arc volcanism
  Volcanic arc
  A volcanic arc is a chain of volcanoes positioned in an arc shape as seen from above. Offshore volcanoes form islands, resulting in a volcanic island arc. Generally they result from the subduction of an oceanic tectonic plate under another tectonic plate, and often parallel an oceanic trench...
  
   or in intra-plate hotspots
  Hotspot (geology)
  The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...
  
  .
• Metamorphic rock
  Metamorphic rock
  Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
  
  s are rocks which once existed as igneous or sedimentary rocks but have been subjected to varying degrees of pressure and heat within the Earth's crust. The processes involved will change the composition and fabric of the rock and their original nature is often hard to distinguish. Metamorphic rocks are typically found in areas of mountain building.

Rock synthesis

The synthetic investigation of rocks proceeds by experimental work that attempts to reproduce different rock types and to elucidate their origins and structures. In many cases no experiment is necessary. Every stage in the origin of clays, sands and gravels can be seen in process around us, but where these have been converted into coherent shales, sandstone and conglomerates, and still more where they have experienced some degree of metamorphism, there are many obscure points about their history upon which experiment may yet throw light. Attempts have been made to reproduce igneous rocks, by fusion of mixtures of crushed minerals or of chemicals in specially contrived furnaces. The earliest researches of this sort are those of Faujas St Fond

Barthélemy Faujas de Saint-Fond

Barthélemy Faujas de Saint-Fond , French geologist and traveller, was born at Montélimar. He was educated at the Jesuit's College at Lyon; afterwards he went to Grenoble where he studied law and was admitted as an advocate to the parlement.He rose to be president of the seneschal's court in...

 and of de Saussure

Horace-Bénédict de Saussure

200px|thumb|Portrait of Horace-Bénédict de Saussure Horace-Bénédict de Saussure was a Genevan aristocrat, physicist and Alpine traveller, often considered the founder of alpinism, and considered to be the first person to build a successful solar oven.-Life and work:Saussure was born in Conches,...

, but Sir James Hall really laid the foundations of this branch of petrology. He showed (1798) that the whinstones (diabase

Diabase

Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...

s) of Edinburgh were fusible and if rapidly cooled yielded black vitreous masses closely resembling natural pitchstones and obsidian

Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...

s, if cooled more slowly they consolidated as crystalline rocks not unlike the whinstones themselves and containing olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

, augite

Augite

Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...

 and feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 (the essential minerals of these rocks).

Many years later Daubrée

Gabriel Auguste Daubrée

Gabriel Auguste Daubrée was a French geologist.Daubrée was born at Metz, and educated at the Ecole Polytechnique in Paris...

, Delesse

Achille Ernest Oscar Joseph Delesse

Achille Ernest Oscar Joseph Delesse was a French geologist and mineralogist.He was born at Metz. At the age of twenty he entered the Ecole Polytechnique, and subsequently passed through the Ecole des Mines...

 and others carried on similar experiments, but the first notable advance was made in 1878, when Fouqué

Ferdinand André Fouqué

Ferdinand André Fouqué was a French geologist and petrologist.He was born at Mortain, in the Manche département....

 and Lévy

Auguste Michel-Lévy

Auguste Michel-Lévy was a French geologist. He was born in Paris.He became inspector-general of mines, and director of the Geological Survey of France. He was distinguished for his researches on extrusive rocks, their microscopic structure and origin; and he employed the polarizing microscope...

 began their researches. They succeeded in producing such rocks as porphyrite, leucite

Leucite

Leucite is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi2O6]. Crystals have the form of cubic icositetrahedra but, as first observed by Sir David Brewster in 1821, they are not optically isotropic, and are therefore pseudo-cubic. Goniometric measurements made by...

-tephrite

Tephrite

Tephrite is an igneous, volcanic rock, with aphanitic to porphyritic texture. Mineral assembly is usually abundant feldspathoids , plagioclase, and lesser alkali feldspar. Pyroxenes are common accessory minerals. Quartz and olivine are absent. Occurrences include phonolite-tephrite Mt...

, basalt

Basalt

Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

 and dolerite, and obtained also various structural modifications well known in igneous rocks, e.g. the porphyritic

Porphyritic

Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

 and the ophitic. Incidentally they showed that while many basic rocks (basalts, etc.) could be perfectly imitated in the laboratory, the acid rocks could not, and advanced the explanation that for the crystallization of the latter the gases never absent in natural rock magmas were indispensable mineralizing agents. It has subsequently been proved that steam, or such volatile substances as certain borates, molybdates, chlorides, fluorides, assist in the formation of orthoclase

Orthoclase

Orthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. Alternate names are alkali feldspar and potassium feldspar...

, quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 and mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 (the minerals of granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

). Sir James Hall also made the first contribution to the experimental study of metamorphic rocks by converting chalk

Chalk

Chalk is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates shed from micro-organisms called coccolithophores....

 into marble

Marble

Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...

 by heating it in a closed gun-barrel, which prevented the
escape of the carbonic acid

Carbonic acid

Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 at high temperatures. In 1901 Adams

Frank Dawson Adams

Frank Dawson Adams was a Canadian geologist.He was born into a prosperous, middle-class family in Montreal, Quebec. At that time modern Canada did not exist : "Canada" consisted of Canada West and Canada East...

 and Nicholson carried this a stage further by subjecting marble to great pressures in hydraulic presses and have shown how the foliated structures, frequent in natural marbles, may be produced artificially.