[[File:GabbroRockCreek1.jpg|thumb|Sample of igneous [[gabbro]]]]
[[Igneousఅగ్ని rockశిలలు]]sareఅగ్నిపర్వతాల formedనుండి whenవిడుదలై moltenచల్లబడిన [[magmaమాగ్మా]] coolsనుండి ఏర్పడతాయి. and are divided into two main categories: [[Intrusion|plutonic rock]] and [[volcanic]]. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the [[Earth]]'s crust (example [[granite]]), while volcanic or extrusive rocks result from magma reaching the surface either as [[lava]] or fragmental ejecta (examples [[pumice]] and [[basalt]]) .<ref name=Blatt>Blatt, Harvey and Robert J. Tracy, 1996, ''Petrology'', W. H. Freeman, 2nd ed. ISBN 0-7167-2438-3</ref>
[[File:SandstoneUSGOV.jpg|thumb|Sandstone with iron oxide bands]]
[[Sedimentaryఅవక్షేప rockశిలలు]]s are formed by deposition of either [[clastic]] [[sediment]]s, organic matter, or chemical precipitates ([[evaporite]]s), followed by compaction of the particulate matter and cementation during [[diagenesis]]. Sedimentary rocks form at or near the Earth's surface. Mud rocks comprise 65% ([[mudstone]], [[shale]] and [[siltstone]]); [[sandstone]]s 20 to 25% and [[carbonate rock]]s 10 to 15% ([[limestone]] and [[dolostone]]).<ref name=Blatt />
[[Metamorphicరూపాంతర rockప్రాప్త శిలలు]]s are formed by subjecting any rock type (including previously-formed metamorphic rock) to different [[temperature]] and [[pressure]] conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth's surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by [[recrystallisation]]).<ref name=Blatt />
The three classes of rocks — the igneous, the sedimentary and the metamorphic — are subdivided into many groups. There are, however, no hard and fast boundaries between allied rocks. By increase or decrease in the proportions of their constituent minerals they pass by every gradation into one another, the distinctive structures also of one kind of rock may often be traced gradually merging into those of another. Hence the definitions adopted in establishing rock nomenclature merely correspond to selected points (more or less arbitrary) in a continuously graduated series.<ref name=EB1911>{{1911|article=Petrology}}</ref>
