Structured Geological Glossary: Igneous Rocks

petrophile	Lover of rocks. May be a petrologist, a stone mason, a rock climber.
geologic cycle	The sequence through which rock material passes in going from its sedimentary form, through diastrophism and deformation of sedimentary rock, then through metamorphism and eventual melting and magma formation, then through volcanism and plutonism to igneous rock formation, and finally through erosion to form new sediments.
petrology	The study of rocks.
igneous	Rock formed when molten rock (magma) that has cooled and solidified (crystallized). See intrusive (plutonic) and extrusive (volcanic) igneous rock.
rock cycle	"The geologic cycle, with emphasis on the rocks produced; sedimentary rocks are metamor-phosed to metamorphic rocks, or melted to create igneous rocks, and all rocks may be uplifted and eroded to make sediments, which lithify to sedimentary rocks. "
magma	Naturally occurring liquid rock often containing volatile, such as water or carbon dioxide. Igneous rocks form by solidification of magma, usually by freezing. Molten silicate rock with dissolved gases. If erupted and degassed it is termed lava.
primary magma	Magma produced by melting a solid precursor. Magma only slightly differentiated in composition from primary magma is called primitive.
accumulative minerals	Minerals that make up the extract assemblage during fractional crystallization of a magma.
differentiation (fractionation)	"Process by which one magma gives rise to another of different composition. Differentiated or fractionated magmas are said to be evolved from a more primitive parent. Processes by which magmas evolve from basaltic to more silicic compositions; includes crystal fractionation, magma mixing, and crustal contamination."
extract assemblage	Assemblage of accumulative mineral extracted from a parent magma duting fractional crystallization to produce a daughter magma.
enrichment factor	Ratio of chemical concentration of an element in a weathered material to that in its fresh parent material.
rock	Rocks are made of different kinds of mineral, or broken pieces of crystal, or broken pieces of rocks. Some rocks are made of the shells of once-living animals, or of compressed pieces of plants. Rocks are divided into three basic types, igneous, sedimentary and metamorphic, depending upon how they were formed.
outgassing	The release of juvenile gases to the atmosphere and oceans by volcanism.
magmatic water	Water that is dissolved in a magma or that is derived from such water.
granitization	The formation of metamorphic granite from other rocks by recrystallization with or without complete melting.
orbicular rocks	Ellipsoidal-shaped masses of rock consisting of successive shells of dark mineral (biotite) and light minerals (feldspar). 200 known localities.
laminar flow	A flow regime in which particle paths are straight or gently curved and parallel.
cogenetic (comagmatic)	Igneous rocks related by differenntiation to a common parental source.
intrusion	Emplacement of magma (molten rock) into preexisting rock. Dikes, sill, and batholith are intrusion.
intrusive rock	Igneous rock that is interpreted as a former intrusion from its cross-cutting contacts, chilled margins, or other field relations.
magma chamber	The subterranean cavity containing the gas-rich liquid magma which feeds a volcano. When this chamber cools and solidifies, it is called a pluton.
plutonic rock	Any igneous rock that cools beneath the surface. (=intrusive rock).
pluton	A large body of intrusive igneous rock that solidified within the crust. Batholiths and Stocks are types of pluton.
root of a volcano	Plutonic igneous rock formed from magma that crystallized beneath the volcano it once fed.
batholith	A great irregular mass of coarse-grained igneous rock with an exposed surface of more than 100 square kilometers, which has either intruded the country rock or been derived from it through metamorphism.
laccolith	A sill-like igneous intrusion that forces apart two strata and form a round, lens-shaped body many times wider than it is thick.
stock	Relatively small globular or columnar-shaped pluton. Like a batholith only smaller.
xenolith	Rock sample with mineralogy foreign to the igneous host in which it occurs. Thought to be a fragment of wall rock from volcanic conduits in the deep lithosphere rapidly transported to the surface during eruption or a piece of country rock engulfed in an intrusion
stope block	Stope blocks form when injection of intrusive igneous rock weakens the solid rock surrounding it, causing blocks to loosen and sink into the molten mass.
sill	A sill is a flat, roughly horizontal, igneous intrusion that lies parallel to the enclosing layer of rock.
lopolith	A large laccolith that is bowl-shaped and depressed in the center, possibly by subsidence of an emptied magma chamber beneath the intrusion.
dike	A sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic foliation, or other texture of a pre-existing rock.
ring dike	A dike in the form of a segment of a cone or cylinder, having an arcuare outcrop.
volcano	Any opening through the crust that has allowed magma to reach the surface, including the deposits immediately surrounding this vent.
active volcano	"A volcano that is erupting; or one that while not erupting at the present has erupted within geologically recent time and is considered likely to do so in the geologically near future. "
dormant volcano	A volcano that is presently inactive but which may erupt again. The major volcanic cone of the Cascade Range in Washington, Oregon, and California, for example, are believed to be dormant rather than extinct.
extinct volcano	A volcano that is not presently erupting and is not likely to do so for a very long time in the future.
Agung	Active volcano, elevation 3141 m (10380 ft) in Bali, Indonesia (8.342ø S 115.508ø E). Agung is known to have erupted three times, the last in 1964.
El Chichon	Active volcano in Mexico (17.360ø N, 93.228ø W), elevation 2225 m (7300 ft). El Chichon is known to have erupted five times, most recently in 1983.
Krakatoa	Active volcano, elevation 813 m (2667 ft), which forms an island in West Indonesia between Java and Sumatra (6.102 S, 105.423 E). Its eruption in 1883, which was one of the most violent in modern times, scattered debris and darkened skies over vast areas. Additional eruptions occurred in late 1927 and in the 1960s.
eruption	Occurs when solid, liquid, or gaseous volcanic materials are ejected into the Earth’s atmosphere or surface by volcanic activity. Eruptions may occur as quiet lava flows or violent explosive events.
vent	The opening at the earth's surface through which volcanic materials reach the surface.
central vent	The largest vent of a volcano, situated at the center of its cone.
volcanic pipe	"The vertical chamber along which magma and gas ascend to the surface; also, a formation of igneous rock that cooled in a pipe and remains after the erosion of the volcano. "
volcanic cone	The deposit of lava and pyroclastic materials that has settled close to the volcano's central vent.
composite volcano	A steep volcanic cone built by both lava flows and pyroclastic eruption.
composite cone	The volcanic cone of a stratovolcano, composed of both cinder and lava flows.
stratovolcano	A volcano composed of both lava flows and pyroclastic material. Also called Composite volcanoes. Common at convergent boundaries. Excellent examples in the U.S. include Mt. St. Helens, Mt. Shasta, and the other peaks of the Cascade Range of California, Oregon and Washington.
cinder cone	A steep, conical hill built up about a volcanic vent and composed of coarse pyroclasts expelled from the vent by escaping gases.
volcanic rock	Igneous rock that cools and solidifies at or very near the Earth’s surface. Volcanoes produce volcanic rock.
cinder	A bubbly (vesicular) volcanic rock fragment that form when molten, gas-filled lava is thrown into the air, then solidifies as it falls.
tuya	A tuya is a volcano that erupts initially beneath a glacier, melts through the ice, and develops an upper, subaerial part, which commonly consists of a flat-topped form capped by a lava flow.
shield volcano	A gently sloping volcano in the shape of a flattened dome built almost exclusively of mafic lava flows. The Hawaiian Islands are a good example.
maar volcano	A volcanic crater without a cone, believed to have been formed by an explosive eruption of trapped gases.
Mauna Loa	"An intermittently active volcano, elevation 4170 m (13,680 ft) on the island of Hawaii (19.475 N, 155.608 W); the last eruption was in 1984. Also see Mauna Loa record."
plug	Solidified lava that fills the conduit of a volcano. Plugs (also called volcanic neck) are usually more resistant to erosion than the material making up the surrounding cone and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
plug dome	The steep-sided, rounded mound formed when viscous lava well up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.
volcanic neck	Solidified lava that fills the conduit of a volcano. Volcanic necks (also called plug) are usually more resistant to erosion than the material making up the surrounding cone and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
caldera	"From the Spanish for cauldron. A basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades. "
crater	A steep-sided usually circular depression formed by either explosion or collapse at a volcanic vent or surface impact of a meteorite, asteroid, or comet.
volcanic dome	"A rounded accumulation around a volcanic vent of congealed lava too viscous to flow away quickly; hence usually rhyolite lava. Volcanic dust"
volcanic dome	A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent often circular in plane view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome. In structural geology, a round or elliptical upwarp of strata resembling a short anticline.
diatreme	A volcanic vent filled with breccia by the explosive escape of gases.
extrusive	Igneous rocks that cool and solidify rapidly at or very near the Earth’s surface. Also known as volcanic rock.
pyroclastic flow	A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500° C, sufficient to burn and carbonize wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight of the overlying material.
pyroclastic texture	The unsorted, angular, and un-rounded texture of the fragments in a pyroclastic rock.
pyroclastic	Igneous material erupted explosively as finely pulverized lava (bombs, blocks, pumice, ash and hot gases).
pyroclastic rock	A rock formed by the accumulation of fragments of volcanic rock scattered by volcanic explosions.
eruption column	A mixture of hot volcanic ash, gases and entrained atmospheric air that rises vertically above an erupting volcano to heights of several tens of kilometers in the earth's atmosphere. The rise of the column is in part due to kinetic energy of the jet of gases and ash issuing from the volcano, but primarily due to the buoyant rise of the low-density mixture in the cooler atmosphere.
phreatic eruption (explosion)	An explosive volcanic eruption caused when water and heated volcanic rock interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.
eruption plume	Dispersla of an eruption column downwind from the volcano, due to the effects of prevailing atmospheric circulation in the atratosphere and troposphere.
volcanic ejecta blanket	A collective term for all the pyroclastic rocks deposited around a volcano, especially by a volcanic explosion.
ejecta	Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes. lava bomb.
fallout	Settling and deposition of particulate matter such as tephra and volcanic aerosol out of an eruption plume and onto the earth's surface.
tephra	General term for all sizes of particles ejected into the air during volcanic eruption. Includes particles as tiny as volcanic ash and as large as bomb and blocks (= pyroclastic).
volcanic aerosol	Very small (microns to fraction of micron in diameter) particles or droplets, composed mainly of sulfuric acid and water, produced by the gas-to-particle conversion of sulfur dioxide gas to sulfuric acid in the volcanic eruption plume.
ash flow	A turbulent mixture of gas and rock fragments most of which are ash-sized particles ejected violently from a crater or fissure. The mass of pyroclastic is normally of very high temperature and moves rapidly down the slope or even along a level surface.
volcanic ash flow	A mixture of volcanic ash and gases that moves downhill as a density current in the atmosphere.
volcanic ash	A volcanic sediment of rock fragments, usually glass, less than 4 millimeters in diameter that is formed when escaping gases force out a fine spray of magma.
ashfall (subaerial)	Volcanic ash that has fallen through the air. The resulting deposit is usually well sorted and exhibits a finely layered structure.
volcanic ash fall	A deposit of volcanic ash resting where it was dropped by eruption and winds.
ash	Fine particles of rock material ejected during an explosive volcanic eruption (commonly intermediate to felsic events). Ash may be either solid or molten when first erupted and generally measures less than 0.10 inch in size (larger particles have other names).
lapilli	A fragment of volcanic rock formed when magma is ejected into the air by expanding gases. The size of the fragments ranges from sand- to cobble-size.
bomb	Fragments of molten or semi-molten rock several inches to several feet in diameter which are blown out during an explosive volcanic eruption. Because of their semi-plastic condition bomb are often modified in shape during their flight or upon impact.
dust veil index	A quantitative method developed by H. H. Lamb for comparing the magnitude of volcanic eruption. The formulae use observations either of the depletion of the solar beam, temperature lowering in the middle latitude, or the quantity of solid matter dispersed as dust. The reference dust veil index is 1000, assigned to the Krakatoa 1883 eruption and the index is calculated using all three methods, where the information is available for statistical comparison purposes. Abbreviated D.V.I.
volcanic bomb	A pyroclastic rock fragment that shows the effects of cooling in flight in its streamlined or bread-crust surface.
tuff	A consolidated rock composed of pyroclastic volcanic rock made up of rock and mineral fragments in a volcanic ash matrix. Tuffs commonly are composed of much shattered volcanic rock glass--chilled magma blown into the air and then deposited. If volcanic particles fall to the ground at a very high temperature, they may fuse together, forming a welded tuff.
volcanic block	A pyroclastic rock fragment ranging from about fist- to car-sized.
aggregation	The process of combination of very small volcanic ash or dust particles in volcanic plumes in the atmosphere to form larger particles which settle out rapidly to the earth's surface due to their large diameter.
volcanic breccia	A pyroclastic rock in which all fragments are more than 2 millimeters in diameter.
pumice	Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava and so filled with vesicle that it resembles a sponge and is very light. . Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.
scoria	Very bubbly (vesicular) basalt or andesite. Both scoria and pumice develop their bubbly textures when escaping gas is trapped as lava solidifies. Scoria is more dense and darker than pumice.
lava	Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.
lava flow	An outpouring of lava onto the land surface from a vent or fissure. Also a solidified tongue-like or shee- like body formed by outpouring lava.
a'a'	Hawaiian term used to describe a lava flow whose surface is broken into rough angular fragments. Aa flows commonly develop from pahoehoe flows as they cool and lose gas.
pahoehoe	Hawaiian term for a fluid volcanic eruption resulting in broad basaltic shield volcanoes. The highly fluid magma flows readily and hardens into ropey form as it cools.
lava tube	A tunnel formed when the surface of a lava flow cools and solidifies, while the still-molten interior flows through and drains away.
kipuka	A Hawaiian term for an island of land completely surrounded by one or more younger lava flows. A kipuka form when lava encircles a hill or a slight rise in the ground as it moves downslope or across relatively flat ground. Because they are surrounded by more recent flows, kipukas are often covered with mature vegetation.
nuée ardente	"A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force down the mountainide; a significant volcanic hazard."
ignimbrite	An igneous rock formed by the lithification of volcanic ash and volcanic breccia.
lahar	A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow. Also known as a glowing avalanche.
pillow lava	When basalt erupt underwater, they commonly form pillow lava, which are mounds of elongate lava pillows formed by repeated oozing and quenching of the hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited.
fumarole	A small vent in the ground from which volcanic gases and heated groundwater emerge, but not lava.
volcanic emanations	Gases, especially steam, emitted from a vent or released from lava.
fumarolic emission	Low-level and generally steady output of low-temperature gases such as sulfur, halogens, water vapor and carbon dioxide from dormant volcanoes and geothermal fields in volcanic regions.
geyser	A hot spring that throws hot water and steam into the air. The heat is thought to result from the contact of groundwater with magma bodies.
geothermal power	Power generated by using the heat energy of the earth.
spines	Horn-like projections formed upon a lava dome.
cotectic	Line curve or surface on a phase diagram depicting crystal-melt equilibria along which two crytals co-precipitate.
liquid of descent	In a graph of chemical composition, a line connecting magma related by differentiation to a common parent. The chemical expression of magmatic evolution.
partitioning	Process by which a given element distributes itself to different extents in two ohases in equilibrium with each other. The ratio of the concentration of the element in one phase limits and is known as the Nernst partition coefficient.
reaction series	A series of chemical reactions occurring in a cooling magma by which a mineral formed at high temperature becomes unstable in the melt and reacts to form another mineral (see also Discontinuous reaction series).
primary phase field	Area or volume on a phase diagram depicting crystal-melt equilibria in which one crystalline phase precipitates.
texture, rock	The rock characteristics of grain or crystal size, size variability, rounding or angularity, and preferred orientation.
phaneritic	An igneous rock texture in which the mineral grain are large enough to be seen with the unaided eye and are of approximately equal size.
aphanitic	An igneous rock texture in which individual mineral grain are too small to be distinguished with the naked eye.
glassiness	The content of extent of glass in an igneous rock.
porphyrictic	A rock texture having large crystal or phenocryst in a finer groundmass.
porphyritic	An igneous rock texture characterized by larger crystal (phenocrysts) in a matrix of distinctly finer crystals (groundmass).
phenocryst	A term used to describe large crystal embedded in a mass of finer crystals (groundmass) in an igneous rock.
matrix	Fine-grained material surrounding larger grains in a sedimentary rock.
equigranular	A rock texture in which all the mineral grain have similar size.
juvenile gas	Gases that come to the surface for the first time from the deep interior.
vesicle	A small cavity (bubble) in an igneous rock that was formerly occupied by a bubble of escaping gas.
vesicular basalt	Holes and other openings in basaltic flow which are the result of trapped gas bubbles. Vesicles are often filled at a later date with a wide variety of materials, including quartz, agate, zeolite, and many other mineral.
basic rock	Any igneous rock containing mafic mineral rich in iron and magnesium, but containing no quartz and little sodium rich plagioclase feldspar.
ultramafic rock	An igneous rock consisting dominantly of mafic mineral, containing tiron and magnesium and with much less silicon and aluminum than most crustal rocks. Most come from the Earth’s mantle. Includes dunite, peridotite, amphibolite, and pyroxenite.
principle of opposition	Graphical technique used to model fractional crystallization of a magma that determines the composition of the extract assemblage.
mafic	A term used to describe mineral or igneous rocks that are rich in iron and/or magnesium. Mafic igneous rocks have a high percentage of dark-colored (mafic) minerals.
mafic mineral	A dark-colored mineral rich in iron and magnesium, especially a pyroxene, amphibole, or olivine.
felsic	An adjective used to describe a light-colored igneous rock poor in iron and magnesium content, abundant in feldspar and quartz.
granite	A coarse-grained intrusive igneous rock with at least 65% silica. Quartz, plagioclase feldspar and potassium feldspar make up most of the rock and give it a fairly light color. Granite has more potassium feldspar than plagioclase feldspar. Usually with biotite, but also may have hornblende.
granitic	"A general term for intrusive igneous rocks that look similar to granite but may range in composition from quartzdiorite to granite. All granitic rocks are light colored; feldspar and quartz are visible in hand specimen. "
aplite	A light-colored igneous rock with the same mineral composition as granite: quartz, plagioclase feldspar, and potassium feldspar, but with a fine-grained, almost sugary texture
granodiorite	An intrusive igneous rock imilar to granite, but contains more plagioclase than potassium feldspar.
diorite	Intrusive igneous rock made of plagioclase feldspar and amphibole and/or pyroxene. Similar to gabbro only not as so dark, and containing less iron and magnesium.
tonalite	Intrusive igneous rock made of plagioclase feldspar, quartz, and amphibole or biotite. May be similar to diorite but contains considerable quartz and is not as dark, and chemically has less calcium, iron and magnesium.
gabbro	A dark, coarse-grained intrusive igneous rock. Gabbro is made of calcium-rich plagioclase, with amphibole and/or pyroxene, and is chemically equivalent to basalt.
rhyolite	A volcanic rock (or lava) chemically equivalent to granite with 69 percent silica or more and is rich in potassium and sodium. Usually light colored, very fine-grained or glassy-looking. May have tiny visible crystal of quartz and/or feldspar dispersed in a glassy white, green, or pink groundmass.
dacite	Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.
andesite	A find-grained extrusive igneous rock intermediate in composition between rhyolite which is felsic and basalt which is mafic. Intermediate volcanic rock containing 54 to 62 percent silica and moderate amounts of iron and magnesium. Andesite mineral commonly include plagioclase and hornblende with lesser amounts of mica, pyroxene and various accessory minerals. Andesites are aphanitic in texture and are usually medium dark in color. They occur with composite volcanic cone associated with convergent plate margins.
norite	An igneous rock with mafic composition.
basalt	A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene. Volcanic rock (or magma) that is generally dark in color, contains 45 to 54 percent silica, and is rich in iron and magnesium. An eruption of basaltic magma is generally quiet and results in flows (both vesicular and non-vesicular) and breccia. Undersea eruptions commonly result in the formation of pillow lava. Basalt represents the initial differentiated material erupted by the earth at spreading center.
flood basalt	A plateau basalt extending many kilometers in flat, layered flows originating in fissure eruption.
tholeiite	Basalt with a characteristic chemical composition found in an island or continental volcanic arc (also a high-K tholeiite).
tholelite	Relatively silica-rich and alkali-poor basaltic rock containing intermediate to calcic, plagioclase, clinopyroxene, and low-Ca pyroxene.
obsidian	A black or dark-colored volcanic glass usually of rhyolitic (felsic) composition.
xenocryst	An inclusion in a rock of material not formed in the rock in which it is found.
vitrophyre	A volcanic rock with larger crystal (phenocrysts) embedded in a glassy groundmass.
glass	A rock formed when magma is too rapidly cooled (quenched) to allow crystal growth.
porphyry	An igneous rock, usually a dike or sill, with larger, generally conspicuous, early-formed crystal contained within a matrix of much smaller crystals.
ophiolite	"Assemblage of rock units including pillow basalt, diabase sheeted dike, gabbro and serpentized periodites lying uncomformably on continental lithosphere, which likely represent sections of oceanic lithosphere accreted to the continent during plate collision. Body of rock composed of some or all of the following principal rock units: basaltic lava; sheeted diabase dikes; cumulate gabbro, norite, pyroxenite, and periodite; metamorphic-textured herzolite, harzburgite and dunite. Layered suite of basic and ultrabasic silicate rocks consisting of pillow basalts, basaltic dikes, gabbros and periodites that are thought to represent segments of oceanic lithosphere that have been thrust onto a convergent plate margins."
ophiolite suite	An assemblage of mafic and ultra-mafic igneous rocks with deep-sea sediment supposedly associated with divergence zones and the sea-floor environment.
komatilte	Volcanic rock mainly found in Precambrian continental shield and containing more than 18 perceny by weight MgO.
peridotite	A coarse-grained mafic igneous rock composed of olivine with accessory amounts of pyroxene and amphibole but little or no feldspar.
kimberlite	A peridotite containing garnet and olivine and found in volcanic pipe, through which it may come from the upper Mantle.
pegmatite	A very coarse-grained igneous rock, commonly with a granitic composition. Usually form from molten rock rich in water or other volatile that facilitate the growth of large crystal. Forms sill and dike.
pegmatold	Name given to a coarse-grained rock in which mineral have a crystalline outline and range from one to several centimeters in size, as in pegmatite, but which lacks the large proportions of quartz and orthoclase that characterize pegmatite.
pyroclastic eruption	A volcanic eruption that produces a large volume of solid volcanic fragments (pyroclastics) rather than fluid lava. This type of eruption is typical of volcanoes with high silica, viscous, gas-rich magma.
rock cycle	The rock cycle refers to the many ways that rocks are generated, deformed, eroded, uplifted, buried, and eventually re-generated as new rocks. The Earth is always changing. Igneous, metamorphic, and sedimentary rocks erode to form sediment. Sediment undergoes burial, lithification, and deformation to form new rock. Sedimentary rock undergoes metamorphism at greater depths or is uplifted and exposed to erosion. Metamorphic rock may become buried to a depth at which melting occurs and the magma generated makes its way to the surface to form new igneous rock and to renew the cycle.