Dacite is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.
Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.
A xenolith is a rock fragment that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock entrained during magma ascent, emplacement and eruption. Xenoliths may be engulfed along the margins of a magma chamber, torn loose from the walls of an erupting lava conduit or explosive diatreme or picked up along the base of a flowing body of lava on the Earth's surface. A xenocryst is an individual foreign crystal included within an igneous body. Examples of xenocrysts are quartz crystals in a silica-deficient lava and diamonds within kimberlite diatremes. Xenoliths can be non-uniform within individual locations, even in areas which are spatially limited, e.g. rhyolite-dominated lava of Niijima volcano (Japan) contains two types of gabbroic xenoliths which are of different origin - they were formed in different temperature and pressure conditions.
A phenocryst is an early forming, relatively large and usually conspicuous crystal distinctly larger than the grains of the rock groundmass of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts. In metamorphic rocks, crystals similar to phenocrysts are called porphyroblasts.
A dike or dyke, in geological usage, is a sheet of rock that is formed in a fracture of a pre-existing rock body. Dikes can be either magmatic or sedimentary in origin. Magmatic dikes form when magma flows into a crack then solidifies as a sheet intrusion, either cutting across layers of rock or through a contiguous mass of rock. Clastic dikes are formed when sediment fills a pre-existing crack.
A volcano tectonic earthquake is caused by the movement of magma beneath the surface of the Earth. The movement results in pressure changes where the rock around the magma has experienced stress. At some point, this stress can cause the rock to break or move. This seismic activity is used by scientists to monitor volcanoes. The earthquakes may also be related to dike intrusion or occur as earthquake swarms.
A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma rising through the Earth's crust begins to spread out horizontally, prying apart the host rock strata. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith its dome-like form.
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a sill complex and a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.
In geology, an igneous intrusion is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and compositions, illustrated by examples like the Palisades Sill of New York and New Jersey; the Henry Mountains of Utah; the Bushveld Igneous Complex of South Africa; Shiprock in New Mexico; the Ardnamurchan intrusion in Scotland; and the Sierra Nevada Batholith of California.
A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed to be primarily caused by internal and gravitational stresses generated by magma emplacement within and across various regions of the volcano, rift zones allow the intrusion of magmatic dykes into the slopes of the volcano itself. The addition of these magmatic materials usually contributes to the further rifting of the slope, in addition to generating fissure eruptions from those dykes that reach the surface. It is the grouping of these fissures, and the dykes that feed them, that serves to delineate where and whether a rift zone is to be defined. The accumulated lava of repeated eruptions from rift zones along with the endogenous growth created by magma intrusions causes these volcanoes to have an elongated shape. Perhaps the best example of this is Mauna Loa, which in Hawaiian means "long mountain", and which features two very well defined rift zones extending tens of kilometers outward from the central vent.
A sheeted dyke complex, or sheeted dike complex, is a series of sub-parallel intrusions of igneous rock, forming a layer within the oceanic crust. At mid-ocean ridges, dykes are formed when magma beneath areas of tectonic plate divergence travels through a fracture in the earlier formed oceanic crust, feeding the lavas above and cooling below the seafloor forming upright columns of igneous rock. Magma continues to cool, as the existing seafloor moves away from the area of divergence, and additional magma is intruded and cools. In some tectonic settings slices of the oceanic crust are obducted (emplaced) upon continental crust, forming an ophiolite.
The Anahim hotspot is a hypothesized hotspot in the Central Interior of British Columbia, Canada. It has been proposed as the candidate source for volcanism in the Anahim Volcanic Belt, a 300 kilometres long chain of volcanoes and other magmatic features that have undergone erosion. This chain extends from the community of Bella Bella in the west to near the small city of Quesnel in the east. While most volcanoes are created by geological activity at tectonic plate boundaries, the Anahim hotspot is located hundreds of kilometres away from the nearest plate boundary.
A ring dike or ring dyke is an intrusive igneous body that is circular, oval or arcuate in plan and has steep contacts. While the widths of ring dikes differ, they can be up to several thousand meters. The most commonly accepted method of ring dike formation is directly related to collapse calderas.
The North Atlantic Igneous Province (NAIP) is a large igneous province in the North Atlantic, centered on Iceland. In the Paleogene, the province formed the Thulean Plateau, a large basaltic lava plain, which extended over at least 1.3 million km2 (500 thousand sq mi) in area and 6.6 million km3 (1.6 million cu mi) in volume. The plateau was broken up during the opening of the North Atlantic Ocean leaving remnants preserved in north Ireland, west Scotland, the Faroe Islands, northwest Iceland, east Greenland, western Norway and many of the islands located in the north eastern portion of the North Atlantic Ocean. The igneous province is the origin of the Giant's Causeway and Fingal's Cave. The province is also known as Brito–Arctic province and the portion of the province in the British Isles is also called the British Tertiary Volcanic Province or British Tertiary Igneous Province.
La Pacana is a Miocene age caldera in northern Chile's Antofagasta Region. Part of the Central Volcanic Zone of the Andes, it is part of the Altiplano-Puna volcanic complex, a major caldera and silicic ignimbrite volcanic field. This volcanic field is located in remote regions at the Zapaleri tripoint between Chile, Bolivia and Argentina.
The magma supply rate measures the production rate of magma at a volcano. Global magma production rates on Earth are about 20–25 cubic kilometres per year (4.8–6.0 cu mi/a).
Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon. The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ago, Archean felsic volcanic rocks provide clues on the Earth's first volcanic activities on the Earth's surface started 500 million years after the Earth's formation.
The geology of Nicaragua includes Paleozoic crystalline basement rocks, Mesozoic intrusive igneous rocks and sedimentary rocks spanning the Cretaceous to the Pleistocene. Volcanoes erupted in the Paleogene and within the last 2.5 million years of the Quaternary, due to the subduction of the Cocos Plate, which drives melting and magma creation. Many of these volcanoes are in the Nicaraguan Depression paralleled by the northwest-trending Middle America Trench which marks the Caribbean-Cocos plate boundary. Almost all the rocks in Nicaragua originated as dominantly felsic continental crust, unlike other areas in the region which include stranded sections of mafic oceanic crust. Structural geologists have grouped all the rock units as the Chortis Block.
Volcanic and igneous plumbing systems (VIPS) consist of interconnected magma channels and chambers which are responsible for the production, storage and transportation of magma in Earth's crust. Volcanic plumbing systems can be found in all active tectonic settings, such as mid-oceanic ridges, subduction zones, and mantle plumes, when magmas generated in continental lithosphere, oceanic lithosphere, and in the sub-lithospheric mantle are transported. Magma is first generated by partial melting, followed by segregation and extraction from the source rock to separate the melt from the solid. As magma propagates upwards, a self-organised network of magma channels develops, transporting the melt from lower crust to upper regions. Channelled ascent mechanisms include the formation of dykes and ductile fractures that transport the melt in conduits. For bulk transportation, diapirs carry a large volume of melt and ascent through the crust. When magma stops ascending, or when magma supply stops, magma emplacement occurs. Different mechanisms of emplacement result in different structures, including plutons, sills, laccoliths and lopoliths.
Catherine Jeanne Annen is a French geologist at the Czech Academy of Sciences. Her research considers igneous bodies, volcanic eruptions. and exploration for geothermal energy. She was awarded the 2022 Geological Society of London Bigsby Medal.
