Misplaced Pages

#408591

71-527: Volcanic activity has occurred repeatedly in the AVB for the last 15 million years, during which time three major magmatic episodes took place 15–13, 9–6 and 3–1 million years ago. These major magmatic episodes are represented by plutons , dike swarms , volcanic fields and large shield volcanoes. Volcanic activity in the last 1 million years has been relatively minor and localized, having produced small lava flows and cones. The last eruptive period took place at

142-400: A chilled margin is often found on the intrusion side of the contact, while a contact aureole is found on the country rock side. The chilled margin is much finer grained than most of the intrusion, and may be different in composition, reflecting the initial composition of the intrusion before fractional crystallization, assimilation of country rock, or further magmatic injections modified

213-535: A bimodal suite of basalt, trachyte and comendite . Basaltic and comenditic dikes are the most abundant, distributed in about equal proportions. Near the centre of each dike swarm are erosional remnants of rhyolite breccia with locally occurring rhyolite flows that comprise the Bella Bella Formation. The breccias contain a high content of coarse basement clasts , indicating that they are the product of explosive volcanism . Rhyolites associated with

284-402: A layered intrusion . The ultimate source of magma is partial melting of rock in the upper mantle and lower crust . This produces magma that is less dense than its source rock. For example, a granitic magma, which is high in silica, has a density of 2.4 Mg/m , much less than the 2.8 Mg/m of high-grade metamorphic rock. This gives the magma tremendous buoyancy, so that ascent of the magma

355-503: A stratiform volcanic pile. The basal unit comprises a series of aphyric trachyte flows and domes with minor flow-banded rhyolite, rhyolitic tuffs and thin hawaiite flows. This unit is overlain by a sequence of alkali- feldspar porphyritic trachytes which occur as pyroclastic deposits as well as small lava flows and domes. These trachytes are overlain and intruded by alkali-feldspar porphyritic quartz -trachytic and trachytic plugs, lava flows, pyroclastic flows, debris flows and dikes in

426-407: A terrane was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form underplating , where the new igneous rock forms a layer on the underside of the crust. The majority of continental crust on the planet is around 1 to 3 billion years old, and it is theorised that there

497-459: A weak zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced. In Andean geology the basement refers to the Proterozoic , Paleozoic and early Mesozoic ( Triassic to Jurassic ) rock units as the basement to the late Mesozoic and Cenozoic Andean sequences developed following

568-506: A contact aureole, and often contain xenolithic fragments of country rock suggesting brittle fracturing. Such intrusions are interpreted as occurring at shallow depth, and are commonly associated with volcanic rocks and collapse structures. An intrusion does not crystallize all minerals at once; rather, there is a sequence of crystallization that is reflected in the Bowen reaction series . Crystals formed early in cooling are generally denser than

639-518: A crystallized magma chamber . A pluton that has intruded and obscured the contact between a terrane and adjacent rock is called a stitching pluton . Intrusions are broadly divided into discordant intrusions , which cut across the existing structure of the country rock, and concordant intrusions that intrude parallel to existing bedding or fabric . These are further classified according to such criteria as size, evident mode of origin, or whether they are tabular in shape. An intrusive suite

710-477: A former cinder cone . To the east lies the Ilgachuz Range , a slightly smaller and less deeply dissected shield volcano that formed 6.1 to 4.0 million years ago. Its nearly circular and gently sloping structure, 25 km (16 mi) in diameter, consists of four assemblages that were deposited in six major episodes of activity. The lower shield assemblage represents the oldest exposed rocks in

781-492: A late stage of relatively minor mafic volcanism. Although the felsic lavas produced during the early evolutionary stage were high in silica content, they remained fluid enough to build felsic shields. The volcanic fields consist of lavas that range from basanites and trachybasalts to evolved trachytes and phonolites. They comprise several small volcanoes that are in the form of cones, domes and plugs, most of which likely experienced episodic and short-lived activity. In contrast,

SECTION 10

#1732766108409

852-473: A plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it causes an upwelling of magma that can cause volcanism along the subduction front on the overriding plate. This produces an oceanic volcanic arc , like Japan . This volcanism causes metamorphism , introduces igneous intrusions , and thickens the crust by depositing additional layers of extrusive igneous rock from volcanoes. This tends to make

923-500: A series of earthquakes measuring up to 2.9 magnitude occurred in the Nechako Basin about 20 km (12 mi) west of Nazko Cone. Most of them occurred 25 to 31 km (16 to 19 mi) below the surface, indicating they originated within the lowermost crust . Analysis of seismic waves suggest that the earthquake swarm was caused by brittle failure and fracturing of rock at depth from magma intrusion. No volcanic eruption

994-402: A small central caldera in which the intracaldera assemblage was deposited. This assemblage consists of a 150 m (490 ft) thick sequence of tuffs that are overlain by a ponded single cooling unit of trachyte more than 165 m (541 ft) in thickness. The postcaldera assemblage comprises alkali basalt and hawaiite flows that issued from flank vents and from a few small vents within

1065-450: A volcanic neck, suggesting that necks tend to form at intersections of dikes where passage of magma is least obstructed. Diatremes and breccia pipes are pipe-like bodies of breccia that are formed by particular kinds of explosive eruptions . As they have reached the surface they are really extrusions, but the non erupted material is an intrusion and indeed due to erosion may be difficult to distinguish from an intrusion that never reached

1136-537: Is Anahim Peak , a trachyte plug surrounded by a pile of hawaiite flows that are four to eight times thicker than those of the Rainbow Range. The abnormal thicknesses of the Anahim Peak hawaiite flows coupled with a significantly coarser-grained groundmass and higher percentage of phenocrysts than the upper portions of the flows suggest that they were ponded during eruption, possibly as lava lakes within

1207-482: Is 1,100 kilometers (680 mi) long and 50 kilometers (31 mi) wide. They are usually formed from magma rich in silica , and never from gabbro or other rock rich in mafic minerals, but some batholiths are composed almost entirely of anorthosite . A sill is a tabular concordant intrusion, typically taking the form of a sheet parallel to sedimentary beds. They are otherwise similar to dikes. Most are of mafic composition, relatively low in silica, which gives them

1278-501: Is a group of intrusions related in time and space. Dikes are tabular discordant intrusions, taking the form of sheets that cut across existing rock beds. They tend to resist erosion, so that they stand out as natural walls on the landscape. They vary in thickness from millimeter-thick films to over 300 meters (980 ft) and an individual sheet can have an area of 12,000 square kilometers (4,600 sq mi). They also vary widely in composition. Dikes form by hydraulic fracturing of

1349-534: Is an excellent insulator , cooling of the magma is extremely slow, and intrusive igneous rock is coarse-grained ( phaneritic ). Intrusive igneous rocks are classified separately from extrusive igneous rocks, generally on the basis of their mineral content. The relative amounts of quartz , alkali feldspar , plagioclase , and feldspathoid is particularly important in classifying intrusive igneous rocks. Intrusions must displace existing country rock to make room for themselves. The question of how this takes place

1420-399: Is called the room problem , and it remains a subject of active investigation for many kinds of intrusions. The term pluton is poorly defined, but has been used to describe an intrusion emplaced at great depth; as a synonym for all igneous intrusions; as a dustbin category for intrusions whose size or character are not well determined; or as a name for a very large intrusion or for

1491-466: Is defined by coalescing pyroclastic cones and associated lava flows that formed subaerially during the Holocene . Radiocarbon dating of bog peat above and below a tephra layer near Nazko Cone suggest that the latest eruption took place about 7,200 years ago. This is the only known Holocene eruption from Nazko Cone as no other tephra layers have been identified. From 9 October 2007 to 15 May 2008,

SECTION 20

#1732766108409

1562-404: Is inevitable once enough magma has accumulated. However, the question of precisely how large quantities of magma are able to shove aside country rock to make room for themselves (the room problem ) is still a matter of research. The composition of the magma and country rock and the stresses affecting the country rock strongly influence the kinds of intrusions that take place. For example, where

1633-638: Is the largest and one of the oldest volcanoes in the Baldface Mountain volcanic field. It consists of a 2.37 million year old cone of aphanitic phonolite and porphyritic trachyte with feldspar phenocrysts. A basaltic lava flow near the head of Moore Creek has an age of 3.91 million years, similar to the oldest rocks known from the Itcha Range or the Chilcotin Group basalts of the surrounding plateau. The relation of this flow to

1704-647: The Chilcotin Group , which in turn overlie Devonian to Jurassic rocks of the Stikinia volcanic arc terrane . In some places the AVB lavas imperceptibly merge with the Chilcotin Group basalts as the two volcanic zones were active simultaneously from the Neogene to Quaternary. This west–east trending volcanic belt is about 330 km (210 mi) long, extending from the Central Coast through

1775-483: The Juan de Fuca Plate , or a propagating crack controlled by stress fields related to large-scale plate tectonics of western North America . The generally preferred tectonic model is a mantle hotspot as the AVB decreases in age from west to east, a trend that is consistent with the westward motion of North America at the rate of about 2.5 cm (0.98 in) per year. There are also observed chemical differences between

1846-411: The Baldface Mountain volcanic field is in question as its source and extent have not been identified. Nazko Cone is the main eruptive centre forming the eastern segment of the AVB. It consists of overlapping piles of basaltic flows and cones that were formed during at least three eruptive periods. The first volcanic episode is represented by a subaerial basalt flow that forms the base of the volcano. It

1917-741: The Bella Bella dike swarm are distributed on Denny Island whereas rhyolites belonging to the Gale Passage dike swarm are found on the northern ends of Athlone Island and Dufferin Island . Exposed in 1,000 m (3,300 ft) high cliffs along the Dean and Burke channels is the King Island Pluton . This easterly trending intrusion has a length of over 20 km (12 mi) and a width of 2.5 km (1.6 mi), cutting across

1988-757: The Coast Mountains to near the community of Nazko on the Interior Plateau. Its orientation is unique among the Neogene–Quaternary volcanic provinces of British Columbia in that it is perpendicular to the major geomorphological , structural and tectonic elements of the Canadian Cordillera . Three segments comprise the AVB: the western segment, which has been reduced to remnants of eruptive breccia , high-level plutons and dike swarms;

2059-547: The Earth's continents being accreted into one giant supercontinent . Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents. In European geology , the basement generally refers to rocks older than the Variscan orogeny . On top of this older basement Permian evaporites and Mesozoic limestones were deposited. The evaporites formed

2130-468: The Ilgachuz Range. It comprises pyroclastic breccias, domes and lava flows of alkali rhyolite that have been hydrothermally altered . The overlying upper shield assemblage, which forms the bulk of the Ilgachuz Range, encompasses a series of comendite and pantellerite flows and domes that are interbedded with flows of alkali basalt and hawaiite. Formation of the shield was followed by collapse of

2201-449: The Itcha Range to just north of Chantslar Lake. Volcanism along the entire length of the volcanic field 2.5 to 1.4 million years ago was characterized by the eruption of generally small volumes of magma that erupted at various times and at various locations. The eruption rates were high enough to build a north–south trending ridge upon which most of the volcanoes are located. This elongated feature, known as Satah Ridge, rises above

Anahim Volcanic Belt - Misplaced Pages Continue

2272-745: The ages is visible as well. The basement rocks of the continental crust tend to be much older than the oceanic crust. The oceanic crust can be from 0–340 million years in age, with an average age of 64 million years. Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at subduction and oceanic rifting areas. The basement rocks are often highly metamorphosed and complex, and are usually crystalline . They may consist of many different types of rock – volcanic, intrusive igneous and metamorphic. They may also contain ophiolites , which are fragments of oceanic crust that became wedged between plates when

2343-442: The basement can be called a "cover" or "sedimentary cover". Crustal rocks are modified several times before they become basement, and these transitions alter their composition. Basement rock is the thick foundation of ancient, and oldest, metamorphic and igneous rock that forms the crust of continents , often in the form of granite . Basement rock is contrasted to overlying sedimentary rocks which are laid down on top of

2414-417: The basement rocks after the continent was formed, such as sandstone and limestone . The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 5 kilometres (3 mi) thick. The basement rock of the crust can be 32–48 kilometres (20–30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at

2485-429: The central caldera. These are the youngest lavas of the Ilgachuz Range. The Itcha Range is the easternmost and youngest of the three AVB shield volcanoes, having formed between 3.8 and 0.8 million years ago. With a diameter of 15 km (9.3 mi), the Itcha Range is also the smallest AVB shield. Its structure is unique among the AVB shields in that it consists of small coalescing volcanic units rather than

2556-407: The central segment, which consists of predominantly shield volcanoes; and the eastern segment, which consists of several small cinder cones and is the location of all modern volcanic activity. Several tectonic models have been proposed to explain the origin of the AVB. These include rifting , a mantle hotspot , a plate -edge effect or slab window leading to magma ascent along the northern edge of

2627-485: The classification is meaningful for bodies which do not change much in area with depth and that have other features suggesting a distinctive origin and mode of emplacement. Batholiths are discordant intrusions with an exposed area greater than 100 square kilometers (39 sq mi). Some are of truly enormous size, and their lower contacts are very rarely exposed. For example, the Coastal Batholith of Peru

2698-423: The composition of the rest of the intrusion. Isotherms (surfaces of constant temperature) propagate away from the margin according to a square root law, so that if the outermost meter of the magma takes ten years to cool to a given temperature, the next inward meter will take 40 years, the next will take 90 years, and so on. This is an idealization, and such processes as magma convection (where cooled magma next to

2769-461: The contact between intrusion and country rock give clues to the conditions under which the intrusion took place. Catazonal intrusions have a thick aureole that grades into the intrusive body with no sharp margin, indicating considerable chemical reaction between intrusion and country rock, and often have broad migmatite zones. Foliations in the intrusion and the surrounding country rock are roughly parallel, with indications of extreme deformation in

2840-416: The contact is given by the relationship T / T 0 = 1 2 + 1 2 erf ⁡ ( x 2 k t ) {\displaystyle T/T_{0}={\frac {1}{2}}+{\frac {1}{2}}\operatorname {erf} ({\frac {x}{2{\sqrt {kt}}}})} where T 0 {\displaystyle T_{0}} is the initial temperature of

2911-468: The contact sinks to the bottom of the magma chamber and hotter magma takes its place) can alter the cooling process, reducing the thickness of chilled margins while hastening cooling of the intrusion as a whole. However, it is clear that thin dikes will cool much faster than larger intrusions, which explains why small intrusions near the surface (where the country rock is initially cold) are often nearly as fine-grained as volcanic rock. Structural features of

Anahim Volcanic Belt - Misplaced Pages Continue

2982-451: The country rock by magma under pressure, and are more common in regions of crustal tension. Ring dikes and cone sheets are dikes with particular forms that are associated with the formation of calderas . Volcanic necks are feeder pipes for volcanoes that have been exposed by erosion . Surface exposures are typically cylindrical, but the intrusion often becomes elliptical or even cloverleaf -shaped at depth. Dikes often radiate from

3053-510: The country rock. Such intrusions are interpreted as taking placed at great depth. Mesozonal intrusions have a much lower degree of metamorphism in their contact aureoles, and the contact between country rock and intrusion is clearly discernible. Migmatites are rare and deformation of country rock is moderate. Such intrusions are interpreted as occurring at medium depth. Epizonal intrusions are discordant with country rock and have sharp contacts with chilled margins, with only limited metamorphism in

3124-501: The crust is undergoing extension, magma can easily rise into tensional fractures in the upper crust to form dikes. Where the crust is under compression, magma at shallow depth will tend to form laccoliths instead, with the magma penetrating the least competent beds, such as shale beds. Ring dikes and cone sheets form only at shallow depth, where a plug of overlying country rock can be raised or lowered. The immense volumes of magma involved in batholiths can force their way upwards only when

3195-416: The crust thicker and less dense, making it immune to subduction. Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcanic arc close to a continent, with which it may collide. When this happens, instead of being subducted, it is accreted to the edge of the continent and becomes part of it. Thin strips or fragments of

3266-603: The easternmost end of the volcanic belt 7,200 years ago, with magma-induced earthquakes having occurred as recently as 2007–2008. The AVB is one of the six volcanic provinces in British Columbia that formed during the Neogene – Quaternary period. It consists mainly of alkaline to peralkaline rocks that range from oversaturated , highly evolved phonolites and rhyolites to more undersaturated lavas. Several AVB centres overlie Miocene flood basalts of

3337-511: The edge of the continent. There are exceptions, however, such as exotic terranes . Exotic terranes are pieces of other continents that have broken off from their original parent continent and have become accreted to a different continent. Continents can consist of several continental cratons – blocks of crust built around an initial original core of continents – that gradually grew and expanded as additional newly created terranes were added to their edges. For instance, Pangea consisted of most of

3408-535: The extreme western end of the King Island Pluton, as well as satellite stocks west of the main pluton. Three large shield volcanoes and two locally extensive volcanic fields form the central AVB. They are chemically bimodal, consisting mainly of mafic and felsic lavas with almost no intermediate lavas having been verified. The shield volcanoes share a similar evolution, having formed by an early stage of predominately voluminous felsic volcanism and

3479-556: The few larger structures in each volcanic field experienced multiple eruptive events over a longer timespan. Both fields were active contemporaneously throughout much of the Pleistocene . The Rainbow Range is a 30 km (19 mi) diameter, moderately dissected shield volcano that was active 8.7 to 6.7 million years ago. Four volcanic episodes characterized by highly fluid alkaline and peralkaline lava flows created an 845 m (2,772 ft) thick assemblage exposed on

3550-401: The field, there is geochemical evidence. Zircon zoning provides important evidence for determining if a single magmatic event or a series of injections were the methods of emplacement. Large felsic intrusions likely form from melting of lower crust that has been heated by an intrusion of mafic magma from the upper mantle. The different densities of felsic and mafic magma limit mixing, so that

3621-407: The hot material, k is the thermal diffusivity (typically close to 10 m s for most geologic materials), x is the distance from the contact, and t is the time since intrusion. This formula suggests that the magma close to the contact will be rapidly chilled while the country rock close to the contact is rapidly heated, while material further from the contact will be much slower to cool or heat. Thus

SECTION 50

#1732766108409

3692-536: The largest. Smaller centres include Jorgensen Hill, Sugarloaf Mountain, Turbo Lake, White Creek and Holte Creek. The Baldface Mountain volcanic field is a group of scattered cones 25 km (16 mi) east of the Itcha Range. Argon–argon dating of seven cones indicate the volcanic field was active 2.5 to 0.91 million years ago, during which time the Itcha Range and the Satah Mountain volcanic field were also areas of volcanicity. Baldface Mountain

3763-541: The low viscosity necessary to penetrate between sedimentary beds. A laccolith is a concordant intrusion with a flat base and domed roof. Laccoliths typically form at shallow depth, less than 3 kilometers (1.9 mi), and in regions of crustal compression. Lopoliths are concordant intrusions with a saucer shape, somewhat resembling an inverted laccolith, but they can be much larger and form by different processes. Their immense size promotes very slow cooling, and this produces an unusually complete mineral segregation called

3834-517: The magma is highly silicic and buoyant, and are likely do so as diapirs in the ductile deep crust and through a variety of other mechanisms in the brittle upper crust. Igneous intrusions may form from a single magmatic event or several incremental events. Recent evidence suggests that incremental formation is more common for large intrusions. For example, the Palisades Sill was never a single body of magma 300 meters (980 ft) thick, but

3905-411: The mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The basement rocks lie below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin. In the same way, the sediments or sedimentary rocks on top of

3976-474: The middle of the shield. The uppermost shield-forming unit consists of megacrystic trachyte flows that are distributed throughout the western part of the Itcha Range. Formation of the shield was followed by the eruption of basanites, alkali basalts and hawaiites from small cinder cones, tuff rings and fissures in the eastern half of the shield. The Satah Mountain volcanic field is a linear region of cones and domes extending 50 km (31 mi) south from

4047-419: The north flank. A basal sequence of comenditic trachyte flows are unconformably overlain by flows and flow breccias of mugearite which in turn are overlain by a 40 to 60 m (130 to 200 ft) thick sequence of columnar-jointed comendite flows. Late stage volcanism resulted in the creation of scattered hawaiite dikes, plugs and minor capping flows over the north flank. Accompanying the northeastern flank

4118-658: The oldest (western) and youngest (eastern) basalts. The youngest basalts are undersaturated alkaline lavas and are classified as basanite , suggesting a deeper source zone for magma in the Nazko area compared to the rest of the AVB. The Bella Bella and Gale Passage dike swarms on the Central Coast of British Columbia form the westernmost extent of the AVB. They are both over 10 km (6.2 mi) wide, with individual dikes measuring up to 20 m (66 ft) thick. Both swarms attain K–Ar ages of 12.5 to 14.5 million years and consist of

4189-738: The onset of subduction along the western margin of the South American Plate . When discussing the Trans-Mexican Volcanic Belt of Mexico the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero terranes respectively. The term basement is used mostly in disciplines of geology like basin geology , sedimentology and petroleum geology in which

4260-503: The predominantly northwest structural trend of the Coast Plutonic Complex . Two distinct phases of magmatism created the King Island Pluton 10.3 to 13 million years ago. The first and principal phase formed the core of the pluton which comprises coarse-grained syenite . These rocks form most of the pluton exposures at sea level . The second phase resulted in the creation of a marginal zone of alkali granite at

4331-761: The remaining magma and can settle to the bottom of a large intrusive body. This forms a cumulate layer with distinctive texture and composition. Such cumulate layers may contain valuable ore deposits of chromite . The vast Bushveld Igneous Complex of South Africa includes cumulate layers of the rare rock type, chromitite, composed of 90% chromite, Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Basement (geology) In geology , basement and crystalline basement are crystalline rocks lying above

SECTION 60

#1732766108409

4402-405: The silicic magma floats on the mafic magma. Such limited mixing as takes place results in the small inclusions of mafic rock commonly found in granites and granodiorites. An intrusion of magma loses heat to the surrounding country rock through heat conduction. Near the contact of hot material with cold material, if the hot material is initially uniform in temperature, the temperature profile across

4473-942: 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 . Because the solid country rock into which magma intrudes

4544-401: The surface when magma/lava. The root material of a diatreme is identical to intrusive material nearby, if it exists, that never reached the then surface when formed. A stock is a non-tabular discordant intrusion whose exposure covers less than 100 square kilometers (39 sq mi). Although this seems arbitrary, particularly since the exposure may be only the tip of a larger intrusive body,

4615-618: The surface. Basement rock is visible, for example, at the bottom of the Grand Canyon , consisting of 1.7- to 2-billion-year-old granite ( Zoroaster Granite ) and schist ( Vishnu Schist ). The Vishnu Schist is believed to be highly metamorphosed igneous rocks and shale , from basalt , mud and clay laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through

4686-456: The surrounding plateau surface with an average topographic prominence of 200 m (660 ft). The alignment of the Satah Mountain volcanic field corresponds with the orientation of two sets of normal faults in the Itcha Range, suggesting that volcanism there may have been controlled by a fault system. More than 20 volcanoes have been identified throughout the Satah Mountain volcanic field, with Satah Mountain and Mount Punkutlaenkut being

4757-414: The underthrusting oceanic plate may also remain attached to the edge of the continent so that they are wedged and tilted between the converging plates, creating ophiolites . In this manner, continents can grow over time as new terranes are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages. As such, the basement rock can become younger going closer to

4828-521: Was at least one period of rapid expansion and accretion to the continents during the Precambrian. Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted into continental crust . It is possible for oceanic crust to be subducted down into the Earth's mantle , at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust. When

4899-581: Was deposited during a Pleistocene interglacial stage 0.34 million years ago and is extensively eroded. The second episode is constituted by a subglacial mound of hyaloclastite that formed under the Cordilleran Ice Sheet , perhaps during the Fraser Glaciation which took place between 25,000 and 10,000 years ago. It forms the western side of Nazko Cone and is dominated by blocks of highly vesicular , glassy basalt. The third episode

4970-401: Was formed from multiple injections of magma. An intrusive body is described as multiple when it forms from repeated injections of magma of similar composition, and as composite when formed of repeated injections of magma of unlike composition. A composite dike can include rocks as different as granophyre and diabase . While there is often little visual evidence of multiple injections in

5041-554: Was likely as the number and size of the tremors were too small. Although these volcano tectonic earthquakes were too small to be felt, they generated substantial local interest as they represented a significant concentration of seismic activity within the Anahim Volcanic Belt. Pluton In geology , an igneous intrusion (or intrusive body or simply intrusion ) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below

#408591