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89-524: 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

178-400: 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

267-526: 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

356-544: A hydrofracture breccia. Hydrothermal clastic rocks are generally restricted to those formed by hydrofracture , the process by which hydrothermal circulation cracks and brecciates the wall rocks and fills them in with veins. This is particularly prominent in epithermal ore deposits and is associated with alteration zones around many intrusive rocks, especially granites . Many skarn and greisen deposits are associated with hydrothermal breccias. A fairly rare form of clastic rock may form during meteorite impact. This

445-402: 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

534-503: 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

623-431: A biased view of the original mineralogy of the rock. Porosity can also be affected by this process. For example, clay minerals tend to fill up pore space and thereby reducing porosity. In the process of burial, it is possible that siliciclastic deposits may subsequently be uplifted as a result of a mountain building event or erosion . When uplift occurs, it exposes buried deposits to a radically new environment. Because

712-410: A considerably lesser portion of framework grains and minerals. They only make up about 15 percent of framework grains in sandstones and 5% of minerals in shales. Clay mineral groups are mostly present in mudrocks (comprising more than 60% of the minerals) but can be found in other siliciclastic sedimentary rocks at considerably lower levels. Accessory minerals are associated with those whose presence in

801-502: 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

890-518: 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

979-470: 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

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1068-481: A great resistance to decomposition are categorized as stable, while those that do not are considered less stable. The most common stable mineral in siliciclastic sedimentary rocks is quartz (SiO 2 ). Quartz makes up approximately 65 percent of framework grains present in sandstones and about 30 percent of minerals in the average shale. Less stable minerals present in this type of rocks are feldspars , including both potassium and plagioclase feldspars. Feldspars comprise

1157-492: 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,

1246-602: A logarithmic size scale. Siliciclastic rocks are clastic noncarbonate rocks that are composed almost exclusively of silicon, either as forms of quartz or as silicates. The composition of siliciclastic sedimentary rocks includes the chemical and mineralogical components of the framework as well as the cementing material that make up these rocks. Boggs divides them into four categories; major minerals, accessory minerals, rock fragments, and chemical sediments. Major minerals can be categorized into subdivisions based on their resistance to chemical decomposition. Those that possess

1335-546: A muddy matrix that leaves little space for precipitation to occur. This is often the case for mudrocks as well. As a result of compaction, the clayey sediments comprising mudrocks are relatively impermeable. Dissolution of framework silicate grains and previously formed carbonate cement may occur during deep burial. Conditions that encourage this are essentially opposite of those required for cementation. Rock fragments and silicate minerals of low stability, such as plagioclase feldspar, pyroxenes , and amphiboles , may dissolve as

1424-437: A result of increasing burial temperatures and the presence of organic acids in pore waters. The dissolution of frame work grains and cements increases porosity particularly in sandstones. This refers to the process whereby one mineral is dissolved and a new mineral fills the space via precipitation. Replacement can be partial or complete. Complete replacement destroys the identity of the original minerals or rock fragments giving

1513-491: A sample's environment of deposition . An example of clastic environment would be a river system in which the full range of grains being transported by the moving water consist of pieces eroded from solid rock upstream. Grain size varies from clay in shales and claystones ; through silt in siltstones ; sand in sandstones ; and gravel , cobble , to boulder sized fragments in conglomerates and breccias . The Krumbein phi (φ) scale numerically orders these terms in

1602-498: A sediment is deposited, it becomes subject to cementation through the various stages of diagenesis discussed below. Eogenesis refers to the early stages of diagenesis. This can take place at very shallow depths, ranging from a few meters to tens of meters below the surface. The changes that occur during this diagenetic phase mainly relate to the reworking of the sediments. Compaction and grain repacking, bioturbation , as well as mineralogical changes all occur at varying degrees. Due to

1691-493: 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

1780-402: 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

1869-450: 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

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1958-524: 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

2047-482: 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

2136-635: Is a fragment of geological detritus , chunks, and smaller grains of rock broken off other rocks by physical weathering . Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport , whether in suspension or as bed load , and in sediment deposits. Clastic sedimentary rocks are rocks composed predominantly of broken pieces or clasts of older weathered and eroded rocks. Clastic sediments or sedimentary rocks are classified based on grain size , clast and cementing material ( matrix ) composition, and texture. The classification factors are often useful in determining

2225-501: 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

2314-534: 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

2403-399: 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

2492-544: Is cemented together and lithified it becomes known as sandstone. Any particle that is larger than two millimeters is considered gravel. This category includes pebbles , cobbles and boulders. Like sandstone, when gravels are lithified they are considered conglomerates. Conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix. These rocks are often subdivided into conglomerates and breccias. The major characteristic that divides these two categories

2581-412: Is compaction. As sediment transport and deposition continues, new sediments are deposited atop previously deposited beds, burying them. Burial continues and the weight of overlying sediments causes an increase in temperature and pressure. This increase in temperature and pressure causes loose grained sediments become tightly packed, reducing porosity, essentially squeezing water out of the sediment. Porosity

2670-461: 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,

2759-514: Is further reduced by the precipitation of minerals into the remaining pore spaces. The final stage in the process is diagenesis and will be discussed in detail below. Cementation is the diagenetic process by which coarse clastic sediments become lithified or consolidated into hard, compact rocks, usually through the deposition or precipitation of minerals in the spaces between the individual grains of sediment. Cementation can occur simultaneously with deposition or at another time. Furthermore, once

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2848-404: 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

2937-456: Is reserved for mudrocks that are laminated, while mudstone refers those that are not. Siliciclastic rocks initially form as loosely packed sediment deposits including gravels, sands, and muds. The process of turning loose sediment into hard sedimentary rocks is called lithification . During the process of lithification, sediments undergo physical, chemical and mineralogical changes before becoming rock. The primary physical process in lithification

3026-429: Is the amount of rounding. The gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. Conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass. In terms of origin and depositional mechanisms they are very similar to sandstones. As a result, the two categories often contain

3115-617: 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

3204-623: 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

3293-450: The Dott scheme , which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of muddy matrix between these larger grains. Rocks that are classified as mudrocks are very fine grained. Silt and clay represent at least 50% of the material that mudrocks are composed of. Classification schemes for mudrocks tend to vary, but most are based on the grain size of

3382-479: 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

3471-481: The chemical and mineralogic make-up of the single or varied fragments and the cementing material ( matrix ) holding the clasts together as a rock. These differences are most commonly used in the framework grains of sandstones. Sandstones rich in quartz are called quartz arenites , those rich in feldspar are called arkoses , and those rich in lithics are called lithic sandstones . Siliciclastic sedimentary rocks are composed of mainly silicate particles derived from

3560-411: 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

3649-713: 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

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3738-730: 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;

3827-461: 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

3916-449: 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

4005-547: The activity of organisms. Despite being close to the surface, eogenesis does provide conditions for important mineralogical changes to occur. This mainly involves the precipitation of new minerals. Mineralogical changes that occur during eogenesis are dependent on the environment in which that sediment has been deposited. For example, the formation of pyrite is characteristic of reducing conditions in marine environments. Pyrite can form as cement, or replace organic materials, such as wood fragments. Other important reactions include

4094-429: 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

4183-407: 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

4272-477: 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

4361-581: The composition of sandstone. They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of mudrocks . Though they sometimes are, rock fragments are not always sedimentary in origin. They can also be metamorphic or igneous . Chemical cements vary in abundance but are predominantly found in sandstones. The two major types are silicate based and carbonate based. The majority of silica cements are composed of quartz, but can include chert , opal , feldspars and zeolites . Composition includes

4450-423: 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

4539-461: 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

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4628-416: 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

4717-468: 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

4806-451: 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

4895-510: 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

4984-501: 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

5073-590: 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

5162-527: 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

5251-550: 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

5340-866: The field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from drilling information. Sedimentary breccias are an integral host rock for many sedimentary exhalative deposits . Clastic igneous rocks include pyroclastic volcanic rocks such as tuff , agglomerate and intrusive breccias , as well as some marginal eutaxitic and taxitic intrusive morphologies. Igneous clastic rocks are broken by flow, injection or explosive disruption of solid or semi-solid igneous rocks or lavas . Igneous clastic rocks can be divided into two classes: Clastic metamorphic rocks include breccias formed in faults , as well as some protomylonite and pseudotachylite . Occasionally, metamorphic rocks can be brecciated via hydrothermal fluids, forming

5429-401: 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

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5518-654: The formation of chlorite , glauconite , illite and iron oxide (if oxygenated pore water is present). The precipitation of potassium feldspar, quartz overgrowths, and carbonate cements also occurs under marine conditions. In non marine environments oxidizing conditions are almost always prevalent, meaning iron oxides are commonly produced along with kaolin group clay minerals. The precipitation of quartz and calcite cements may also occur in non marine conditions. As sediments are buried deeper, load pressures become greater resulting in tight grain packing and bed thinning. This causes increased pressure between grains thus increasing

5607-407: 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

5696-529: 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

5785-541: 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

5874-510: 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

5963-431: The major constituents. In mudrocks, these are generally silt, and clay. According to Blatt, Middleton and Murray mudrocks that are composed mainly of silt particles are classified as siltstones. In turn, rocks that possess clay as the majority particle are called claystones. In geology, a mixture of both silt and clay is called mud. Rocks that possess large amounts of both clay and silt are called mudstones. In some cases

6052-474: 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

6141-419: 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

6230-654: 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

6319-405: The pores between grain of sediment. The cement that is produced may or may not have the same chemical composition as the sediment. In sandstones, framework grains are often cemented by silica or carbonate. The extent of cementation is dependent on the composition of the sediment. For example, in lithic sandstones, cementation is less extensive because pore space between framework grains is filled with

6408-493: 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

6497-424: The process brings material to or closer to the surface, sediments that undergo uplift are subjected to lower temperatures and pressures as well as slightly acidic rain water. Under these conditions, framework grains and cement are again subjected to dissolution and in turn increasing porosity. On the other hand, telogenesis can also change framework grains to clays, thus reducing porosity. These changes are dependent on

6586-763: 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 Clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast

6675-410: The rock are not directly important to the classification of the specimen. These generally occur in smaller amounts in comparison to the quartz, and feldspars. Furthermore, those that do occur are generally heavy minerals or coarse grained micas (both muscovite and biotite ). Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10–15 percent of

6764-433: The same sedimentary structures. Sandstones are medium-grained rocks composed of rounded or angular fragments of sand size, that often but not always have a cement uniting them together. These sand-size particles are often quartz but there are a few common categories and a wide variety of classification schemes that classify sandstones based on composition. Classification schemes vary widely, but most geologists have adopted

6853-399: The shallow depths, sediments undergo only minor compaction and grain rearrangement during this stage. Organisms rework sediment near the depositional interface by burrowing, crawling, and in some cases sediment ingestion. This process can destroy sedimentary structures that were present upon deposition of the sediment. Structures such as lamination will give way to new structures associated with

6942-405: 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

7031-542: The solubility of grains. As a result, the partial dissolution of silicate grains occurs. This is called pressure solutions. Chemically speaking, increases in temperature can also cause chemical reaction rates to increase. This increases the solubility of most common minerals (aside from evaporites). Furthermore, beds thin and porosity decreases allowing cementation to occur by the precipitation of silica or carbonate cements into remaining pore space. In this process minerals crystallize from watery solutions that percolate through

7120-479: The specific conditions that the rock is exposed as well as the composition of the rock and pore waters. Specific pore waters, can cause the further precipitation of carbonate or silica cements. This process can also encourage the process of oxidation on a variety of iron bearing minerals. Sedimentary breccias are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They may form either: In

7209-887: 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

7298-401: 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,

7387-456: 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

7476-419: The term can also be used to refer to a family of sheet silicate minerals. Silt refers to particles that have a diameter between .062 and .0039 millimeters. The term mud is used when clay and silt particles are mixed in the sediment; mudrock is the name of the rock created with these sediments. Furthermore, particles that reach diameters between .062 and 2 millimeters fall into the category of sand. When sand

7565-405: The term shale is also used to refer to mudrocks and is still widely accepted by most. However, others have used the term shale to further divide mudrocks based on the percentage of clay constituents. The plate-like shape of clay allows its particles to stack up one on top of another, creating laminae or beds. The more clay present in a given specimen, the more laminated a rock is. Shale, in this case,

7654-421: The weathering of older rocks and pyroclastic volcanism. While grain size, clast and cementing material (matrix) composition, and texture are important factors when regarding composition, siliciclastic sedimentary rocks are classified according to grain size into three major categories: conglomerates , sandstones , and mudrocks . The term clay is used to classify particles smaller than .0039 millimeters. However,

7743-564: 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

7832-401: 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

7921-545: 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

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