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109-656: The main glaciers in the Silverthrone area are the Pashleth , Kingcome , Trudel , Klinaklini and Silverthrone glaciers. Most of the caldera lies in the Ha-Iltzuk Icefield , which is the largest icefield in the southern half of the Coast Mountains; it is one of the five icefields in southwestern British Columbia that thinned between the mid-1980s and 1999 due to global warming . Nearly half of

218-424: A cirque landform (alternatively known as a corrie or as a cwm ) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by arêtes ) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and the weight of the snow falling above compacts it, forming névé (granular snow). Further crushing of the individual snowflakes and squeezing

327-793: A 10 kilometres (6 mi) long andesitic lava flow in the Pashleth Creek and Machmell River valleys. There is also evidence lava flows may have once partly blocked or at least altered the course of the Machmell River. Renewed activity in this area could disrupt the course of the river and have a serious impact on people living or working downstream. Volcanic gas includes a variety of substances. These include gases trapped in cavities ( vesicles ) in volcanic rocks , dissolved or dissociated gases in magma and lava , or gases emanating directly from lava or indirectly through ground water heated by volcanic action . The volcanic gases that pose

436-522: A glacier is usually assessed by determining the glacier mass balance or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area is snow-covered at the end of the melt season, and they have a terminus with a vigorous flow. Following the Little Ice Age 's end around 1850, glaciers around the Earth have retreated substantially . A slight cooling led to

545-601: A glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water. On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions , but glaciers may be found in mountain ranges on every continent other than the Australian mainland, including Oceania's high-latitude oceanic island countries such as New Zealand . Between latitudes 35°N and 35°S, glaciers occur only in

654-411: A glacier via moulins . Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface. Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick. The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of

763-823: A history of significant explosive behavior, such as the Silverthrone Caldera. Volcanic eruptions in Canada rarely cause fatalities because of their remoteness and low level of activity. The only known fatality due to volcanic activity in Canada occurred at the Tseax Cone in 1775, when a 22.5-kilometre-long (14.0 mi) lava flow traveled down the Tseax and Nass Rivers , destroying a Nisga'a village and killing approximately 2,000 people by volcanic gases . Towns and cities south of Silverthrone are home to well over half of British Columbia's human population, and there

872-408: A kilometer per year. Eventually, the ice will be surging fast enough that it begins to thin, as accumulation cannot keep up with the transport. This thinning will increase the conductive heat loss, slowing the glacier and causing freezing. This freezing will slow the glacier further, often until it is stationary, whence the cycle can begin again. The flow of water under the glacial surface can have

981-404: A large effect on the motion of the glacier itself. Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometers can be transported between lakes over the course of a couple of years. This motion is thought to occur in two main modes: pipe flow involves liquid water moving through pipe-like conduits, like a sub-glacial river; sheet flow involves motion of water in

1090-402: A level approaching that in other established countries with historically active volcanoes. Active or restless volcanoes are usually monitored using at least three seismographs all within approximately 15 kilometres (9.3 mi), and frequently within 5 kilometres (3 mi), for better sensitivity of detection and reduced location errors, particularly for earthquake depth. Such monitoring detects

1199-460: A lower heat conductance, meaning that the basal temperature is also likely to be higher. Bed temperature tends to vary in a cyclic fashion. A cool bed has a high strength, reducing the speed of the glacier. This increases the rate of accumulation, since newly fallen snow is not transported away. Consequently, the glacier thickens, with three consequences: firstly, the bed is better insulated, allowing greater retention of geothermal heat. Secondly,

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1308-439: A proportion of the total potassium present, as only relative, not absolute, quantities are required. To obtain the content ratio of isotopes Ar to K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum. The potassium is quantified by flame photometry or atomic absorption spectroscopy . The amount of K

1417-493: A quick and productive determination of priority areas for further efforts. The existing network of seismographs to monitor tectonic earthquakes has existed since 1975, although it remained small in population until 1985. Apart from a few short-term seismic monitoring experiments by the Geological Survey of Canada, no volcano monitoring has been accomplished at the Silverthrone Caldera or at other volcanoes in Canada at

1526-555: A rate of 40 millimetres (1.6 in) per year. Hot magma upwelling above the descending oceanic plate creates volcanoes, each of which erupts for a few million years. It is estimated that the subduction zone has existed for at least 37 million years; in that time it has created a line of volcanoes, called the Cascade Volcanic Arc , that stretches over 1,000 kilometres (620 mi) along the subduction zone from Northern California to Vancouver Island . Several volcanoes in

1635-404: A result of emptying the magma chamber beneath the volcano. If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice above it. A roughly circular fracture—a "ring fault"—develops around the edge of the chamber. These ring fractures serve as feeders for fault intrusions that are also known as ring dikes . Secondary volcanic vents may form above

1744-488: A thin layer. A switch between the two flow conditions may be associated with surging behavior. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream. The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes. The speed of glacial displacement is partly determined by friction . Friction makes

1853-410: A tremendous impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than other glaciers. Thermally, a temperate glacier is at a melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below the freezing threshold from the surface to its base, although

1962-782: A volcanic front that was active in the Miocene , during early stages of subduction of the Juan de Fuca Plate . With the notable exception of King Island , all the intrusive and eruptive rocks are calc-alkaline, mainly granodioritic bodies and dacite ejecta. On a broader scale, the intrusive and eruptive rocks are part of the Coast Plutonic Complex , which is the single largest contiguous granite outcropping in North America. The intrusive and metamorphic rocks extend approximately 1,800 kilometres (1,100 mi) along

2071-401: Is a function of the purity of the sample, the composition of the mother material, and a number of other factors. These factors introduce error limits on the upper and lower bounds of dating, so that the final determination of age is reliant on the environmental factors during formation, melting, and exposure to decreased pressure or open air. Time since recrystallization is calculated by measuring

2180-655: Is a likelihood that future eruptions will cause damage to populated areas, making Silverthrone and other Garibaldi belt volcanoes further south a major potential hazard. For this reason, additional projects to study Silverthrone and other Garibaldi belt volcanoes to the south are being planned by the Geological Survey of Canada . There are significant hazards from almost all Canadian volcanoes that require hazard maps and emergency plans. Volcanoes which exhibit significant seismic activity, such as Silverthrone, appear to be most likely to erupt. A significant eruption of any of

2289-468: Is a persistent body of dense ice that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries . It acquires distinguishing features, such as crevasses and seracs , as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques , moraines , or fjords . Although

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2398-645: Is about 30 kilometres (19 mi) long and 20 kilometres (12 mi) wide while the Crater Lake caldera in Oregon , United States is 10 kilometres (6 mi) long and 8 kilometres (5 mi) wide. Such calderas are usually formed by large cataclysmic eruptions reaching 7 on the Volcanic Explosivity Index (described as "super-colossal"). Silverthrone Caldera is one of the eleven Canadian volcanoes associated with recent seismic activity :

2507-456: Is above or at freezing at the interface and is able to slide at this contact. This contrast is thought to a large extent to govern the ability of a glacier to effectively erode its bed , as sliding ice promotes plucking at rock from the surface below. Glaciers which are partly cold-based and partly warm-based are known as polythermal . Glaciers form where the accumulation of snow and ice exceeds ablation . A glacier usually originates from

2616-407: Is affected by factors such as slope, ice thickness, snowfall, longitudinal confinement, basal temperature, meltwater production, and bed hardness. A few glaciers have periods of very rapid advancement called surges . These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous movement state. These surges may be caused by the failure of the underlying bedrock,

2725-464: Is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar). Potassium is a common element found in many materials, such as feldspars , micas , clay minerals , tephra , and evaporites . In these materials, the decay product Ar is able to escape the liquid (molten) rock but starts to accumulate when the rock solidifies ( recrystallizes ). The amount of argon sublimation that occurs

2834-411: Is because these peaks are located near or in the hyperarid Atacama Desert . Glaciers erode terrain through two principal processes: plucking and abrasion . As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands. This expansion causes

2943-406: Is by basal sliding, where meltwater forms between the ice and the bed itself. Whether a bed is hard or soft depends on the porosity and pore pressure; higher porosity decreases the sediment strength (thus increases the shear stress τ B ). Porosity may vary through a range of methods. Bed softness may vary in space or time, and changes dramatically from glacier to glacier. An important factor

3052-434: Is called glaciology . Glaciers are important components of the global cryosphere . Glaciers are categorized by their morphology, thermal characteristics, and behavior. Alpine glaciers form on the crests and slopes of mountains. A glacier that fills a valley is called a valley glacier , or alternatively, an alpine glacier or mountain glacier . A large body of glacial ice astride a mountain, mountain range, or volcano

3161-416: Is called rock flour and is made up of rock grains between 0.002 and 0.00625 mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by glacial striations . Glaciers produce these when they contain large boulders that carve long scratches in

3270-668: Is classified as acidic , having high to intermediate levels of silica , as in rhyolite , dacite , and andesite . Andesitic and rhyolitic magma are commonly associated with the two forms of explosive eruptions called Plinian and Peléan eruptions . Silverthrone is considerably younger than its nearest prominent neighbour Franklin Glacier Complex to the east-southeast. Most of the caldera's eruptive products have been heavily eroded by alpine glaciers and are now exposed in precipitous slopes extending from near sea level to elevations less than 3,000 metres (9,800 ft). The bulk of

3379-400: Is clearly much younger than the potassium-argon date, and high-energy glacial streams have only begun to etch a channel along the margin of the lava flow. The younger andesitic rocks issued from a cluster of vents, now mostly ice-covered, ranged around the periphery of the caldera. At high elevations, proximal breccia and cinders from several eroded cones rest on coarse colluvium derived from

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3488-410: Is expected to be similar to its previous eruptions. But this would likely be abandoned in part because of the volcano's remoteness. A likelihood of Canada being critically affected by local or close by volcanic eruptions argues that some kind of improvement program is required. Benefit-cost thoughts are critical to dealing with natural hazards. However, a benefit-cost examination needs correct data about

3597-647: Is higher, and the mountains above 5,000 m (16,400 ft) usually have permanent snow. Even at high latitudes, glacier formation is not inevitable. Areas of the Arctic , such as Banks Island , and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold. Cold air, unlike warm air, is unable to transport much water vapor. Even during glacial periods of

3706-569: Is likely to rise considerably with a temporary mapping and monitoring project. Knowledge at the Silverthrone Caldera and other volcanoes in the Garibaldi Volcanic Belt is not as established, but certain contributions are being done at least Mount Cayley . An intensive program classifiying infrastructural exposure near all young Canadian volcanoes and quick hazard assessments at each individual volcanic edifice associated with recent seismic activity would be in advance and would produce

3815-844: Is part of the Pemberton Volcanic Belt , which is circumscribed by a group of epizonal intrusions . At another deeply eroded caldera complex called Franklin Glacier Complex , the Pemberton Volcanic Belt merges with the Garibaldi Volcanic Belt , a northwest-trending belt of volcanic cones and fields extending from near the Canada–United States border east of Vancouver on the British Columbia Coast . The intrusions are thought to be subvolcanic bodies associated with

3924-498: Is possible that small precursor earthquake swarms might go undetected, particularly if no events were observed; more significant events in larger swarms would be detected but only a minor subdivision of the swarm events would be complex to clarify them with confidence as volcanic in nature, or even associate them with an individual volcanic edifice. Glacier A glacier ( US : / ˈ ɡ l eɪ ʃ ər / ; UK : / ˈ ɡ l æ s i ər , ˈ ɡ l eɪ s i ər / )

4033-487: Is rarely measured directly. Rather, the more common K is measured and that quantity is then multiplied by the accepted ratio of K / K (i.e., 0.0117%/93.2581%, see above). The amount of Ar is also measured to assess how much of the total argon is atmospheric in origin. According to McDougall & Harrison (1999 , p. 11) the following assumptions must be true for computed dates to be accepted as representing

4142-411: Is rarely useful in dating because calcium is so common in the crust, with Ca being the most abundant isotope. Thus, the amount of calcium originally present is not known and can vary enough to confound measurements of the small increases produced by radioactive decay. The ratio of the amount of Ar to that of K is directly related to the time elapsed since

4251-897: Is termed an ice cap or ice field . Ice caps have an area less than 50,000 km (19,000 sq mi) by definition. Glacial bodies larger than 50,000 km (19,000 sq mi) are called ice sheets or continental glaciers . Several kilometers deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70 m (230 ft). Portions of an ice sheet or cap that extend into water are called ice shelves ; they tend to be thin with limited slopes and reduced velocities. Narrow, fast-moving sections of an ice sheet are called ice streams . In Antarctica, many ice streams drain into large ice shelves . Some drain directly into

4360-413: Is the region where there is a net loss in glacier mass. The upper part of a glacier, where accumulation exceeds ablation, is called the accumulation zone . The equilibrium line separates the ablation zone and the accumulation zone; it is the contour where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. In general, the accumulation zone accounts for 60–70% of

4469-402: Is the underlying geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes. Further, bed roughness can also act to slow glacial motion. The roughness of the bed is a measure of how many boulders and obstacles protrude into the overlying ice. Ice flows around these obstacles by melting under the high pressure on their stoss side ; the resultant meltwater

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4578-552: Is then forced into the cavity arising in their lee side , where it re-freezes. As well as affecting the sediment stress, fluid pressure (p w ) can affect the friction between the glacier and the bed. High fluid pressure provides a buoyancy force upwards on the glacier, reducing the friction at its base. The fluid pressure is compared to the ice overburden pressure, p i , given by ρgh. Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (p i – p w ) of 30 kPa; i.e. all of

4687-512: Is too far away to provide a good indication of what is happening beneath the caldera. It may sense an increase in activity if the volcano becomes very restless, but this may only provide a warning for a large eruption. It might detect activity only after the volcano has started erupting. A possible way to detect an eruption is studying Silverthrone's geological history since every volcano has its own pattern of behavior, in terms of its eruption style, magnitude and frequency, so that its future eruption

4796-451: Is unlikely that enough Ar will have had time to accumulate to be accurately measurable. K–Ar dating was instrumental in the development of the geomagnetic polarity time scale . Although it finds the most utility in geological applications, it plays an important role in archaeology . One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below

4905-993: The Andes , the Himalayas , the Rocky Mountains , the Caucasus , Scandinavian Mountains , and the Alps . Snezhnika glacier in Pirin Mountain, Bulgaria with a latitude of 41°46′09″ N is the southernmost glacial mass in Europe. Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period . In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya . Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya , and in

5014-644: The Faroe and Crozet Islands were completely glaciated. The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds. Glaciers can be found in all latitudes except from 20° to 27° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above 6,500 m (21,330 ft). Between 19˚N and 19˚S, however, precipitation

5123-760: The Himalayas , Andes , and a few high mountains in East Africa, Mexico, New Guinea and on Zard-Kuh in Iran. With more than 7,000 known glaciers, Pakistan has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly 13 million km (5 million sq mi) or about 98% of Antarctica 's 13.2 million km (5.1 million sq mi), with an average thickness of ice 2,100 m (7,000 ft). Greenland and Patagonia also have huge expanses of continental glaciers. The volume of glaciers, not including

5232-486: The Machmell River and other local water sources. Because the Silverthrone region is in a remote and exceptionally rugged part of the Coast Mountains, danger from lava flows would be low to moderate. Magma with high to intermediate levels of silica (as in andesite , dacite or rhyolite ) commonly move slowly and typically cover small areas to form steep-sided mounds called lava domes . Lava domes often grow by

5341-562: The Quaternary , Manchuria , lowland Siberia , and central and northern Alaska , though extraordinarily cold, had such light snowfall that glaciers could not form. In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high (4,500 to 6,900 m or 14,800 to 22,600 ft) and cold, but the relative lack of precipitation prevents snow from accumulating into glaciers. This

5450-535: The Rwenzori Mountains . Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far north, New Zealand and the subantarctic islands of Marion , Heard , Grande Terre (Kerguelen) and Bouvet . During glacial periods of the Quaternary, Taiwan , Hawaii on Mauna Kea and Tenerife also had large alpine glaciers, while

5559-448: The 1990s and 2000s. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year. Ogives or Forbes bands are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals

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5668-459: The Coast Mountains. The still largely unexplained tectonic causes of the volcanism that has produced the Silverthrone Caldera are a matter of ongoing research. Silverthrone is not above a hotspot as are Nazko or Hawaii . However, it may be a product of the Cascadia subduction zone because andesite , basaltic andesite, dacite and rhyolite can be found at the volcano and elsewhere along

5777-487: The Earth's ozone layer . Because carbon dioxide gas is heavier than air, the gas may flow into low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation. Currently Silverthrone is not monitored closely enough by the Geological Survey of Canada to ascertain how active the volcano's magma system is. The existing network of seismographs has been established to monitor tectonic earthquakes and

5886-628: The Garibaldi Volcanic Belt are a few kilometers, and in more isolated northern regions they are up to 10 kilometres (6 mi). The location magnitude level in the Garibaldi Volcanic Belt is about magnitude 1 to 1.5, and elsewhere it is magnitude 1.5 to 2. At "carefully monitored volcanoes both the located and noticed events are recorded and surveyed immediately to improve the understanding of a future eruption. Undetected events are not recorded or surveyed in British Columbia immediately, nor in an easy-to-access process. In countries like Canada it

5995-551: The Garibaldi belt volcanoes would significantly impact Highway 99 and communities like Pemberton , Whistler and Squamish , and possibly Vancouver . The explosive nature of past eruptions at Silverthrone Caldera suggests that this volcano poses a significant long-distance threat to communities across Canada. A large explosive eruption can produce large amounts of ash that could significantly affect communities across Canada. Ash columns could rise to several hundred meters above

6104-486: The Silverthrone Caldera because the level of knowledge is insufficient due to its remoteness. A large volcanic hazard program has never existed within the Geological Survey of Canada. The majority of information has been collected in a lengthy, separate way from the support of several employees, such as volcanologists and other geologic scientists . Current knowledge is best established at the Mount Meager massif and

6213-412: The advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous. Glaciers move downhill by the force of gravity and the internal deformation of ice. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the amount of strain (deformation) is proportional to

6322-520: The air from the snow turns it into "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy, such as a gap between two mountains. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure. On steeper slopes, this can occur with as little as 15 m (49 ft) of snow-ice. In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called firn . Under

6431-430: The amount of melting at surface of the glacier, the faster the ice will flow. Basal sliding is dominant in temperate or warm-based glaciers. The presence of basal meltwater depends on both bed temperature and other factors. For instance, the melting point of water decreases under pressure, meaning that water melts at a lower temperature under thicker glaciers. This acts as a "double whammy", because thicker glaciers have

6540-713: The annual movement of the glacier. Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer. This creates a swale and space for snow accumulation in the winter, which in turn creates a ridge. Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives. Glaciers are present on every continent and in approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories. Extensive glaciers are found in Antarctica, Argentina, Chile, Canada, Pakistan, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in

6649-665: The arc are potentially active. All of the known historic eruptions in the arc have been in the United States . Two of the most recent were Lassen Peak in 1914 to 1921 and the major eruption of Mount St. Helens in 1980 . It is also the site of Canada's most recent major eruption, about 2,350 years ago at the Mount Meager massif . Very little is known about Silverthrone's eruptive history. However, as at other calderas , eruptions at Silverthrone are explosive in nature, involving viscous magma, glowing avalanches of hot volcanic ash and pyroclastic flows . The source magma of this rock

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6758-432: The bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow. Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching

6867-445: The bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are chatter marks , lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock. The rate of glacier erosion varies. Six factors control erosion rate: When

6976-419: The breccia is part of a caldera-fill succession. The presence of irregular subvolcanic intrusions and a profusion of dikes within the breccia—but not in adjacent country rock —provide further evidence of the Silverthrone Caldera. Potassium-argon dates of 750,000 and 400,000 years on rhyolitic lava domes above the basal breccia are consistent with the high rates of uplift and erosion recorded elsewhere in

7085-658: The coast of British Columbia, the Alaska Panhandle and southwestern Yukon . This is a remnant of a once vast volcanic arc called the Coast Range Arc that formed as a result of subduction of the Farallon and Kula Plates during the Jurassic -to- Eocene periods. In contrast, Garibaldi , Meager , Cayley and Silverthrone areas are of recent volcanic origin. Like other calderas, Silverthrone formed as

7194-408: The complex appears to have been erupted between 100,000 and 500,000 years ago, but postglacial andesitic and basaltic andesite cones and lava flows are also present. Anomalously old potassium-argon dates of 1,000,000 and 1,100,000 years were obtained from a large lava flow at least 10 kilometres (6.2 mi) long in the postglacial Pashleth Creek and Machmell River valleys. This blocky lava flow

7303-571: The created ice's density. The word glacier is a loanword from French and goes back, via Franco-Provençal , to the Vulgar Latin glaciārium , derived from the Late Latin glacia , and ultimately Latin glaciēs , meaning "ice". The processes and features caused by or related to glaciers are referred to as glacial. The process of glacier establishment, growth and flow is called glaciation . The corresponding area of study

7412-467: The deep profile of fjords , which can reach a kilometer in depth as ice is topographically steered into them. The extension of fjords inland increases the rate of ice sheet thinning since they are the principal conduits for draining ice sheets. It also makes the ice sheets more sensitive to changes in climate and the ocean. Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as

7521-483: The deformation to become a plastic flow rather than elastic. Then, the glacier will begin to deform under its own weight and flow across the landscape. According to the Glen–Nye flow law , the relationship between stress and strain, and thus the rate of internal flow, can be modeled as follows: where: The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing

7630-401: The deposits. It has also been indispensable in other early east African sites with a history of volcanic activity such as Hadar, Ethiopia . The K–Ar method continues to have utility in dating clay mineral diagenesis . In 2017, the successful dating of illite formed by weathering was reported. This finding indirectly led to the dating of the strandflat of Western Norway from where

7739-418: The essentially correct explanation in the 1840s, although it was several decades before it was fully accepted. The top 50 m (160 ft) of a glacier are rigid because they are under low pressure . This upper section is known as the fracture zone and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in

7848-462: The extrusion of many individual flows less than 30 metres (98 ft) thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour. But lava eruptions at Silverthrone Caldera can be more intense than those at other Cascade volcanoes. Lava flows with high to intermediate levels of silica rarely extend more than 8 kilometres (5 mi) from their source while Silverthrone has produced

7957-475: The fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 46 m (150 ft) deep but, in some cases, can be at least 300 m (1,000 ft) deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in

8066-450: The glacial base and facilitate sediment production and transport under the same moving speed and amount of ice. Material that becomes incorporated in a glacier is typically carried as far as the zone of ablation before being deposited. Glacial deposits are of two distinct types: Potassium-argon dating Potassium–argon dating , abbreviated K–Ar dating , is a radiometric dating method used in geochronology and archaeology . It

8175-453: The glacier to melt, creating a water source that is especially important for plants, animals and human uses when other sources may be scant. However, within high-altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater. Since glacial mass is affected by long-term climatic changes, e.g., precipitation , mean temperature , and cloud cover , glacial mass changes are considered among

8284-428: The glacier will be accommodated by motion in the sediments, or if it'll be able to slide. A soft bed, with high porosity and low pore fluid pressure, allows the glacier to move by sediment sliding: the base of the glacier may even remain frozen to the bed, where the underlying sediment slips underneath it like a tube of toothpaste. A hard bed cannot deform in this way; therefore the only way for hard-based glaciers to move

8393-510: The glacier's surface area, more if the glacier calves icebergs. Ice in the accumulation zone is deep enough to exert a downward force that erodes underlying rock. After a glacier melts, it often leaves behind a bowl- or amphitheater-shaped depression that ranges in size from large basins like the Great Lakes to smaller mountain depressions known as cirques . The accumulation zone can be subdivided based on its melt conditions. The health of

8502-411: The greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide , carbon dioxide and hydrogen fluoride . Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from the volcano. Globally, large explosive eruptions that inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote weakening of

8611-399: The hazard types, magnitudes and occurrences. These do not exist for volcanoes in British Columbia or elsewhere in Canada in the detail required. Other volcanic techniques, such as hazard mapping, displays a volcano's eruptive history in detail and speculates an understanding of the hazardous activity that could possibly be expected in the future. At present no hazard maps have been created for

8720-619: The ice at the bottom of the glacier move more slowly than ice at the top. In alpine glaciers, friction is also generated at the valley's sidewalls, which slows the edges relative to the center. Mean glacial speed varies greatly but is typically around 1 m (3 ft) per day. There may be no motion in stagnant areas; for example, in parts of Alaska, trees can establish themselves on surface sediment deposits. In other cases, glaciers can move as fast as 20–30 m (70–100 ft) per day, such as in Greenland's Jakobshavn Isbræ . Glacial speed

8829-420: The ice sheets of Antarctica and Greenland, has been estimated at 170,000 km . Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater. Many glaciers from temperate , alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause

8938-556: The ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a rock glacier , like the Timpanogos Glacier in Utah. Abrasion occurs when the ice and its load of rock fragments slide over bedrock and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces

9047-488: The ice, called seracs . Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from

9156-606: The icefield is drained by the Klinaklini Glacier, which feeds the Klinaklini River . The Silverthrone Caldera is very remote and rarely visited or studied by geoscientists, such as volcanologists . It can be reached by helicopter or — with major difficulty — by hiking along one of the several river valleys extending from the British Columbia Coast or from the Interior Plateau . Silverthrone

9265-411: The idea that meltwater, refreezing inside glaciers, caused the glacier to dilate and extend its length. As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow. James Forbes came up with

9374-418: The increased pressure can facilitate melting. Most importantly, τ D is increased. These factors will combine to accelerate the glacier. As friction increases with the square of velocity, faster motion will greatly increase frictional heating, with ensuing melting – which causes a positive feedback, increasing ice speed to a faster flow rate still: west Antarctic glaciers are known to reach velocities of up to

9483-423: The infrared OH stretching mode of the water molecule. (Liquid water appears blue for the same reason. The blue of glacier ice is sometimes misattributed to Rayleigh scattering of bubbles in the ice.) A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or terminus . Glaciers are broken into zones based on surface snowpack and melt conditions. The ablation zone

9592-679: The most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below. Because ice can flow faster where it is thicker, the rate of glacier-induced erosion is directly proportional to the thickness of overlying ice. Consequently, pre-glacial low hollows will be deepened and pre-existing topography will be amplified by glacial action, while nunataks , which protrude above ice sheets, barely erode at all – erosion has been estimated as 5 m per 1.2 million years. This explains, for example,

9701-445: The most sensitive indicators of climate change and are a major source of variations in sea level . A large piece of compressed ice, or a glacier, appears blue , as large quantities of water appear blue , because water molecules absorb other colors more efficiently than blue. The other reason for the blue color of glaciers is the lack of air bubbles. Air bubbles, which give a white color to ice, are squeezed out by pressure increasing

9810-415: The older parts of the volcanic complex. The presence of unconsolidated glacial fluvial deposits under the flow suggest that it is less than 1,000 years old. Although the particular Volcanic Explosivity Index (VEI) of the Silverthrone Caldera is unknown, the chemistry and structure of the volcano can be compared to other calderas that have a history of producing some of the world's most violent eruptions. It

9919-683: The others are Castle Rock , Mount Edziza , Mount Cayley , Hoodoo Mountain , The Volcano , Crow Lagoon , Mount Garibaldi , Mount Meager massif , Wells Gray-Clearwater Volcanic Field and Nazko Cone . Seismic data suggests that these volcanoes still contain live magma plumbing systems, indicating possible future eruptive activity. Although the available data does not allow a clear conclusion, these observations are further indications that some of Canada's volcanoes are potentially active, and that their associated hazards may be significant. The seismic activity correlates both with some of Canada's most youthful volcanoes, and with long-lived volcanic centers with

10028-725: The pooling of meltwater at the base of the glacier  — perhaps delivered from a supraglacial lake  — or the simple accumulation of mass beyond a critical "tipping point". Temporary rates up to 90 m (300 ft) per day have occurred when increased temperature or overlying pressure caused bottom ice to melt and water to accumulate beneath a glacier. In glaciated areas where the glacier moves faster than one km per year, glacial earthquakes occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1. The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in

10137-410: The pressure of the layers of ice and snow above it, this granular ice fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles. Glacial ice has a distinctive blue tint because it absorbs some red light due to an overtone of

10246-413: The ratio of the amount of Ar accumulated to the amount of K remaining. The long half-life of K allows the method to be used to calculate the absolute age of samples older than a few thousand years. The quickly cooled lavas that make nearly ideal samples for K–Ar dating also preserve a record of the direction and intensity of the local magnetic field as

10355-400: The recrystallization of magma, more K will decay and Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified. Despite Ca being the favored daughter nuclide, it

10464-413: The remaining 10.7% of decay events. Argon, being a noble gas , is a minor component of most rock samples of geochronological interest: It does not bind with other atoms in a crystal lattice. When K decays to Ar ; the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal. But it can escape into

10573-469: The ring fracture. As the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds of thousands of square kilometers. Steep contacts between the thick basal breccia of Mount Silverthrone and older crystalline rocks of adjacent peaks suggest that

10682-454: The risk of an eruption, offering a forecasting capability which is important to mitigating volcanic risk. Currently the Silverthrone Caldera does not have a seismograph closer than 124 kilometres (77 mi). With increasing distance and declining numbers of seismographs used to indicate seismic activity, the prediction capability is reduced because earthquake location and depth measurement accuracy decreases. The inaccurate earthquake locations in

10791-481: The rock was cool enough to trap the Ar by the equation: where: The scale factor 0.109 corrects for the unmeasured fraction of K which decayed into Ca ; the sum of the measured K and the scaled amount of Ar gives the amount of K which was present at the beginning of the elapsed time period. In practice, each of these values may be expressed as

10900-722: The sample cooled past the Curie temperature of iron. The geomagnetic polarity time scale was calibrated largely using K–Ar dating. Potassium naturally occurs in 3 isotopes: K (93.2581%), K (0.0117%), K (6.7302%). K and K are stable. The K isotope is radioactive; it decays with a half-life of 1.248 × 10  years to Ca and Ar . Conversion to stable Ca occurs via electron emission ( beta decay ) in 89.3% of decay events. Conversion to stable Ar occurs via electron capture in

11009-514: The sea, often with an ice tongue , like Mertz Glacier . Tidewater glaciers are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin , Devon , and Ellesmere Islands in Canada, Southeast Alaska , and the Northern and Southern Patagonian Ice Fields . As the ice reaches the sea, pieces break off or calve, forming icebergs . Most tidewater glaciers calve above sea level, which often results in

11118-409: The stagnant ice above, forming a bergschrund . Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges . Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of

11227-423: The stress being applied, ice will act as an elastic solid. Ice needs to be at least 30 m (98 ft) thick to even start flowing, but once its thickness exceeds about 50 m (160 ft) (160 ft), stress on the layer above will exceeds the inter-layer binding strength, and then it'll move faster than the layer below. This means that small amounts of stress can result in a large amount of strain, causing

11336-467: The subduction zone. At issue are the current plate configuration and rate of subduction but Silverthrone's chemistry indicates that Silverthrone is subduction related. The Cascadia subduction zone is a long convergent plate boundary that separates the Juan de Fuca , Explorer , Gorda and North American Plates . Here, the oceanic crust of the Pacific Ocean sinks beneath North America at

11445-438: The surface snowpack may experience seasonal melting. A subpolar glacier includes both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier. In a similar way, the thermal regime of a glacier is often described by its basal temperature. A cold-based glacier is below freezing at the ice-ground interface and is thus frozen to the underlying substrate. A warm-based glacier

11554-406: The surrounding region when the right conditions are met, such as changes in pressure or temperature. Ar atoms can diffuse through and escape from molten magma because most crystals have melted and the atoms are no longer trapped. Entrained argon – diffused argon that fails to escape from the magma – may again become trapped in crystals when magma cools to become solid rock again. After

11663-496: The true age of the rock: Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample. Ar–Ar dating is a similar technique that compares isotopic ratios from the same portion of the sample to avoid this problem. Due to the long half-life of K , the technique is most applicable for dating minerals and rocks more than 100,000 years old. For shorter timescales, it

11772-460: The volcano which would make this a hazard for air traffic along the coastal airway between Vancouver and Alaska . Volcanic ash reduces visibility and can cause jet engine failure as well as damage to other aircraft systems. In addition, pyroclastic fall could also have a deleterious effect on the Ha-Iltzuk Icefield surrounding the volcano. Melting of glacial ice could cause lahars or debris flows . This in turn could endanger water supplies on

11881-417: The weight of the ice is supported by the underlying water, and the glacier is afloat. Glaciers may also move by basal sliding , where the base of the glacier is lubricated by the presence of liquid water, reducing basal shear stress and allowing the glacier to slide over the terrain on which it sits. Meltwater may be produced by pressure-induced melting, friction or geothermal heat . The more variable

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