Radiometric dating, radioactive dating or radioisotope dating is a technique used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay; the use of radiometric dating was first published in 1907 by Bertram Boltwood and is now the principal source of information about the absolute age of rocks and other geological features, including the age of fossilized life forms or the age of the Earth itself, can be used to date a wide range of natural and man-made materials. Together with stratigraphic principles, radiometric dating methods are used in geochronology to establish the geologic time scale. Among the best-known techniques are radiocarbon dating, potassium–argon dating and uranium–lead dating. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change.
Radiometric dating is used to date archaeological materials, including ancient artifacts. Different methods of radiometric dating vary in the timescale over which they are accurate and the materials to which they can be applied. All ordinary matter is made up of combinations of chemical elements, each with its own atomic number, indicating the number of protons in the atomic nucleus. Additionally, elements may exist in different isotopes, with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide; some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide; this transformation may be accomplished in a number of different ways, including alpha decay and beta decay. Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product. In many cases, the daughter nuclide itself is radioactive, resulting in a decay chain ending with the formation of a stable daughter nuclide. In these cases the half-life of interest in radiometric dating is the longest one in the chain, the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years to over 100 billion years. For most radioactive nuclides, the half-life depends on nuclear properties and is a constant, it is not affected by external factors such as temperature, chemical environment, or presence of a magnetic or electric field. The only exceptions are nuclides that decay by the process of electron capture, such as beryllium-7, strontium-85, zirconium-89, whose decay rate may be affected by local electron density.
For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present; the basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created, it is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron.
This can reduce the problem of contamination. In uranium–lead dating, the concordia diagram is used which decreases the problem of nuclide loss. Correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.6 ± 0.05 million years ago using uranium–lead dating and 3.56 ± 0.10 Ma using lead–lead dating, results that are consistent with each other. Accurate radiometric dating requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement, the half-life of the parent is known, enough of the daughter product is produced to be measured and distinguished from the initial amount of the daughter present in the material; the procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This involves isotope-ratio mass spectrometry; the precision of
The Trans-Hudson orogeny or Trans-Hudsonian orogeny was the major mountain building event that formed the Precambrian Canadian Shield, the North American Craton, the forging of the initial North American continent. It gave rise to the Trans-Hudson orogen, or Trans-Hudson Orogen Transect, the largest Paleoproterozoic orogenic belt in the world, it consists of a network of belts that were formed by Proterozoic crustal accretion and the collision of pre-existing Archean continents. The event occurred 2.0-1.8 billion years ago. The Trans-Hudson orogen sutured together the Hearne-Rae and Wyoming cratons to form the cratonic core of North America in a network of Paleoproterozoic orogenic belts; these orogenic belts include the margins of at least nine independent microcontinents that were themselves sections of at least three former major supercontinents, including Laurasia and Kenorland, contain parts of some of the oldest cratonic continental crust on Earth. These old cratonic blocks, along with accreted island arc terranes and intraoceanic deposits from earlier Proterozoic and Mesozoic oceans and seaways, were sutured together in the Trans-Hudson Orogen and resulted in extensive folding and thrust faulting along with metamorphism and hundreds of huge granitic intrusions.
The THO is a right-angled suture zone that extends eastward from Saskatchewan through collisional belts in the Churchill province, through northern Quebec, parts of Labrador and Baffin Island, all the way to Greenland as the Rinkian belt and Nagssugtodidian Orogen. Westward it goes across Hudson Bay through Saskatchewan and extends 90 degrees south through eastern Montana and the western Dakotas, downward through eastern Wyoming and western Nebraska, is cut off by the Cheyenne belt - the northern edge of the Yavapai province (see Trans-Hudson Orogen map and the THOT Transect map. To the south, the orogen contributed to the subsurface Phanerozoic strata in Montana and the Dakotas that created the Great Plains; the Trans-Hudson orogeny was the culminating event of the Paleoproterozoic Laurentian assembly, which occurred after the Wopmay orogeny. The Trans-Hudson orogeny resulted from the collision of the Superior Craton of eastern Canada with the Hearne Craton in northern Saskatchewan and the Wyoming Craton of the western United States, with the Archean microcontinent Sask Craton trapped in the THO western interior.
Similar to the Himalayas, the Trans-Hudson orogeny was the result of continent-continent collision along a suture zone. Only the roots of this mountain chain remain, but these can be seen in northeastern Saskatchewan and in the Black Hills of South Dakota; the Trans-Hudson orogeny and the consequent upheaval of the continental crust in the middle Proterozoic eon caused the area around the Great Lakes to become a flattened plain, which in turn led to the creation of the intercontinental basin and the interior and central plains of the United States. The Black Hills of South Dakota is one of the few remaining exposed portions of the Trans-Hudson orogenic belt; the peaks of the Black Hills are 3,000 to 4,000 feet above the surrounding plains, while Black Elk Peak - the highest point in South Dakota - has an altitude of 7,242 feet above sea level. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form the core of the uplift; the nature and timing of this portion of the THO event in southern Laurentia is poorly understood, when compared to the exposed northern segments in Canada.
The Black Hills offer the only surface exposure of the deformed and metamorphosed belt of Paleoproterozoic continental margin rocks in the collisional zone between the Archean Wyoming and Superior provinces. Based on geophysical evidence, this zone has been broadly interpreted to be the southern extension of the THO, truncated by the ~1.680 Ga. Central Plains orogen. Marine evidence indicates that the area opened to form an ocean called the Manikewan Ocean. Faulting and igneous rocks all indicate that divergence formed a rift valley that continued to spread until it resulted in a passive margin in which there was no tectonic activity. Shallow marine deposits formed on the continental shelves, oceanic crust formed on the margins of the continental cratons as the divergence continued; the divergence stopped reversed direction, collision occurred between continental land masses. During the Wopmay orogeny, subduction occurred as oceanic crust of the Slave Craton was subducted beneath an eastward moving continental plate.
During the Trans-Hudson orogeny, rifting at first separated the Superior craton from the rest of the continent. The Superior Craton reversed its direction and the ocean basin began to close. A subduction zone formed as the oceanic crust of the Superior Craton was subducted beneath the Hearne and Wyoming Craton with the Sask Craton in the middle. Volcanic arcs developed as the cratons collided resulting in the THO mountain building; the Northwestern hinterland zone is a complex tectonically deformed region that includes the Peter Lake and Seal River domains, other parts of the Cree Lake Zone, now included in Hearne Province. The Reindeer zone to the north is a 500 km wide collage of Paleoproterozoic arc volcanic rocks, volcanogenic sediments, younger molasse, divisible into several lithostructural domains. Most of these rocks evolved in an oceanic to transitional
An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges. Orogeny is the primary mechanism; the word "orogeny" comes from Ancient Greek. Although it was used before him, the term was employed by the American geologist G. K. Gilbert in 1890 to describe the process of mountain building as distinguished from epeirogeny; the formation of an orogen can be accomplished by the tectonic processes such as oceanic subduction or continental subduction convergence of two or more continents for collisional orogeny). Orogeny produces long arcuate structures, known as orogenic belts. Orogenic belts consist of long parallel strips of rock exhibiting similar characteristics along the length of the belt. Although orogenic belts are associated with subduction zones, subduction tectonism may be ongoing or past processes.
The subducting tectonism would consume crust, thicken lithosphere, produce earthquake and volcanoes, build island arcs in many cases. Geologists attribute the arcuate structure to the rigidity of the descending plate, island arc cusps relate to tears in the descending lithosphere; these island arcs may be added to a continental margin during an accretionary orogeny. On the other hand, subduction zones may be reworked at a time due to lithospheric rifting, leading to amphibolite to granulite facies metamorphism of the thinned orogenic crust; the processes of orogeny can take tens of millions of years and build mountains from plains or from the seabed. The topographic height of orogenic mountains is related to the principle of isostasy, that is, a balance of the downward gravitational force upon an upthrust mountain range and the buoyant upward forces exerted by the dense underlying mantle. Rock formations that undergo orogeny are deformed and undergo metamorphism. Orogenic processes may push buried rocks to the surface.
Sea-bottom and near-shore material may cover all of the orogenic area. If the orogeny is due to two continents colliding high mountains can result. An orogenic event may be studied: as a tectonic structural event, as a geographical event, as a chronological event. Orogenic events: cause distinctive structural phenomena related to tectonic activity affect rocks and crust in particular regions, happen within a specific period In general, there are two main types of orogens at convergent plate margins: accretionary orogens, which were produced by subduction of one oceanic plate beneath one continental plate to result in either continental arc magmatism or the accretion of island arc terranes to continental margins. An orogeny produces an orogen, but a range-foreland basin system is only produced on passive plate margins; the foreland basin forms ahead of the orogen due to loading and resulting flexure of the lithosphere by the developing mountain belt. A typical foreland basin is subdivided into a wedge-top basin above the active orogenic wedge, the foredeep beyond the active front, a forebulge high of flexural origin and a back-bulge area beyond, although not all of these are present in all foreland-basin systems.
The basin migrates with the orogenic front and early deposited foreland basin sediments become progressively involved in folding and thrusting. Sediments deposited in the foreland basin are derived from the erosion of the uplifting rocks of the mountain range, although some sediments derive from the foreland; the fill of many such basins shows a change in time from deepwater marine through shallow water to continental sediments. Although orogeny involves plate tectonics, the tectonic forces result in a variety of associated phenomena, including crustal deformation, crustal thickening, crustal thinning and crustal melting as well as magmatism and mineralization. What happens in a specific orogen depends upon the strength and rheology of the continental lithosphere, how these properties change during orogenesis. In addition to orogeny, the orogen is subject to other processes, such as erosion; the sequence of repeated cycles of sedimentation and erosion, followed by burial and metamorphism, by crustal anatexis to form granitic batholiths and tectonic uplift to form mountain chains, is called the orogenic cycle.
For example, the Caledonian Orogeny refers to a series of tectonic events due to the continental collision of Laurentia with Eastern Avalonia and other former fragments of Gondwana in the Early Paleozoic. The Caledonian Orogen resulted from these events and various others that are part of its peculiar orogenic cycle. In summary, an orogeny is an episode of deformation and magmatism at convergent plate margins, during which many geological processes play a role at convergent plate margins; every orogeny has its own orogenic cycle, but composite orogenesis is common at convergent plate margins. Erosion represents a subsequent phase of the orogenic cycle. Erosion removes much of the mountains
The Jurassic period was a geologic period and system that spanned 56 million years from the end of the Triassic Period 201.3 million years ago to the beginning of the Cretaceous Period 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era known as the Age of Reptiles; the start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, the Tithonian event at the end; the Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations; the Jurassic is named after the Jura Mountains within the European Alps, where limestone strata from the period were first identified. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north, Gondwana to the south; this created more coastlines and shifted the continental climate from dry to humid, many of the arid deserts of the Triassic were replaced by lush rainforests.
On land, the fauna transitioned from the Triassic fauna, dominated by both dinosauromorph and crocodylomorph archosaurs, to one dominated by dinosaurs alone. The first birds appeared during the Jurassic, having evolved from a branch of theropod dinosaurs. Other major events include the appearance of the earliest lizards, the evolution of therian mammals, including primitive placentals. Crocodilians made the transition from a terrestrial to an aquatic mode of life; the oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs, while pterosaurs were the dominant flying vertebrates. The chronostratigraphic term "Jurassic" is directly linked to the Jura Mountains, a mountain range following the course of the France–Switzerland border. During a tour of the region in 1795, Alexander von Humboldt recognized the limestone dominated mountain range of the Jura Mountains as a separate formation that had not been included in the established stratigraphic system defined by Abraham Gottlob Werner, he named it "Jura-Kalkstein" in 1799.
The name "Jura" is derived from the Celtic root *jor via Gaulish *iuris "wooded mountain", borrowed into Latin as a place name, evolved into Juria and Jura. The Jurassic period is divided into three epochs: Early and Late. In stratigraphy, the Jurassic is divided into the Lower Jurassic, Middle Jurassic, Upper Jurassic series of rock formations known as Lias and Malm in Europe; the separation of the term Jurassic into three sections originated with Leopold von Buch. The faunal stages from youngest to oldest are: During the early Jurassic period, the supercontinent Pangaea broke up into the northern supercontinent Laurasia and the southern supercontinent Gondwana; the Jurassic North Atlantic Ocean was narrow, while the South Atlantic did not open until the following Cretaceous period, when Gondwana itself rifted apart. The Tethys Sea closed, the Neotethys basin appeared. Climates were warm, with no evidence of a glacier having appeared; as in the Triassic, there was no land over either pole, no extensive ice caps existed.
The Jurassic geological record is good in western Europe, where extensive marine sequences indicate a time when much of that future landmass was submerged under shallow tropical seas. In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the epicontinental Sundance Sea left marine deposits in parts of the northern plains of the United States and Canada during the late Jurassic, most exposed sediments from this period are continental, such as the alluvial deposits of the Morrison Formation; the Jurassic was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate. Carbonate hardgrounds were thus common, along with calcitic ooids, calcitic cements, invertebrate faunas with dominantly calcitic skeletons; the first of several massive batholiths were emplaced in the northern American cordillera beginning in the mid-Jurassic, marking the Nevadan orogeny. Important Jurassic exposures are found in Russia, South America, Japan and the United Kingdom.
In Africa, Early Jurassic strata are distributed in a similar fashion to Late Triassic beds, with more common outcrops in the south and less common fossil beds which are predominated by tracks to the north. As the Jurassic proceeded and more iconic groups of dinosaurs like sauropods and ornithopods proliferated in Africa. Middle Jurassic strata are neither well studied in Africa. Late Jurassic strata are poorly represented apart from the spectacular Tendaguru fauna in Tanzania; the Late Jurassic life of Tendaguru is similar to that found in western North America's Morrison Formation. During the Jurassic period, the primary vertebrates living in the sea were marine reptiles; the latter include ichthyosaurs, which were at the peak of their diversity, plesiosaurs and marine crocodiles of the families Teleosauridae and Metriorhynchidae. Numerous turtles could be found in rivers. In the invertebrate world, several new groups appeared, including rudists (a reef-formi
The Klamath Mountains are a rugged and populated mountain range in northwestern California and southwestern Oregon in the western United States. They have a varied geology, with substantial areas of serpentinite and marble, a climate characterized by moderately cold winters with heavy snowfall and warm dry summers with limited rainfall in the south; as a consequence of the geology and soil types, the mountains harbor several endemic or near-endemic trees, forming one of the largest collections of conifers in the world. The mountains are home to a diverse array of fish and animal species, including black bears, large cats, owls and several species of Pacific salmon. Millions of acres in the mountains are managed by the United States Forest Service; the northernmost and largest sub-range of the Klamath Mountains are the Siskiyou Mountains. Physiographically, the Klamath Mountains include the Siskiyou Mountains, the Marble Mountains, the Scott Mountains, the Trinity Mountains, the Trinity Alps, the Salmon Mountains, the northern Yolla-Bolly Mountains.
They are a section of the larger Pacific Border province, which in turn is part of the Pacific Mountain System physiographic division. These are the ten highest points in the Klamath Mountains: 1. Mount Eddy 2. Thompson Peak 3. Mount Hilton 4. Caesar Peak 5. Sawtooth Mountain 6. Wedding Cake Mountain 7. Caribou Mountain 8. China Mountain 9. Gibson Peak 10. Boulder Peak A large portion of the Klamath Mountains is managed by the United States Forest Service. Several national forests lie in the Klamath Mountains region, including the Shasta-Trinity National Forest, Siskiyou National Forest, Klamath National Forest, Six Rivers National Forest, Mendocino National Forest; the Klamath Mountains contain 11 wilderness areas in both Oregon and California: There are extensive hiking trail systems, recreation areas, campgrounds both primitive and developed in the Klamaths. A 211-mile stretch of the Pacific Crest Trail passes through these mountains as well; this section of the PCT is known locally as "The Big Bend" and is the transition from the California Floristic Province to the Cascades.
The Bigfoot Trail is a 400-mile trail through the Klamath Mountains from the Yolla Bolly-Middle Eel Wilderness to Crescent City, California. Klamath Mountains is the name given to one of California's eleven geomorphic provinces; the rocks of the Klamath Mountains originated as island arcs and continental fragments in the Pacific Ocean. The island masses consisted of rifted fragments of pre-existing continents and volcanic island masses created over subduction zones; these island masses contain rocks as old as 500 million years, dating to the early Paleozoic Era. A succession of eight island terranes moved eastward on the ancient Farallon plate and collided with the North American plate between 260 and about 130 million years ago; each accretion left a terrane of rock of a single age. During the accretion, subduction of the plate metamorphosed the overlying rock and produced magma which intruded the overlying rock as plutons. Serpentinite, produced by the metamorphism of basaltic oceanic rocks, intrusive rocks of gabbroic to granodiorite composition are common rocks within the Klamath terranes.
Subsequent lava flows from active volcanoes in the Cascade Range and the erosion of the Oregon Coast Range to the north covered these rocks with basalt and sediments. As a consequence of the geology, the mountains harbor rich biodiversity, with several distinct plant communities, including temperate rain forests, moist inland forests, oak forests and savannas, high elevation forests, alpine grasslands; these communities form the Klamath Mountains ecoregion. One of the principal plant communities in the Klamath Mountains is Mediterranean California Lower Montane Black Oak-Conifer Forest; the ecoregion includes several endemic or near-endemic species, such as Port Orford cedar or Lawson's cypress, foxtail pine, Brewer's spruce, forming one of the largest collections of different conifers in the world. The flowering plant Kalmiopsis leachiana endemic to the Klamaths, is limited to the Siskiyou sub-range in Oregon. ConifersA large concentration of diverse coniferous species of trees exists in these mountains.
Thirty conifer species inhabit the area, including two endemic species, the Brewer's spruce and the Port Orford cedar, making the Klamath Mountains one of the richest coniferous forest regions of the world in terms of concentrated species diversity. The region has several edaphic plant communities, adapted to specific soil types, notably serpentine outcrops. In 1969, Drs. John O. Sawyer and Dale Thornburgh discovered 17 species of conifers in 1 square mile around Little Duck Lake and Sugar Creek in the Russian Wilderness, they called this diverse area the Miracle Mile. In 2013 Richard Moore identified western juniper, in the Sugar Creek canyon; this is now considered the richest assemblage of conifers per unit area in any temperate region on Earth. Conifer species in the Klamath Mountains include coast Douglas-fir, Port Orford cedar, ponder
The Farallon Plate was an ancient oceanic plate that began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart during the Jurassic period. It is named for the Farallon Islands, which are located just west of California. Over time, the central part of the Farallon Plate was subducted under the southwestern part of the North American Plate; the remains of the Farallon Plate are the Juan de Fuca and Gorda Plates, subducting under the northern part of the North American Plate. The Farallon Plate is responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate; these fragments from elsewhere are called terranes. Much of western North America is composed of these accreted terranes; the understanding of the Farallon Plate is evolving as details from seismic tomography provide improved details of the submerged remnants. Since the North American west coast shows a convoluted structure, significant work has been required to resolve the complexity.
In 2013 a new and more nuanced explanation emerged, proposing two additional now-subducted plates which would account for some of the complexity. As data accumulated, a common view developed that one large oceanic plate, the Farallon plate, acted as a conveyor belt, conveying terranes to North America's west coast, where they accreted; as the continent overran the subducting Farallon plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent; as of 2013, it is accepted that the western quarter of North America consists of accreted terrane accumulated over the past 200 million years as a result of the oceanic Farallon plate moving terranes onto the continental margin as it subducts under the continent. However this simple model was unable to explain many terrane complexities, is inconsistent with seismic tomographic images of subducting slabs penetrating the lower-mantle.
In April 2013 Sigloch and Mihalynuk noted that under North America these subducting slabs formed massive vertical walls of 800 km to 2,000 km deep and 400–600 km wide, forming "slab walls". One such large "slab wall" runs from north-west Canada to the eastern U. S. and extends to Central America. Sigloch and Mihalynuk proposed that the Farallon should be partitioned into Northern Farallon, Angayucham and Southern Farallon segments based on recent tomographic models. Under this model, the North American continent overrides a series of subduction trenches and incorporates microcontinents as it moves west in the following sequence: 165–155 Myr ago the Mezcalera promontory strikes land and begins to be overridden; the overridden segment is replaced by an incipient South Farallon trench. 160–155 Myr ago the Rocky Mountain deformation begins, recorded by a synorogenic clastic wedge. The Franciscan subduction complex on the South Farallon plate begins. 125 Myr ago the collision of the North America margin with an archipelago of terranes begins.
This broad expanse causes strong deformations and creates the Sevier Mountains and the Canadian Rocky Mountains. 124–90 Myr ago the Omenica magmatic belts are formed in the Pacific Northwest along with a gradual override of the Mezcalera promontory by the Pacific Northwest. 85 Myr ago the South Farallon trench moves westward after accretion of the Shatsky Rise Conjugate plateau. Sonora volcanism results from the slab sinking; the Tarahumara ignimbrite province is formed. 85–55 Myr ago Strong transpressive coupling of Farallon plate to terranes produces the buoyant Shatsky Rise. The Laramide orogeny results from basement uplift more than 1,000 km inland. 72–69 Myr ago the Angayucham arc, is overridden by North America and Carmacks volcanic episode results. 85–55 Myr ago Conjugate subducts. Northward shuffle of Insular terrane, Intermontane terrane, Angayucham terranes along margin. 55–50 Myr ago saw the override of the Cascadia Root arc by the Pacific Northwest along with accretion of the Siletzia and Pacific Rim terranes.
55–50 Myr ago Final override of westernmost Angayucham occurred, with an explosive end of Coast Mountain arc volcanismWhen the final archipelago, the Siletzia archipelago lodged as a terrane, the associated trench stepped west as the terrane accreted, converting an intra-oceanic subduction trench into the current Cascadia subduction zone and creating a slab window. Izanagi Plate Kula Plate Kula–Farallon Ridge Pacific-Farallon Ridge San Andreas Fault Notes Bibliography USGS Professional Paper 1515: images of Farallon Plate Demise of the Farallon Plate Beneath North America – California Institute of Technology The Farallon Plate – Goddard Space Flight Center
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source; the rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations. Sills parallel beds and foliations in the surrounding country rock.
They can be emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills into near vertical orientations. Sills can be confused with solidified lava flows. Intruded sills will show partial incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed. Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will show evidence of vesicles where gases escaped into the atmosphere; because sills form at shallow depths below the surface, the pressure of overlying rock prevents this from happening much, if at all. Lava flows will typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock do not. Certain layered intrusions are a variety of sill that contain important ore deposits. Precambrian examples include the Bushveld and the Great Dyke complexes of southern Africa, the Duluth intrusive complex of the Superior District, the Stillwater igneous complex of the United States.
Phanerozoic examples are smaller and include the Rùm peridotite complex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusions contain concentrations of gold, platinum and other rare elements. Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by short dike-like segments; such sills are known as transgressive, examples include the Whin Sill and sills within the Karoo basin. The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3D seismic reflection data; such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive. Aquatic sill Batholith Dike Laccolith Sheet intrusion Sill swarm Stock