Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of the Earth's lithosphere, since tectonic processes began on Earth between 3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century; the geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s. The lithosphere, the rigid outermost shell of a planet, is broken into tectonic plates; the Earth's lithosphere is composed of many minor plates. Where the plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, oceanic trench formation occur along these plate boundaries; the relative movement of the plates ranges from zero to 100 mm annually. Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust.
Along convergent boundaries, subduction, or one plate moving under another, carries the lower one down into the mantle. In this way, the total surface of the lithosphere remains the same; this prediction of plate tectonics is referred to as the conveyor belt principle. Earlier theories, since disproven, proposed gradual expansion of the globe. Tectonic plates are able to move because the Earth's lithosphere has greater mechanical strength than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Plate movement is thought to be driven by a combination of the motion of the seafloor away from spreading ridges due to variations in topography and density changes in the crust. At subduction zones the cold, dense crust is "pulled" or sinks down into the mantle over the downward convecting limb of a mantle cell. Another explanation lies in the different forces generated by tidal forces of the Moon; the relative importance of each of these factors and their relationship to each other is unclear, still the subject of much debate.
The outer layers of the Earth are divided into the asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat; the lithosphere is more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction, whereas the asthenosphere transfers heat by convection and has a nearly adiabatic temperature gradient; this division should not be confused with the chemical subdivision of these same layers into the mantle and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times depending on its temperature and pressure. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Plate motions range up to a typical 10–40 mm/year, to about 160 mm/year; the driving mechanism behind this movement is described below. Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust and continental crust.
Average oceanic lithosphere is 100 km thick. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km thick at mid-ocean ridges to greater than 100 km at subduction zones. Continental lithosphere is about 200 km thick, though this varies between basins, mountain ranges, stable cratonic interiors of continents; the location where two plates meet is called a plate boundary. Plate boundaries are associated with geological events such as earthquakes and the creation of topographic features such as mountains, mid-ocean ridges, oceanic trenches; the majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and known today. These boundaries are discussed in further detail below; some volcanoes occur in the interiors of plates, these have been variously attributed to internal plate deformation and to mantle plumes.
As explained above, tectonic plates may include continental crust or oceanic crust, most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans; the distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is fo
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.063 millimetres with individual grains too small to be distinguished without a microscope. With increased pressure over time, the platy clay minerals may become aligned, with the appearance of fissility or parallel layering; this finely bedded material that splits into thin layers is called shale, as distinct from mudstone. The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in the sediment prior to lithification. Mud rocks such as mudstone and shale account for some 65% of all sedimentary rocks. Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit. Mudstone can be separated into these categories: Siltstone — more than half of the composition is silt-sized particles. Claystone — more than half of the composition is clay-sized particles.
Mudstone — hardened mud. Mudstone can include: Shale -- exhibits fissility. Argillite — has undergone low-grade metamorphism. In the Dunham classification system of limestones, a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud component. A recent study by Lokier and Al Junaibi has highlighted that the most common problems encountered when describing a mudstone is to incorrectly estimate the volume of'grains' in the sample - in consequence, misidentifying mudstone as wackestone and vice versa; the original Dunham classification defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan to a grain size of less than or equal to 30 μm. Wright proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt.
On December 13, 2016, NASA reported further evidence supporting habitability on the planet Mars as the Curiosity rover climbed higher, studying younger layers, on Mount Sharp. Reported, the soluble element boron was detected for the first time on Mars. In June 2018, NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks 3.5 billion years old. Mudstone on planet Mars Aeolis quadrangle Composition of Mars Timeline of Mars Science Laboratory Tonstein – A hard, compact sedimentary rock, composed of kaolinite or, less other clay minerals
Continental crust is the layer of igneous and metamorphic rocks that forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in silicates and aluminium minerals and has a lower density compared to the oceanic crust, called sima, richer in magnesium silicate minerals and is denser. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, more mafic in character; the continental crust consists of various layers, with a bulk composition, intermediate. The average density of continental crust is about 2.83 g/cm3, less dense than the ultramafic material that makes up the mantle, which has a density of around 3.3 g/cm3. Continental crust is less dense than oceanic crust, whose density is about 2.9 g/cm3. At 25 to 70 km, continental crust is thicker than oceanic crust, which has an average thickness of around 7–10 km.
About 40% of Earth's surface is occupied by continental crust. It makes up about 70% of the volume of Earth's crust; because the surface of continental crust lies above sea level, its existence allowed land life to evolve from marine life. Its existence provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion. All continental crust is derived from mantle-derived melts through fractional differentiation of basaltic melt and the assimilation of pre-existing continental crust; the relative contributions of these two processes in creating continental crust are debated, but fractional differentiation is thought to play the dominant role. These processes occur at magmatic arcs associated with subduction. There is little evidence of continental crust prior to 3.5 Ga. About 20% of the continental crust's current volume was formed by 3.0 Ga. There was rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga.
During this time interval, about 60% of the continental crust's current volume was formed. The remaining 20% has formed during the last 2.5 Ga. In contrast to the persistence of continental crust, the size and number of continents are changing through geologic time. Different tracts rift apart and recoalesce as part of a grand supercontinent cycle. There are about 7 billion cubic kilometers of continental crust, but this quantity varies because of the nature of the forces involved; the relative permanence of continental crust contrasts with the short life of oceanic crust. Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is subducted back into the mantle. Continental crust is subducted. For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in recycled oceanic crust. Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history.
The height of mountain ranges is related to the thickness of crust. This results from the isostasy associated with orogeny; the crust is thickened by the compressive forces related to continental collision. The buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion; this forms a keel or mountain root beneath the mountain range, where the thickest crust is found. The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and severed, replaced by oceanic crust; the edges of continental fragments formed. The high temperatures and pressures at depth combined with a long history of complex distortion, cause much of the lower continental crust to be metamorphic - the main exception to this being recent igneous intrusions. Igneous rock may be "underplated" to the underside of the crust, i.e. adding to the crust by forming a layer beneath it. Continental crust is produced and destroyed by plate tectonic processes at convergent plate boundaries.
Additionally, continental crustal material is transferred to oceanic crust by sedimentation. New material can be added to the continents by the partial melting of oceanic crust at subduction zones, causing the lighter material to rise as magma, forming volcanoes. Material can be accreted horizontally when volcanic island arcs, seamounts or similar structures collide with the side of the continent as a result of plate tectonic movements. Continental crust is lost through erosion and sediment subduction, tectonic erosion of forearcs and deep subduction of continental crust in collision zones. Many theories of crustal growth are controversial, including rates of crustal growth and recycling, whether the lower crust is recycled differently from the upper crust, over how much of Earth history plate tectonics has operated and so could be the dominant mode of continental crust formation and destruction, it is a mat
Thin-skinned deformation is a style of deformation in plate tectonics at a convergent boundary which occurs with shallow thrust faults that only involves cover rocks, not deeper basement rocks. The thin-skinned style of deformation is typical of many fold and thrust belts developed in the foreland of a collisional zone or back arc of a continental volcanic arc; this is the case where a good basal decollement exists in a weaker layer like a shale, evaporite, or a zone of high pore fluid pressure. This was first described as part of the Sevier Orogeny. In the rock record, this will increase the influence of more surficial rocks, which includes sedimentary rocks. You will see repeated sections of the same rock over and over as thrust faults, coming up from the decollement, stack the same layer on top of itself; the sediments that are created by this type of deformation are lithic sandstones. Thick-skinned deformation Fold and thrust belts
John Horne PRSE FRS FRSE FEGS LLD was a Scottish geologist. He served as President of the Royal Society of Edinburgh from 1915 to 1919. Horne was born on 1 January 1848, in Campsie, the son of Janet and James Horne of Newmill, a farmer, he was educated at the High School and the University of Glasgow where he studied under Lord Kelvin. He left university without graduating at the age on 19. In 1867 he joined the Scottish Branch of HM Geological Survey as an assistant and became an apprentice to Ben Peach; the two soon became good collaborators. Horne was involved in mapping the Central Lowlands. Horne was a logical thinker and writer, complementing Peach's skills of resolving the internal structure of mountains by looking at the surface rocks. Thia approach allowed them to resolve a long-running debate on the geological formation of the Highlands. After their work in the Highlands and Peach wrote'Northwest Highlands Memoir' in 1907; the work is regarded as one of the most important geological memoirs.
Horne wrote most of the memoir himself. From 1901 until 1911, John Horne was the Director of the Scottish Branch of the Survey. Horne was elected a Fellow of the Royal Society of Edinburgh in 1881, upon the proposal of Sir Archibald Geikie, Sir Charles Wyville Thomson, Peter Tait and Robert Gray, won the Society's Neill Prize for 1889-92. Horne was active in the affairs of the RSE and served as Councillor, Vice-President and President. Horne was elected a Fellow of the Royal Society of London in 1900 and was a Fellow of the Royal Scottish Geographical Society, he served as President of the Edinburgh Geological Society. In life he lived at 12 Keith Crescent in Blackhall, Edinburgh, he died on 30 May 1928 in Edinburgh. He was married to Anna Leyland Taylor, he was grandfather to the psychologist Thomas Arthur Munro. A monument to the work of Peach and Horne was erected at Inchnadamph, close to the Moine Thrust where they did some of their best-known work; the inscription reads: "To Ben N Peach and John Horne who played the foremost part in unravelling the geological structure of the North West Highlands 1883-1897.
An international tribute. Erected 1980." Knockan Crag Inchnadamph North West Highlands Geopark Geology of Scotland profile at www.scottishgeology.com
Moine Thrust Belt
The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 kilometres south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to 10 kilometres, although it is wider on Skye; the presence of metamorphic gneisses and schists lying stratigraphically above sedimentary rocks of lower Paleozoic age in the Northwest Highlands had been known since the early 19th century, convincing Roderick Murchison that the change was a purely metamorphic effect and that the upper gneiss was younger than the sediments beneath.
He was supported in this interpretation by Archibald Geikie and James Nicol. After further fieldwork, Nicol changed his mind and advocated instead that the contact at the base of the upper gneisses was tectonic, starting what was known as the Highlands Controversy. A tectonic interpretation was supported by, amongst others, Charles Lapworth who had corresponded with Albert Heim on similar structures in the Alps. In 1883 and 1884 the survey geologists Ben Peach and John Horne were sent into the area by the survey's director Archibald Geikie to carry out detailed mapping; the results of the mapping proved conclusively to Peach and Horne that the contact was tectonic and they were able to persuade Geikie when he visited them in the field in October 1884. In November that year Peach and Horne's preliminary results were published and Geikie published a paper in the same issue of Nature in which he coined the term "thrust-plane" for these low-angle faults, although the term was already in use before then.
By 1888 the term "Moine Thrust" was being used for the tectonic break at the base of Moine schists. The recognition of the Moine Thrust Belt in the early 1880s was a milestone in the history of geology as it was one of the first thrust belts discovered and where the importance of large scale horizontal rather than vertical movements became apparent. Detailed mapping of the Moine Thrust Belt by the survey continued for another two decades, culminating in the classic survey memoir The Geological Structure of the Northwest Highlands of Scotland, published in 1907; the Moine Thrust Belt was formed during the late stages of the Caledonian Orogeny as part of the collision between Laurentia and Baltica. It is the most westerly Caledonian structure in Scotland apart from the Outer Isles Fault in the Outer Hebrides, developed within the Hebridean Terrane; the Moine Thrust Belt defines the boundary between the Hebridean Terrane to its northwest and the Northern Highlands Terrane to its southeast. The thrust carried metamorphic material over 200 km across Scotland masking the geology of the previous terrane.
However, small windows, such as the Assynt window and the Glen Achall imbricated thrust system, allow geologists to estimate what the geology of Scotland was like before the Caledonian Orogeny. The relationship between the Moine Thrust Belt and other Scandian age structures in Scandinavia and East Greenland remains unclear, due to uncertainties associated with the Great Glen Fault zone; this major sinistral strike-slip fault was active during the late stages of the orogeny, but continued to move during the early Devonian and appears to truncate the southern end of the thrust belt. The total late Caledonian displacement on the Great Glen Fault is poorly constrained, making reconstruction of the southern part of the orogenic belt difficult; the stratigraphic sequence of the footwall of the Moine Thrust is the full sequence characteristic of the Hebridean Terrane. The Lewisian complex consists of granitic gneisses that are of Archaean and Paleoproterozoic age, they form the basement to both the Torridonian Supergroup and the Moine Supergroup of the Northern Highlands Terrane, in the hanging wall of the Moine Thrust.
The Torridonian Supergroup is of Neoproterozoic age and consists of sandstone with a maximum preserved thickness of over 8 km. It is divided into three groups, the Stoer and Torridon groups; the unconformity at the base of this unit is irregular, showing that it was deposited on an eroded land surface. The Cambrian to lower Ordovician rocks consist of two groups, the Ardvreck Group and the Durness Group; the Ardvreck Group lies above an angular unconformity over various parts of the Torridon Group and locally over the Lewisian. It is a sequence of quartz arenites; the lowermost part of the Eriboll Formation, the Basal Quartzite Member, is pebbly at its base. The overlying Pipe Rock Member is a distinctive quartz arenite with many white weathering skolithos trace fossils that act as strain markers in areas of more ductile deformation; the uppermost two parts of the Ardvreck Group form the An t-Sron Formation, with the dolomitic Fucoid Beds Member being overlain by the quartz arenites of the Salterella Grit Member.
The succeeding Durness Group consists of dolostones, with some limestone and chert. The distinctive character of this sequence enabled detailed mapping in areas of poor exposure and allowed sections repeated by thrusting to be recognised; the Moine Supergroup, like the Torridonian, is of Neoproterozoic age and a lateral equivalent of that unit. Near the Moine Thrust all of the Moine rocks form pa
1994 Northridge earthquake
The 1994 Northridge earthquake was a magnitude of 6.7, blind thrust earthquake that occurred on January 17 at 4:30:55 a.m. PST in the San Fernando Valley region of the County of Los Angeles, its epicenter was in a neighborhood in the north-central Valley. The quake had a duration of 10–20 seconds, its peak ground acceleration of 1.8g was the highest instrumentally recorded in an urban area in North America. Strong ground motion was felt as far away as Las Vegas, about 220 miles from the epicenter; the peak ground velocity at the Rinaldi Receiving Station was 183 cm/s, the fastest recorded. Two 6.0 Mw aftershocks followed, the first about one minute after the initial event and the second 11 hours the strongest of several thousand aftershocks in all. The death toll was 57, with more than 8,700 injured. In addition, property damage was estimated to be $13–50 billion, making it one of the costliest natural disasters in U. S. history. The earthquake struck in the San Fernando Valley about 20 miles northwest of downtown Los Angeles.
Although given the name "Northridge", where the quake was believed to have been centered and substantial damage occurred, the actual epicenter was pinpointed in the neighboring community of Reseda within several days. The National Geophysical Data Center placed the hypocenter's geographical coordinates at 34°12′47″N 118°32′13″W and at a depth of 11.4 miles. It occurred on a undiscovered fault, now named the Northridge blind thrust fault. Several other faults experienced minor rupture during the main shock and other ruptures occurred during large aftershocks, or triggered events. Damage occurred up to 85 miles away, with the most damage in the west San Fernando Valley, the cities of Santa Monica, Simi Valley and Santa Clarita; the exact number of fatalities is unknown, with sources estimating it at 60 or "over 60", to 72, where most estimates fall around 60. The "official" death toll was placed at 57; some counts factor in related events such as a man's suicide inspired by the loss of his business in the disaster.
More than 8,700 were injured including 1,600. The Northridge Meadows apartment complex was one of the well-known affected areas in which sixteen people were killed as a result of the building's collapse; the Northridge Fashion Center and California State University, Northridge sustained heavy damage—most notably the collapse of parking structures. The earthquake gained worldwide attention because of damage to the vast freeway network, which serves millions of commuters everyday; the most notable was to the Santa Monica Freeway, Interstate 10, known as the busiest freeway in the United States, congesting nearby surface roads for three months while the freeway was repaired. Farther north, the Newhall Pass interchange of Interstate 5 and State Route 14 collapsed as it had 23 years earlier in the 1971 Sylmar earthquake though it had been rebuilt with minor improvements to the structural components. One life was lost in the Newhall Pass interchange collapse: LAPD motorcycle officer Clarence Wayne Dean fell 40 feet from the damaged connector from southbound 14 to southbound I-5 along with his motorcycle.
Because of the early morning darkness, he did not realize that the elevated roadway below him had collapsed, was unable to stop in time to miss the fall and died instantly. When the interchange was rebuilt again one year it was renamed the Clarence Wayne Dean Memorial Interchange in his honor. Additional damage occurred about 50 miles southeast in Anaheim as the scoreboard at Anaheim Stadium collapsed onto several hundred seats; the stadium was vacant at the time. Although several commercial buildings collapsed, loss of life was minimized because of the early morning hour of the quake, because it occurred on a federal holiday; because of known seismic activity in California, area building codes dictate that buildings incorporate structural design intended to withstand earthquakes. However, the damage caused revealed; because of these revelations, building codes were revised. Some structures were not red-tagged until months because damage was not evident; the quake produced unusually strong ground accelerations in the range of 1.0 g.
Damage was caused by fire and landslides. The Northridge earthquake was notable for hitting the same exact area as the Mw 6.6 San Fernando earthquake. Estimates of total damage range between $13 and $40 billion. Most casualties and damage occurred in multi-story wood frame buildings. In particular, buildings with an unstable first floor performed poorly. Numerous fires were caused by broken gas lines from houses shifting off their foundations or unsecured water heaters tumbling. In the San Fernando Valley, several underground gas and water lines were severed, resulting in some streets experiencing simultaneous fires and floods. Damage to the system resulted in water pressure dropping to zero in some areas. Five days it was estimated that between 40,000 and 60,000 customers were still without public water service; as expected, unreinforced masonry buildings and houses on steep slopes suff