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Transcript
Tectonic change 1 To many geographers one of the most amazing things about the subject is that one of the most fundamental ideas affecting the subject has only become apparent to us over the last 45 years, i.e. the complex history of our planet. In that time the theory of plate tectonics has been accepted, refined and widely applied as a remarkably successful framework for understanding the history of the Earth. Theme 2 – Investigating Tectonic and Hydrological Change Key Question 2.1 What are the processes associated with plate tectonics? * * Patterns of plates and plate boundaries. Processes associated with constructive, destructive and conservative plate margins. Key Question 2.2 * * * Hazards associated with tectonic activity. Demographic, economic and social impacts of tectonic hazards. Local and regional impacts of tectonic hazards . Key Question 2.3 managed? * * * What are the hazards associated with tectonic events? How are tectonic hazards perceived and Different perceptions and awareness of tectonic hazards by groups with conflicting interests. Strategies to manage tectonic hazards. The effectiveness of management strategies. Continental Drift The idea that continents can move around on the Earth's surface has been suggested since the 17th century, but serious evidence in support of the idea of Continental Drift was only put forward by the German meteorologist Alfred Wegener in 1912. His ideas were initially greeted with scepticism, but by now the idea is fully accepted and maps of the world at various geological ages have been produced. The big change since the 1950's has been the science of plate tectonics, which shows how the continents move, explains that our Earth is a dynamic entity with a history of turbulence, destruction, creation and upheaval Evidence for plate tectonics Evidence in support for the theory of plate tectonics has come from a number of areas: [1] the analysis of seismic (earthquake) waves telling us about the internal composition of the Earth; [2] the use of sonar and radio-sounding equipment giving us information about the deep ocean floor, information previously denied us by its inaccessibility; [3] the magnetic analysis of deep ocean floor rocks; [4] the existence of rocks formed in tropical climates in polar areas, and of glaciations in tropical areas; The Gondwana glaciation of c.300 mya [5] the anomalous distributions of various plants and animals which are hard to explain except by the process of continental drift; [6] the comparison of the surfaces of the Earth and other planets by the development of spacecraft. Creation of the Solar System The Sun forming out of a cloud of dust and gas Planetisimals colliding and accreting We have long known that the inside of the Earth is hot - the evidence is there in volcanoes and hot geysers. But why is the centre of the earth hot and where does the heat come from? The answer lies in understanding what happened when the Earth was formed. The Earth has existed for approximately 4650 million years. It was probably formed by small pieces of material, known as planetesimals, accumulating or accreting in space. These initially came together to form a homogenous, undifferentiated mass. As the cold planetesimals accumulated, the planet began to heat up and then separate into different layers. Impacts and radioactivity This heating was caused by two different factors: firstly the impact of bodies striking the planet gave out heat, and secondly the process of the Earth contracting and squeezing itself into a smaller space also gave out heat. Since that initial period a third source of heat has also been active, i.e. the disintegration of radioactive elements like uranium and thorium, although scarce, is sufficient to generate high temperatures inside the confines of the Earth. Differentiation and outgassing During the process of differentiation most of the Earth's iron and nickel and other heavy metals sunk to the Earth's core, leaving the lighter elements to make up a disproportionate amount of the upper layers of the Earth. Outgassing Gradually the surface of the Earth cooled until by 4000 million years ago it had cooled sufficiently to allow an ocean to form, an ocean which precipitated from the gases which were outgassed from the crust by volcanoes. The Archean Period At this time continents did not exist. The oldest rocks so far found on the earth date from around 3900 million years ago. This period of the Earth's history is called the Archean period and lasted until 2500 million years ago. As the surface of the earth cooled plates of solid rock formed, but these were dragged down into the hot molten mantle on convection currents. Microplates Gradually some of these plates became cold enough as they were pulled down into the mantle that they stayed solid for longer, and were subjected to greater heat. This produced a lighter material which floated to the surface and formed a different kind of plate. Initially these plates were in the shape of long strips, reflecting the shapes of the fissures from which they emerged. Being lighter they resisted being pulled back into the mantle and stayed on the surface as continents. Over the years partial melting of basaltic crustal material from oceanic plates has formed continental crust which cannot be subducted. Continental crust is lighter and through weathering, erosion and deposition by rivers, the sea , wind and glaciers is now composed of a wide variety of rocks. Stromatolites and blue-green algae Stromatolites at Shark Bay, Western Australia Life started very early on in the Earth's history, although did not progress beyond simple single celled organisms for most of its history. It is likely that it thrived in pools of water near hydrothermal springs. The oldest `fossils' found date from 3550 million years ago in rocks from Western Australia, and are algal structures known as stromatolites. Around 2500 million years ago a radical change took place. Blue-green algae developed the ability to photosynthesise and so free oxygen became much more common in the atmosphere. Life became more diverse, and the free oxygen became broken down high in the atmosphere to form a layer of ozone, which prevented the harmful ultra-violet rays from reaching the surface, and so stimulated the development of more types of life. Cratons The Proterozoic period (2500 - 550 million years ago) saw the establishment of the Earth's continents. Land areas were transformed from narrow strips of crust to continent size chunks. These ancient hearts of continents are known as cratons. The low lying eroded remains of these ancient continents can be seen today in the ancient shields of Canada, Siberia, Australia, central Africa, the Amazon and Antarctica Supercontinents By the end of the Proterozoic period life was plentiful in the sea, although not on land, and the atmosphere was rich in oxygen. It is possible that at one or several times during the Proterozoic era there were supercontinents comprising all of the Earth's land surface, as continental drift brought all of the smaller continents together. Evidence of a glacial period 750 mya is to be found on the Skerries, north of Anglesey Snowball Earth At least twice during this long period in Earth's history the positions of the continents and their effect upon ocean currents caused the Earth to enter an ice age (at about 2300 and 700 million years ago). There is evidence for this second ice age in rocks from the Skerries just to the north of Anglesey. Gradually the supercontinents break up and their parts drift round the Earth until they meet up again and form another supercontinent. This pattern is called the Wilson Supercycle and is thought to occur over periods of c400 million years. This process has been important for the last 2500 million years. Hypsometry So much for the early history of the Earth, but how does continental drift actually occur, according to the theory of plate tectonics? Interest in continental drift began again during the 1950's when evidence from a number of areas began to filter through to geologists. Thorough surveys of the sea floor enabled graphs to be made plotting the relative distribution of surface areas of different heights (a hypsometric graph). For the Earth it shows a bimodal distribution (with two peaks). If the data is plotted as a cumulative frequency curve (a hypsometric curve) it suggests that the break between oceans and continents does not occur at sea level but at a depth of c200m. A map of major global morphological features shows large areas of fairly flat terrain, both on the continents and oceans, and long thin areas of mountains. Structure of the Earth Seismic evidence has given us a picture of the interior of the Earth. There are three main zones, the crust , mantle and core. The boundary between the core and mantle is very sharp and located at a depth of 2900km. The crust-mantle boundary is marked by the Mohorovicic discontinuity (Moho), with an average depth of 35km beneath continents but only 10km beneath oceans. Lithosphere and asthenosphere In the upper part of the mantle is located a semi-rigid layer known as the lithosphere which behaves as if it were part of the crust. The thickness of the lithosphere shows considerable variability and its lower boundary is gradational and shows no sharp change. Underneath the lithosphere is the asthenosphere, where the plastic like properties of the rocks permits them to flow slowly over periods of time. It corresponds to a zone of low velocity seismic waves. Remanent magnetism It was found earlier in this century by Brunhes in France and Matuyama in Japan that rocks displayed remanent magnetism, i.e. they took on the polarity of the magnetic field of the Earth at the time they cooled and hardened. But the polarity of the Earth has changed over time and at certain periods seems to have reversed, i.e. north becoming south etc. Analysis of rocks on either side of the Mid Atlantic Ridge seemed to show a pattern of parallel lines aligned on either side of the ridge itself. Sea-floor spreading In 1962 H.H.Hess of Princeton University in the USA suggested that the mid ocean ridges were where new oceanic crust is generated by the upwelling of hot mantle material. The crust, he suggested, spreads laterally away from these ridges until it reaches an island arc or mountain belt where it descends into the mantle along the adjacent oceanic trench. This `conveyor belt' view of the ocean crust was termed sea-floor spreading. Plate tectonics The plate tectonics theory proposes that the Earth comprises seven major and at least a dozen minor lithospheric plates composed of the crust and the upper more rigid part of the mantle. These plates are constantly in motion with one another and the motion of one plate influences the motion of the others. Rates of movement vary up to 100mm/year (100km/million years) averaging 70mm/year. Much of but not all of the Earth's seismic activity occurs along the boundaries between these plates. The narrow zone marking the relative movement between two plates, usually clearly demarcated by seismic activity is termed a plate boundary, while the peripheral region adjacent to this boundary is called a plate margin. Types of plate boundary There are three types of plate boundary: * at a divergent boundary new crust is formed and attached to the upper part of adjoining lithospheric plates while new upper mantle is accreted to the lower part. Plate movement is laterally away from the spreading ridge. These can also be called constructive boundaries. * at a convergent boundary two plates are in motion towards each other, with one plate slipping down below the other along a subduction zone. The surface area of a plate is reduced at a convergent boundary but increased along a spreading ridge. These can also be called destructive boundaries. • along a transform boundary two plates simply move past each other without any major element of divergence or convergence. These are also called transform faults or conservative boundaries. The most famous example is the San Andreas Fault in California. At some locations three plates meet. These areas are called triple junctions. But how do the convection currents cause sea-floor spreading? Here researchers differ in opinion, although some theories have more adherents than others. One idea is that lateral flow in the asthenosphere, arising from convection currents in the mantle, drag the overlying lithosphere (A-C on next slide). Convection currents rise and diverge below midoceanic ridges and descend along subduction zones. Convection occurs when too much heat is present at depth to be conveyed upwards solely by thermal conduction. Temperatures increase and material begins to rise as it is less dense than surrounding material. Cooler denser material moves laterally to take its place. Most geologists support the idea that convection occurs in the mantle. However whether this is solely or even primarily responsible for plate movement is another matter. The depth of convection currents The depth at which convection currents start is a major bone of contention, with some researchers arguing that it is confined to the asthenosphere (A), others suggesting 700km as a likely depth for convection's lower limit (B). More recently some have suggested whole mantle convection (C), as large scale convection cells such as this fit in with the size of the plates themselves. Ridge-push, Gravity sliding and Slab pull Other processes have also been postulated to account for plate movement. The injection of lava at ocean ridges would push plates apart, it has been argued (D). Others have said that the thickening of the plates as they move away from the hot mid ocean ridge area and add material at their base through accretion would make a plate slide under the force of gravity downwards from a mid-ocean ridge towards a subduction zone (E). Cool, thick, old oceanic lithosphere is gravitationally unstable as it is generally denser than the asthenosphere over which it lies. Therefore a cold, dense lithospheric slab descending up to 700km into the mantle at a subduction zone will tend to pull the remainder of the plate with it (F). As rock warms and cools so slowly the centre of the slab will be up to 1000oC cooler than the surrounding mantle at a depth of several hundred km retaining its high density characteristics. Slab pull The argument over the relative significance of these processes continues today, but the indications are that it is the last process, lithospheric slab pull that is the predominant driving force. Those plates attached to long subduction boundaries tend to move more quickly than those without extensive subducting boundaries The Pacific Ocean The Atlantic Ocean Landforms of divergent boundaries Divergent or constructive boundaries occur in two main places – in the middle of the sea at a MID-OCEAN RIDGE and underneath a continent at a RIFT VALLEY Mid-ocean ridges have many submarine volcanoes where pillow lavas are erupted, a central rift valley, hydrothermal vents and black smokers. The Mid-Atlantic Ridge Pillow lavas Lava is squeezed out of a lava tube like toothpaste. It solidifies immediately forming pillow-like shapes. Local examples dating from 450 mya can be found at Ynys Llanddwyn. Black smokers Water from the seafloor had seeped into the rock, become heated and dissolved minerals from the rock. When meeting deep ocean water at 2°C the minerals precipitate into a chimney. In 1977 a deep sea submarine of the East Pacific Rise discovered chimneys of superheated steam at 300°C escaping from hydrothermal vents, along with whole ecosystems of animal and plant life associated with the minerals in the water. Shield and fissure volcanoes Shield volcanoes are found where the mid-ocean ridge breaks the surface of the sea, e.g. Iceland. They are built up from layer upon layer of fluid basaltic lava. Fissure volcanoes, e.g. Krafla in Iceland are very flat and occur where the crust opens up in a line. Lava is basaltic and fluid. These volcanoes are effusive, i.e. material flows rather than explodes from them. Mid-continental rift valleys In East Africa is located the largest rift valley system on land. The continent is opening apart and the two sides leave a central area lower than the sides. Horst and graben The Great Karas Mountains from Namibia, an example of a horst The uplifted block on either side is called a horst, and the central rift area is called a graben. Below are two views of the East African Rift Valley and the lakes which fill part of the graben. Half graben If one side moves more than the other an asymetrical half-graben can result. Landforms of convergent boundaries Convergent boundaries occur in three places, between one ocean plate and another (an island arc) between an ocean plate and a continental plate between two continental plates (a collision zone) In the first two subduction can take place because oceanic rock is heavy, so a slab of cold oceanic crust is thrust down into the mantle. Earthquakes occur at increasing depths (the Wadati-Benioff Zone). An ocean trench is formed. Some material partially melts and rises as a diapir to crate explosive andesitic volcanoes. Island arc These are numerous in the Pacific Ocean, Indonesia and around the Caribbean. Here an ocean trench is to be found, e.g. the deepest of all the Marianas Trench. Magma rises to the surface and a volcanic arc occurs, e.g. Mt Pelee in the Caribbean. Diapirs cause some spreading in the marginal basin. The WadatiBenioff Zone On destructive margins, where an ocean plate is subducted beneath another oceanic or a continental plate, a line of earthquakes can be detected, getting progressively deeper away from the ocean trench. The two seismologists who first discovered this feature were Egyptians Wadati and Benioff. Soft material on the ocean floor, oozes and muds, are scraped off by subduction to form melange. Some of Anglesey is formed of melange rocks from a long distant subduction episode. Andesitic mountain range When oceanic crust is subducted under continental crust an ocean trench, e.g. Peruvian Trench is formed. Diapirs rise to form many andesitic volcanoes containing an explosive mix of oceanic material (basalt) and continental material. The rock is named after the Andes mountain range. The compression forms folds and fault in the rock, hence a fold mountain range. Stratovolcanoes As magma from partial melting of the oceanic crust forces its way through to the surface through continental crust a wide variety of material can reach the open air. This can vary from steam and gases (in fumaroles and solfarata), to ash and dust, lava and the blasted material of pyroclastic flows. Volcanoes in andesitic mountain chains and island arcs are often strato-volcanoes. These are composed of alternate layers of lava, ash and pyroclastic material (clouds of molten rock droplets and steam). The central vent often falls in to form a crater. Secondary vents may occur on the slopes. These volcanoes are much more explosive than shield volcanoes and often represent a significant hazard to life and property. Pyroclastic flows Many volcanoes in the Ring of Fire, around the Pacific, such as this one, Semeru, in Indonesia, are prone to pyroclastic eruptions Intrusive rocks Part of a granite batholith in Namibia Not all of the magma which rises in diapirs reaches the surface. Magma which cools underground is much more likely to have large crystal sizes than volcanic lavas. If the magma is basaltic the rock is called gabbro. If the rock is of continental crust it is called granite. Granite often forms large features underground called batholiths. If magma can rise through cracks in the rock it can form dikes, sills and laccoliths. Collision zones Some areas of rock may be broken off and forced over other layers. This is called a nappe. When two continents meet both plates are light and thick and cannot be subducted. A collision zone results. Fold mountains are created by the pressure and although earthquakes may occur, volcanoes are much less common. India and the Himalayas The Indo-Australian Plate moved northwards and collided with the Eurasian Plate, creating the Himalayas over the last 50 million years. Transform margins Transform margins, or conservative plate boundaries may occur under the ocean or on land. They are especially common at right angles to mid-ocean ridges, where they are known as fracture zones or transform faults. They split the ridge up into smaller blocks so that it can bend. No volcanoes are found on these boundaries but a number of landforms associated with seismic activity are found. An excellent example is the San Andreas fault in California. Landforms of transform margins The landforms of transform margins are those involved with earthquake fault lines. Hills and valleys are offset, drainage channels are diverted and escarpments and depressions may form. Lakes may form in the depressions. 3-4 metres of rightlateral slip have offset a ridge and created a 'shutter' ridge blocking flow in the dry creek channel. 1999 Hector Mine earthquake California Denali fault in Alaska crossing stream after 2002 earthquake. Stream offset was 4.8m. An escarpment along a fault line. A fault pond or sag pond on the San Andreas Fault in California. Turbidity currents In oceans earthquakes may cause large amounts of mud to flow as a turbidity current. They have great erosive power and may break submarine cables. Many of the largest of the world’s rivers have created submarine canyons which produce turbidity currents fairly regularly.