Tectónica de placas y terremotos


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Una power point realizada por la universidad de Bristol sobre tectónica de placas. Mi aporte fue hacer la traducción y agregar información sobre una publicación científica donde se predice el gran terremoto que ocurrió en Chile el 27 de febrero de 2010. La publicación fue recibida el 20 de marzo del 2007, aceptada el 10 de febrero del 2008 y publicada en 2009, un año antes del 27F

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  • El interior de la tierra está dividido en capas basado en propiedades quínicas y físicas. la tierra tiene una corteza sólida, un Manto muy viscoso y un Núcleo, cuya parte externa es líquida, que es mucho menos viscosa que el Manto y la parte interna es sólida. Working from the centre de la tierra out we have: The Interno Núcleo is a primarily solid sphere about 1220 km in radius situated at Earth's center. basado en the abundance of quínicas elements in the solar system, their físicas propiedades, y other quínicas constraints regarding the remainder of Earth's volume, the Interno Núcleo is believed to be composed primarily of a nickel-iron alloy, with small amounts of some unknown elements. The temperature is estimated at 5,000-6,000 degrees Celsius y the pressure to be about 330 to 360 GPa (which is over 3,000,000 times that of the atmosphere!) The liquid externo Núcleo is 2300 km thick y like the Interno Núcleo composed of a nickel-iron alloy (but with menos iron que el solid Interno Núcleo). Iseismic y other geophysical evidence indicates that the externo Núcleo is so hot that the metals are in a liquid state. The Manto i s approximately 2,900 km thick y comprises 70% of Earth's volume. (the Núcleo makes up about 30% of Earth's volume, with the externo Corteza [where we live] <1%!!). The Manto está dividido into sections based upon changes in its elastic propiedades with depth. In the Manto, temperatures range between 500-900 degrees Celsius at the upper boundary with La corteza to over 4,000 degrees Celsius at the boundary with the Núcleo. Due to the temperature difference between la tierra's surface y externo Núcleo, y the ability of the crystalline rocks at high pressure y temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the Manto (Manto convection cells). Hot material rises up as Manto plumes (like a lava lamp!), while cooler (y heavier) material sinks downward to be reheated y rise up again. - We shall see that this process is very important for Tectónica de Placas motion… The externo most layer is La corteza - this is the most familiar to us as it is where we live. The distinction between Corteza y Manto is basado en chemistry, rock types y seismic characteristics. Presenter: Ask the students to guess what the most abundant element in la Corteza de la tierra is…..they may be surprised to learn that it is actually Oxygen ( 46.6% Oxygen; 27.7% Silica; 8.1% Aluminum; 5.0% Iron; 3.6% Calcium; 2.8% Sodium; 2.6% Potassium; 2.1% Magnesium; plus trace elements) Click to next slide for more on La corteza….
  • la tierra tiene two different types of Corteza: Continental Corteza y Corteza oceánica. Each tiene different propiedades y therefore behaves in different ways. Continental Corteza: Continental Corteza forms the land (the continents, as the name suggests) that we see today. Continental Corteza averages about 35 km thick. Under some mountain chains, Cortezaal thickness is approximately twice that thickness (about 70 km thick). - The mountains we see on earth have deep roots in La corteza that we can’t see. La corteza “floats” on the more densa Manto y, like how only the tip of an iceberg sticks up out of the water, we see only the tip of the continental Corteza - the mountain ranges. Continental Corteza is menos densa y therefore more buoyant than Corteza oceánica Continental Corteza contains some of the oldest rocks on Earth. - Ancient rocks exceeding 3.5 billion years in age are found on all of Earth's continents. The oldest rocks on Earth found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 Ga) [Ga = billion years ago] y the Isua SupraCortezaal rocks in West Greenland (3.7 to 3.8 Ga), but well-studied rocks nearly as old are also found in the Minnesota River Valley in the USA (3.5-3.7 billion years), in Swaziland (3.4-3.5 billion years), y in Western Australia (3.4-3.6 billion years). Corteza oceánica: As the name already suggests, this Corteza is below the oceans. Compared to continental Corteza, Corteza oceánica is thin (6-11 km). It is more densa than continental Corteza y therefore when the two types of Corteza meet, Corteza oceánica will sink underneath continental Corteza. The rocks of the Corteza oceánica are very young compared with most of the rocks of the continental Corteza. They are not older than 200 million years.
  • Presenter: Ask the class if they have heard of Tectónica de Placas before (commonly students will have some idea of the general concept), y ask them if they can explain the theory.
  • If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle…..the shape of Africa y South America are a good example. This is because they DID used to fit together! la tierra as we see it today was not always like it is now. Land masses have pulled apart y joined together by the process we call Tectónica de Placas….
  • There are 12 major Placas on Earth, each of which slide around at a rate of centimetres per year, pulling away from, scraping against or crashing into each other. Each type of interaction produces a characteristic “ tectónica feature ” , like mountain ranges, Volcanes y (or) rift valleys, that we will discuss during this lecture.
  • This diagram shows the major tectónica Placas. Presenter: Point out the UK, sitting on the Eurasian Placa. Also the Placa boundary between Africa y South America (note that it tiene the same shape as the coastlines in these countries).
  • Placas are made of rigid Litósfera – formed of La corteza y the extreme upper Manto (point out these layers on the figure).
  • The Astenósfera, beneath the Litósfera, is part of the upper Manto y is so hot that it is 1 – 5% liquid (I.e. 95 – 99% solid). This liquid, usually at the junctions of the crystals, allow it to flow – which is why ‘astheno’ means weak.’ Beneath the Astenósfera is the rest of the Manto, which is completely solid – but can also flow (on geological time scales) because of the intense temperatures y pressures involved. The base of the Litósfera-Astenósfera boundary corresponds approximately to the depth of the melting temperature in the Manto.
  • How y Why do tectónica Placas move around? The question of how tectónica Placas are moved around the globe is answered by understanding Manto convection cells. In the Manto hot material rises towards the Litósfera (like hot air rising out of an open oven - ever opened an oven door y felt the blast of hot air coming past your face?). The hot material reaches the base of the Litósfera where it cools y sinks back down through the Manto. The cool material is replaced by more hot material, y so on forming a large “convection cell” (as pictured in the diagram). This slow but incessant movement in the Manto causes the rigid tectónica Placas to move (float) around la tierra surface (at an equally slow rate).
  • Firstly, there are three types of Placa boundary, each related to the movement seen along the boundary. Divergent boundaries are where Placas move away from each other Convergent boundaries are where the Placas move towards each other Transform boundaries are where the Placas slide past each other. Presenter: See diagrams for each - it is important to remember the names of the boundary types y the motion involved.
  • In Tectónica de Placas, a divergent boundary is a linear feature that exists between two tectónica Placas that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the Placas se separen, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity. Presenter: Reiterate the process by going through the diagram, including the presence of Manto convection cells causing the Placas to break apart y also as a source for new molten material. Where a divergent boundary forms on a continent it is called a RIFT or CONTINENTAL RIFT, e.g. African Rift Valley. Where a divergent boundary forms under the ocean it is called an OCEAN RIDGE.
  • When continental Corteza pushes against continental Corteza both sides of the convergent boundary have the same propiedades (think back to the description of continental Corteza: thick y buoyant). Neither side of the boundary wants to sink beneath the other side, y as a result the two Placas push against each other y La corteza buckles y cracks, pushing up (y down into the Manto) high mountain ranges. For example, the European Alps y Himalayas formed this way.
  • Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental Corteza was pushing against continental Corteza the Himalayan mountain belt was pushed up. “ Mountains ” were also pushed down into the Manto as the normally 35 km thick Corteza is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 metres high. This means that below the surface at the foot of the mountain La corteza is a further 61 km deep!!
  • At a convergent boundary where continental Corteza pushes against Corteza oceánica, the Corteza oceánica which is thInterno y more densa que el continental Corteza, sinks below the continental Corteza. This is called a Subduction Zone. The Corteza oceánica descends into the Manto at a rate of centimetres per year. This Corteza oceánica is called the “Subducting Slab” (see diagram). When the subducting slab reaches a depth of around 100 kilometres, it dehydrates y releases water into the overlying Manto wedge (Presenter: explain all of this using the diagram). The addition of water into the Manto wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental Corteza forming Volcanes. Subduction is a way of recycling the Corteza oceánica. Eventually the subducting slab sinks down into the Manto to be recycled. It is for this reason that the Corteza oceánica is much younger que el continental Corteza which is not recycled.
  • The Andes mountain range along the western edge of the South American continent is an example of a mountain belt formed by subduction. The continental Corteza of the South American Placa tiene buckled under the compressional strain of converging with the Nasca y Antarctic Placas. Additionally there are many Volcanes, the result of melting of the subducting slab y the production of new material that tiene risen through La corteza to the surface.
  • When two oceánica Placas converge, because they are densa, one runs over the top of the other causing it to sink into the Manto y a subduction zone is formed. The subducting Placa is bent down into the Manto to form a deep depression in the seafloor called a trench. Trenches are the deepest parts of the ocean y remain largely unexplored.
  • Manned or unmanned submersible vehicles (top right photo) have explored small parts of trenches discovering new species (like the fish photographed here) y amazing ecosystems.
  • This map summarises all the known Placa boundaries on Earth, showing whether they are divergent, convergent or transform boundaries.
  • Volcanes can be formed in three ways: Via subduction. The subducting slab dehydrates to form new melt that will rise through La corteza to be erupted at the surface. Via rifting. When two Placas se separen magma rises, producing volcanic eruptions at the surface. At “Hotspots”….hotspot do not necessarily occur along a Placa boundary. So hotspot Volcanes can form in the middle of tectónica Placas….Click for example.
  • We have seen the connection between Volcanes y Tectónica de Placas. Is there a connection between earthquakes y Tectónica de Placas?
  • The black dots on this map of the world depict where earthquake activity is occurring. You can see that, as with Volcanes, la tierraquakes are NOT randomly distributed around the globe. Instead they occur in linear patterns associated with Placa boundaries. (Note this diagram of earthquake distribution closely resembles the Pacific Ring of Fire distribution of volcanism).
  • We know there are three types of Placa boundaries: Divergent, Convergent y Transform. Movement y slipping along each of these types of boundaries can form an earthquake. Depending on the type of movement, la tierraquakes occur in either a shallow or deep level in La corteza. The majority of tectónica earthquakes originate at depths not exceeding tens of kilometers. In subduction zones, where old y cold Corteza oceánica descends beneath another tectónica Placa, “ Deep Focus Earthquakes ” may occur at much greater depths (up to seven hundred kilometers!). These earthquakes occur at a depth at which the subducted Corteza should no longer be brittle, due to the high temperature y pressure. A possible mechanism for the generation of deep focus earthquakes is faulting. Earthquakes may also occur in volcanic regions y are caused there both by tectónica faults y by the movement of magma (hot molten rock) within the volcano. Such earthquakes can be an early warning of volcanic eruptions.
  • The take home messages from this lecture: See if the students remember what each layer, type of Placa y type of boundary are called. Can they describe the motion of the three Placa boundary types? Where does volcanism occur in relation to Tectónica de Placas? Where do earthquakes occur in relation to Tectónica de Placas?
  • Tectónica de placas y terremotos

    1. 1. La Estructura de la tierra y la Tectónica de Placas
    2. 2. Estructura de la tierra • la tierra está hecha de 3 principales capas: – Núcleo – Manto – Corteza Interno Núcleo externo Núcleo externo Corteza
    3. 3. La corteza • ¡Es donde vivimos! • la Corteza de la tierra está hecha de: Corteza Continental -Gruesa(10-70km) - flotante (menos densa que la Corteza oceánica) - En su mayor parte es más vieja Corteza oceánica - Delgada (~7 km) - densa (se unde bajo la Corteza continental) - Más joven
    4. 4. ¿Qué es la Tectónica de Placas?
    5. 5. • Si observas un mapa de la tierra, puedes darte cuenta que los continentes podrían encajar como piezas de un rompecabezas.
    6. 6. Tectónica de Placas • La Corteza de la tierra está dividida en 12 placas principales las cuales se están moviendo en varias direcciones. • Este movimiento de placas provoca que ellas colisionen, se separen o friccionen unas contra otras. • Cada tipo de interacción provoca un ciertas características de la estructura de la tierra o caracteres “tectónicos”. • La palabra, tectónica, se refiere a la deformación de la corteza como una consecuencia de la interacción de las placas.
    7. 7. Placas terrestres
    8. 8. ¿De qué están hechas las placas tectónicas? • Las Placas están hechas de Litósfera rígida. La Litósfera está hecha de la corteza y de la parte superior del Manto.
    9. 9. ¿Qué hay debajo de las Placas tectónicas? • Debajo de la Litósfera (la cual está hecha de placas tectónicas) está la astenósfera.
    10. 10. Movimiento de las Placas • Las “Placas” de la Litósfera se mueven, como promedio, 5 cm al año, por las celdas calientes de convección del Manto subyacente.
    11. 11. • Divergentes • Convergentes • Transformantes Hay 3 tipos de límites de Placas
    12. 12. • Dorsales oceánicas – A medida que las placas se separan, material nuevo es expulsado para llenar el vacío Límites divergentes
    13. 13. • Forma montañas, ej. Los alpes europeos, Himalayas Colisión Continente- continente
    14. 14. Himalayas
    15. 15. • Llamada SUBDUCCIÓN Colisión Continente-Corteza oceánica
    16. 16. • La Litósfera Oceánica subside por debajo de la Litósfera continental. • La Litósfera oceánica se calienta y deshidrata a medida que subside . • El material fundido se eleva formado una cadena de montañas, ej. Los Andes Subducción
    17. 17. • Cuando dos placas, como la de Nazca y la Sudamericana colisionan, una se hunde debajo de la otra. En nuestro caso es la de NAZCA la que se hunde bajo la Placa sudamericana formando una zona de subducción. • La placa de subducción se dobla hacia abajo formado una depresión muy profunda en el piso oceánico llamada TRINCHERA. • Las partes oceánicas más profundas se encuentran en lo largo de las trincheras. – El. La Trinchera de las Marianas, La trinchera Chile-perú
    18. 18. - Subducción Una de las tres maneras de formarse los volcanes es por:
    19. 19. …¿cuál es la conexión? Terremotos y Tectónica de Placas…
    20. 20. • Así como los Volcanes, los terremotos no están distribuidos al azar en el globo terrestre. • En los límites entre placas, la fricción provoca que ellas puedan adherirse o pegarse. Cuando se desprenden, la liberación de energía provoca que ellas se Figura que muestra la distribución de los terremotos alrededor de la tierra.
    21. 21. ¿Donde se forman los terremotos? La figura muestra la tectónica que origina nuestros sismos.
    22. 22. Tectónica de Placas:resumen • la tierra está hecha de 3 principales capas (Núcleo, Manto, Corteza) • Sobre la superficie de la tierra están las placas tectónicas que lentamente se mueven alrededor del globo. • Las Placas están hechas de Corteza y de Manto superior (Litósfera) • Hay dos tipos de Placas • Hay 3 tipos de límites de Placa • Los volcanes y los terremotos están estrechamente ligados a los márgenes de las Placas tectónicas
    23. 23. Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile J.C. Ruegga, , A. Rudloff b, C. Vignyb, R. Madariagab, J.B. de Chabaliera, J. Camposc,∗ E. Kauselc, S. Barrientosc, D. Dimitrovd. Paper publicado el 2009 en la revista “Physics of the Earth and Planetary Interiors” donde se pronostica un posible terremoto de magnitud 8-8,5 grados en la zona entre Concepción y Constitución. Les dejo el abstract. • The Concepción–Constitución area [35–37◦S] in South Central Chile is very likely a mature seismic gap, • since no large subduction earthquake has occurred there since 1835. Three campaigns of global positioning • system (GPS) measurements were carried out in this area in 1996, 1999 and 2002. We observed • a network of about 40 sites, including two east–west transects ranging from the coastal area to the • Argentina border and one north–south profile along the coast. Our measurements are consistent with • the Nazca/South America relative angular velocity (55.9◦N, 95.2◦W, 0.610◦/Ma) discussed by Vigny et al. • (2008, this issue) which predicts a convergence of 68mm/year oriented 79◦N at the Chilean trench near • 36◦S. With respect to stable South America, horizontal velocities decrease from 45mm/year on the coast • to 10mm/year in the Cordillera. Vertical velocities exhibit a coherent pattern with negative values of • about 10mm/year on the coast and slightly positive or near zero in the Central Valley or the Cordillera. • Horizontal velocities have formal uncertainties in the range of 1–3mm/year and vertical velocities around • 3–6mm/year. Surface deformation in this area of South Central Chile is consistent with a fully coupled • elastic loading on the subduction interface at depth. The best fit to our data is obtained with a dip of 16±3◦, • a locking depth of 55±5 km and a dislocation corresponding to 67mm/year oriented 78◦N. However in • the northern area of our network the fit is improved locally by using a lower dip around 13◦. Finally a • convergence motion of about 68mm/year represents more than 10m of displacement accumulated since • the last big interplate subduction event in this area over 170 years ago (1835 earthquake described by Darwin). • Therefore, in a worst case scenario, the area already has a potential for an earthquake of magnitude • as large as 8–8.5, should it happen in the near future.