Earth Structure & Plate Tectonics PDF

Earth Structure & Plate Tectonics  Structure of the Earth Mantle The Earth is made up of 3 main Outer core layers: Inner core – Core – Mantle – Crust Crust The Layers of the Earth  The Crust This is...

Earth Structure & Plate Tectonics  Structure of the Earth Mantle The Earth is made up of 3 main Outer core layers: Inner core – Core – Mantle – Crust Crust The Layers of the Earth  The Crust This is where we live! The Earth’s crust is made of: Continental Crust Oceanic Crust - thick (10-70km) - thin (~7 km) - less dense than - Dense oceanic crust - young - mostly old  The Earth has two different types of crust: Continental crust and Oceanic crust. Each has different properties and therefore behaves in different ways. Continental crust: Continental crust forms the land (the continents, as the name suggests) that we see today. Continental crust averages about 35 km thick. Under some mountain chains, crustal thickness is approximately twice that thickness (about 70 km thick). - Continental crust is less dense and therefore more buoyant than oceanic crust - Continental crust contains some of the oldest rocks on Earth. - Oceanic crust: - this crust is below the oceans. - Compared to continental crust, Oceanic crust is thin (6-11 km). - It is more dense than continental crust and therefore when the two types of crust meet, oceanic crust will sink underneath continental crust. - The rocks of the oceanic crust are very young compared with most of the rocks of the continental crust. They are not older than 200 million years.  The Lithospheric Plates The crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, semi-rigid asthenosphere.  The Lithosphere The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The Asthenosphere The asthenosphere is the semi-rigid part of the middle mantle that flows like hot asphalt under a heavy weight. The Mantle The Mantle is the largest layer of the Earth. The middle mantle is composed of very hot dense rock that flows like asphalt under a heavy weight. The movement of the middle mantle (asthenosphere) is the reason that the crustal plates of the Earth move. Convection Currents The middle mantle "flows" because of convection currents. Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling and sinking again --repeating this cycle over and over. The Outer Core The core of the Earth is like a ball of very hot metals. The outer core is so hot that the metals in it are all in the liquid state. The outer core is composed of the melted metals of nickel and iron. The Inner Core The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place like a solid.  How do we know what the Earth is made of? Geophysical surveys: seismic, gravity, magnetics, electrical, geodesy – Acquisition: land, air, sea and satellite – Geological surveys: fieldwork, boreholes, mines If we can’t go to the centre of the Earth (except in fictional movies!) how do we know what the internal structure of the Earth is like? We need to use geophysical imaging techniques to model what is going on below our feet. For example, when there is an earthquake it sends out seismic waves (shock waves) through the Earth. Seismologists can measure the time it takes for these waves to reach seismic monitoring stations set up around the globe. (The machine that measures seismic waves is called a seismometer). The different layers in the earth have been inferred using the time of travel of refracted and reflected (bent backward angularly) seismic waves created by the earthquakes (see left diagram). That is, changes in the seismic velocity occur as the waves pass through different materials. Measuring these changes tell seismologists how many layers there are and the thickness and physical properties of each layer.  We need not wait for earthquakes to occur, on a local scale on land (cheap but slow methods) and at sea (more expensive but quicker) explosions can be set to cause shock waves to pass through the crust (simulating an earthquake) that can be measured in the same way. Other geophysical methods, for example measuring different gravity, magnetic and electrical anomalies by air and (or) satellite can help to reconstruct shallow crustal features. We can also go and examine rocks at and near the surface of the crust, through fieldwork, drilling boreholes and mining. What is Plate Tectonics?  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.  Plate Tectonics The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction. World Plates  Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells What happens at tectonic plate boundaries?  Three types of plate boundary Divergent Convergent Transform  Divergent Boundaries Spreading ridges – As plates move apart new material is erupted to fill the gap  In plate tectonics, a divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity. 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.  Convergent Boundaries Convergent boundaries are where the plates move towards each other. There are three types of convergent boundary, each defined by what type of crust (continental or oceanic) is coming together. Three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision  Continent-Continent Collision Forms mountains, e.g. European Alps, Himalayas  When continental crust pushes against continental crust both sides of the convergent boundary have the same properties. Neither side of the boundary wants to sink beneath the other side, and as a result the two plates push against each other and the crust buckles and cracks, pushing up (and down into the mantle) high mountain ranges. For example, the European Alps and Himalayas formed this way. Himalayas  Continent-Oceanic Crust Collision Called SUBDUCTION  At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and more dense than the continental crust, sinks below the continental crust. This is called a Subduction Zone. The oceanic crust descends into the mantle at a rate of centimetres per year. This oceanic crust is called the “Subducting Slab” (see diagram). When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge. The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes. Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled. Subduction Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes  Ocean-Ocean Plate Collision When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep!  Transform Boundaries Where plates slide past each other Above: View of the San Andreas transform fault  The third type of boundary are transform boundaries, along which plates slide past each other. The San Andreas fault, adjacent to which the US city of San Francisco is built is an example of a transform boundary between the Pacific plate and the North American plate. Volcanoes and Plate Tectonics… …what’s the connection? Volcanoes are formed by: - Subduction - Rifting - Hotspots  Volcanoes can be formed in three ways: 1. Via subduction. The subducting slab dehydrates to form new melt that will rise through the crust to be erupted at the surface. 2. Via rifting. When two plates pull apart magma rises, producing volcanic eruptions at the surface. 3. At “Hotspots”….hotspot do not necessarily occur along a plate boundary. So hotspot volcanoes can form in the middle of tectonic plates.  What are Hotspot Volcanoes? Hot mantle plumes breaching the surface in the middle of a tectonic plate The Hawaiian island chain are examples of hotspot volcanoes. Photo: Tom Pfeiffer / www.volcanodiscovery.com  Firstly, what are hotspot volcanoes and how do they form? A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time. The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface (see left diagram). A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries. The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).  The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other. Earthquakes and Plate Tectonics… …what’s the connection?  As with volcanoes, earthquakes are not randomly distributed over the globe Figure showing the distribution of earthquakes around the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Where do earthquakes form? Figure showing the tectonic setting of earthquakes  Plate Tectonics Summary The Earth is made up of 3 main layers (core, mantle, crust) On the surface of the Earth are tectonic plates that slowly move around the globe Plates are made of crust and upper mantle (lithosphere) There are 2 types of plate There are 3 types of plate boundaries Volcanoes and Earthquakes are closely linked to the margins of the tectonic plates