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Lecture 4

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Environmental Science
Nick Eyles

EESA06 – Introduction to Planet Earth Lecture 4 • How do we tell the story of planet earth? It is history, a long story of geohistory • There is an easy way to ‘divide up’ the story of earth’s history • The supercontinent cycle idea that continents come together to form a supercontinent which has very brief existence then they split up, form oceans and continents move apart and then oceans die. This is the basic principle of earth’s history, but it is a bit ironic because you explain earth history in terms of what oceans are doing. Most of earth’s history is based on the ocean cycle, it reflects planet’s geography and history • Look at the life cycle of oceans: how they are created, how they grow up, and how they eventually die; once you understand this, you can go back through earth’s history and understand these complicated rocks in a simple narrative • East Africa is where you go today to see an ocean being born • Geologists go look at rocks, volcanic activity and apply it to ancient past, this process we call it ‘uniformitarianism.’ This is a guiding principle in geology that we look at modern environments, learn from them and then go back into the past. Sometimes called ‘actualism’ (same meaning – present is the guide to the past) • There are some exceptions to actualism; this process is important to the evolution of the planet but we have not seen ourselves – meteorite impacts. These huge impacts have profoundly changed the history of life on the planet and some people argue that some of these supercontinents break up when large meteorite impact occurs. Economically, they are important too (mining impacts) but we have not seen one ourselves. • Idea of actualism can only take us so far, ancient past informs the present; understanding the ancient past informs us of how the planet works today. We only know meteorite impacts occur because we have looked at ancient rocks. • There have been different schools of thought that have said there have been catastrophic events • Ex. There used to be a school of thought called catastrophism (big floods, huge earthquakes), but now it is a blend of two • Start from East Africa, then Jordon, then end up in Turkey • If you move one plate, you start effecting the surrounding plates • Major earthquakes threaten Istanbul; the timing of past earthquakes is well known and their location is known. There is a strange phenomenon called cascading earthquakes where the positions of the earthquakes get closer and closer to the city. It is estimated that in the next 60 years or so Istanbul is in for a very major devastating earthquake, but the point is Istanbul is threatened because of what is going on in East Africa • Review: see notes, computer simulation of the earth’s interior. Mantle (largest component of the earth) is made up of rock, but it is not fluid and is at temperature where the rock is deforming and subject to these large scale convection currents. Plumes of hot rock are coming up from the outer surface of core, and these plumes are going to feed volcanic activity at the earth’s surface; this is key when we look at the break up and formation of continents and supercontinents. When plumes rise underneath a supercontinent and tears it apart, it also provides all the raw materials for volcanic rocks. What happens on the surface is driven by what happens under the earth’s surface. If we want to know more about what happens tectonically at the earth’s surface then we need to know more about the earth’s mantle • The core rotates, at a slightly different rate than the mantle which is why we have the earth’s magnetic field • How do these plates move around? How are they actually driven by convection currents in the mantle? • Oceanic crust (rock that underlies the oceans), it is more dense than continental crust. Continental crust rises, it is buoyant whereas the ocean crust sinks back down into the mantle – subducting process (this is the reason why we have ocean basins on planet earth). This creates these broad topographic lows which are flooded by water • Oceanic crust, basalt (very important rock type, full of iron, magnesium, it is dark coloured rock) is produced at mid ocean ridges and it is the principle component of oceanic crust • Basalt is 3 g/cm^3 in density whereas continental crust is 2.6-2.7 g/cm^3 (lighter) • Layer in which the oceanic crust plates slide over is called asthenosphere – which means weak sphere in Greek; this allows these relatively rigid plates to move around the planet • Mohorovicic discontinuity is actual base for the continental crust • Lithosphere(it means ‘rocky’ sphere) – is underlying weak material • Lithosphere is on top of the asthenosphere • Aid to learning  to remember something, it is good to write notes so it aides memory; same with figure , if you sketch it roughly it is easier to remember • History of Pangea  continents breaking up and oceans form (which really start as cracks called rifts); they break up precisely because there were plumes underneath. So the supercontinent was sitting on several hot plumes that were coming up from the mantle and those exerted extensional force (something that pulls something apart to extend something as it gets together to compress something). Pangea was torn apart by plumes. Can’t create these broad areas of oceanic crust out of nothing, have to have huge plumes feeding these mid ocean ridges otherwise the oceans don’t widen. About 60 000 km all around the planet (mid ocean ridges) that are fed by mantle plumes that provide raw materials out of which we form basalt, out of which we form oceanic crust. Therefore, no plumes mean no oceans. • Very early in the history of the planet, there would have been one ocean (planet was completely covered in water), so what is the origin of the continents? As far as we know, water would have cooled the crust underneath so for the first time you have old, thick, wet crust and this in term can be subducted down into the mantle. Early earth was completely molten (red hot ball); it has a crust that was repeated broken up by large meteorites because at that stage the earth is still gaining mass and gravity. This is a result of de-gassing from the mantle. Water was slowly accumulating on the earth’s surface that would have locally cooled the hot crust, which would have started the subduction process (created the first continent) and after that can’t get rid of continents because it is buoyant. Small volcanic islands were produced also. • Oldest rocks of Canada are in Yellowknife, and are 4.1 billion years old. You can map then within the slave craton which was the original North American and then additional crusts joint through time to form larger continents. Continents are always evolving and are growing by colliding with things • Terrains – blocks of geology that are distinct from other blocks (different fossils, different rock types). Terrains occur when continents collide with things (ex. stuck onto western part of North America) • Much of maritime Canada today has the same geology, landscape and rock types as north western Africa because a lot of African rocks got left behind. Famous red rocks of Maritime is in Morocco also • There is heat flow to the earth’s surface from the deep interior of the planet. This is like putting a lid on a sauce pan. Building a large supercontinent prevents heat from escaping in that area so the mantle warms up (since heat isn’t released to the surface) which triggers the plumes. Supercontinents are the architect of their own demise. • For any given time interval, take a look at the width of the colour in the ocean floor diagrams which indicates the power of the plumes. Pacific Ocean is wider because the plumes are more powerful than Atlantic and it produces much more stuff (oceanic crust) • Today’s lecture: • How oceans originate: at rifts, plumes rise and impinge on the bottom of the crust, which pushes continental crust in opposite directions. It is also melting at its tip and producing volumes of lava, basalt • Rifts can be underwater, along a mid ocean ridge, east Africa, and in Iceland you can actually walk across rift because it is raised above sea level • Stage 1  East African rift has several huge faults; where this initial rigid slab/plate is being warmed and pulled. Warming a slab makes it much more easier to break • Basalt is relatively heavy. If we build up great thicknesses of basalt, it is going to want to subside, sink down into softer rock under plume. We might be producing huge amounts of basalt, rift doesn’t fill up, but it gets deeper because that enormous amount of basalt is pushing its way down into the underlying mantle • Stage 2  as the hole gets deeper and deeper, the ocean will break through so you get a flooded rift. Ex. Red Sea is an ocean because it has a spreading center along its middle. Technically, it isn’t a sea because oceans are defined by having spreading centers and it has one. Red Sea is widening because the plume tip is melting, new material is being added all the time so in order to accommodate that, the crust has to move apart (about 5 cm/year). Sea floor spreading is occurring so it results in widening of the ocean. It is known as an embryonic ocean, then becomes young, adolescent ocean. All the modern oceans at one time went through this evolution during the break up of pangea • Stage 3  After embryonic stage, it becomes a mature ocean. Where the process above has gone on for millions of years and the continent is being pushed this far apart such that there is a huge expansion of ocean crust underlying a mature ocean. Passive margins of continents: is the trailing edge or active margin of a continent. Divergent plate margins (plates that are moving apart) and converging plate margins (plates being pushed together and subduction occurs along with other things) • The next phase is the death of that ocean (subject of a later lecture) where the continents will be pulled back together again and the cycle will repeat itself; hence, the name ocean life cycle. Cycle only occurs if there is a plume that can sufficiently feed the ocean floor. We need plumes • Plume  a column of hot (solid mantle) rock (like toffee, lava lamp) rising, it impacts the base of the continent. Extensional forces warm the base of the continent and pull it apart. During a plume’s passage from the earth’s deep interior it is moving into areas of lower pressure so that hot rock will begin to melt (referred to as decompression melting). This rock is dry, there is some water in it but not much. As it rises, the pressure comes up, it wants to melt. Melting causes magma intrusions (intrudes) and they happen all the time so you get sea floor spreading (analogy of finger trick, take hand and push other hand’s fingers through it from underneath, so spreads). • Geologists give the name of dykes  vertical wall like feature that is produced by intrusion. This is a very important process that creates oceanic crust and pushes crust apart. It is the ‘fuel’ of plate tectonics (drives the spreading) • When red hot magma is moving up the dykes, it is cooling and it also sometimes reaches a surface where the magma floods (known as flood basalts which is very fluid magma) • Then there is a wait because the basalts are heavy. Then you get subsidence because part of the continent is being pulled in opposite directions. This is how rifts originate • Rocks that have some water in them and rocks that have a lot
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