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Plate Tectonics and Life

  1. Sep 19, 2006 #1
    Hey guys

    Im doing research for an assignment on plate tectonics and life. The question is basically how it works and also how it helps maintain earth as a habitable enviroment. Now i am in no way looking just for answers, more an understanding so i can sound more intelligent in my project and also understand it better just for myself because i do find certain parts interesting (im not doing a geology course, its just an assignment for a unit im doing, long story :P)

    ok, so heres what i know (or think i know anyway :P)

    You have the mantle and the crust. The upper part of the mantle is the asthenosphere with a bit of the lithosphere. The upper part of the lithosphere is the crust. Now this lithosphere moves around on the mechanically weaker and softer asthenosphere for a number of reasons.

    a.) Convection currents in the mantle driven by the core
    b.) Subduction pulling
    c.) mid ocean ridge 'pushing'

    There are two kinds of crust, continental and oceanic. Oceanic is denser and so sinks and is made of basalt. The continental crust is however made of andesite and granite igneous rocks and their metamorphic and sedimentary companions.

    As plumes of hot magma rise in certain parts they partially melt the lithosphere and spew more material up, either through a volcano or mid ocean ridge (undersea volcano). If the oceanic version of this occurs you get seafloor spreading where new oceanic crust is created as the lava (magma underground) flows and cools to form new rock.

    Continental crust is on the other hand formed by subduction. If an oceanic plate subducts under an existing continental plate or other oceaninc plate (that can happen right?) the plate undergoing subduction dehydrates. This water then rises up and loweres the melting point of the above plate, causing it to melt and mix with surrounding materials to form the andesite and granite materials that make up the continental crust. Thus without water, you cant have continental crust. And so the crust slowly grows along this zone.

    I also understand the whole supercontinent and contiental drift thing, and that supercontinents spell their own doom because they dont allow the earth to lose heat

    Now in terms of making earth habitable (im sure ive missed heaps in the above section, i didnt go through my notes with a fine toothcomb but the basics are there i think). For this i just had a few general sections

    1.) Errosion vs mountain building
    If there was no plate tectonics, and thus no mountain building, errosion would eventually weather away everything and remove all sediment, effectively covering the world in water.
    2.) The carbon cycle
    Volcanoes and the rock cycle play an important role in the carbon cycle and thus the regulation of the earths temperature. plate tectonics affect both these cycles
    3.) Life was thought to first originate near oceanic fissures where magma was superheating the water. You also find that the water at mid ocean ridges circulates through the material and brings back up with it carbonacous material and metals that are essential for life

    Am i on the right track?
    Keep in mind this is only a first year assignment for a non-geology unit (its called our solar system so it includes some geology but its not a indepth thing)

  2. jcsd
  3. Sep 19, 2006 #2


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    Sounds like excellent work so far. WRT the carbon cycle; some treatment of the role of geological carbon sequestration in the regulation of atmospheric chemical composition might not be out of line. Perhaps the role of techtonic activity in the hydrologic cycle could also be mentioned (subduction and volcanism, as well as "juvinile water").

    Looks like a very good paper.
  4. Sep 21, 2006 #3
    cool thanks!

    i just have one question. i dont understand the bit (i know i wrote it but just regujitating from notes rly) about how continental crust is formed. I mean i get that as subduction occurs the plater underneath gets dehydrated and the water goes up lowereing the melting point of the mantle under the above plate and thus melting it. supposedly this rock which i assume is basalt then interacts with the plate (very general wording :S) and makes more continental crust

    so if you need continental crust to make continental crust.... can someone elaobrate on this process please?
  5. Sep 25, 2006 #4
  6. Sep 26, 2006 #5


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    Did you use a style for your references?
  7. Oct 4, 2006 #6
    Nope, just typed em. why
  8. Oct 13, 2006 #7
    Not sure what you don't understand here, basically "new" continental crust is just recycled "old" continental crust. It melts at depth lowering its density and causing it to become buoyant, it rises, differentiates and eventually cools to form new igneous rock at or near the surface. No continental crust is lost or gained in this process.
    I suppose what you're asking is how the continental crust got there in the first place? In which case I'd hesitantly tell you that at some point large depositional basins probably opened up, upon which low density sediments were deposited, eventually evolving into continental crust - particularly if the plate tectonic regime had compressed the sediments causing them to uplift.
  9. Oct 5, 2010 #8
    Plate tectonics shift.
  10. Oct 5, 2010 #9
    when magma cools very slowly the heavier crystals sink to the bottom and lighter materials float to the surface.
    That is my understanding of how granite forms.
  11. Oct 6, 2010 #10


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  12. Oct 7, 2010 #11


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    The notion that Granite is formed thru some type of dehydration process is wrong.

    Basalt is denser than Granite due to predominance of heavier oxides such as CaO, FeO & MgO at the expense of lighter SiO2.
  13. Oct 9, 2010 #12
    Seems very well written, the only thing I would add is the effect of the moon in the process of life formation. Back in the early days the moon was much closer, and therefore its tidal forces much greater.

    What effect the moon has on magma and plate techtonics I do not know but the regular periodic very large tides has been proposed to be a driving force for the formation of life.
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