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structure of oceanic lithosphere
ophiolite
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layers that make up ophiolite:
- ocean seds
- pillow basalts [extrusive mafic]
- sheeted dikes of basalt [extrusive mafic]
- gabbro [intrusive mafic]
- ...............................................................crust
- peridotite [intrusive ultra-mafic]
- .............................................................mantle
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pillow basalt
- a structure that forms when lava erupts underwater
- due to rapid cooling because water has high heat capacity
- any incidence of pillow lavas, on continent or in the ocean, indicates the lava erupted under water
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dike
igneous intrusion that forms when magma intrudes other rocks
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sheeted dikes
- dikes of basaltic magma
- intrusion continues through the weakest point
- weakest point will be the seams of dikes or hot regions
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magma chamber
- find xenoliths
- differentiation occurs
- leave behind the makings of gabbro
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ridge-push
volcanic eruptions at the mid-ocean ridge are pushing the plates apart
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slab-pull
metamorphism increases the density of subducted rocks which pulls the plate along
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plate-sliding
plates are dragged around on top of moving convection cells in the mantle
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black smokers
- hydrothermal vents found on the seabed cause mineralization
- responsible for ore-formation
- pyrite, chalcopyrite, sphalerite, etc.
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how continents grow:
- magmatic addition; the most buoyant, silica-rich crust is formed over subduction zones; both continental and island arcs play a role
- continental accretion; buoyant portions of crust are attached during plate movements; individual fragments are called accreted terranes
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the Wilson cycle
- describes the plate movements that open and close basins and also describes the effect on continents
- 1. rifting splits the continent...
- 2. ...leading to the creation of new oceanic crust
- 3. passive margin -- oceanic and continental lithosphere are in direct contact with no active tectonic boundary
- 4. convergence begins; an oceanic plate subducts beneath a continental plate, creating a volcanic chain
- 5. terrane accretion welds material to the continent
- 6. orogeny thickens the crust and builds mountains forming a new super continent
- 7. the continent erodes, thinning the crust. rifting may begin the process again
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