platetectonics or are we dancing on an eggshell? is there a magma ocean below? …and much more…
TRANSCRIPT
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PLATETECTONICS
Or
Are we dancing on an eggshell?
Is there a magma ocean below?
…and much more…
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PLATETECTONICS
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What causes plate movement?
Heat transfer by mantle convection!
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Earth’s mantle: peridotite!!!
geotherm
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olivine
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Olivine (cut gems)
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Olivine: thin section
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Olivine with rims altered to iddingsite
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Pyroxenes:
Orthopyroxenes: (Mg,Fe2+)2[SiO6]
Clinopyroxenes: (Ca,Na,Li)(Mg,Fe2+,Fe3+,Mn,Al,Ti)[(Si,Al)2O6]
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Clinopyroxene: thin section
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Plagioclase: albite
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Plagioclase: thin section
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Olivine (coloured) in plagioclase (grey)
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Garnet: (Ca,Mg,Fe,Mn)3(Al,Fe,Cr)3[SiO4]3
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Garnet (cut gem)
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Garnet: thin section (polarized light)
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Earth’s mantle: peridotite!!!
geotherm
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Earth’s geotherm is well-below the solidus!
So why does magma form?
i.e., why does the mantle melt?
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geotherm
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…now back to the surface…
moving oceanic and continental plates
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plate movement in cm/yr
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earthquakes:at plateboundaries (in particular along subduction zones)
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…creation of oceanic crust…
SPREADING ZONES
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mantle convection drives plate tectonics!
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heat causes convection - heat transport by convection
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mid-oceanic ridges / spreading zones:
“constructive” and “divergent” plate boundaries
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Structure of the oceanic crust
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Recent (modern) pillow basalts at the seafloor off the US west-coast
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Ancient pillow basalt
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Ancient pillow basaltin a road cutblack pillow rims are basaltic glass
formed by quenching of hot lava during contact with cold seawater
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hydrothermal vent:black smoker: hot, acidic, reducing(colour due to formation of black sulphide particles)
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hydrothermal vent:
white smoker: <200oC
(colour due to formation of silica and sulphate particles)
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hydrothermal vents:
black and white smokers
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most recently discovered (!!!):
new type of (cold) hydrothermal vent(heat produced by alteration of oceanic crust, white minerals are Ca carbonate)
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Structure of the oceanic crust
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Ophiolites:
- pieces of oceanic crust thrusted (obducted) onto the edge of continental plates;
- assemblage of mafic (basalt) and ultramafic (perodotite) lavas found in association with sedimentary rocks;
- found in Cyprus, New Guinea, Newfoundland, California, Turkey, and Oman.
Semail ophiolite in southeastern Oman probably formed in the Cretaceous not far from what is now the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle) westward onto the Arabian shield.
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Ophiolites in the middle east
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marine sediment:ferrugineousradiolarian chert(biogenic quartz)
igneous rock:
pillow basalt
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pillow basalt
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sheeted dikes (basalt)
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sheeted dikes (basalt)
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layered gabbros
(gabbro is the coarse-grained equivalent of a basalt)
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close-up view of layered gabbro
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Moho
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Layered harzburgite (ultramafic mantle rock)
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SUBDUCTION ZONES
…and now for something completely different:
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“destructive” and “converging” plate boundaries
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Two types ofdestructive plate boundaries:- oceanic crust subducted under oceanic crust- oceanic crust subducted under continental crust[oceanic (basaltic) crust is denser than continental (granitic) crust]
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Mount Fuji, Japan
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Mount Mayon, Philippines
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stratovolcanoesin the Andes at the Chile-Bolivia border
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Andesite:
dark pyroxene phenocrysts* in grey groundmass of plagioclase and minor pyroxene and olivine
*large crystals that formed in the magma prior to eruption
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viscous andesite lava flowson the flanks of Colima stratovolcano, Mexico
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channelled andesite lava flows at Lascar volcano, Chile
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What causes island-arc volcanism?
What does actually melt and how?
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!!!
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slab pull causes convection in the mantle wedge
back-arc spreading
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back-arc spreading causes basin formation and volcanism
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Sediments and plate tectonics
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Chemical evolution of mantle and crust
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bulk Earth Earth’s crust
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Crustal growth throughout Earth’s history was episodic!
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two models for recycling of Earth’s crust
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subducted slab “stored” at 650km discontinuity
subducted slab “stored” in the Lower Mantle
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so much for
PLATE TECTONICS…