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PLATETECTONICS
Or
Are we dancing on an eggshell?
Is there a magma ocean below?
…and much more…
PLATETECTONICS
What causes plate movement?
Heat transfer by mantle convection!
geotherm
Earth’s mantle: peridotite!!!
olivine
Olivine (cut gems)
Olivine: thin section
Olivine with rims altered to iddingsite
Pyroxenes:
Orthopyroxenes: (Mg,Fe2+)2[SiO6]
Clinopyroxenes: (Ca,Na,Li)(Mg,Fe2+,Fe3+,Mn,Al,Ti)[(Si,Al)2O6]
Clinopyroxene: thin section
Plagioclase: albite
Plagioclase: thin section
Olivine (coloured) in plagioclase (grey)
Garnet: (Ca,Mg,Fe,Mn)3(Al,Fe,Cr)3[SiO4]3
Garnet (cut gem)
Garnet: thin section (polarized light)
geotherm
Earth’s mantle: peridotite!!!
Earth’s geotherm
is well-below the
solidus!
So why does
magma form?
i.e., why does
the mantle melt?
geotherm
…now back to the surface…
moving oceanic and continental plates
plate movement in cm/yr
earthquakes:
at plateboundaries (in particular along subduction zones)
…creation of oceanic crust…
SPREADING ZONES
mantle convection drives plate tectonics!
heat causes convection - heat transport by convection
mid-oceanic ridges / spreading zones:
“constructive” and “divergent” plate boundaries
Structure of the oceanic crust
Recent (modern) pillow basalts at the seafloor
off the US west-coast
Ancient pillow basalt
black pillow rims are basaltic glass
formed by quenching of hot lava
during contact with cold seawater
Ancient pillow basalt
in a road cut
hydrothermal vent:
black smoker: hot, acidic, reducing
(colour due to formation of black sulphide particles)
hydrothermal vent:
white smoker: <200oC
(colour due to formation of
silica and sulphate
particles)
hydrothermal vents:
black and white smokers
most recently discovered (!!!):
new type of (cold) hydrothermal vent
(heat produced by alteration of oceanic crust,
white minerals are Ca carbonate)
Structure of the oceanic crust
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.
Ophiolites in the middle east
marine sediment:
ferrugineous
radiolarian chert
(biogenic quartz)
igneous rock:
pillow basalt
pillow basalt
sheeted dikes (basalt)
sheeted dikes (basalt)
layered gabbros
(gabbro is the coarse-grained equivalent of a basalt)
close-up view of layered gabbro
Moho
Layered harzburgite (ultramafic mantle rock)
…and now for something completely different:
SUBDUCTION ZONES
“destructive” and “converging” plate boundaries
Two types of
destructive plate boundaries:
- oceanic crust subducted under oceanic crust
- oceanic crust subducted under continental crust
[oceanic (basaltic) crust is denser than continental (granitic) crust]
Mount Fuji, Japan
Mount Mayon, Philippines
stratovolcanoes
in the Andes at the Chile-Bolivia border
Andesite:
dark pyroxene phenocrysts* in grey groundmass of
plagioclase and minor pyroxene and olivine
*large crystals that formed in the magma prior to eruption
viscous andesite lava flows
on the flanks of Colima stratovolcano, Mexico
channelled andesite lava flows at Lascar volcano, Chile
What causes island-arc volcanism?
What does actually melt and how?
!!!
slab pull causes convection in the mantle wedge
back-arc spreading
back-arc spreading causes basin formation and volcanism
Sediments and plate tectonics
Chemical evolution of mantle and crust
bulk Earth
Earth’s crust
Crustal growth throughout Earth’s history was episodic!
two models for recycling of Earth’s crust
subducted slab “stored”
at 650km discontinuity
subducted slab “stored”
in the Lower Mantle
so much for
PLATE TECTONICS…