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!

Earth’s mantle: peridotite!!! geotherm

olivine

Olivine (cut gems)

Olivine: thin section

Olivine with rims altered to iddingsite

Pyroxenes: Orthopyroxenes: (Mg,Fe 2+ ) 2 [SiO 6 ] Clinopyroxenes: (Ca,Na,Li)(Mg,Fe 2+,Fe 3+,Mn,Al,Ti)[(Si,Al) 2 O 6 ]

Clinopyroxene: thin section

Plagioclase: albite

Plagioclase: thin section

Olivine (coloured) in plagioclase (grey)

Garnet: (Ca,Mg,Fe,Mn) 3 (Al,Fe,Cr) 3 [SiO 4 ] 3

Garnet (cut gem)

Garnet: thin section (polarized light)

Earth’s mantle: peridotite!!! geotherm

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

in a road cut black pillow rims are basaltic glass formed by quenching of hot lava during contact with cold seawater

hydrothermal vent: black smoker: hot, acidic, reducing (colour due to formation of black sulphide particles)

hydrothermal vent: white smoker: <200 o C (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. S 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

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)

SUBDUCTION ZONES …and now for something completely different:

“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 EarthEarth’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…