ESS105H1 Lecture Notes - Lecture 3: Mantle Convection, Continental Crust, Ultramafic Rock

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Published on 31 Jan 2019
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Lecture 3: Plate Tectonics II & Minerals
The Earths Structure and Plate Tectonics
The unique physical properties of the Earth's interior allow for tectonic plates to move above the mantle
The crust, mantle, and core boundaries are defined by chemical composition
The crust:
o Oceanic (10km): mafic composition and avg rock type is basal/gabbro
o Continental crust (20-90km): felsic composition and avg rock type = granite
The mantle is a Solid rock layer between the crust and the core
o Below ~100-150 km, the rock is hot enough to flow
o It convects: hot mantle rises, cold mantle sinks
o Mantle composition = ultramafic rock called peridotite
o The Moho discontinuity is the lower boundary that separates the crust from the upper mantle
Marked by a change in the velocity of seismic P waves
Lithosphere-Asthenosphere
Lithosphere = brittle portion of Earth's interior
o Behaves as a non-flowing, rigid material
o Comprises both crust and upper mantle
o Broken into plates which move in response to mantle convection
o It does not take part in mantle convection
Asthenosphere ductile portion of Earth’s interior
o Behaves as a soft ductile solid
o Contains a small percentage of melt (< 2%)
o Shallower under oceanic lithosphere
o Deeper under continental lithosphere
Boundaries Between Layers
The Crust-Mantle boundary = Moho
o defined by seismic discontinuity indicating significant change in composition
Brittle-ductile transition
o Defined by a significant change in rock physical properties (viscosity)
Also defined as the depth below which earthquakes do not occur
Convection
The motion of tectonic plates is partly driven by convection within the Earth
o Hot material expands, becomes less dense and rises
o Cold material contracts, becomes denser and sinks
Hot material rises at the mid-ocean ridges
Cold material sinks at oceanic trenches
We know from laboratory experiments that the Earth’s mantle rocks would behave as a viscous liquid under high
pressure and temperature, while still remaining crystalline (solid)
There is still debate over whole mantle vs layered mantle convection
Mantle Convection
Mantle convection and plate tectonic movements are inseparable manifestations of the heat-driven dynamic cooling
process of the Earth
o Slab pull: horizontal pressure gradient associated with the sinking of a slab - the dense subducting slab
“pulls” the rest of the plate after it
o Ridge push: gravitational sliding of a plate off the elevated ridge (not an active push by the ascending
mantle)
Proof of Plate Movement
Paleomagnetism: the study of the Earth’s ancient magnetic field preserved in rocks
o Many rocks record the strength and direction of the Earth's magnetic field at the time that the rocks formed,
especially iron-rich, volcanic rocks, such as basalt, which is very common on the ocean floors
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