CAS ES 107 Chapter Notes - Chapter 7: Lithosphere, Asthenosphere, Palaeogeography
7 Jun 2018
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Chapter 7: Part 2
The Mantle
-samples of the deeper mantle material that are available at the surface were brought up during
rare geological events, like the formation of kimberlites→ long-pipe shaped igneous bodies that
passed rapidly from the upper mantle to the near-surface, they contain diamonds that formed
under high pressures in mantle
-most of what we know about the mantle is inferred from seismology
-velocity structure indicates that the mantle is uniform in composition and formed of silicate
materials
-as depth increases so does pressure, so this causes changes in structural and mineralogical
structures
-it’s mostly molten rock but there’s a small amount of melt because it allows the crust and upper
mantle to move relative to the underlying mantle (plate tectonics)
-a transition zone of rapidly increasing seismic wave velocities separates the upper and lower
mantle
The Core
-the core is made of iron and nickel
-much denser than mantle
-although the core is farther from the surface, it affects surface conditions because it the source
of earth’s magnetic field
-magnetic dynamo→ a mechanism that transforms energy from fluid motions (convection) into
electrical currents that create a magnetic field
-so the liquid iron in the core is a conductor of electricity
-the outer core convects because a temp gradient is established across it by heat loss from the
solid, inner core
-the energy required to drive convection in the outer core is derived from the gradual growth of
the inner core
-as earth’s interior cools, liquid iron slowly freezes into solid iron, this freezing process releases
heat and the new solid iron particles also heat the outer core as they settled down to join the inner
core again
-the heat released by this drives outer core convection, aka magnetic dynamo
Seafloor Spreading
-the resurgence in interest in Wegener’s theory about continental drift arose when new info was
obtained by investigating the topographic and magnetic features of the seafloor
-topography→ the configuration of a surface in particular the position and elevation
-mid-ocean ridges→ chains of subsea volcanic mountains running down the centers of the ocean
basins
-seafloor spreading→ magma pushed up from the mantle and pushed the seafloor to the sides of
the ridges and is replaced by new seafloor. This occurs symmetrically
-magnetic polarity→ the geographic orientation of the north and south poles
-we know the magnetic poles have flipped numerous times because of the complex behavior of
the convecting liquid outer core
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Document Summary
Most of what we know about the mantle is inferred from seismology. Velocity structure indicates that the mantle is uniform in composition and formed of silicate materials. As depth increases so does pressure, so this causes changes in structural and mineralogical structures. It"s mostly molten rock but there"s a small amount of melt because it allows the crust and upper mantle to move relative to the underlying mantle (plate tectonics) A transition zone of rapidly increasing seismic wave velocities separates the upper and lower mantle. The core is made of iron and nickel. Although the core is farther from the surface, it affects surface conditions because it the source of earth"s magnetic field. Magnetic dynamo a mechanism that transforms energy from fluid motions (convection) into electrical currents that create a magnetic field. So the liquid iron in the core is a conductor of electricity.
