• Wegener’s principle of continental drift was often dismissed
because he failed to produce a mechanism by which the
continents moved
• In the late 1950’s, a geologist named
Harry Hess suggested a new
hypothesis
• Proposed that the valley at the center of
a mid-ocean ridge was a crack, or rift.
• At this rift, magma deep within earth rose to
fill the crack
• As the ocean floor moves away from the
ridge, rising magma cooled and solidified to
form new crust
• Known as sea-floor spreading
• Hess suggested that if the ocean
floor is moving, the continents might
be moving too
• The evidence for seafloor spreading is found
in paleomagnetism
• The study of the magnetic
properties of rocks
• Earth acts as a giant magnet due to the activity of the
inner and outer core
• As magma solidifies to form rock, iron-rich minerals in the magma
align with Earth’s magnetic field in the same way that a compass
needle does
• When the rock hardens the magnetic orientation of the minerals
becomes permanent
• Magnetic Reversals
• Geologic evidence shows that Earth’s
magnetic field has not always pointed
north as it does now
• Scientists have dated rocks from different points in
Earth’s history which show differing magnetic properties
• Classified by these properties
• Help to determine how quickly the crust is moving, and for
how long
• Some rocks show normal polarity, meaning they
formed when magnetic north was “normal”, and some
rocks show reversed polarity, meaning they formed
when magnetic north actually pointed south
• Information is used to create a pattern known as the
geomagnetic reversal time scale
• Magnetic Symmetry
• Scientists use the geomagnetic reversal
time scale to study the ocean floor
• They observed that the patterns that
occurred on one side of the ridge were
mirrored on the other side
• Information gathered from this
symmetry is used to assign ages to the
sea-floor rocks
• The rocks that were located closer to the
ridge were younger than rocks that were
located further way from the ridge
• Supports Hess’s claim of sea-floor
spreading
• Mantle Convection
• Convection is the movement of heated material due to differences
in density that is caused by differences in temperature
• Energy generated by Earth’s core and radioactivity within the
mantle heat the mantle material
• Results in the rising of hot materials and the sinking of cool
materials
• As the mantle material moves, the overlying tectonic plates move
along with it.
• There are two forces that result from mantle convection: ridge push
& slab pull
• Ridge Push
• Newly formed rock at a mid-ocean ridge is warmer and less
dense than older rock nearby
• As the new rock cools, it becomes denser and sinks down away
from the ridge
• As it slides down it exerts a force on the rest of the plate
• Known as ridge push
• Pushes the plate away from the ridge
• Slab Pull
• Where plates pull away from each other at a midocean ridge, magma from the asthenosphere rises to the
surface
• As the new lithosphere moves away from the mid-ocean
ridge, the lithosphere cools and becomes denser
• When the lithosphere becomes dense enough, it begins to
subduct into the asthenosphere
• As the leading end of the plate sinks, it pulls the rest of the
plate along behind it
• Known as slab pull