Transcript Chapter 4.1 Seafloor Spreading

Seafloor Spreading
(still continuing ch. 4.1)
 Underwater
mountain range called
the Mid-Atlantic Ridge discovered in
1947.
 Part of an 80,000-km-long midocean ridge system around the whole
earth.
 Rocks from the ocean floor were
young compared to most continental
rocks.
 Princeton
geologist Harry Hess
hypothesized that the mid-ocean
ridges were breaks in the earth’s
crust, where magma was rising up
and spreading away from the ridge in
both directions, forming new rock.
 If the ocean floor was moving, then
maybe the continents were also
moving !?!
Paleomagnetism of the Ocean
Floor
 Earth
acts as a giant magnet.
 Compass needles align with the
magnetic lines of force that extend
from pole to pole.
 When magma cools, iron-rich
minerals line up with the magnetic
field.
 When
rock hardens, magnetic
orientation of the minerals become
permanent and point to north.
 Geologists have discovered some
rocks with magnetic orientations
pointed south…
 All north-pointing rocks (normal
polarity) were from similar time
periods.
 All
south-pointing rocks (reverse
polarity) were also from similar time
periods.
 This led to the discovery that Earth’s
magnetic field has reversed itself
many times throughout history.
 Magnetic patterns on the ocean floor
show alternating bands of normal
and reverse polarity.
 Identical
magnetic striped patterns
on either side of the mid-ocean
ridges.