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Earth Science
Historical Introduction
Sea Floor Spreading & the
Earth’s Magnetic Field
During WWII
►Maps
of ocean
floor using
 Sonar
 Magnetometers
Harry Hess, a Captain during
WWII, gathered data while
cruising from battle to battle
Surprising Data
►Deepest
areas (trenches) were
close to the continents
►Underwater mountain ranges ran
through every ocean
 Each mid-ocean range had a
central valley running down the
middle (rift valley)
Dark blue – deepest areas
Yellow – shallowest areas
Surprising Data
►Symmetrical
patterns of magnetic
reversals centered on ocean ridges
The Earth’s Magnetic Field
Reverses!
►Reversals
are “recorded” as ironrich lava cools
►Reversals occur every ~200,000
years
 We haven’t had one for 700,000
years!
Reversals recorded in lava
flow
Age Evidence
crust is much younger than
continental crust
►Youngest rocks are found closest
to ocean ridges
►Ocean
Data provides support for
“Sea Floor Spreading”
►Lava
erupts at mid-ocean ridges
and cools, creating new sea floor
►Sea floor dives back under
continental crust at trenches
(Subduction Zones)
Process is driven by convection currents within
asthenosphere