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Earth Science Notes
Seafloor Spreading
PT and Seafloor Spreading
Recall:
• The PT Theory was not accepted at first
because there was no explanation for how
the continents moved.
– Scientist did not know about convection cells
yet.
• After a while, evidence began to mount for
seafloor spreading. This provided an
explanation for why continents moved.
Evidence for Seafloor Spreading:
Magnetic Reversals
• During World War II, sensitive instruments
called magnetometers were developed to help
detect steel-hulled submarines.
We use these
instruments
today to
measure earths
magnetic fields
and to find
ferrous
materials (like
the cannon)
Evidence for Seafloor Spreading:
Magnetic Reversals
• When research scientists
used magnetometers to
study the ocean floor, they
discovered a surprising
pattern.
• Measurements of magnetic
variations showed that
alternating bands of rocks
recording normal and
reversed polarity were
arranged symmetrically
about mid-ocean ridges
Data from
magnetometer
reading.
Evidence for Seafloor Spreading
Evidence for Seafloor Spreading:
Magnetic Reversals
• Magnetic Reversals
– The Earth has a magnetic
north pole and a magnetic
south pole.
– These poles have flipped
periodically in Earth’s past.
• This means the north pole has
become the south pole and vise
versa.
– The record of the reversal of
the magnetic field is preserved
in magnetic rocks which lie
along the ocean floor.
What Creates Earth’s Magnetic
Field?
• The inner core of the Earth is solid – this is due to high
pressure keeping it that way.
• The outer core, a 2,000 km thick layer of iron, nickel, and
small quantities of other metals. Lower pressure than the
inner core means the metal here is fluid.
• Convections cells circulate this molten iron. The Coriolis
force also influences the movement of this fluid material.
• The flow of iron generates electric currents, which
produce magnetic fields. Charge metal atoms passing
through these fields go on to create electric currents of
their own. This self-sustaining loop is called the
geodynamo.
Magnetic Reversals
Some measurements
put the magnetic north
pole moving at 4 miles
a year.
Evidence for Seafloor Spreading
• Compare the
two pictures
we see here.
• Hypothesize
about how the
East African
Rift Valley
might look in
the distant
future.
Evidence for Seafloor Spreading:
Age of Rock
• Rocks samples taken near the Mid-Ocean
ridges show that rock to be the youngest
on Earth.
• Rocks increase in age as the get farther
away from the mid-ocean ridges.
Evidence for Seafloor Spreading:
Age of Rock