Seafloor Spreading
Discovering the Mid Ocean Ridge
• In 1925, Germany outfitted a boat and set out for
two years to systematically and scientifically look
at the oceans -- and to show the flag, since the
Versailles Treaty had banned German navy boats
from foreign ports.
• This expedition was the first use of closely spaced
echo sounders to map deep sea topography and
the first to reveal the extent of the sea floor's
rugged terrain. The expedition also found that a
continuous mountain-like ridge runs through the
Atlantic to the southwest of Africa.
• The expedition also found that a continuous
mountain-like ridge runs through the Atlantic to
the southwest of Africa.
• That ridge, it was later discovered, extended
through the major oceans of the world. It is now
called the Mid-Ocean Ridge.
• In some places the canyon, called the Great Global
Rift, came very close to land. The rift appeared to
be breaks in the earth's crust, but perfectly fitted
breaks, like joints made by a carpenter. The rift
outlined chunks of the earth's crust, which were
named tectonic (from a Greek word for
"carpenter") plates.
Seafloor Spreading
• Harry Hess was a geologist and Navy submarine
commander during World War II.
• Part of his mission had been to study the
deepest parts of the ocean floor. In 1946 he had
discovered that hundreds of flat-topped
mountains shape the Pacific floor.
• The discovery of the Great Global Rift in the
1950s inspired him to look back at his data from
years before.
• After much thought, he proposed in 1960 that
the movement of the continents was a result of
sea-floor spreading. In 1962, he added a
geologic mechanism (seafloor spreading) to
account for Wegener's moving continents.
Seafloor Spreading
• It was possible, he said, that molten magma
from beneath the earth's crust could ooze up
between the plates in the Rift Valley.
• As this hot magma cooled in the ocean water,
it would expand and push the plates on either
side of it -- North and South America to the
west and Eurasia and Africa to the east.
• This way, the Atlantic Ocean would get wider
but the coastlines of the landmasses would
not change.
(Fig 1.11) Sea floor Spreading.
Seafloor Spreading Video Clip
Seafloor Spreading
•The weaknesses between the diverging plates fill
with molten rock from below. Sea water cools the
molten rock, which quickly turns to rock, and
pushes the older rock to the outside.
•This continuous process builds a chain of
volcanoes and rift valleys called a Mid Ocean Ridge.
Subduction:
process in which one
lithospheric plate collides with and is forced down under
another plate and moves back into the Earth's mantle.
• If new oceanic lithosphere is
created at mid-ocean ridges,
where does it go?
• The lithosphere arrives at the
edge of a continent, where it is
subducted or sinks into the
asthenosphere
• Thus, oceanic lithosphere is
created at mid-ocean ridges and
consumed at subduction zones,
areas where the lithosphere sinks
into the asthenosphere
Subduction
• Earthquakes are
generated in the rigid
plate as it is subducted
into the mantle
• Magma generated along
the top of the sinking
slab rises to the surface
to form volcanoes.
Paleomagnetism
• lava gets erupted at the midocean ridge axis it cools and
turns into hard rock
• As it cools it becomes
permanently magnetized in
the direction of the Earth’s
magnetic field
• Earth’s magnetic field has
reversed its polarity
(direction) hundreds of times
during the past several
hundred million years
Paleomagnetism
• A polarity reversal means that the magnetic
North flips to where we know the South Pole is
• At the mid-ocean ridge spreading axis, these
flips in the direction of the Earth’s magnetic field
are recorded in the magnetization of the lava
• This creates a symmetrical pattern of magnetic
stripes of opposite polarity on either side of midocean ridges
Polar Reversal Video Clip
Paleomagnetism
Review of Seafloor Spreading
New Rock is created
by rising magma at
the mid-ocean ridge
Old rock gets
pushed away and
sinks back into
mantle