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9.2: Sea-Floor Spreading
in the early 1900s, scientists using
sonar (SOund Navigation And
Ranging) discovered deep-ocean
trenches
deep-ocean trenches: long, curved
valleys along the edges of some
ocean basins
trenches form the deepest parts of
the ocean
deepest is the Mariana Trench
in the late 1950s, scientists
discovered mid-ocean ridges
mid-ocean ridges: long, undersea
mountain chains that have a steep,
narrow valley at its center
ridges form as magma rises to
create new ocean floor made of
basalt
at the center of some mid-ocean
ridges are a crack in Earth’s crust
where magma can rise
this crack is a rift valley
Harry Hess (1963) proposed the idea
of sea-floor spreading where
a) magma rises at mid-ocean ridges to
create new ocean floor
b) ocean floor slowly moves outward
away from the rift
c) ocean floor sinks back into the
mantle at deep-ocean trenches
when ocean plates sink back into
the mantle, it is called subduction
rift and then cools to form new rock.
trenches where the ocean plates
sink back into the mantle are called
subduction zones
rift and then cools to form new rock.
1960s brought more data to support
the idea of sea-floor spreading
scientists learned that Earth’s
magnetic field occasionally reverses
polarity
north magnetic pole becomes the
south magnetic pole – called
reversed polarity
paleomagnetism: study of the
alignment of magnetic minerals that
rock gains during formation
scientists discovered
a striped magnetic
pattern on the
ocean floor on
each side of a midocean ridge
pattern on one side of the ridge is a
mirror image of the pattern on the
other side
mid-ocean ridges contain young
rocks – none older than 180 million
years
rocks get older towards the trenches
thin rocks – covered with less
sediment – at top of ridge, getting
thicker with more sediment as
move towards the trenches
sea-floor spreading was the
mechanism to explain continental
drift!
rift and then cools to form new rock.