Sea-Floor
Spreading
Was Wegener Right?
Recently, new technology
has given us new clues into
drifting continents.
Wegener’s theory of
drifting continents is
supported by this new
evidence.
Mapping the MidOcean Ridge
In the mid-1900s, scientists
found the mid-ocean ridge
underwater, which is the
longest chain of mountains
in the world.
Scientists used sonar to
map the ridge.
Evidence for SeaFloor Spreading
Finding the mid-ocean ridge
supported Wegener’s theory
of continental drift.
Harry Hess was a scientist
who suggested that the
ocean floors move like
conveyor belts.
Evidence for SeaFloor Spreading
Remember, there are two types
of crust.
Continental Crust ( “land”)
Oceanic Crust (the land under
the oceans
There is a big crack in the
oceanic crust, and this is where
the mid-ocean ridge forms.
Evidence for SeaFloor Spreading
Hess suggested that along
the mid-ocean ridge, magma
rises from the mantle and
erupts.
The magma spreads out and
pushes older rocks to both
sides of the ridge.
Evidence for SeaFloor Spreading
This process is called seafloor spreading.
Evidence for SeaFloor Spreading
There are three key areas of
evidence that support the
theory of sea-floor spreading.
Evidence from molten
material (magma)
Evidence from magnetic
stripes
Evidence from drilling
samples
Evidence from Molten
Material
Using a submarine,
scientists found rocks that
can only be formed when
magma hits water.
This proved that molten
material has erupted from
the mid-ocean ridge.
Evidence from
Magnetic Stripes
Remember how Earth is a
big magnet?
Scientists found out that
Earth’s magnet has reversed
itself in recent history.
Evidence from
Magnetic Stripes
When Earth’s magnetic field
pointed north, the iron in the
magma that erupted into the
ocean faced one direction as it
hardened.
When Earth’s magnetic field
reversed, the iron faced the
opposite direction. This formed
stripes.
Evidence form
Drilling Samples
Scientists sent a ship to drill
deep into the ocean floor
and collect many rock
samples.
They then tested the age of
the rocks using carbon
dating.
Evidence from
Drilling Samples
Scientists found out that
the farther away from the
mid-ocean ridge the
samples were taken, the
older the oceanic crust.
The oceanic crust near the
mid-ocean is youngest.