Transcript Sea-Floor Spreadingx

Marie Tharp

Oceanographer

First to map details of
the ocean floor on a
global scale

Her observations
became crucial to the
eventual acceptance of
the theories of plate
tectonics and
continental drift
Marie Tharp's "World Ocean Floor Map”
1977
Seafloor Spreading
In the 1960’s, a scientist
named Harry Hess made a
discovery that would vindicate
Wegener
Using new technology, radar,
he discovered that the
seafloor has both trenches and
mid-ocean ridges
Harry Hess proposed the seafloor spreading theory
Echo Sounding Device Used By Hess
Seafloor Spreading
Hess proposed that hot,
less dense material below
Earth’s crust rises toward
the surface at the midocean ridges
Then, it flows sideways,
carrying the seafloor away
from the ridge in both
directions
Seafloor Spreading
As the seafloor spreads apart at a mid-ocean ridge, new
seafloor is created
The older seafloor moves away from the ridge in
opposite directions
This helped explain how the crust could move —
something that the continental drift hypothesis could not
do
Sea Floor Spreading
Seafloor Spreading - The process by which
molten material adds new oceanic crust to
the ocean floor
Evidence from Molten Material
Strange rocks shaped like
pillows or like toothpaste
squeezed from a tube have
been found
These rocks can form only
when molten material
hardens quickly after
erupting under water
These rocks showed that
molten material has
erupted again and again
from cracks along the
central valley of the midocean ridge
Mid Atlantic Ridge – Molten Material
Evidence from Magnetic Stripes
Scientists discovered that the rock that
makes up the ocean floor lies in a
pattern of magnetized “stripes”
780,000 years ago, magnetic poles
reversed themselves
If they reversed today, the needle in a
compass would point south instead of north
The rock in the ocean is made of iron,
which began as molten material
Magnetic Reversals
Evidence from Magnetic Stripes
Plates move apart
Magma rises and cools
forming new sea-floor
youngest rock is near the
ridge, it gets older as you
move away
The earth’s magnetic field is
recorded in the rock as it
forms
We have a record of the
earth’s magnetic reversals on
either side of the ridge
100 Greatest
Discoveries:
Magnetic Reversals
Polarity Reversal
•Scientists later discovered that the earth’s
magnetic field changes polarity every few
thousand years
Magnetic Reversals in the Seafloor
As the rock cools, it records these magnetic
Magnetization of the sea floor
reversals in the seafloor
Evidence of Sea Floor Spreading
Key Idea
As distance from the ridge
increases, the age of the seafloor increases
Evidence from Drilling Samples
When scientists sampled the rocks, they found that the
further away from the ridge the rocks were the older
they were
The younger rocks were always in the center of the ridges
Important Things to Remember
Theory of sea floor spreading - hot, less
dense material below the Earth’s crust
rises toward the surface at the midocean ridges. Then it flows sideways,
carrying the seafloor away from the
ridge in both directions
As distance from the ridge increases,
the age of the sea-floor increases
Important Things to Remember
Henry Hess proposed the seafloor
spreading theory
He proposed that hot, less dense material
below Earth’s crust rises toward the
surface at the mid-ocean ridges. Then, it
flows sideways, carrying the seafloor
away from the ridge in both directions
Quick Check
Turn and tell your neighbor the three pieces of
evidence that support the theory of seafloor
spreading
Sea Floor Spreading
Hydrothermal Vents
Ocean Exploration