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Science
• I can explain sea floor spreading
• Entry task- what is sea floor spreading?
Agenda:
Check pangaea
Sea floor spreading notes
Video on sea floor spreading
Start sea floor spreading lab
Poor Wegener
For 50 years, Wegener was not believed!
This was because he could not explain
HOW the continents could drift.
In the 1950’s, new data from
studies of the ocean floor
provided an explanation of
how continents moved.
Wegener was proven right!
Recall
Wegener’s continental drift idea wasn’t accepted.
This was because he could not explain
HOW the continents could drift.
In the 1950’s scientists began
mapping the ocean floor
using sonar.
They expected it to be smooth and
level…….
Mapping the Floor
Scientists found huge mountain ranges!
These mountains, called mid-ocean ridges,
were found in all oceans.
Each mid-ocean ridge featured a deep rift valley in
between the two ridges.
Looking closely at the ridges, scientists
found the mechanism that could move
continents.
Mid-Ocean Ridges
Evidence
Scientists found three pieces of evidence:
1. Sea Floor Spreading
Mid-ocean ridges form a
long a crack in the oceanic crust.
Molten magma rises through the crack,
cools, and forms new oceanic crust.
New magma pushes the crust apart,
causing the sea floor to spread apart.
1. Sea Floor Spreading
continental
crust
lithosphere
mantle
oceanic
crust
Mid Atlantic Ridge
Spreading Center
Evidence
2. Age of the Sea Floor
Scientists found that the youngest
rock was closest to the ridge and
the oldest rock was farthest away.
This showed that oceanic crust is
constantly forming and moving away
from the ridges, like a conveyor belt.
As the sea floor moves, so do the
tectonic plates and the continents!
2. Age of the Sea Floor
youngest
rocks
oldest
rocks
Evidence
3. Ocean Trenches
The ocean floor does not
keep on spreading forever!
It sinks into deep underwater
canyons called oceanic trenches.
Here, the oceanic crust sinks down
into the mantle and is destroyed.
The old crust gets recycled in the mantle.
3. Ocean Trenches
oceanic trench
continental
crust
oceanic
crust
mantle
subduction zone
3. Ocean Trenches
Sea Floor Spreading
Objective: Build a model of the ocean floor to explore the movements of the
plates and look at the evidence that suggests the movements the model
demonstrates.
Background- The theory of Continental Drift didn’t really catch on
at first. During World War two Harry Hess had a ship equipped
with sonar. They mapped out the shape of the ocean floor and
found a big surprise. This surprise and other evidence was the
basis for the Theory of Seafloor Spreading proposed by Hess in
1960. The Theory of Seafloor Spreading gave a great deal of
credibility to Wegener’s Theory of Continental Drift. Both theories
are accepted today