Transcript Sea-Floor Spreading

Sea-Floor Spreading
Sea-Floor Spreading
Tubeworms have
no mouth, eyes,
or stomach.
These bacteria
convert the
chemicals that
shoot out of the
deep sea vents
into food for
the worm.
What do they have to do with sea-floor spreading?
1. Mid-Ocean Ridge – the longest chain of
underwater mountains in the world
How did scientist discovered them?
Sonar
• A device that bounces sound waves off underwater
objects and then records the echoes of these sound
waves.
Who mapped the ocean floor?
Harry Hess
• An American
•
geologist who studied
mid-ocean ridges.
He suggested that the
ocean floors move
like conveyor belts,
carrying the
continents along with
them.
Conveyor belt
What is seafloor spreading?
Sea-floor spreading
• The process that continually adds new material to the
•
ocean floor.
What’s the evidence?
• Evidence: molten material, magnetic stripes, and
drilling samples.
How does it happen?
1. At the mid-ocean ridge, molten material
rises from the mantle and erupts.
2. The molten material then spreads out,
pushing older rock to both sides of the
ridge.
3. As the molten material cools, it forms a
strip of solid rock in the center of the
ridge.
4. Then more molten material flows into the
crack.
Let’s see it in action!
Evidence #1 - Molten Material
• The submersible,
Alvin, found strange
rocks shaped like
pillows or like
toothpaste squeezed
from a tube. Such
rocks can form only
when molten material
hardens quickly after
erupting under water.
Evidence from molten material
Rocks shaped like
pillows (rock
pillows) show that
molten material
has erupted again
and again from
cracks along the
mid-ocean ridge
and cooled quickly
Evidence #2 - Magnetic Stripes
compass
Scientists
discovered that the
rocks that make up
the ocean floor lie in
a pattern of
magnetized
“stripes”. They hold
a record of
reversals in Earth’s
magnetic field.
Evidence #3 - Drilling Samples
• The Glomar Challenger
did a drilling sample
and found that the
older rocks were the
ones farther away
from the center. The
younger ones were in
the center of the ridge.
Stop Here
Summarize
Subduction at Deep-Ocean
Trenches
• Wider & wider? Deep•
•
ocean trenches
Ocean floor plunges into
deep underwater
canyons are deep-ocean
trenches.
Subduction is the process
by which the ocean floor
sinks beneath a deepocean trench and back
into the mantle.
Subduction
• At deep-ocean trenches, subduction
allows part of the ocean floor to sink back
into mantle, over tens of millions of years.
• Subduction and Earth’s Oceans
• Earth’s ocean floor is renewed about every
200 million years.
Subduction in the Pacific &
Atlantic
• Deep ocean trenches are swallowing more
oceanic crust than the mid-ocean ridge
can produce. Thus, the width of the
Pacific will shrink.
• The Atlantic is expanding. It has short
trenches. In some places, the oceanic
crust is attached to the continental crust
which moves the continents.