Transcript Earth`s Spreading Ocean Floor

Sea-Floor Spreading
Key Concepts
What is the process of sea-floor
spreading?
What is the evidence for sea-floor
spreading?
What happens at deep-ocean trenches?
Key Terms
Mid-ocean ridge
Sonar
Sea-floor spreading
Deep-ocean trench
subduction
Mapping the Ocean Floor
Sonar was used in the mid 1900’s to
map the ocean floor
Sonar bounces sound waves off
objects and records the echoes of
these waves.
Time it takes for echo to come back
indicates the distance from the
object
Sonar discovered the mid-ocean
ridges
Mid-Ocean Ridges
Mid-ocean ridge system is a chain of
underwater mountains that extend
into all of Earth’s oceans.
They are more than 50,000 km long
Most are under hundreds of meters
of water
They reach the surface in a few
places such as Iceland.
A steep sided valley splits the top of
some mid-ocean ridges.
MID-OCEAN RIDGES
Mid- Ocean Ridges
Sea Floor Spreading
In 1960 Harry Hess, an American
geologist, suggested that the sea floor
spread apart along both sides of a midocean ridge as new crust is added.
As a result the ocean floors move like
conveyor belts carrying continents with
them.
He called this sea-floor spreading
Molten material erupts through the valley
in the center of the ridge. Hardens and
cools to form new sea floor.
Sea Floor Spreading
Evidence for sea floor spreading
From Molten Material
– Alvin, The world’s first deep ocean
submersible used for deep sea
exploration, found strange rocks shaped
like pillows in the mid-ocean ridge. This
was evidence that molten material came
out of the ridge and hardened quickly
Bulbous Pillow Lava
Evidence from Magnetic
Stripes
Scientists studied the rock patterns in the
ocean floor .
They found that the rock on the ocean
floor lies in a pattern of magnetized
stripes
Magnetic stripes in ocean floor rocks are
formed by the reversal of the Earth’s
magnetic poles.
The last reversal happened 780,000 years
ago.
The pattern of stripes matches on each
side of the ocean ridge.
Magnetic stripes
Evidence from drilling samples
Glomar Challenger gathered samples of
rocks by drilling into the ocean floor
Drilled through 6 km of water into ocean
floor
Scientists then determined the age of the
rocks
They found that the rocks next to ocean
ridges are younger than rocks farther
away.
As the ocean floor spreads the older rocks
move farther away.
Glomar Challenger
Is the Earth’s surface
getting larger from sea-floor
spreading?
Subduction at Trenches
Deep ocean trenches are deep
underwater canyons
In a process that takes tens of
millions of years, the ocean floor
sinks back into the mantle at deep
ocean trenches.
Subduction is the process where
the ocean floor sinks beneath a
deep ocean trench.
Subduction zones
Oceanic crust and
Continental crust
Oceanic and Oceanic
Crust
The Process of Subduction
Sea-floor spreading and subduction
work together like a conveyor belt.
New oceanic crust is hot.
As it moves away from the ridge it
cools, and becomes more dense
At a trench, the older, more dense
crust is pulled by gravity and sinks
down beneath the trench
Sea floor spreading and
subduction
Subduction and the Earth’s
Oceans
Sea-Floor spreading and subduction can
change the size and shape of the oceans.
Ocean floor is renewed every 200 million
years – the time it takes for the floor to
travel from ridge to trench.
The Pacific Ocean is shrinking. More crust
is being subducted than is being formed.
The Atlantic Ocean is expanding. More
crust is being formed than is being
subducted.
References
http://www.whoi.edu/page.do?pid=8422
http://oceanridge.ldeo.columbia.edu/courses/subgeol/
mid_ocean_landscape.html
http://oceansjsu.com/105/exped_commoti
on/8.html