Transcript Sea Floor Spreading Barrows

FQ: How could continents move
apart?
1. Think, pair, share
2. Write your ideas in your scientist
notebook
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
Sonar Animation
Mid-Ocean Ridges
The Mid-ocean Ridge 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.
Rift Valley - A steep sided valley splits
the top of some mid-ocean ridges.
Iceland Rift Valley
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 rift
valley in the center of the ridge. Hardens
and cools to form new sea floor.
Sea Floor Spreading
Animation
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
Animation
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.
Age of Ocean Rocks
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 animation
Subduction zones
Oceanic crust and
Continental crust
Oceanic and Oceanic
Crust
Seafloor Spreading and
Subduction (animation)
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 (animation)
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