Bellringer
What three specific types of evidence did Wegener
study that led to him creating the idea that the
continents had moved?
Sea-Floor Spreading
Notes
Mid-Ocean Ridges
• Since the mid-1900s, scientists have been using
sonar to study the ocean floor. Sonar is a device
that bounces sound waves off underwater
objects. The longer it takes the sound waves to
bounce back, the farther away the objects are.
• Using sonar, scientists found long mountain
ranges on the ocean floor. They called these
mid-ocean ridges because they run through the
middle of all oceans.
Mid-Ocean Ridges
• In a few places, mid-ocean ridges poke
above the surface and form islands.
Iceland is the top of a mid-ocean ridge
in the North Atlantic Ocean.
What is Sea-Floor
Spreading?
• Sea-Floor Spreading is a process that
slowly adds new rock to the ocean
floors. Scientist Harry Hess came up
with the idea of sea-floor spreading in
1960.
Sea-Floor Spreading
• Here’s how sea-floor spreading works. In the
center of a mid-ocean ridge, melted rock
pushes up through cracks in the ocean floor.
The melted rock pushes older, solid, more
dense rock away from both sides of the ridge.
The melted rock cools and forms new solid
rock at the center of the ridge, called a rift
valley.
http://education.sdsc.edu/optiputer/flash/seafloorsprea
d.htm
Sea-Floor Spreading
• This process keeps repeating. Slowly,
the ocean floor is pushed farther away
from both sides of the mid-ocean ridge.
At the same time, new rock keeps
adding to the ocean floor in the rift
valley.
• As a result, the ocean floors move like
conveyor belts, carrying the continents
along with them.
http://www.mysciencebox.org/files/images/seafloor_ani
mation.gif
Evidence for Sea-Floor
Spreading
• In the 1960s, scientists began to look for and
uncover evidence supporting sea-floor
spreading.
• Using a submarine named Alvin, scientists were
able to look into a rift valley and examine
something called pillow lava, which is a special
type of solid rock that only forms on the ocean
floor when magma cools very rapidly. This
proved that new molten material was being
added to the ocean floor at these ridges.
Evidence for Sea-Floor
Spreading
• As you already know, convection in Earth’s
outer core causes the Earth to have a
magnetic field. Well, occasionally this field
flips and the south pole and north pole switch
polarities. The last time this happened was
about 780,000 years ago.
• Measuring the magnetic iron “stripes” on both
sides of a mid-ocean ridge, scientists
discovered that on both sides of the ridge,
there are matching “stripes” that flip direction
each time the poles reverse.
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_
paleomag.html
Click the “Normal” button in the upper-right to flip the
polarity of Earth’s magnetic field.
Evidence for Sea-Floor
Spreading
• In 1968, the Glomar Challenger used special
equipment to drill into the ocean floor six
kilometers beneath the ocean’s surface.
• This feat has been compared to using a sharpended wire to dig a hole into a sidewalk from the
top of the Empire State Building.
• They successfully brought up rock samples from
the ocean floor. After analyzing the rocks’ ages,
they were able to determine that the rocks got
older the farther away they moved from the ridge.
Subduction at
Trenches
• Sea-floor spreading makes the ocean floors get
wider. New rock keeps forming at mid-ocean
ridges. Old rock keeps getting pushed farther
away from both sides of the ridges.
• After millions of years, the oldest rocks will begin
to form ocean trenches. At a trench, the dense
basalt of the oceanic crust is forced to bend
downward and begins to sink into the mantle.
This process is known as subduction.
•
•
•
Subduction at
Trenches
Sea-floor spreading and subduction work together to
create a balance in the amount of ocean that covers
the planet.
In the Atlantic Ocean, very little subduction occurs,
meaning that the Atlantic Ocean is continuously getting
larger at the Mid-Atlantic Ridge.
Conversely, there is a massive amount of subduction
in the Pacific Ocean on the western coast of Asia and
the eastern coast of the Americas. This means that the
Pacific Ocean (currently the largest ocean on the
planet) is shrinking.