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/
seafloorspread.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_animation.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 sharp-ended
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.