While you watch….look for
How did they find the mountain ranges?
Ridge is the longest
chain of mountains
in the world and are
located on the
The Mid-Ocean Ridge system, shown above
snaking its way between the continents, is more
than 56,000 kilometers (35,000 mi) long. It circles
the earth like the stitching on a baseball!
This is a model of sea floor spreading at a divergent
boundary called a mid ocean ridge.
B. Sonar is a device that
bounces sound waves off
underwater objects and
then records the echoes of
these sound waves.
Scientists use sonar to
study the ocean floor.
C. Evidence for Sea-Floor Spreading
The mid-ocean ridge forms along cracks in the
2. At the mid-ocean ridge, molten material rises
from the mantle and erupts. The molten
material then spreads out, pushing older rock to
both sides of the ridge
3. Sea floor spreading is the process by which new
material is continually added to the ocean floor.
4. Several types of evidence is used to support seafloor spreading – magnetic stripes, molten
material, and drilling samples
As the sea floor spreads, the lava cools according to
the magnetic poles at the time. The rocks on the
ocean floor have proved that the earth’s magnetic
field sometimes reverses. The inner core flips and
so the north pole moves to the southern
hemisphere! The earth itself does not flip.
D. Subduction at Deep-Ocean
1. Deep ocean trenches are where the ocean
floor plunges into deep underwater
canyons. They are formed where the
oceanic crust bends toward the mantle.
2. Subduction – process by which the ocean
floor sinks beneath a deep ocean trench
and back into the mantle
3. At deep ocean trenches, subduction allows
part of the ocean floor to sink back into
the mantle, over tens of millions of years.
The PACIFIC ocean
is where most of
the crust is getting
into the Earth.
Here, there are lot
of volcanoes and
known as the Ring
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