Download Mapping the Ocean Floor

Chapter 4: Plate Tectonics
Lesson 2: Development of a Theory
Using an echo sounder, scientists were able to determine the depth of the ocean and create
a topographic map of it in the 1940’s.
These maps revealed vast mountain ranges that stretched for miles on the ocean floor.
Mid-ocean ridges are mountain ranges in the middle of the oceans, and are much longer
than mountains on land.
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By the 1960’s, scientists discovered a new process that helped explain continental drift.
Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean
ridge and older oceanic crust moves away from the ridge.
When the seafloor spreads, the mantle below melts and forms magma.
Magma erupts on Earth’s surface as lava, and crystallizes on the seafloor, forming Basalt.
Because the lava erupts under water, it cools rapidly and forms rounded structures called
pillow lava.
As the seafloor continues to spread, the older oceanic crust moves away from the midocean ridge.
Topography of the Seafloor
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The rugged mountains that make-up mid-ocean ridges form in two different ways:
1. Large amounts of lava can erupt from the center of the ridge, cool, and build-up around
the ridge.
2. As the lava cools and forms new crust, it cracks and the rocks move up or down these
cracks in the seafloor, forming jagged mountain ranges.
Over time, sediment accumulates in the ocean basins. Close to the mid-ocean ridge there is
almost no sediment. Far from the mid-ocean ridge, the layer of sediment becomes thick
enough to make the seafloor smooth.
The smooth part of the seafloor is called the abyssal plain.
Moving Continents Around
Continents move as the seafloor spreads along a mid-ocean ridge.
The first evidence used to support seafloor spreading was discovered by studying the
magnetic signature of the rocks on the seafloor.
Magnetic Reversals
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Normal polarity is a state in which magnetized objects, such as compass needles, will orient
themselves to point north. (Earth today)
Sometimes a magnetic reversal occurs, and the magnetic field reverses directions.
The opposite of normal polarity is reversed polarity- a state in which magnetized objects
would reverse themselves and orient themselves to point south.
Magnetic reversals occur naturally every few hundred thousand to few million years.
Rocks Reveal Magnetic Signature
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Basalt on the seafloor contains iron-rich minerals that are magnetic.
When magnetic minerals in cooling lava crystallize, they record the direction of Earth’s
magnetic field at the time.
Evidence to Support the Theory
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Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to
measure and record the magnetic signature of the rocks.
They discovered parallel magnetic stripes on either side of the mid-ocean ridge.
Each pair of stripes had a similar composition, age, and magnetic character.
The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is
carried away from the center of the ridges in opposite directions.
Other measurements made on the seafloor also confirm seafloor spreading.
Measurements have confirmed that more thermal energy leaves Earth near mid-ocean
ridges than is released from beneath abyssal plains.
Sediment collected from the seafloor can be dated to show that the sediment closest to the
mid-ocean ridge is younger than the sediment farther away from the ridge.
Pillow Lava
Oldest Basalt
Older Basalt
Young Basalt
Mid-ocean
Ridge
Oceanic Crust
Continental Crust
Magma
Asthenosphere