"Sea Floor Spreading Worksheet"

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Name ____________________________________________________________
Period _____
Sea-Floor Spreading
The longest chain of mountains in the world is the system of mid-ocean ridges. In the
mid-1900s, scientists mapped the mid-ocean ridges using sonar. Sonar is a device that bounces
sound waves off underwater objects and then records the echoes of these sound waves. The mid-
ocean ridges curve along the sea floor, extending into all of Earth’s oceans. Most of the
mountains in the mid-ocean ridges lie hidden under hundreds of meters of water. A steep-sided
valley splits the top of some mid-ocean ridges. The Earth’s ocean floors move like conveyor
belts, carrying the continents along with them, as they move. This movement begins at a mid-
ocean ridge. A ridge forms along a crack in the oceanic crust. At a 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. As the molten material cools, it forms a strip of solid rock in the
center of the ridge. Then more molten material splits apart the strip of solid rock that formed
before, pushing it aside. This process, called sea-floor spreading, continually adds new material to
the ocean floor.
Scientists have found strange rocks shaped like pillows in the central valley of mid-ocean
ridges. Such rocks can form only if molten material hardens quickly after erupting under water.
The presence of these rocks supports the theory of sea-floor spreading. More support came when
scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized
“stripes”. The pattern is the same on both sides of the ridge. These stripes hold a record of
reversals in Earth’s magnetic field. The final proof of sea-floor spreading came from rock samples
obtained by drilling into the ocean floor. Scientists found that the farther from a ridge the rocks
were taken, the older they were. The ocean floor does not just keep spreading. Instead, it sinks
beneath deep underwater canyons called deep-ocean trenches. Where there are trenches,
subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep-
ocean trench and back into the mantle. At deep-ocean trenches, subduction allows part of the
ocean floor to sink back into the mantle, over tens of millions of years.
The processes of subduction and sea-floor spreading can change the size and shape of the
oceans. Because of these processes, the ocean floor is renewed about every 200 million years. The
Pacific Ocean is shrinking. Its many trenches are swallowing more ocean crust than the mid-
ocean ridge is producing. The Atlantic Ocean is expanding. In most places, the oceanic crust of
the Atlantic Ocean is attached to continental crust. As the Atlantic’s floor spreads, the continents
along its edges also move.
Name ____________________________________________________________
Period _____
Sea-Floor Spreading
The longest chain of mountains in the world is the system of mid-ocean ridges. In the
mid-1900s, scientists mapped the mid-ocean ridges using sonar. Sonar is a device that bounces
sound waves off underwater objects and then records the echoes of these sound waves. The mid-
ocean ridges curve along the sea floor, extending into all of Earth’s oceans. Most of the
mountains in the mid-ocean ridges lie hidden under hundreds of meters of water. A steep-sided
valley splits the top of some mid-ocean ridges. The Earth’s ocean floors move like conveyor
belts, carrying the continents along with them, as they move. This movement begins at a mid-
ocean ridge. A ridge forms along a crack in the oceanic crust. At a 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. As the molten material cools, it forms a strip of solid rock in the
center of the ridge. Then more molten material splits apart the strip of solid rock that formed
before, pushing it aside. This process, called sea-floor spreading, continually adds new material to
the ocean floor.
Scientists have found strange rocks shaped like pillows in the central valley of mid-ocean
ridges. Such rocks can form only if molten material hardens quickly after erupting under water.
The presence of these rocks supports the theory of sea-floor spreading. More support came when
scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized
“stripes”. The pattern is the same on both sides of the ridge. These stripes hold a record of
reversals in Earth’s magnetic field. The final proof of sea-floor spreading came from rock samples
obtained by drilling into the ocean floor. Scientists found that the farther from a ridge the rocks
were taken, the older they were. The ocean floor does not just keep spreading. Instead, it sinks
beneath deep underwater canyons called deep-ocean trenches. Where there are trenches,
subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep-
ocean trench and back into the mantle. At deep-ocean trenches, subduction allows part of the
ocean floor to sink back into the mantle, over tens of millions of years.
The processes of subduction and sea-floor spreading can change the size and shape of the
oceans. Because of these processes, the ocean floor is renewed about every 200 million years. The
Pacific Ocean is shrinking. Its many trenches are swallowing more ocean crust than the mid-
ocean ridge is producing. The Atlantic Ocean is expanding. In most places, the oceanic crust of
the Atlantic Ocean is attached to continental crust. As the Atlantic’s floor spreads, the continents
along its edges also move.
Name ____________________________________________________________
Period _____
Questions:
1. Name and describe the feature of the ocean floor shown at A.
2. Describe the process shown occurring at B, and explain what results from this.
3. What happens to old oceanic crust as new molten material rises from the mantle?
4. The arrows on the figure show the ocean floor spreading from the ridge. What are three kinds
of evidence scientists have found to support this idea?
1.
2.
3.
5. What process is shown occurring at C, and why does it occur?
6. A device that scientists use to map the ocean floor is ___________________________________________.
7. The feature on the ocean floor at C is called a(n) _______________________________________________.
8. The process that continually adds new material to the ocean floor is called ___________________
_________________________________ _____________________________________________________.
9. The process by which the ocean floor sinks into the mantle is called _______________________________.
10. A chain of underwater mountains along which sea-floor spreading occurs is a
_____________________ _________________________________________ _____________________________________.
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