Sea Floor Spreading Worksheet - Reading Comprehension Worksheets printable pdf download

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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.

Sea Floor Spreading Worksheet - Reading Comprehension Worksheets