Name___________________________________________ Date_________________
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.
