Mid-Ocean Ridge Is an undersea mountain chain that is part of a long system of mountains that winds beneath Earth’s oceans.

Sonar Is a device that bounces sound waves off underwater objects and then records the echoes of these sound waves. The time it takes for the echo to arrive indicates the distance to the object

A ship using sonar to map the Ocean’s floor.

Sea Floor Spreading Continually adds new material to the ocean floor.

What is the process of sea-floor spreading? In sea-floor spreading, the sea floor spreads apart along both sides of the mid-ocean ridge as new crust is added. As a result, the ocean floor moves like conveyor belts, carrying the continents along with them.

What is the evidence for sea-floor spreading? Several types of evidence supported the theory of sea-floor spreading: eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves

Evidence from Molten Material discovered by Alvin (a small submarine built to withstand the crushing pressure 4 KM down) In the ridges central valley, Alvin’s crew found rocks shaped like pillows and other rocks looked like toothpaste squeezed from a tube. Such rocks form only when molten material hardens quickly under water

Evidence From Magnetic Stripes Patterns in the rocks provide more support for sea-floor spreading. Earth’s magnetic poles have reversed themselves many times during the Earth’s history. Scientists used instruments to record the magnetic memory of rock on both sides of the ridge.

Evidence From Drilling Samples Final proof came from rock sample obtained through drilling into the ocean floor. After samples were brought up through the pipe, scientists tested the rocks to determine the age. The farther away from the ridge, the older the rock.

Deep-Ocean Trenches A deep valley along the ocean floor beneath which oceanic crust slowly sinks towards the mantle.

What happens at deep ocean trenches? In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep ocean trenches.

Subduction The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle. During Subduction, ocean floor closer to a mid ocean ridge move away from the ridge and closer to the trench. New ocean floor is hot, but as it moves away from the ridge, it cools and becomes more dense.