|Mapping the Ocean Floor
SOund NAvigation and Ranging or SONAR is used to find and identify objects in water and to find water depth. It is used in water because sound waves taper off less in water than radar (radio) or light waves. Sonar was first used in World War I to detect submarines. Later, in the 1920’s, sonar was used to map deep water areas.
Active sonar emits a pulse of sound into the water. When the sound wave encounters an object, the sound bounces off the object and returns an “echo”. Since the speed of sound in water is known (about 1524 m/sec), and the time it takes for the sound to travel from the ship to the object and back is recorded by the ship, the distance (depth) to the object can be determined. This method of determining depth is known as echo-sounding.
Usually, a ship makes a series of paths across an area. As the ships moves across the area, it continually sends and records sound waves. In this way, many depth readings are collected to use for making a topographic map of the sea floor.
There are two areas used in this investigation. The first is the Mariana Trench located just east of the Mariana Islands in the western Pacific Ocean basin. The trench is about 2,550 km (1,580 mi) long and on average 70 km (40mi) wide. Its deepest part is about 11,000 meters (close to 7 miles). The trench lies along a subduction zone, where the Pacific Plate is being subducted beneath the Phillippine Plate.
The second is the Mid Atlantic Ridge, which is an underwater mountain range found in the Atlantic Ocean. The Ridge sits on a bulge caused by convection currents in the asthenosphere. The range runs from the Arctic Ocean to the Southern tip of Africa. The Mid Atlantic Ridge results from divergent plate boundaries (North American from the Eurasian and South American from the African) where magma rises from the mantle between the plates. The heat from the magma causes the crust on either side of the rift to expand, forming the ridges. The average rate at which the plates move apart is about 2.5 cm per year, a little faster than the rate at which fingernails grow.