Stephanie Lauder, age 16, of Great Falls, Mont., for her question:
HOW DID THE MEDITERRANEAN SEA FORM?
The Mediterranean Sea is one of the world's chief trade routes. It is about 2,200 miles long and has Europe to the north, Asia to the east and Africa to the south. Some earth scientists believe that the Mediterranean Sea was formed by what they call the desert theory.
These scientists say that the Mediterranean may have dried up about a dozen times between 7.5 million and 5.5 million years ago. During this period, the movement of the European and African continents repeatedly closed and reopened the Strait of Gibraltar.
Each time the Strait of Gibraltar was closed, the Mediterranean Sea began to dry up. After about 1,000 years of evaporation, only a large desert remained. A few salty lakes, like the Great Salt Lake in Utah, dotted this desert.
The earth scientists say that when the strait reopened, water from the Atlantic rushed in, forming a huge waterfall. The waterfall carried about 1,000 times as much water as does Niagara Falls, and it refilled the Mediterranean Sea in about 100 years.
In 1970, scientists found evidence that supported the desert theory. That year, geologists aboard a research vessel called the Glomar Challenger drilled cores of rock from the Mediterranean Sea floor. The cores contained minerals, known as evaporites, that are formed by the evaporation of salty water.
Some scientists explain the formation of the Mediterranean Sea by the theory of plate tectonics. They say that over 200 million years ago, the continents formed a single land mass called Pangaea. The Tethys Sea, a huge bay that developed into the Mediterranean, indented the east coast of Pangaea.
Through the centuries, the theory says, Pangaea broke up into continents that began to drift slowly toward their present location. As they drifted, Africa turned counterclockwise and Eurasia turned clockwise. Their movement opened a waterway at the western end of the sea.
The plate tectonics theory explains that about 65 million years ago, the rotation of Africa and Eurasia had almost closed the eastern end of the Tethys Sea. The sea thus acquired its present shape.
The Latin word "mediterranean" means "in the middle of land."
The Mediterranean Sea has been important since ancient times. Many early civilizations, including those of Egypt, Greece, Phoenicia and Rome, developed along its shores.
The Mediterranean Sea has an average depth of 4,926 feet. It reaches its greatest depth, 16,302 feet, in a depression called the Hellenic Trough that lies between Italy and Greece.
Earthquakes occur frequently throughout the Mediterranean region, especially in Greece and western Turkey. Volcanic action formed many of the islands in the sea. A few volcanoes in the region still erupt. They Include Mount Etna, Stromboli and Vesuvius.