The Tethys Ocean (Greek: Τηθύς) was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.
About 250 million years ago, during the Triassic, a new ocean began forming in the southern end of the Paleo-Tethys Ocean. A rift formed along the northern continental shelf of Southern Pangaea (Gondwana). Over the next 60 million years, that piece of shelf, known as Cimmeria, traveled north, pushing the floor of the Paleo-Tethys Ocean under the eastern end of Northern Pangaea (Laurasia). The Tethys Ocean formed between Cimmeria and Gondwana, directly over where the Paleo-Tethys used to be.
During the Jurassic Period (150 Ma), Cimmeria finally collided with Laurasia. There it stalled, the ocean floor behind it buckling under, forming the Tethyan Trench. Water levels rose and the western Tethys came to shallowly cover significant portions of Europe, forming the (first) Tethys Sea. Around the same time, Laurasia and Gondwana began drifting apart, opening an extension of the Tethys Sea between them that today is the part of the Atlantic Ocean between the Mediterranean and Caribbean. As North and South America were still attached to the rest of Laurasia and Gondwana, respectively, the Tethys Ocean in its widest extension was part of a continuous oceanic belt running around the Earth between about latitude 30° N and the Equator. Thus, ocean currents at that time—around the Early Cretaceous—ran radically differently from the way they do today.
Between the Jurassic and the Late Cretaceous (which started about 100 Ma), even Gondwana began breaking up, pushing Africa and India north, across the Tethys and opening up the Indian Ocean. As these land masses pushed in on it from all sides, up until as recently as the Late Miocene (15 Ma), the Tethys ocean continued to shrink, becoming the Tethys Seaway or (second) Tethys Sea.
Today, India, Indonesia, and the Indian Ocean cover the area once occupied by the Tethys Ocean, and Turkey, Iraq, and Tibet sit on Cimmeria. What was once the Tethys Sea has become the Mediterranean Sea. Other remnants are the Black, Caspian, and Aral Seas (via a former inland branch known as the Paratethys). Most of the floor of the Tethys Ocean disappeared under Cimmeria and Laurasia. Geologists like Suess have found fossils of ocean creatures in rocks in the Himalayas, indicating that those rocks were once underwater, before the Indian continental shelf began pushing upward as it smashed into Cimmeria. We can see similar geologic evidence in the Alpine orogeny of Europe, where the movement of the African plate raised the Alps.
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