Tethys Ocean: a vanished realm now lying in the mountains.

08/06/2020

The Tethys Ocean was a vast marine realm that no longer exists. Is it all gone? Not quite. There are still remnants of this ocean on Earth today. The Mediterranean Sea, for example, is the last vestige of the Tethys Ocean. Other remains can be found on land and high up in the mountains.

Author: Kathelijne Bonne.

There are five oceans: the Atlantic, Pacific, Indian, Southern and Arctic oceans. In the geological past, the continents lay in different locations, and oceans were different as well. Tethys is the most famous vanished ocean. 

In this article, we briefly explain what an ocean is, when the Tethys ocean existed, which creatures it boasted, and how the existence of the Tethys was first suspected. Let's start with the basics.

What is an ocean?

There are oceans because the outer 'shell' of the earth is fragmented, as a puzzle, with the continents are large pieces, as explained in the article on Plate Tectonics. The oceans separate the continents from each other. The ocean basins are not static but slowly widen. An ocean basin is low-lying depression and is therefore flooded by seawater. But oceans are more than just submerged lowlands. Beneath the water column, on average 4 km deep, lies the oceanic crust, which consists of basalt rock. Basalt is solidified lava that flows out of large cracks in the middle of the oceans and forms 'new' ocean crust when it solidifies. The continental crust, contrarily, consists mainly of granite, a completely different rock. 

Let's get back to the Tethys Ocean. At the start of the Mesozoic, the time span from 250 to 65 million years ago, the southern continents, including Africa, lay together as part of one large supercontinent, Pangaea. It was separated from the Eurasian continent by a very large ocean, the Tethys. 

Strictly speaking, the Tethys Sea was not a single, vast, uninterrupted expanse of water all the time. It was dotted by smaller continents, volcanoes, plateaus and other landforms. Pieces of Italy, Iberia, Corsica and Sardinia, Anatolia, and a few others, lay for some time as islands in the Tethys Ocean. You can see the islands in the above map in the middle of the northern hemisphere.

Marine reptiles in warm subtropical waters

The Equator ran across the Tethys Ocean, and there was an overall warm, pleasant climate, especially in the Cretaceous Period (last part of the Mesozoic). The Tethys Sea teemed with life, which thrived in the warm, subtropical waters. There were many ammonites, sea lilies, bivalves, sea cucumbers and corals. They were part of a food pyramid and were preyed upon by marine reptiles with big, scary beaks, such as the ichthyosaurus (picture below) and the plesiosaurus (title picture). 

Ichthyosaur fossil, note the huge eye sockets. (photo: Viridiflavus / CC BY-SA 3.0)
Ichthyosaur fossil, note the huge eye sockets. (photo: Viridiflavus / CC BY-SA 3.0)

These reptiles patrolled the world's oceans. But there were also fish, including sharks and coelacanths. Like today, a myriad of microorganisms was at the base of the food chain. Their skeletons (little shells) are made of chalk or silica and have good chances of being preserved in the sedimentary layers. When they die, their skeletons slowly settle on the seafloor. There, they form thick layers. One of the greatest examples of such chalk layers are the White Cliffs of Dover. Its white rocks consist almost entirely of the remains of organisms known as cocolithophores. Britain's old name, Albion, refers to the color you see as you approach the island.

Occasionally, remains of larger animals and plants were also preserved, for example, the ichthyosaur (see photo above) that can be found along the famous Jurassic Coast, also in the UK.

And at the same time, marine remains have been found in more bizarre places such as high mountain peaks. Let's zoom in.

White Cliffs of Dover (picture: Immanuel Giel / CC BY-SA 3.0)
White Cliffs of Dover (picture: Immanuel Giel / CC BY-SA 3.0)

Himalayas and Mount Everest

During the Cretaceous Period, some parts of Pangaea (Arabia and India) drifted northward, on a trajectory to collide with Eurasia. The Tethys Ocean became smaller as these continents closed in on Eurasia. Eventually, Arabia and India collided with the great northern continent and formed the vast transcontinental mountain range stretching from the Pyrenees, the Alps and the Zagros Mountains all the way to the Himalayas. The sediments deposited previously on the seafloor of the Tethys are now scattered throughout this huge mountain range and can be found at high altitudes. 

The summit of the world's highest mountain, Mount Everest, is made up of old Tethyan marine rocks. At more than eight kilometers above sea level, you can find trilobites, sea lilies and shellfish that once crawled on a long-gone seabed. 

Mount Everest, brown lines showing the rock layers (picture: Rdevany / GFDL license)
Mount Everest, brown lines showing the rock layers (picture: Rdevany / GFDL license)

In the late nineteenth century, the marine fossils found in mountains were a source of great consternation for scientists, including professor Edward Suess (read about his discoveries in our article on Plate Tectonics). In the nineteenth century, the Earth was still considered static and immobile, and the idea of moving pieces of Earth was for dreamers. Suess was the first to suggest that there had been a sea between Europe and Africa, based on his observations of marine creatures high in the Swiss Alps. He chose an apt name, Tethys, after the Greek goddess of the sea. However, it would take another 100 years before the evidence for the plate tectonics came.

The Tethys was also the home of ancient whales that now lie under the blazing sun in the Sahara.

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Title picture: Elasmosaurus.

Visit the link of WorldOceansDay and think about how you can take action for the benefit of healthier oceans.