|St. John's Island off the Egyptian Red Sea coast|
St. John's Island (also known as Zabargad, Zebirget, Topazios) is the largest of a group of islands in Foul Bay, Red Sea in Southern Egypt. It covers an area of 4.50 square kilometres (approx.).Zabargad is thus a rare spot where the thin "skin" of planet Earth's surface, the crust, has been wounded by massive tectonic plate upheavals and a little of the underlying "flesh", upper mantle, has become visible.
It is not a quaternary volcanic island, but rather is believed to be an upthrusted part of upper mantle material. The nearest island is Rocky Island. The island is slightly north of the Tropic of Cancer, and its highest point is 235 metres.
The island is considered geologically unique as it is uplifted mantle, a fragment of the sub-Red Sea lithosphere. Rocks on the island are mainly lower crustal metamorphic rocks. The island became present above sea level after African and Asiatic continental plates converged to cause rocks in the lower crust to be uplifted.
Today geologists do know about many other locations where the mantle is exposed. By far the largest area is under the Atlantic Ocean where more than thousand square kilometres of the mantle is exposed. This area is so deep over 3 km under water that its exploration is very difficult and expensive. In the New World there is a large region in eastern Canada, the Tableu at Newfoundland. Mantle rocks are at the reach of humans also in several other places.