Continents on the Move

The world's land masses have not always been in the place where they are today. During the past 700 million years they have moved, broken apart and joined together. The earth's crustal plates include the land masses - continental plates - and the material beneath the oceans - oceanic plates. This movement of continental plates continues today and some plates, such as the Arabian continental plate drifting away from Africa, travel at a rate of 5 centimetres a year. At the margins of these plates there are two types of phenomena; "active" events caused by colliding plates and "passive" events where the plates are spreading apart. Material of an oceanic plate is more dense than that of a continental plate, so that when the two collide the oceanic plate is forced beneath the continental plate. This process, called subduction, can result in the formation of mountain ranges such as the Zagros mountains of Iran and activities associated with this process are deep-seated earthquakes and intense volcanicity, sometimes with catastrophic results.

When the "active" and "passive" margins have been identified, and by using data from the earth's past magnetic fields that have been fossilised within the iron minerals of basalts found at the margins of spreading oceanic plates, the positions of the land masses can be plotted for various parts of geological time. The results from this plotting is called palaeogeography.

• 140 million years ago, the massive continental plate of Africa, the northeastern part of which was to become Arabia, steadily moved northeastwards. This movement was due to sea floor spreading along the mid-Atlantic ridge - a "passive" plate margin.
• 70 million years ago the rocks of ancient Nubia, linking Arabia with Egypt at that time, began to drift apart to form the beginnings of what is now the Red Sea rift. At this time Arabia was still isolated from both Europe and Asia by a seaway named the Tethys that once connected the ancient Mediterranean with the Arabian Gulf and the Indian Ocean.
• 23 million years ago, approximately, the pace of movement of Arabia away from Africa increased by an anticlockwise motion to the northeast that closed the Tethys sea. This event prompted the formation of the Zagros and Taurus Mountains of Iran and Turkey respectively and, in Arabia, most of the volcanicity linked with this movement was confined to the southern Red Sea area, namely the Republic of Yemen and the Kingdom of Saudi Arabia.

A land connection to Africa was still in place in southwestern Arabia; the Ethiopian-Yemen isthmus existed until Pliocene times (about 5 million years ago) when the Red Sea was connected to the Mediterranean but cut off from the Gulf of Aden. This land bridge allowed migrations of terrestrial animals to and from Africa and Asia via Arabia.