Geological transformation of Sharjah, United Arab Emirates

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Khursheed Dinshaw (India)

In this article, I will briefly deal with the fascinating and relatively recent geological transformation of the Sharjah region of the United Arab Emirates (UAE). Sharjah needs no introduction in terms of it being a popular tourist destination, especially for families. However, very few know how it was formed and subsequently transformed. In this article, I hope to explain this fascinating aspect of Sharjah.

From the beginning

At the beginning of the Miocene Period, 23 Ma, Arabia finally split from Africa along the Red Sea and the Gulf of Aden became a separate plate. This new plate moved in a northerly direction and collided with, and was subducted under, the Eurasian continent (Fig. 1). The Strait of Hormuz also closed as the remains of the Tethys Ocean formed a rapidly subsiding basin in which thick layers of salt were deposited. Large scale folding and faulting took place in the UAE producing hills of folded rock, such as Jebel Fai’yah and Jebel Hafit.

Fig. 1. Granite from continental drift.

In the eastern part of the UAE, uplift of the Al-Hazar Mountains began. This continued into the Pliocene Period, from 5 to 2 Ma. In the late Miocene and Pliocene, the Sharjah region finally rose above sea level and the landscape we see today was formed.

Fig. 2. Various rock exhibits at the Sharjah Natural History and Botanical Museum.

When the region known as Sharjah rose above sea level, it allowed the area to be covered by the moving sands of the Rub’al Khali Desert. The sands come from rocks worn down and transported by wind, heat and rain along the southern edge of the Arabian Peninsula. When the sea level was lower, most of the Arabian Gulf was dry land. Large sand dunes developed on the floor of the Gulf. Until 8,000 years ago and sand could be blown across from Saudi Arabia by way of Qatar and the Great Pearl Bank area to form the desert in Abu Dhabi. Other important sources included sand deposited beneath the sea. This was blown ashore when the Arabian Gulf dried out. Sand also came from local sources, such as the erosion of Al-Hazar Mountains on Sharjah’s east coast.

Fig. 3. Fossiliferous limestone showing several coiled molluscs known as ammonites.

Climate change

During the last two million years, major changes in the earth’s climate have affected Sharjah. During much of this time, the major control on Sharjah’s climate was the development of glaciers in the northern hemisphere. As the ice sheets grew, regions near the equator became wetter. When the glaciers melted, the warmer equatorial climatic belts expanded northwards and the climate became drier.

However, Arabia’s arid climate has been interrupted by much wetter conditions twice during the last 100,000 years. The first period was between 37,000 and 17,000 years ago, and the second between 9,000 and 6,000 years ago, when lakes formed even in the Rub’al Khali desert. Since then, Arabia’s climate has become steadily drier over the last 6,000 years.

Salt

During the Pliocene Period, Jebel Dhanna, and the islands of Sir Bani Yas and Sir Abu Nu’air were formed as rising domes of salt pushed up the overlying rocks above sea level. When the thick layers of Cambrian salt were put under pressure by Pliocene earth movements, the salt flowed like a liquid and rose to the surface to form a sand dome or diaper as it is known in Arabic.

Along the plains of the Arabian Gulf, coastal salt flats were formed, when the sea level dropped and the waters dried up and left extensive salt flats. Salt flats, which are known as sabkhas in Arabic, also form in the desert when waters from torrential rains pick up salt from the land and form temporary lakes in low areas between sand dunes. When the heat and the sun return after the rains, the water evaporates to leave salt.

Wetter times

Fig. 4. Another view of the ammonites in Fig. 3.

During wetter times in the Pleistocene Period from 1.8 million to 11,700 years ago, rivers carrying rock debris cut deep valleys through the mountains. These rivers continued to cut down into the valley floors until they were at the same level as the sea. As the climate became drier, the rivers dried out and the valleys were left filled with sediments (including the gypsum shown in Fig. 5).

Fig. 5. Gypsum – calcium sulphate.

In the last few thousand years, sea level has fallen and the rivers began to cut channels, known as wadis, into river sediments. Today, the wadis carry water only after heavy rain has fallen in the mountains (Fig. 6).

Fig. 6. Schist from a wadi in Sharjah.

Sharjah and the UAE are located in the north eastern corner of the Arabian plate.

Current times – tectonics

Although surrounded by areas of active earth movements most of the region lies in a quiet segment of the earth’s crust free from earthquakes. To the north of Sharjah, the Arabian and Eurasian plates are colliding. This produces many large earthquakes and has formed the Zagros Mountains of Iran.

Fig. 7. Schist from the crust.

All photographs are by Khursheed Dinshaw.

Acknowledgements

I gratefully acknowledge the support of Sharjah Natural History and Botanical Museum.

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