Restless Earth - Formative Forces & Processes
Formative Forces & Processes
From its earliest beginnings, the physical geography of Abu Dhabi displays the consequences of plate tectonics as a fundamental formative process. Although the movements of the Earth's plates are incredibly slow, the sheer scale and massive forces involved over millions of years have come together to form the geological foundations and dramatic features of the landscape we see today. Plate tectonics can be viewed as a ‘mega formative' process resulting in large scale features such as mountain ranges, rift valleys and basins, fault escarpments and visibly folded, warped and faulted surface and sub surface rock formations. These physical features are not static; rather they are dynamic and continually changing over time. They are not only subjected to additional formative forces but influence the processes themselves, which, in turn, continue to reshape the landscape and constituent landforms of Abu Dhabi and of the rest of the region. While these continual and relative Earth movements generally go largely unnoticed, except by scientists, and only involve millimetres per year, there remains, though, the potential for bigger dislocations. The Arabian Plate, on which AbuDhabi is located, is gradually being subducted (moving underneath) the Eurasian Plate in the area of the Zagros Mountains in southern Iran. Sudden jolts cause small scale earthquakes, such as one which caused the evacuation of some tall buildings in Abu Dhabi City in 2008, and much more destructive earthquakes remain an ever present hazard across the region, especially in Iran. These sudden and violent movements can cause changes to the natural landscape, as well as damage to man-made structures, including bridges and buildings.
The Consequences of Collision
The movement and collision of the Earth's plates have had profound impacts on the physical geography of Abu Dhabi and the wider region. As noted earlier, during the Cretaceous, the South Atlantic oceanic basin formed, forcing the Afro-Arabia Plate north eastwards. By the Late Cretaceous (about 100 70 million years ago) this movement was so rapid that it overwhelmed the capacity of subduction to effectively swallow the plate margins in a process whereby denser oceanic crust sinks below lighter continent crust.
Consequently, the oceanic crust and overlying sediments of the ancient Tethys Sea were thrust upwards onto the north east margins of the Arabian Plate to form the unique geological structures of the Semail Ophiolite and the underlying Hawasina Nappes. The collision between the smaller Arabian and massive Eurasian Plates buckled and uplifted the plate margins to form the Zagros Mountains.
More recently, in the Oligo-Miocene, the opening of the Gulf of Aden and Red Sea split the Arabian Plate away from the African Plate, continuing its journey alone. The forces associated with this resulted in further uplift along the north-eastern margin to produce the Hajar Mountains, which are still rising, depending on location, at 2–6 millimetres per year.
This movement received a powerful push in a north-easterly direction about four to five million years ago. Opposing forces down-warped the Gulf area, although it is unclear when it was first flooded by ocean waters. What is certain is that the Gulf has been subjected to cyclical changes in sea level for at least the past 350,000 years and probably longer.
This caused alternating flooding and drying episodes, creating the unique landforms of the sabkhas and extensive dune fields blown southwards by winds like the shamal (northerly wind). While the geological building blocks of the Emirate have remained fairly stable for the past 500million years, powerful tectonic forces continue to re shape the geology and landscape.
Today, the Arabian Plate with Abu Dhabi as its ‘passenger' continues its ancient journey northwards relative to the African Plate at 5–14 millimetres per year and to Eurasia at 27 millimetres per year.
The Geology of Jeopardy
Tectonic movements apply immense forces to sub-surface rocks. Under such stresses, rocks either break and fault or fold. The type and degree of response is controlled by the nature of the stress and the characteristics of the rock. Faults may then become the focus for earthquakes. The ongoing compression of the Zagros Range causes numerous relatively weak earthquakes.
Occasional large movements along vertical linear faults can be accompanied by devastating earthquakes. One such fault lies beneath Dibba on the UAE East Coast. Abu Dhabi has no known major faults although those flanking Jebel Hafit and underlying sabkha Matti have been associated with moderate earthquakes.
What Lies Beneath? - Ophiolite Formation
Ophiolites are rare sequences of rocks where a section of the Earth's upper mantle, oceanic crust and overlying sediments have been thrust upwards by tectonic forces. The Hajar Mountains include the world's largest and best exposed ophiolite complex which can be seen outcropping between Dhaid and Dibba. It provides a unique opportunity to examine rocks from deep within the Earth and to understand plate tectonics and related processes.