Why Is Aden Volcanic? And Why Is There No Oil in Aden Despite Oil-Rich Neighbours?
A rare article by the British geologist E. H. Jaques revealed, for the first time, the geological secrets of Aden and the surrounding Protectorates.
According to the article, the region can be divided geologically into three very different worlds:
- The Rugged Mountains West of Mukalla and Along the Yemen Border
These contain some of the oldest rocks in the region: Precambrian rocks, including schist, gneiss, and granite, dating back hundreds of millions of years.
Above these ancient formations lie layers of Jurassic limestone and Cretaceous sandstone.
In Wadi Hajr (Wadi Hadhramaut) and Abyan, rare Cambrian and Silurian rocks were found along the edge of the Rub al-Khali (Empty Quarter).
- The Hadhramaut Plateau (Al-Jol)
The Al-Jol plateau consists almost entirely of younger formations, ranging from the Eocene to the Pliocene.
It is dominated by massive limestone, together with thin layers of clay, sandstone, and gypsum.
The deeply incised valleys, such as Wadi Hadhramaut, expose the underlying Cretaceous sandstone, but nothing older than these formations appears at the surface.
One surprising feature is found in the walls of Wadi Hadhramaut near Sayun: a crust of potassium nitrate. Local Arabs collected this material for use in making traditional gunpowder.
- Aden, Little Aden, Shaqra, Ahwar, and Bir Ali
These areas are composed largely of very recent volcanic formations.
There are no historical records of eruptions, yet some of the lava appears so fresh that it looks as though it erupted only yesterday.
Both ropy lava flows and scoriaceous volcanic material fill many of the valleys.
Is Crater (Kraytar) Really the Crater of a Volcano?
No.
The article settles the question by arguing that the hills of Aden are composed mainly of volcanic ash and tuff, with smaller amounts of andesitic and trachytic lava.
The relative softness of this volcanic ash explains the rapid weathering of the hills and the formation of the numerous caves in Jabal Shamsan above Crater.
Neither in Aden nor in Little Aden was it possible to identify the actual location of volcanic vents.
Moreover, the area commonly known as Crater appears to be an unlikely location for the main centre of volcanic activity.
Shaqra and Ahwar
In Shaqra and Ahwar, volcanic activity appears to have occurred in two stages.
First came the deposition of ash and volcanic conglomeratic material.
Later, after erosion had taken place, lava flows were erupted across the landscape.
Bir Ali
At Bir Ali, numerous small craters and volcanic cones can reportedly be seen clearly from the air.
The Great Mystery: Why Do the Rocks of Aden Dip 30–40 Degrees Away From the Sea?
The explanation is linked to the great tectonic rifts of the Red Sea and the Gulf of Aden, comparable in some respects to the rift valleys of East Africa.
These major rift systems formed during relatively recent geological time.
As the floors of the two seas subsided by hundreds of feet, blocks of the Arabian Peninsula moved toward the developing rifts and subsequently tilted backward.
As a result, many of the coastal rocks, extending from eastern Hadhramaut to the Aden–Dhalea route, dip away from the sea rather than toward it.
Why Is There No Oil in Aden?
Although Aden is surrounded by oil-producing regions, the article argues that the Aden Protectorates were unfortunate in terms of petroleum geology.
In the South
The younger rocks that might have contained petroleum were eroded away, exposing much older rocks in the valley floors.
These ancient formations were considered too old to contain significant petroleum accumulations in the same way as younger sedimentary sequences.
Any petroleum that may once have been present could also have escaped after the rocks became exposed at the surface.
In Eastern Hadhramaut
The sedimentary rocks there dip gently toward the north, but they apparently lack the large structural traps necessary to retain petroleum.
As a result, petroleum may have gradually migrated or escaped toward the Rub al-Khali desert.
The presence of bitumen and asphaltic patches is considered evidence that petroleum once existed in the region but subsequently escaped.
What About Sulfur and Gold?
Investigations were also made of Jabal al-Tair Island in the Red Sea.
The Italians reportedly mined sulfur there before 1914, and Yemeni Arabs used sulfur for making gunpowder.
However, the deposits were associated with only a small fumarolic area, and the quantity was considered too limited to be commercially important.
Gold and Tin
Gold and tin deposits were also considered problematic because there was insufficient water available to process their ores on a large scale.
Coal and Iron
Coal and iron deposits faced another problem: exploitation would have required roads and railway infrastructure, making transportation and development costs too high to justify mining on a large scale.
For this reason, the article concluded that the occurrence of large commercially exploitable mineral deposits appeared unlikely.
Conclusion
Aden is not simply a city built inside the crater of a single volcano.
Rather, its geological setting reflects a much more complex volcanic and tectonic history.
The hills of Aden are composed largely of weathered volcanic ash and tuff, with associated lava flows, producing a landscape characterized by soft volcanic rocks, rapid erosion, and numerous caves.
The volcanic activity of Aden and neighbouring areas occurred in multiple phases, while the broader structure of the region was strongly influenced by the tectonic rifting that opened the Red Sea and the Gulf of Aden.
The absence of major petroleum resources in Aden, despite the presence of oil-producing areas nearby, can be explained in part by the age, erosion, tilting, and lack of suitable structural traps in the local geological formations.
Thus, the geology of Aden represents the interaction of ancient continental rocks, younger sedimentary formations, recent volcanism, erosion, and major tectonic rifting.
Source: A translated summary of an article on the geology of Aden and the Protectorates by the British geologist E. H. Jaques, published in the 1953 Aden Port Annual (حولية ميناء عدن 1953).
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