When one hears about Arabia, one pictures a desolate land of sand dunes, filled with oil refineries. But the origin of that landscape is not frequently questioned: how did it come to be, and why does it look as it does? The answer to that question lies in the region's tectonic history, its climate, and the weathering that therefore ensues.
One of the persistent topics that must be addressed is the presence of oil on the Arabian peninsula. Before deformation, these oil reserves were estimated to be 2,000km wide, 4,000km long, and 3,000m thick. (Alnaji) Most of these come from carbonates deposited on a continental shelf during the Mesozoic, next to a passive margin. (Alnaji) (A passive margin is a tectonic plate boundary that is neither subducting or colliding. It leads to a flat landscape, upon which quite a lot of sedimentary material can accumulate. (Strickler, M.)) Carbonates form from the skeletons of algae, invertebrate shells, or coralline reefs, or precipitated out of coralline reefs. Common carbonates on the Arabian peninsula are limestone (CaCO3) and dolomite (CaMg(CO3)2). Oil reservoirs are formed in the following fashion: hydrocarbons leak out of sedimentary, organic-rich rocks. Due to their light density, they float to above the interstitial watery sediments around them. The oil rises upward, and, if a layer of impervious rock (called a seal, usually concave from below) lies above the oil, it is contained, forming an “oil trap.” Eventually, most of the water is forced out, leaving behind a reservoir containing oil, and possibly natural gas as well.(Shelton, J.)
To understand the high propensity of oil reservoirs on the peninsula, a look at the geologic history of the area is necessary. During the Precambrian time, a collection of island arcs and small crustal fragments that formed an accretion against a segment of older continental crust, forming the continent Gondwana. This continent was partially covered by glaciers, some reaching as far as Western Arabia, during the Lower Ordovician. (Alnaji) During the Silurian, the glaciers melted, raising the sea level. While the peninsula was underwater, various sediments accumulated and compressed, creating some shale sedimentation. (Alnaji, N.) The peninsula was completely connected to Africa, and formed part of the coastline of a large landmass called Gondwana. (See Fig. 1)
Fig. 1: Gondwana and the Paleo-Tethys ocean can be easily seen. Arabia's present position is outlined, and its Silurian position in Gondwana can be interpolated. (Credit: Scotse,C. )
Eventually, Gondwana rotated and moved northward, where it intercepted Eurmerica and formed Pangea. (Blakey, R., 2006) The pressure of these two continents colliding created the Hercynian orogeny (mountain building event,) during the early Permian. (Fig. 2) This orogeny is not only responsible for several mountain belts, but also a large percentage of the oil reserves found on the peninsula: the compression forced hydrocarbons (many from the Silurian) to move, and also formed some seals over the reservoirs. (Faqira, M.) The Hercynian orogeny contributed to large oil reservoirs by moving hydrocarbons along the edge of the Central Arabian Arch and along faults, and created some new reservoir seals. (Fagira) Another notable aspect of the Carboniferous is the pre-Unayzah Uncomformity (Alnaji) – an unconformity is when deposition is stopped, erosion takes places, and then depostion begins again, leaving a missing section of time. (Shelton, J.) This is important because it shows that deposition had stopped during this portion of time, and erosion was instead occurring.
Fig. 2: By the Early Permian, Pangaea has formed. The Hercynian orogeny is centered. Despite its distance from the actual orogeny, the collision affected the Arabian peninsula greatly. (Credit: Blakey, R.)
Next, during the Middle Jurassic, Pangea broke up into the continents we now know today. (Fig. 3) The Arabian peninsula entered a tectonically stable time period during the Jurassic, and developed a continental shelf near the Neo-Tethys Sea, right next to a passive margin. In addition, at this time several intrashelf basins, including the Gotnia, South Rub' AlKhali, and Arabian Basins, formed, as a result of tectonic differentiation and rising sea level. These basins accumulated a lot of organic-rick rocks during the Late Callovian, when the peninsula was inundated by an oxygen-poor ocean.(Alnaji)
Fig. 3: During the Middle Jurassic, Pangaea has begun to split into its constituent parts. Africa is beginning to rotate, and will soon intercept Asia. (Credit: Blakey, R.)
During the Late Cretaceous, the Neo-Tethys sea was closed due to further tectonic action. Also, at this time, the pre-Aruma Unconformity was created, showing another period of stopped sedimentation. This action also remade the Hercynian Orogeny features and formed the major oil reserves we now use. (Alnaji)
Another orogeny, the Zagros orogeny, happened during the Tertiary when Asia and Arabia were thrust together. When this happened, the Arabian plate was subducted under Iran, where it still lies with one corner underneath. (Alnaji) This series of long, tectonically inactive times and brief periods of compression served to deposit, manipulate, and contain hydrocarbons, leading to major oil reservoirs throughout the peninsula.
The Arabian Peninsula may contain the Arabian Desert: one of the largest deserts on the planet, with an approximate area of one million mi2(Geology.com), however, unlike what one might suppose, it is not entirely sand dunes and desolation. The peninsula is composed of a plateau, sloping north-east from the Red Sea to the eastern lowlands by the Persian Gulf, and its elevation ranges from 37m below sea level, to 3,660m above sea level.(De Pauw, E.) As a result, it has a variety of localized climates within its overall climate, which is very arid. This is mainly because of the large distance between it and major weather systems (like the North Atlantic depression,) and its propensity towards receiving continental air from Africa and China during the winter and summer. (De Pauw, E.) Precipitation (and therefore vegetation) is very sparse and patchy in general, and greatly affected by the terrain.
As the peninsula is located in the Northern Hemisphere, its winter occurs at the same time as winter in North America, Europe, and Asia. The coldest time of the year is between December and Feburary, (De Pauw, E.) when the earth rotates the Northern Hemisphere away from the sun. In addition, during the winter polar continental air blows down from Central Asia, resulting in lowered temperatures, clear skies, and dry weather. “Lowered” temperatures are, of course, relative – average winter temperatures across the peninsula range from 41 to 81.5 degrees Fahrenheit. (Fig. 4) Any winter precipitation comes from moist polar maritime air that moved through North Africa and the Mediterranean. (De Pauw, E.) Snow has occasionally fallen in the Yemeni and Asir highlands, as a result of their increased altitude.
Fig.4: Mean temperatures during the coldest and warmest months of the year. (Credit: De Pauw, E.)
Springtime is when the majority of precipitation falls in the Arabian Peninsula. This is due to the Indian Monsoon's influence: (De Pauw, E.) as the earth rotates around the sun on its tipped axis, the area closest to the sun moves from north to south, which changes weather patterns, and brings some areas of the world – including India – large amounts of unusual precipitation. Some of this precipitation works its way over to the Arabian peninsula, but it is mitigated by the tropical continental area present on the peninsula at that time.
During the summer, which occurs between June and September, average temperatures across the peninsula range from 72.5 to 104 degrees Fahrenheit. (Fig. 4) In addition to the standard temperature increase brought on by the Earth's rotation, tropical continental air blows in from Africa, creating a stable high pressure area. This brings very hot, very dry air, clear skies, and low humidity. (De Pauw, E.) It is this time of year that generates the standard mental picture of the Middle East: deathly hot.
Precipitation on the peninsula is scarce, overall, and varies highly between years. (The Biome Group) Amount of precipitation correlates closely with the elevation, and sometimes the Yemen and Asir highlands, and the Hajar mountains generate their own weather systems, including frequent instances of fog. These mountains also guide the precipitation and wind around the peninsula. (De Pauw, E.) The evaporation rate exceeds the precipitation rate, perpetuating the arid climate. (The Biome Group)
The precipitation is directly the cause of the sparse and patchy vegetation patterns on the peninsula. Areas where water collect, such as wadis (ephemeral stream beds, where water is only present during periods of intense precipitation, frequently canyon-like,) support a greater number of plants more effectively than the rest of the desert. (De Pauw, E.) Vegetation patterns depends more specifically on the frequency of flooding, stream velocity, sediment type, and the local variability of rainfall.
Soil forms very slowly, as most water sluffs right off the exposed bedrock or drains through the limited soil. Without any plants to hold the little soil together, there is much erosion. (Stoffer, P.) Any soils that do form are coarse, shallow, and rocky, with good drainage. Small particulates are blown away, leaving only the larger pieces behind. (The Biome Group)
Since the peninsula is a very dry area, chemical breakdown (i.e. the dissolution of minerals from water) is very low. However, mechanical breakdown (i.e. the physical subdivision of large rocks into smaller rocks) is very high. This is a result of the heating and cooling of rocks, roots forcing their way through cracks, wind, precipitation, and any ice wedging that might occur. (Stoffer, P.) Different types of rock have varying resistances to erosion, creating differential erosion and carving curious shapes into the rocks, such as pillars, ledges, etc. (Taylor, S.)
Erosion through precipitation is intense. Frequently, rain drops in still air strike the ground with a velocity of 30ft/s, and, by the time one inch of rain has fallen, the ground will have been hit by a total mass of 113 tons. (Shelton, J.) Erosion consists frequently on a small-scale – sand thrown up by the impact of rain, small rocks jostled. (Shelton, J.) However, large-scale erosion also happens - if the slopes become saturated with rain, rock falls or landslides will occur. (Stoffer, P.) This material will be moved along in a debris flow, consisting of a large amount of rock, plant material, etc., held together with just enough water to keep it moving. (Stoffer, P.) A flash flood usually consists of a greater percentage of water in the debris flow. In a flash flood, rocks are smashed together, decreasing their surface area, and increasing the rate of chemical breakdown. (Stoffer, P.) (This is because, with a greater surface area, there are more exposed atoms to be dissolved by the water.) In general, the rocks become smaller and more rounded the farther they are away from their source. (Stoffer, P.) When this material exits a canyon, it spreads out significantly, creating what is called an alluvial fan. (Shelton, J.)
Aeolian processes (wind erosion) does not erode a desert landscape nearly as much as one would think, however it does play a part. Small particles of rock or sand are moved through the air or along the ground, in a process called deflation. (Taylor, S.) Frequently, this material slams into other rocks and “sand blasts” them, or abrades them, further increasing the amount of material being carried by the wind. (Taylor, S.) If there is a strong wind, and it carries a lot of material, it can become a sand or dust storm. These storms can cover several countries, as seen in Fig. 5.
Fig. 5: A dust storm crossing Iraq, Kuwait, Saudi Arabia, and Iran in 2003. Dust storms differ from sand storms in that their particulate size is smaller, meaning it can be blown farther and spread more widely. (Shelton, J.) (Credit: NASA Earth Observatory.)
Looking back through the history of the Arabian peninsula, it is possible to see how a series of depositional and compressional periods contributed to the formation of large oil reservoirs, and the general topology of the region. If the same material was placed in a more temperate location, the equivalent of the American South would have occurred. Instead, the region's tectonic activity has placed it in its current position, resulting in a highly arid climate. The scorching temperatures and lack of precipitation have resulted in a carved landscape, complete with sand dunes, wadis, and dust storms. This combination of factors has resulted in one of the hottest, sandiest, and most desolate places on the globe.
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