Editing Geography of Saudi Arabia (section)

==Topography and natural regions==
[[Image:Saudi Arabia Topography.png|thumb|right|Topography of Saudi Arabia (Altitude color coded)]]
[[File:Arabia Saudi physical.jpg|thumb|right|Saudi Arabia physical features]]
[[File:Koppen-Geiger Map v2 SAU 1991–2020.svg|alt=Köppen–Geiger climate classification map at 1-km resolution for Saudi Arabia (1991–2020)|thumb|Köppen–Geiger climate classification map at 1-km resolution for Saudi Arabia (1991–2020)]]
The [[Arabian Peninsula]] is an ancient [[massif]] composed of stable crystalline rock whose geologic structure developed concurrently with the [[Alps]]. Geologic movements caused the entire mass to tilt eastward and the western and southern edges to tilt upward. In the valley created by the fault, called the [[Great Rift Valley|Great Rift]], the Red Sea was formed. The Great Rift runs from the Mediterranean along both sides of the Red Sea south through [[Ethiopia]] and the lake country of [[East Africa]], gradually disappearing in the area of [[Mozambique]], [[Zambia]], and [[Zimbabwe]]. Scientists analyzing photographs taken by United States astronauts on the joint United States-Soviet space mission in July 1975 detected a vast fan-shaped complex of cracks and fault lines extending north and east from the [[Golan Heights]]. These fault lines are believed to be the northern and final portion of the Great Rift and are presumed to be the result of the slow rotation of the Arabian Peninsula counterclockwise in a way that will, in approximately ten million years, close off the Persian Gulf and make it a lake.

On the peninsula, the eastern line of the Great Rift fault is visible in the steep and, in places, high [[escarpment]] that parallels the Red Sea between the Gulf of Aqaba and the [[Gulf of Aden]]. The eastern slope of this escarpment is relatively gentle, dropping to the exposed shield of the ancient landmass that existed before the faulting occurred. A second lower escarpment, the Jabal Tuwayq, runs north to south through the area of [[Riyadh]].

In the south, a coastal plain, the [[Tihamah]], rises gradually from the sea to the mountains. [[Hejaz]] extends southward to the borders of mountainous [[Yemen]]. The central plateau, [[Najd]], extends east to the Jabal Tuwayq and slightly beyond. A long, narrow strip of desert known as Ad Dahna separates Najd from eastern Arabia, which slopes eastward to the sandy coast along the Persian Gulf. North of Najd a larger desert, An Nafud, isolates the heart of the peninsula from the steppes of northern Arabia. South of Najd lies one of the largest sand deserts in the world, the [[Rub al Khali]].

A recent global remote sensing analysis suggested that there were 706km² of [[Mudflat|tidal flats]] in Saudi Arabia, making it the 37th ranked country in terms of tidal flat area.<ref>{{cite journal |last1=Murray |first1=N.J. |last2=Phinn |first2=S.R. |last3=DeWitt |first3=M. |last4=Ferrari |first4=R. |last5=Johnston |first5=R. |last6=Lyons |first6=M.B. |last7=Clinton |first7=N. |last8=Thau |first8=D. |last9=Fuller |first9=R.A. |title=The global distribution and trajectory of tidal flats |journal=Nature |date=2019 |volume=565 |issue=7738 |pages=222–225 |doi=10.1038/s41586-018-0805-8 |pmid=30568300 |s2cid=56481043 |url=https://www.nature.com/articles/s41586-018-0805-8}}</ref>

===Hejaz===

[[Image:Saudi-desert.gif|thumb|right|A view of the [[Tuwaiq]] escarpment from the west.]]
[[Image:Harrat Al Birk, Saudi Arabia.jpg|thumb|[[Harrat al Birk]] volcanic field.]]
The western coastal escarpment can be considered two mountain ranges separated by a gap in the vicinity of [[Mecca]] in Tihamah. The northern range in the Hejaz seldom exceeds 2,100 meters, and the elevation gradually decreases toward the south to about 600 meters. The rugged mountain wall drops abruptly to the sea with only a few intermittent coastal plains. There are virtually no natural harbors along the Red Sea. The western slopes have been stripped of soil by the erosion of infrequent, but turbulent rainfalls that have fertilized the plains to the west. The eastern slopes are less steep and are marked by dry river beds ([[wadi]]s) that trace the courses of ancient rivers and continue to lead the rare rainfalls down to the plains. 

Scattered oases, drawing water from springs and wells in the vicinity of the wadis, permit some agriculture. Of these oases, the largest and most important is [[Medina]]. South of Hejaz, the mountains exceed 2,400 meters in several places with some peaks nearing 3,000 meters. 
The eastern slope of the mountain range in Asir is gentle, melding into a plateau region that drops gradually into the Rub al Khali. Although rainfall is infrequent in this area, several fertile wadis, of which the most important are the Wadi Bishah and the Wadi Tathlith, make oasis agriculture possible on a relatively large scale. 

Extensive lava fields (harrat) scar the surfaces of the plateaus east of the mountain ranges in the Hijaz and give evidence of fairly recent volcanic activity. The largest of these beds is Khaybar, north of Medina; another is [[Al Harrah, Saudi Arabia|Al Harrah]], part of the large volcanic field [[Harrat Ash Shamah]].<ref name="Hamawi, Mu'jam Al-Buldan p. 219">Yaqut al-Hamawi, Mu'jam Al-Buldan, vol. 2, p. 219</ref>

===Tihamah===
The rugged western face of the escarpment drops steeply to the coastal plain, the [[Tihamah]] lowlands, whose width averages 65 kilometers. Along the seacoast is a salty tidal plain of limited agricultural value, backed by potentially rich alluvial plains. The relatively well-watered and fertile upper slopes and the [[Dhorm Mountain|mountains]] behind are extensively [[Terrace (earthworks)|terraced]] to allow maximum land use. This coastal plain is part of the [[Arabian Peninsula coastal fog desert]] [[ecoregion]]. Both Mecca and Jeddah lie within the northern part of Tihamah.<ref name="Hamawi, Mu'jam Al-Buldan p. 219"/>

===Najd===
East of the Hejaz and Asir lies the great plateau area of [[Najd]]. This region is mainly rocky plateau interspersed by small, sandy deserts and isolated mountain clumps. The best known of the mountain groups is the [[Shammar Mountains|Jabal Shammar]], northwest of [[Riyadh]] and just south of the [[Nafud desert|An Nafud]]. This area is the home of the pastoral [[Shammar]] tribes, which under the leadership of the [[Rashidi dynasty|Al Rashid]] were the most implacable foes of the [[House of Saud|Al Saud]] in the late 19th and early 20th centuries. Their capital was the large oasis of [[Ḥaʼil|Ha'il]], now a flourishing urban center.

Across the peninsula as a whole, the plateau slopes toward the east from an elevation of 1,360 meters in the west to 750 meters at its easternmost limit. Wadis cross the region in an eastward direction from the Red Sea escarpment toward the Persian Gulf. There is little pattern to these remains of ancient riverbeds; the most important of them are [[Wadi Hanifa]], [[Wadi ar Rummah]], Wadi as Surr, and [[Wadi ad-Dawasir]].

The heart of Najd is the area of the Jabal Tuwayq, an arc-shaped ridge with a steep west face that rises between 100 and 250 meters above the plateau. Many oases exist in this area, the most important of which are [[Buraydah]], [[Unayzah]], [[Riyadh]], and [[Al Kharj]]. Outside the oasis areas, Najd is sparsely populated. Large salt marshes ([[Sabkha|sabkah]]) are scattered throughout the area.

===Northern Arabia===
[[File:Buraida desertification 02.jpg|thumb|arean with shallow lakes in the Qasim region]]
The area north of the An Nafud is geographically part of the [[Syrian Desert]]. It is an upland plateau scored by numerous wadis, most tending northeastward toward Iraq. This area, known as Badiyat ash Sham and covered with grass and scrub vegetation, is extensively used for pasture by nomadic and seminomadic herders. The most significant feature of the area is the [[Wadi Sirhan|Wadi as Sirhan]], a large basin as much as 300 meters below the surrounding plateau, which is the vestige of an ancient inland sea. For thousands of years, some of the heavily traveled caravan routes between the Mediterranean and the central and southern peninsula have passed through the Wadi as Sirhan. The most important oases in the area are Al Jawf and [[Sakakah]], just north of the An Nafud.

=== Eastern Arabia ===
East of the Ad Dahna lies the As Summen Plateau, about 120 kilometers wide and dropping in elevation from about 400 meters in the west to about 240 meters in the east. The area is generally barren, with a highly eroded surface of ancient river gorges and isolated buttes.

Farther east the terrain changes abruptly to the flat lowlands of the coastal plain. This area, about sixty kilometers wide, is generally featureless and covered with gravel or sand. In the north is the Ad Dibdibah graveled plain and in the south the 'Al Jafurah sand desert, which reaches the gulf near Dhahran and merges with the Rub al Khali at its southern end. The coast is extremely irregular, merging sandy plains, marshes, and salt flats almost imperceptibly with the sea. As a result, the land surface is unstable; in places water rises almost to the surface, and the sea is shallow, with shoals and reefs extending far offshore. Only the construction of long [[Mole (architecture)|moles]] at Ras Tanura has opened the Saudi coast on the gulf to seagoing tankers.

Eastern Arabia is sometimes called [[Al-Ahsa Oasis|'Al-Hasa]] or 'Al Ahsa after the great oasis, one of the more fertile areas of the country. 'Al-Hasa, the largest oasis in the country, actually comprises two neighbouring oases, including the town of Al-[[Hofuf]].

=== Great deserts ===
[[File:JUBBAH 16.jpg|thumb|Highway crossing the Nafud desert]]
Three great deserts isolate the great plateau area Najd of Saudi Arabia from the north, east, and south, as the Red Sea escarpment does from the west. In the north, the [[An Nafud]]—sometimes called the Great Nafud because An Nafud is the term for desert—covers about 55,000 square kilometers at an elevation of about 1,000 meters. Longitudinal dunes—scores of kilometers in length and as much as 90 meters high and separated by valleys as much as 16 kilometers wide—characterize the An Nafud. Iron oxide gives the sand a red tint, particularly when the sun is low. Within the area are several watering places, and winter rains bring up short-lived succulent grasses that permit nomadic herding during the winter and spring.

Stretching more than 125 kilometers south from the An Nafud in a narrow arc is the [[Ad-Dahna Desert]], a narrow band of sand mountains also called the river of sand. Like the An Nafud, its sand tends to be reddish, particularly in the north, where it shares with the An Nafud the longitudinal structure of sand dunes. The Ad Dahna furnishes the Bedouin with winter and spring pasture, although water is scarcer than in the An Nafud.

The southern portion of the Ad Dahna curves westward following the arc of the Jabal Tuwayq. At its southern end, it merges with the [[Rub' al Khali]], one of the truly forbidding sand deserts in the world and, until the 1950s, one of the least explored. The topography of this huge area, covering more than 550,000 square kilometers, is varied. In the west, the elevation is about 600 meters, and the sand is fine and soft; in the east, the elevation drops to about 180 meters, and much of the surface is covered by relatively stable sand sheets and salt flats. In places, particularly in the east, longitudinal sand dunes prevail; elsewhere sand mountains as much as 300 meters in height form complex patterns. Most of the area is totally waterless and uninhabited except for the few wandering Bedouin tribes.