Is Rub' al Khali the largest sand desert in the world?

Question

This sentence appears in the Wikipedia article on Rub' al Khali.

The Rub' al Khali desert [note 1] (Arabic: الربع الخالي‎, i.e., "the Empty Quarter") is the largest continuous sand desert (erg) in the world (...)

I was wondering how this sentence can be true, especially when combined with:

It is part of the larger Arabian Desert.

The Rub' al Khali has 650,000 square kilometres, Arabian Desert has 2,330,000 square kilometres and Sahara has 9,200,000 square kilometres.

If Rub' al Khali is part of the larger Arabian Desert then isn't the Arabian Desert the largest sand desert in the world?

I was always told (and taught) that "Sahara is the largest sand desert in the world", so I must admit that I am a little bit confused by above cited Wikipedia articles.

Answer 1

These rankings are all a matter of what counts and what doesn’t. For examples, I’ve constructed three similar rankings, each one a little stricter—which forces out some contenders, and gives those below them that are still left higher rankings. This way we can see how Rub' al Khali can be largest in one ranking, while larger-still deserts are found in the more permissive rankings:

Any desert

The largest deserts (and select desert subsets) on Earth are...

1. Antarctica—5,500,000 sq mi or 14,000,000 km²

2. Arctic—5,400,000 sq mi or 13,985,000 km²

3. Sahara—3,300,000 sq mi or 9,000,000 km²

4. Arabian—900,000 sq mi or 2,330,000 km²

5. Gobi—500,000 sq mi or 1,000,000 km²

6. Kalahari—360,000 sq mi or 900,000 km²

7. Rub' al Khali—250,000 sq mi or 650,000 km²

   • Part of the Arabian

8. Great Victoria—220,000 sq mi or 647,000 km²

9. Great Sandy–Gibson—210,000 sq mi or 556,000 km²

   • Authorities vary on whether this is one desert or two; see below

[...]

14. Great Sandy—150,000 sq mi or 400,000 km²

    • As a separate desert from the Gibson

[...]

18. Erg Chech—123,000 sq mi or 320,000 km²

    • Part of the Sahara

[...]

22. Gibson—60,000 sq mi or 156,000 km²

    • As a separate desert from the Great Sandy

Hot desert

The largest hot deserts (and select desert subsets) on Earth are...

1. Sahara

   • Antarctica and the Arctic are left off because they are very cold

2. Arabian

3. Kalahari

   • Gobi is kind of a mixed case, getting cold in the winter and hot in the summer; leaving it off here since this is defined as “hot” deserts

4. Rub' al Khali

   • Still a subset of the Arabian

5. Great Victoria

6. Great Sandy–Gibson

   • Still arguably two deserts

[...]

9. Great Sandy

   • Separately from the Gibson

   • The Patagonian and Great Basin dropped out of that ellipsis

[...]

11. Erg Chech

    • Part of the Sahara

    • Karakum and Colorado Plateau removed from the list in that ellipsis

[...]

13. Gibson

    • Separately from the Great Sandy

    • Kyzylkum and Taklamakan removed from the list in that ellipsis

Hot, sandy desert

The largest continuous areas of hot, sandy desert on Earth are...

1. Rub' al Khali

   • Per the Wikipedia article, the rest of the Arabian consists of

     • A corridor of sandy terrain known as the Ad-Dahna desert connects the large An-Nafud desert (65,000 km² or 40,389 square miles) in the north of Saudi Arabia to the Rub' Al-Khali in the south-east.^{[citation needed]}

     • The Tuwaiq escarpment is a region of 800 km (500 mi) arc of limestone cliffs, plateaux, and canyons.^{[citation needed]}

     • Brackish salt flats: the quicksands of Umm al Samim^{[citation needed]}

     • The Wahiba sands of Oman: an isolated sand sea bordering the east coast[citation needed]

     Since there are non-sand areas within the Arabian, it cannot count as a whole in this list. Rub' al Khali is the largest sandy section, and thus tops this list.

   • “The Sahara is mainly rocky hamada (stone plateaus), Ergs (sand seas - large areas covered with sand dunes) form only a minor part,” quoth Wikipedia.

   • “The Kalahari Desert is a large semi-arid sandy savanna,” “A semi-desert, with huge tracts of excellent grazing after good rains,” and so on, all per Wikipedia again. Low rainfall, but not so low; you don’t get the “sand sea” effect here due to the presence of some plant life.

2. Great Sandy–Gibson

   • Again, if we treat them as one desert.

   • The Great Victoria Desert “consists of many small sandhills, grassland plains, areas with a closely packed surface of pebbles (called desert pavement or gibber plains) and salt lakes,” from yet more Wikipedia, so that’s why that’s gone.

3. Great Sandy

   • Without Gibson

   • Dropped from the list here are the Syrian (“The land is open, gravely desert pavement, cut with occasional wadis.”) and the Chihuahuan (“Several larger mountain ranges [...] create ‘sky islands’ of cooler, wetter, climates adjacent to, or within the desert, and such elevated areas have both coniferous and broadleaf woodlands, including forests along drainages and favored exposures,” “According to the World Wide Fund for Nature the Chihuahuan Desert may be the most biologically diverse desert in the world,” “it receives more precipitation than other warm desert ecoregions.”)

4. Erg Chech

   • After having dropped the Sonoran Desert, because, well, this:

     

     which is the image on the top-right of the Wikipedia page.

[...]

6. Gibson

   • The Gibson Desert is still behind the Thar Desert, as “Most of the desert is covered by huge shifting sand dunes that receive sediments from the alluvial plains and the coast.”

---

All numbers come from Wikipedia’s list of largest deserts, Wikipedia’s article on Rub' al Khali, and DavePhD’s answer, for Erg Chech, though I have created my own lists to emphasize various points.

Note that these rankings are fairly arbitrary: I just used Wikipedia’s list, added in Rub' al Khali, Great Sandy–Gibson as a single desert, and Erg Chech, to address the desert subsets that have been discussed in answers here, and applied my own (fairly arbitrary) filtering to the first list to produce the second and third. There is nothing special about my rankings; you could easily come up with different rules for what does or doesn’t count, and achieve different rankings. The point is that you have to pay very close attention to exactly what competition someone is coming in 1st in—qualifiers on it may make the achievement less than it first appears.

Also note that the numbers don’t always agree: DavePhD’s answer has a larger area for the Great Sandy–Gibson than Wikipedia does (possibly by including areas between the two deserts that would not be considered part of either when separated), as well as a smaller area for Rub' al Khali. Natural borders are not always fixed, and are not always clear, hard lines—differences of opinion in where the limit is, what does or doesn’t count as still being part of the same desert, etc. all can come into play, to say nothing of the possibility that they can and do shift over time.

Answer 2

The Arabian Desert is not only a sand desert

All sand deserts are deserts, but not all deserts are sand deserts. And you do not call an entire desert a sand desert just because a part of it — in this case only 1/5 of the total area — is a sand desert.

This is a desert. The image is taken at the Dome C Station in Antartica _{(image source)}

The Arabian Desert is not just a sand desert, it consists of several types of desert. To say that The Arabian Desert is "the largest sand desert" implies that the Arabian Desert is a sand desert in its entirety, which is not true. A part of it is the world's largest sand desert, but not all of it.

By comparison: you would never say that the Arabian Desert is the world's largest oasis, just because one part of it — Al-Hasa (الأحساء) — is the world's largest oasis.

Answer 3

Table 5.1 of Aeolian Sand and Sand Dunes lists all sand deserts (ergs) that are larger than 12,000 km².

The second largest at 560,000 km² is Rub' al Khali.

The largest at 630,000 km² is the Great Sandy-Gibson Desert.

The book also states "The largest of the Saharan ergs is the Erg Chech in southern Algeria, which has an area of 319,000 km²."