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.

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    I don't think I understand your question. The article already says that it is the largest "erg" desert. For comparison: the Sahara is the largest "hot" desert (according to Wikipedia). So what is it that your skeptical of? – Jordy Mar 19 at 8:48
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    From the Wikipedia article on Sahara: "The Sahara is mainly rocky hamada (stone plateaus), Ergs (sand seas - large areas covered with sand dunes) form only a minor part" – IMil Mar 19 at 9:01
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    But it seems to be less a question about the veracity of the source, and more a lack of understanding of the terminology involved. It might be a better fit on a different SE. Is there one for geography? – Sebastian Redl Mar 19 at 10:40
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    @SebastianRedl Seems like this might be on topic at Earth Science. – called2voyage Mar 19 at 12:51
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    I'm voting to close this question as off-topic because this would probably be better in Earth Science. It seems like the phrasing hinges on the phrase "continuous sand desert" which would be a good thing for the Earth Science guys to tackle but not us. – DenisS Mar 19 at 15:16
up vote 31 down vote accepted

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

[...]

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

    • As a separate desert from the Gibson

[...]

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

    • Part of the Sahara

[...]

  1. 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

[...]

  1. Great Sandy

    • Separately from the Gibson

    • The Patagonian and Great Basin dropped out of that ellipsis

[...]

  1. Erg Chech

    • Part of the Sahara

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

[...]

  1. 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:

      Sonoran Desert image from Wikipedia, depicting many cacti and scrub brush

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

[...]

  1. 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.

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.

enter image description here

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 itAl-Hasa (الأحساء)is the world's largest oasis.

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    The question also raised the issue of the Sahara, so this answer would be improved by mentioning where the Sahara falls in this, i.e. is that too not entirely sandy, or is it actually not as large, or what? – KRyan Mar 19 at 13:45
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    @KRyan The questioner asks why not — since according to their course the largest continuous region of ergs is located in the Arabian Desert — the Arabian Desert is named "the largest sand desert", but this distinction is instead given to a sub-region of the larger desert. This does not concern the Sahara since the Sahara does not contain the largest continuous region of ergs. In any case, I think the question is akin to asking "Why is the Sahara not considered the largest oasis in existence", just because it contains oases. – MichaelK Mar 19 at 15:45
  • @KRyan And — as a fun side note — the largest oasis is not in the Sahara, that too is in the Arabian Desert. – MichaelK Mar 19 at 15:46
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    You do a fine job of correcting the mistake OP was making, but you don't actually answer the question "is it the biggest sand desert?" – user36688 Mar 19 at 16:33
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    @notstoreboughtdirt I am not answering the question posed in the headline, no, because with the added context of the question, it is obvious that OP assumes that the area in question is the largest of its kind; the question is why the Arabian Desert - of which Rub' al Khali is a part - is then not called the largest sand desert. – MichaelK Mar 19 at 16:43

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."

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    @KRyan ok, I added, is that better now ? – DavePhD Mar 19 at 13:58
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    @KRyan That is a different matter because the Sahara does not contain the largest continuous ergs known on Earth. The largest deserts on Earth is the Antarctic and Arctic deserts. Neither of them are sand deserts. – MichaelK Mar 19 at 13:59
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    Some sources treat the Sandy Desert and the Gibson Desert as different (smaller) deserts. – James McLeod Mar 19 at 14:01
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    @JamesMcLeod this book says "Australia has four major desert sand seas, the largest occupying the Great Sandy Desert and part of the Gibson Desert ". So it is named as two deserts, but it is a contiguous region, and the sand bridges the two named areas. – DavePhD Mar 19 at 14:10
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    @DavePhD Wikpedia — referencing these sources: 1 and 2 (dead link, title "Department of the Environment WA - Refugia for Biodiversity". 2009.) — lists Great Sandy is "only" 284,993 square kilometers. It appears that your reference (Pye, Tsoar 2009) references some really old data (Wilson 1970, 1973) and counts both Great Sandy and Gibson into that number. – MichaelK Mar 19 at 14:11

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