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While there are areas on our world that hold vast amounts of black sand like Reynisfjara, they are usually confined to beaches and don't cover a large area.

What I'm wondering is how can a desert with true, magnetite sand be formed, and how different would it be from a regular desert.

Also, while the Karakum Desert does indeed have black sand dunes, these are not caused by the color of the sand, but by the color of the soil underneath the sand. This question is specifically asking how a desert where the sand itself is black can form.

Addendum: I should have put this in the description but I'm referring to deserts that occur on Earth-like worlds if possible.

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    $\begingroup$ Black rust is formed in low moisture low oxygen environments. That means it is slowly forms under what we would usually consider conditions where rust would not be prone to develop. That makes things tricky. It's considered the "good" kind of rust. I would think it would only be a transitional stage since it's probably not going to be replenished with the same black sand. $\endgroup$
    – DKNguyen
    Aug 4 at 2:58
  • $\begingroup$ @DKNguyen - I grew up on a home lot on a hillside with exposed sandy soil in spots that formed deposits of black magnetite sand in spots just like the large deposits mentioned in the first link in the OP. We could easily pick up sand grains using magnets as kids and spent quite a bit of time collecting samples. It was fascinating to us that this was possible to do with sand. The deposits showed no tendency to turn brown over time despite being in the Midwest with plenty of rains over the summers. $\endgroup$ Aug 4 at 19:54
  • $\begingroup$ My mistake, black rust isn't formed by low moisture. It is low oxygen. @CTeegarden The closest I have ever gotten to black sand is while watching Princess Mononoke; which is to say, not very close at all. $\endgroup$
    – DKNguyen
    Aug 4 at 20:02

4 Answers 4

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Iceland has black sand deserts (not only beaches), mainly due to volcanic minerals/rocks and lava sand. One of the sources mention that it can occur through glacial outburst floods (jökulhlaup) which can be triggered by geothermal activities and subglacial volcanic eruptions. Volcaniclastic sediments and debris are carried with this meltwater and it gives the black color to the terrain.

enter image description here enter image description here

The sand is black because many volcanic minerals and rocks are dark-coloured. Common rock types of volcanic islands are basalt, andesite and volcanic glass. Minerals such as pyroxene, amphibole and iron oxide also lend the sand its black colour.

ornosk.com

Around the year 1000, many farmers set up shop on Mýrdalssandur. By the 15th century, however, most of these farmsteads were abandoned due to violent eruptions originating from Katla volcano. Today, the area is a 700 square kilometre desert of black sand, made up of the deposits from Mýrdalsjökull ice cap and the sudden glacial floods (or jökulhaup) that run from the glacier down to the sea. Evidence of these prior floods can be seen throughout Mýrdalssandur, be it the washed out bridges or the peculiar rock erosions. Even today, farmers close to Mýrdalssandur worry that receding glaciers will cause rivers to change direction and overflow. It’s no wonder Icelanders left this region a long time ago.

guidetoiceland.is

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    $\begingroup$ +1 for Jokulhaups! $\endgroup$
    – Atog
    Aug 4 at 15:51
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    $\begingroup$ @ermanen Interesting, I didn’t know that the entire tracks of land where big enough to be considered a desert. Though I wonder if this can only occur near polar regions or if there is a way to have black deserts closer to the equator $\endgroup$
    – Seraphim
    Aug 5 at 2:08
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    $\begingroup$ (+1) As an additional example, the Canary Islands are known among tourists for black sand beaches. The black sand there is of volcanic origin like that in Iceland. $\endgroup$
    – njuffa
    Aug 5 at 7:36
  • $\begingroup$ I've looked up but couldn't find if a black desert can form in other regions. Iceland has a unique geographical feature where there are both volcanoes and glaciers, the land of fire and ice. There are black sand beaches around the world (or possibly small inner terrains) where there is volcanic activity. Iceland has barren black sand areas where it is considered a desert, per the source I've quoted also. It has a potential to be bigger in a volcanic group region. Bonus: Black Desert in Egypt where there are black mounds. $\endgroup$
    – ermanen
    Aug 5 at 7:38
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Basalt based sand.

Black sand is usually due to a basalt source rock, basalts is just rare on earth continental crust so there are no basalt deserts. It is also softer than the much more common quartz so it does not get transported as far. Quartz is very common on earth and quartz sand is very durable and can build up on earth, but if you are not on earth you don't need the same chemical makeup, Mars for instance has large black sand dune fields.

enter image description here

https://slate.com/technology/2015/11/mars-curiosity-now-in-a-dune-field.html

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Black rust is formed in a low oxygen environment. So I suppose requires large amounts of iron to be mostly sealed from the atmosphere and slowly oxidize over a long period of time before eventually becoming exposed to form a desert.

Maybe buried underground? Or at a sea bed where the sea eventually dries up? Or maybe it's in an area where CO2 from volcanoes or something displaces a significant amount of oxygen over a large area for a prolonged period of time.

But once it's exposed, that's it. It is not going to be replenished by more black sand from weathering like another desert might be.

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Did you know that grass incorporates silica into the vacuoles of its leaves and stems? (It's to deter grazers.) This is where most deserts, especially ones that are expanding, get there sand: the grass dies and releases its silica as sand grains.

How does this help? Simple: have the grass analogue incorporate additional or even different minerals to deter grazers. Black silica does exist, it's called obsidian. And if you insist on magnetite, think of what it would do to the grass!

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    $\begingroup$ I don't think phytoliths in grasses are big enough to account for dessert sand. From what I can glean, they're around 10-20 μm in size (which is too small to even count as fine sand, really). And particles of that size or smaller account for only ~10% of dessert sand (and ignores other sources). -- Though none of that really matters for a fictional world. $\endgroup$
    – towr
    Aug 5 at 12:07

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