# Molten salt seas on the future Earth

As the sun gets brighter, the oceans are expected to evaporate by the next billion years or so (the vapor slowly gets stripped off into space). The salt, which is less dense than most rocks, will be left behind. Then wait another couple of billion years. Most rocks melt at a higher temperature than sea salt: basalt melts at around 1000C, quartz melts at 1713C, but NaCl melts at only 801C (adding other salts will lower this). So will a tangerine sun loom large over red-hot waves softly breaking on abyssal beaches?

• This would seem to be an Earth Science question. Jun 19 '19 at 20:19
• Try 3.5-4.5 billion years. See en.wikipedia.org/wiki/Timeline_of_the_far_future
– PM 2Ring
Jun 19 '19 at 22:03
• Note their are plenty of other materials that will melt before you reach that temprature.
– John
Jun 20 '19 at 15:52
• @John: Does "plenty" ever come close to >3.5% the mass of all oceans combined, in cases where "melt" means "liquid" as opposed to the way limestone decomposes? Jun 20 '19 at 18:55
• @Luaan: Molten salt isn't lava. It seems very fluid (at 3:00 it pours out with a braided stream pattern, looks just like water): youtube.com/watch?v=e7sIvcYmuk0. Namely glass and basalt is viscous b/c of polymerized chains, salt doesn't have these. Jul 24 '19 at 22:43

So will a tangerine sun loom large over red-hot waves softly breaking on abyssal beaches?

Probably not. Interesting question, and one might think that yes - you could melt the evaporite deposits that will be left after all the water is gone.

The problem is that molten salts are extremely reactive materials. They will corrode any rock they get into contact with. As most rocks on the ocean floor are silicates (basalts etc) you will just get chloride-silicate minerals. For example, the reaction between halite and nepheline (a fairly common silicate mineral) will lead to the formation of sodalite (another fairly common mineral):

$$\ce{6NaAlSiO4 + 2NaCl = Na8(SiAl)6O24Cl2}$$

There's a long list of known chloride-silicate minerals, and for a more realistic salt–basalt reaction the products can be things like scapolite, adrianite, or wadalite.

You will most likely end up with no salt, overlying a somewhat thin (metres?) layer of these silicate-chlorides. It could be that in some points you might have ephemeral pools of liquid salt, or in places where the chloride-silicate layer is thick enough to effectively isolate the liquid salt from the silicate rocks below, but those will probably be negligible features on the scale of the planet.

One final comment:

quartz melts at 1713C

In isolation. When quartz is mixed with other things (like feldspars in a granite), it melts at a lower, eutectic, temperature. See related question one and two.

• I assume Acarbodavyne where you choose mostly Na and Cl and a small amount of Ca K and SO4? But this may not be "bad": at 3000 degrees solid W and solid C react and form liquid WC. If there is excess W, a pool of WC will form on top of the block of W (I think, not sure if you would get a solid solution with dilute C in W). Conversely, at 100 degrees liquid K and Hg react forming solid KHg/KHg2 amalgam. I can't find Acarbodavyne's melting point is, so it's hard to tell if NaCl and aluminosilicates at 900 degrees behave like the WC system and form pools or the K Hg system and stay solid. Jun 23 '19 at 18:18
• For sodalite, it melts at 1079C: rruff-2.geo.arizona.edu/uploads/CM40_163.pdf. For Nepheline Syenite it's 1100C, so the extra NaCl may slightly lower the melting point. Also, they find that NaCl vaporizes with enough heat driving the reaction backwards (vapor pressure ~1kPa at 1000C, enough for rain clouds). Rocks offgas Na, K, and O2 at these temperatures, but negligible Si or Al. So we should get rain that flows freely like water, but most rocks are semi-molten by then and unable to form river or lake beds for hardy cyborgs to enjoy? Jun 23 '19 at 18:57

I very much doubt whether this is going to happen within the next billion years, but something like it is bound to happen one day. Our sister planet Venus has already reached a surface temperature of 400C,which has boiled off the oceans but is not hot enough to melt salt. There should be some salt deposits there (even Ceres has some),but perhaps they are buried, as is much of the evaporite salt on Earth. To cut a long story short, in several billion years time the Earth will be like a super-hot Venus, but with the occasional pond or lake of molten salt rather than oceans of it, and an enormous tangerine sun will indeed loom large in the sky. Mankind will have gone to join the dinosaurs billions of years earlier, so there's no need to worry about it.