Saturday, October 3, 2015

Gold and Lava

This ask-a-Geologist query began as a science fiction writer trying to make his novel more realistic. His original question was framed around a scene where gold is mixed in a lava flow. Barring the obvious difficulties of how you would (a) concentrate and refine the gold and (b) why would you want to play with gold in a lava flow in the first place...

           Q: Does the gold just melt completely away? Would It sink? Would it meld into the Lava and you couldn't tell the different between the two? Would it rest on top and be visibly different? I'm writing a story and I want to be as scientifically accurate as possible. And considering I don't know what happens when Molten Lava touches a refined Precious metal. I thought i would ask.

– Jeremy A

A: To start with, your hypothesis is a long way removed from any possible reality. This is because gold is rarely found in nature in a form larger than tiny flakes. The rare nugget found in Alaska is just that: extremely rare, and secondary at that (i.e., the nugget is not the original form).

            If you want to modify your story to deal with a refined gold artifact, the following may be helpful:

            The melting temperature of pure, refined gold is 1064 °C. By comparison, the melting point of magma is in the range 700 °C to 1300 °C - depending on its silica content. Gold is also extremely conductive, both electrically and thermally. Gold will thus tend to conduct heat through it very efficiently. 

            The density of pure refined gold is quite high: 19.3 g/cc. A typical magma might range in density from 2.4 g/cc to 3.35 g/cc - again depending on silica content. 

            From these, I can suggest two possible scenarios:

1. If a refined gold object is set on an active flow down-range from Kilauea/Pu'u O'o (on the Big Island, Hawai'i), it will initially start to sink into the magma. Experience has shown (including my personal experience) that magma exposed to air quickly forms a crust and hardens inward - rapidly - from that crust. I've personally walked over active toes of magma flows there, because it was already crusting over. And yes, it was still rough on my boot-soles, but mainly because the crust is really mostly glass. Under these very specific circumstances, the gold object would end up being locked, partially sunk, in the cooling magma crust. 

2. In another possible scenario, the refined gold object might be thrown into an active lava tube (look at the photo in the upper right of this link: for an example of a skylight broken into a lava tube). In this case, the magma temperature is in the 1200 - 1300 °C range, and the gold object would first sink, then slowly turn liquid and begin to disperse in the magma as it continues down the lava tube. 

Hope this helps your book be a bit more realistic.

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