Andrew asked:
Given ancient alien artifacts which take the form of giant stone cubes, made roughly fifteen million years ago, on an airless moon, what sort of information about them or their makers could a geologist infer from analyzing them?
All right, off the top of my head:
Age analysis:
- How old the rock itself is, via radiometric analysis, looking at zircons, etc. but this only tells you when the rock itself cooled. Which is of questionable use if we’re talking a sedimentary rock, since at best that will tell you the age of the parent rock. If it’s metamorphic, what radiometric analysis would tell you age wise really depends on the degree of metamorphism.
- How long the rock has been exposed on the surface. If we’re taking a moon with no atmosphere, then the artifacts could be examined for pitting/scarring caused by micrometeorites. As long as some measurement can be made as to the historical frequency of that sort of impact on the moon in general, then you could do some statistical analysis and get an idea of exposure time.
Why do we care how old the rock is? Well, if it’s a wildly different age from what it’s sitting on, that implies some interesting things. As does knowing how long it’s been sitting out on the surface, since those two numbers might be quite different.
Basic compositional analysis (here I’m assuming igneous or metamorphic rather than sedimentary rocks):
- Are there weird, unknown minerals? What about ones that are incredibly rare on Earth but common elsewhere? Particular sorts of minerals (eg Olivine versus quartz) will tell you about the type of melt the rock came from. Some minerals only occur in certain conditions (eg metamorphic minerals like silliminite) while others indicate a particular, very specific set of formation conditions (like diamonds). This is something you’d learn from x-ray diffraction.
- Textures will also tell you important things, like how rapidly the rock cooled, etc. Spinifex texture, fit example, tends to be seen in things like komatiites, which have a very specific melt composition and literally no longer form on Earth today. And all this you can do with thin sections. If you have a sedimentary rock, you can learn ridiculous amounts about the formation of the rock with thin sections, such as looking at generations of cement or weathering features.
- Even just looking at bulk oxide makeup (via something like xrf analysis) can give you clues about origin and formational conditions. For example, I used XRD analysis of samples from my vertisols to calculate mean annual precipitation during their formation in my master’s thesis. There is a ton of research out there about various sorts of rocks, formation or weathering conditions, and how that relates to their basic chemical makeup.
- At the very least you can use this to figure out if the rock is even native to the area. If there’s something really wild about the composition (for example, there are absolutely no impurities in any of the crystals) that could be a hint that the rocks were manufactured in some way rather than formed in natural conditions.
Visual assessment:
Just by looking at it even, there will be clues about how these things were–or weren’t–made. Tool marks? No tool marks? Or if you look microscopically using some sort of pocket scanning electron microscope, what will you see? Crystals cut cleanly in half? Evidence of flash melting, as if these were shaped using some kind of super heated plasma blade? Or were they made in molds, in which case everything would have crystallized perfectly flat against the mold surface? These visual clues might tell you the most about the makers of the artifacts.
This is obviously a non-exhaustive list. I’m sure there’s a million other things a geologist with a different specialization than mine could think to assess. But hopefully this will get you started!