Darwin and the Kitchen Sink

Last Thursday was Darwin day, and various people have been celebrating his contribution to the understanding of biology. But Darwin was not merely a life scientist. He was also an igneous petrologist. And he made several observations that are crucial to the understanding of basic igneous processes that are important for everything from the moon to the kitchen sink.

In his 1844 publication “Geological Observations on Volcanic Islands”, Darwin observed:

One side of Fresh-water Bay, in James Island, is formed by the wreck of a large crater, mentioned in the last chapter, of which the interior has been filled up by a pool of basalt, about two hundred feet in thickness. This basalt is of a grey colour, and contains many crystals of glassy albite, which become much more numerous in the lower, scoriaceous part. This is contrary to what might have been expected, for if the crystals had been originally disseminated in equal numbers, the greater intumescence of this lower scoriaceous part would have made them appear fewer in number.

He then combines this observation with known experimental petrology:

The sinking of crystals through a viscid substance like molten rock, as is unequivocally shown to have been the case in the experiments of M. Dree, is worthy of further consideration, as throwing light on the separation of the trachytic and basaltic series of lavas.

To deduce that:

In a body of liquified volcanic rock, left for some time without any violent disturbance, we might expect, in accordance with the above facts, that if one of the constituent minerals became aggregated into crystals or granules, or had been enveloped in this state from some previously existing mass, such crystals or granules would rise or sink, according to their specific gravity. Now we have plain evidence of crystals being embedded in many lavas, whilst the paste or basis has continued fluid. I need only refer, as instances, to the several, great, pseudo-porphyritic streams at the Galapagos Islands, and to the trachytic streams in many parts of the world, in which we find crystals of feldspar bent and broken by the movement of the surrounding, semi-fluid matter.

Why is this important? Well, evolution, while useful for explaining the existence and progression of life on Earth, has very little to do with dead planets or kitchen appliances. On the other hand, crystal settling does.

The rock samples brought back from the moon by the Apollo space program show that the Moon formed with a global magma ocean, this ocean, as it cooled, produced a feldspathic crust as a result of the anorthosite crystals floating on the Fe-rich magma. The light colored lunar highlands are composed of this primary anorthositic crust. But more important than the moon is a fine collection of kitchen utensils.

When large basaltic magma chambers cool, the iron and chromium oxides, which are much denser than the magma, settle out to the bottom of the magma chamber when the magmatic evolution allows them to crystallize. An example is shown by Highly Allochthonous here. Where this process concentrates the oxide mineral chromite (FeCr₂O₄) is where the element chromium is generally mined from. And Chromium is a key ingredient in stainless steel, the material from which so many great kitchen utensils and appliances are formed.

So Charles Darwin isn’t just responsible for describing our relationship to monkeys, the spread of anti-biotic resistant TB, and the evolution of the fossil record. He is also responsible for discovering the process that forms chromite deposits, without which fine kitchen appliances, swiss army knives, and fancy car accessories would be much more expensive.