Geologists are all about analogs. Since we don’t have actual time machines, we have to look at modern situations to project into the past and determine what happened.
In a
previous post I mentioned gypsum in Oklahoma, precipitating out of seawater on a hot, arid equatorial coastline 255 million years ago. A good modern analog for this is Umm Sa'id Sabkha, on the east coast of Qatar along the Persian Gulf (or Arabian Gulf if you prefer), in the photo below.
A sabkha (Arabic for salt flat) is a low-lying area occasionally inundated by marine water. When the water withdraws, mineral salts precipitate as the water evaporates. Sabkas are the basis for our understanding of how many ancient deposits of evaporite minerals formed, including salt (halite), gypsum (used in wallboard), and sylvite (a source of potash for fertilizer, mined in New Mexico and imported from Canada and Belarus).
Another analog: We’ve only known about black smokers (photo, left) since 1977. These hydrothermal vents on the abyssal sea floor spew superheated, metal-rich water into the ocean. Metal sulfides precipitate around the vents and provide a possible analog for ancient copper, lead, and zinc deposits exploited on land today. Deep mantle sources may be the origin of some metal concentrations in the crust.
Lots of mineral deposits can’t be explained by direct analogs—for example, we can’t really look at the roots of a modern volcano to observe the processes that may be concentrating ores there. But inferences from modern situations are probably the best way we have of understanding unusual mineral bodies and predicting where to find more.
“The present is the key to the past” – a concept first clearly expressed by Scottish geologist James Hutton in 1795 (but probably not in those exact words).
Images – Sabkha NASA, black smoker NOAA
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