Analogs

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

You've got gypsum!

Virtually every home in America contains a great volume of one mineral: gypsum, the primary constituent of wallboard.

Gypsum (chemically, calcium sulfate) crystallizes from supersaturated sea or lake water when the water evaporates, much like common salt precipitates from such water. In the United States, Oklahoma is the leading producer of gypsum—nearly 3,500,000 tons of it, worth more than $26 million in 2007.

But wait, you say — Oklahoma is nowhere near an evaporating sea bed, or even a lake! True, but 255,000,000 years ago, Permian time, as dinosaurs were about to begin their long reign, western Oklahoma was indeed a shallow, restricted lagoon or arm of the sea, and the climate was arid along a shoreline not far from the equator. Evaporation happened.

In Oklahoma’s Blaine Formation four to six gypsum layers, each as much as 10 feet thick, separate thin red shale beds. Shale solidifies from mud, and its red color reflects exposure to the atmosphere as evaporation proceeded: the iron in the mud oxidized to hematite—red iron oxide, essentially rust.

In today’s arid Oklahoma, when groundwater dissolves ancient gypsum and re-deposits it, beautiful crystals form. Oklahoma’s official state crystal is selenite, a type of gypsum found most notably at Great Salt Plains State Park near Jet, a town in northwestern Oklahoma.

After Oklahoma, U.S. gypsum production leaders are Arkansas, Iowa, California, Nevada, Texas, Indiana, and Michigan. But our total, nearly 13 million tons a year, can’t satisfy U.S. wallboard demand. Imports, mostly from Canada, Mexico, and Spain, account for more than 25% of US gypsum consumption. As with so many minerals, China leads the world with gypsum production more than triple that of the United States.

Based on What Things Are Made Of, Chapter 1: All The Ships of the World.

Public domain gypsum image via Wikipedia.