I am a geologic report writer by trade, and, to my friends’ occasional exasperation, I make no secret of my interest in the earth sciences. It was therefore not entirely surprising when I recently received a small package from a abroad, wrapped with ribbon and containing a pale-green rock about half the size of my pinky finger. The friend who sent it lives in Prague, and we have maintained a written correspondence for the better part of the past two years. The rock was accompanied by this explanation:
“I have enclosed a small example of the scientific—and of course, specifically geologic—life of the Czech Republic. I’m sure you’ll be able to tell more about it, should you examine and research it, but two particularly interesting facts were related to me about this specimen and its ilk. First, its name: internationally known as moldavite, in the Czech language, it’s known as vltavín. Either way, its namesake is the mother river of this nation, the Vltava (German: Moldau), the same that runs through Prague itself. Second, the deposit from which this comes has since been exhausted, so not only is it a mineral that occurs primarily in the [republic], it’s a rare one at that.”
I took my friend at his suggestion to “examine and research” the teardrop-shaped stone and quickly realized that it is a very special type of rock, a tektite. Tektites are pieces of natural glass that form when a meteorite strikes Earth and launches bits of molten rock into the air, where they solidify and rain down upon our planet. Digging deeper, I discovered that a moldavite is a certain type of tektite that, indeed, occurs almost exclusively in southern Bohemia (the westernmost and largest subdivision of the Czech Republic) and is associated with an impact that occurred 15 million years ago in what is now eastern Germany.
My own piece of moldavite is decidedly green, thanks to a high concentration of the element nickel. It is also slightly transparent, especially when held up to a light. The moldavite is unmistakably drop-shaped and slightly arched like a leaping dolphin. It is not at all difficult to imagine it as a droplet of molten glass flying through the air.
But it is also scarred and pitted, and it somewhat resembles the surface of a tub of cookie-dough ice cream from which an inconsiderate roommate has carefully excavated all of the chunks of dough. The best return on a gift is seeing the recipient putting it to use: I therefore resolved to discover as much as I could about tektites and to fit their existence into a larger geologic story.
Tektites are somewhat rare rocks to begin with, requiring a perfect alignment of conditions in order to form. The meteorite must be large enough that it doesn’t burn up in the atmosphere. It must strike Earth at an oblique angle and must strike a surface that lends itself to glass formation, such as loose sand. In fact, tektites are only known to exist in four geographical locations: Central Europe, Ivory Coast, Australasia, and Texas and Georgia in North America. Each is associated with a specific meteorite impact, although a crater corresponding to the Australasian tektites has not been identified—yet.
The impact that created moldavites, although far from the most catastrophic in the history of Earth (there wasn’t even a mass extinction!), would have been a fearsome event to witness. A meteorite more than a kilometer in diameter and traveling at 20 kilometers per second struck the planet, causing an explosion with the force of 1.8 million Hiroshima-size atomic bombs.
The rock itself did not survive the collision; it vaporized upon impact, leaving scientists to speculate on its chemical makeup. The friction of the compressed atmosphere liquefied the earth and splashed it up to 40 kilometers into the air. Today this collision is represented by the Nördlinger Ries crater in southeastern Germany and a scattered field of moldavites to the east, in southern Bohemia.
The area that the meteorite struck was the North Alpine foreland basin, a low point in Earth’s surface where a package of sediments was being deposited and, over geologic timescales, compressed into rock. Rocks created in depositional basins like these are fascinating because they offer geologists an opportunity to learn about the past, and to infer the ancient environments that the rocks were deposited in.
The North Alpine basin was created by the intense weight of the Alps to the south, like a mattress bowing around a hefty cat lying on it. As quickly as the Alps were rising, they were eroding, and massive quantities of sediments were being washed away and deposited in basins like this one. The sediments that became moldavites represent a time when sea level was low, and sand, gravel and clay were being distributed in vast outwash fans where mountain streams emptied onto the plains.
Geologists call this theory of using the present to explain the past “uniformitarianism.” Uniformitarianism suggests that the laws of nature have remained uniform since the beginning of time and that the eternal processes of erosion and deposition have shaped every feature of Earth. Sediments are deposited and become rocks, which are uplifted and eroded into sediments again, which are deposited to become rocks, and so on.
This theory was first suggested in 1785 by Scottish geologist James Hutton, who famously observed of the cyclic nature of geologic time: “We see no vestige of a beginning, no prospect of an end.” Uniformitarianism was the prevailing wisdom of geologic thought until the 19th century, when catastrophism was introduced by paleontologist Georges Cuvier, who wanted to answer the rather uncomfortable question posed by the disappearance of species from the fossil record. Extinction, he argued, was hopefully not a uniform occurrence.
Cuvier proposed that massive brief, and incredibly violent events (catastrophes) had shaped Earth. Although he was a student of the Enlightenment and avoided incorporating religion into his work, his suggestion was nonetheless seized on by those who were uncomfortable with the nonbiblical timescales required in uniformitarianism and welcomed a theory ready-made to accommodate Noah’s flood.
The proponents of uniformitarianism and catastrophism were at odds with each other for much of the past two centuries, with convincing arguments being made by both sides. Geologists today have mostly compromised on these approaches, understanding a world that operates on uniform principles but is punctuated by catastrophic events like massive volcanic eruptions, glacial outburst floods or meteorite impacts.
The piece of moldavite that I have is the perfect encapsulation of the uniformitarianism-catastrophism compromise. A meteorite struck the North Alpine basin, a regional depression collecting sediments washed from the Alps. In this one catastrophic moment, the gradual deposition was interrupted, and liquefied sand was blasted into the air.
But no sooner had the pieces of glass landed than they became beholden to sedimentary processes again, carried by rivers, streams and gravity to be deposited in basins. The moldavites are found in a few distinct layers of Bohemian sediment today, a reminder of the time that a catastrophic meteorite interrupted the uniform accumulation of North Alpine sediment.
Now moldavites are collected and sold at gem and mineral stores, coveted both by rock hounds with an interest in their long story and by members of the patchouli-scented crowd who believe the rocks to have “powerful energy.” And maybe they do; I’ve had a piece of moldavite in my possession for only a few weeks, and I’ve already found myself reading obscure articles in journals of Czech mineralogy and, dare I say, rhapsodizing over this Bohemian stone.
Originally posted by: Michael Barthelmes