Monat: Februar 2022

Nicolas Steno and the Nature Of Fossils

In October 1666, a large shark was captured by a fishing boat in the sea of Livorno (at the time part of the reign of Tuscany). The animal was pulled onto the shore, beaten to death and dismembered. As the body was too heavy to be transported, only the head was saved. It was brought to Florence, where famous Danish anatomist and naturalist Niels Stensen (latinized Nicolas Steno) was asked to dissect the specimen.

Steno later published a detailed anatomical description of the shark, which included a chapter where he compared the shark’s teeth with fossils commonly found in the hills of Tuscany. The fossils of unknown origin were simply referred to as Glossopetrae or tongue stones.

Steno was not the first to speculate about an organic origin of fossils. In his 1565 book De Rerum fossilium, Lapidum et Gemmarum maxime, figuris et similitudinis Liber (On Fossil Objects), Swiss naturalist Conrad Gesner compared fossil sea urchins with living specimens, and argued that some fossils are petrified organisms. In 1616, the Italian naturalist Fabio Colonna argued that glossopetrae were shark teeth.

However, there was a big problem – they couldn’t explain how the supposed remains of sea animals could be embedded in rocks now parts of inland mountains. So most naturalists preferred to explain fossils just as curiously shaped inorganic features growing inside rocks.

Steno was the first to explain why fossilized teeth of a creature from the sea could be found inside rocks that were far distant from the modern sea.  Before dissecting the shark in Florence, Steno had visited the Royal Danish Kunstkammer in Copenhagen and the fossils on display there. He wrote in a private note:

Snails, shells, oysters, fish, etc., found petrified on places far remote from the sea. Either they have remained there after an ancient flood or because the bed of the seas has slowly been changed. On the change of the surface of the earth I plan a book, etc.

During his travels in Tuscany, Steno had studied outcrops of layered rocks, and he had recognized that the sedimentary layers were petrified shores and marine sediment of an ancient, now vanished sea.

He also noted that fossils are found only in layered rocks and never in recent soils. If fossils were of inorganic nature, like many naturalists argued at the time, we should find them in every kind of soil and rock. By combining his observations in the field and the results of the shark’s dissection, Steno formulated a surprisingly modern geo-theory:

  • The layers were formed by deposition in water. The now hard rock was once a soft mud.
  • An animal living in the sea would, after its death, sink slowly into the soft mud. Mud and water would petrify the animal and preserve it.
  • Collapsing cavities in earth´s crust would tilt layers along the borders of the forming crater upwards. Exposed to air the mud – with the fossils still inside – dries and becomes hard rock.
  • Eventually the crater fills with water and a new layer is deposited above the old ones.

In the end, Steno argued, all the layers were uplifted again, this time high enough to form a mountain, where a curious naturalist can find the fossils eroding from the rocks. Unfortunately Steno’s work, like the work of many others before him, was soon forgotten and ignored.

In 1695, amateur physician and naturalist John Woodward published An Essay toward a Natural History of the Earth. This book, intended to prove the veracity of the Biblical account of a large ancient flood, was not well written and mostly based on work copied from other naturalists. Woodward argued that fossils are the remains of animals killed by a flood, and cited Steno to support this idea. However, reading Steno’s original considerations makes it clear that a single flood was not sufficient to explain the thick layers of sedimentary rocks found everywhere.  Woodward’s book was therefore mostly dismissed by contemporary scientists, but it made Steno’s ideas popular again. Scientists then began to actively study and discuss sedimentary rocks and fossils – and the rest is history.

Used References:

KARDEL, T. & MAQUET, P. (eds.) (2013): Nicolaus Steno – Biography and Original Papers of a 17th Century Scientist. Springer Publishing: 739

The Geology Of Jules Verne’s Journey To The Center of the Earth

Figure from the novel „Journey to the Center of the Earth“ published by Jules Verne in 1864.

„Well gentlemen, at one point at least I agree … the materials of the geologists are not charts, chalk and chatter, but the earth itself. We should never know the truth until we are able to make that journey and see for ourselves.“ 

from „Where Time Began,“ a 1976 film based on Jules Verne’s novel Journey to the Center of the Earth.

Novelist Jules Verne was born on February 8, 1828, in the French city of Nantes. Today he is known as a pioneer of the science-fiction genre, imagining a submarine traveling twenty thousand leagues under the sea, a space projectile heading to the moon and a fantastic journey into the depths of our world. One hundred and fifty years after Verne’s visions, humans have walked on the moon, nuclear submarines can travel under the sea and we have started to explore the mysteries of the deep earth.

Journey to the Center of the Earth was published in 1864 and was immediately a critical success, and has remained in publication in both French and English to this day. In the opening chapters of the novel, the German Professor Otto Lidenbrock and his nephew Axel discover an ancient document, written by Snorri Sturluson. This (fictional) 16th-century alchemist described a journey into a large system of volcanic conduits, accessible from the crater of the Icelandic volcano Snæfellsjökull. So Lindenbrock and his nephew traveled to Iceland, employed a local guide, and following the document’s coded directions, entered the volcanic crater.

There, they descended through the sedimentary layers of the crust into its foundation. About 140 kilometers beneath the surface they discovered an underground sea occupying a cavern, roughly the size of Europe, hollowed in the granite of the lower crust. The travelers ventured upon the „Lidenbrock Sea“, as they name the newly discovered ocean, in a raft built out of the logs of “great palm-trees of species no longer existing” growing along the shores.

At sea, they witnessed a battle between Jurassic sea monsters and disembarked on an island with a geyser. Venturing inland they discovered living mastodons and primitive hominids. Verne’s bestseller was a product of rich imagination and research. He likely based his fictional travel account on the works of geologists like Alcide d’Orbigny, who classified rock strata by their fossil content, Elie de Beaumont, who worked on the origin of mountain ranges, and Charles Sainte-Claire Deville, who studied volcanoes.

Geological section, published by German geophysicist August Sieberg in 1914, showing the anatomy of a stratovolcano, with a main conduit, various lateral dikes and a large sill connected to the magma reservoir. In contrast to the sketch, the conduits for magma are in reality only a few meters wide.

An important source of inspiration to Verne were the books by the French scientist and writer Louis Figuier. In 1864 Figuier published La Terre avant le déluge, a popular science book discussing geology and paleontology. From Verne’s surviving correspondence with his publisher, we know that he started to work on his novel sometimes between January to August 1864. Some passages and scenes in Verne’s novel, like the battle between an ichthyosaur and a plesiosaur witnessed by the travelers, was likely inspired by an illustration in Figuier’s book. Verne’s imaginary forest growing along the „Lidenbrock Sea“ was similar to the fossil forests of the Carboniferous period. The heat necessary to keep the forest alive comes from „the excessive heat of the globe. The Earth was still so hot in itself that its innate temperature dominated“ as Figuier writes in his textbook. Before the discovery of radioactive decay, geologists believed that earth’s inner heat was the residual heat of its formation from a molten ball. Over time earth cooled down and a solid crust formed.

Verne’s explorers used the hollow volcanic conduit of Snæfellsjökull as a gateway to earth’s interior. Many geologists at the time believed that volcanic conduits, empty once the volcano erupted, connected a volcanic crater to magma chambers deep underground. Today we know that such conduits are far too small (and obstructed by solid rock) for humans to move through.

However, Verne was right when he described a chamber full of gigantic crystals found deep underground. For crystals to grow, they need the right conditions and a lot of time. In theory, there are no limits to how large a crystal can become, however, perfect conditions for crystal growth are rarely met. That said, such perfect conditions are found in the Cueva de los Cristales, located in the Naica Mine, Chihuahua, Mexico.

The mine of Naica was opened in 1828 to mine for lead, zinc and silver ore. In 1910 a natural cave was discovered, named later Cueva de las Espadas. The name derives from the three-foot long blade-like gypsum (calcium-sulfate) crystals covering the walls of the cave. However, what the miners discovered almost 90 years later, during the construction of a tunnel 0.2 mile below ground, is even more astounding. The Cueva de los Cristales hosts the most incredible crystals ever discovered, mirroring Verne’s fantastic description. Almost perfect conditions made it possible to grow gypsum crystals more than 10-meters in length and with an estimated weight of 40 to 50 tons.

The enormous gypsum crystals of Naica. Note person at bottom right for scale. Credit: Wikipedia/Alexander Van Driessche. CC BY 3.0. Van Driessche

Maybe Verne was right in even a more spectacular way. The largest crystal possible on earth could be indeed found at its center. Earth’s core is a solid ball of superhot iron and nickel alloy about 760 miles in diameter. Modern research suggests that it displays a crystalline structure. Unfortunately, at the moment, there is no way to be sure and visit this place as Verne imagined.