#27: Petrified Forests
Dear Reader,
If you’ve ever been fascinated by fossils — impressions of past organisms left behind in stone — I’d like you to imagine an entire forest that’s turned to stone!
I first encountered petrified wood during a trip to Dholavira, during my college days. The fossil park adjacent to the archaeological site had a few specimens that looked like fallen tree logs but were as hard and cold as rock. These fossilized tree remains from the Jurassic Era date back roughly 175 million years!
A year on, after a job interview in Pondicherry, I found myself in the eerily quiet, half-forgotten petrified forest at Tiruvakkarai — a younger formation from around 20 million years — with a richer assemblage of fallen, fossilised tree trunks.
In this blog, I’d like to explore how petrified forests form, what testimony they bear to ancient vegetation and climate regimes, and how they've lent credence to geoscience theories.
What are petrified forests?
Petrified forests are the remnants of ancient plants or trees that were buried under sediment, and evaded the processes of decay. Over time, mineral-rich groundwater flowed through the layers of sediment, and little by little, replaced the organic plant material with minerals like silica, calcite, and pyrite. Silica crystals occur in a range of structures and colours, and petrified wood may take on an opal-like lustre, or show dark-red opaque jasper rings, with translucent agate pockets. Other trace minerals like iron or manganese can impart red or silvery-gray hues. The resultant fossil exhibits woody details of bark, internal structures and tree rings.
Like all other fossils, petrified forests were formed under a particular set of conditions — where volcanic deposits may have buried plant material under ash, pyroclastic debris, or mudflows or water-borne sediments caused forests to submerge. Occasionally, one can find casts or impressions of wood near coal seams, rather than actual petrification.
Some notable examples include the Petrified Forest National Park (USA), the Petrified Forest of Lesvos (Greece), the Jaramillo Petrified Forest National Park (Argentina), and the Takapuna Fossil Forest (New Zealand).
In India, the Akal Wood Fossil Park (Rajasthan), the Tiruvakkarai and Sattanur National Fossil Wood Parks (Tamil Nadu), are examples of petrified forests. Beyond these, you can find specimens of petrified forests at most fossil parks in India. Other specimens I’ve encountered include a beautiful opalized specimen at the Indian Museum in Kolkata, an ancient, petrified conifer at Lalbagh Gardens in Bangalore, and one, lamentably, serves as a beam at the Qutb Minar complex in Delhi.
Petrified Forests in Legend & Lore
The word ‘petrify’ comes from the Greek root for stone, petra. For Greek mythology buffs, the word ‘petrify’ would conjure up images of Medusa, a snake-haired gorgon, who could turn people to stone if she looked at them. In Norse mythology, the dusky elves, svartálfar, were associated with petrification, as they would turn to stone when exposed to daylight. In popular culture like the film Frozen, or the animated series Hilda, trolls are known to do the same.
In Thailand, spirits are believed to live in petrified tree trunks, and shrines are set up near fossil forests. Some say, touching petrified wood will confer a long life, reflecting some understanding of how ancient these fossils are.
In Navajo legend, the huge silicified tree trunks scattered across Arizona, are the remains of Yetzo, a monstrous giant that had to be slayed by their ancestors when settling in the region. The Paiute tribe, another tribe inhabiting North America’s Great Basin, believe petrified trees to be shafts of arrows shot by Shinauav, the god of thunder, or shattered weapons that fell to earth as gods and giants battled. As revered artefacts, some believe touching or removing the petrified wood, would bring down a curse. These superstitions may have preserved the ancient fossil forests across North America, much like in India, where felling or cutting sacred groves was thought to bring ill fortune.
Darwin’s Fossil Specimens
In 1835, Darwin aboard the Beagle, passed through the Chilean Andes and remarked:
“In the central part of the range, at an elevation probably of seven thousand feet, on a bare slope, I observed some snow-white projecting columns. These were petrified trees, eleven being silicified, and from thirty to forty converted into coarsely-crystallized white calcareous spar. They were abruptly broken off; the upright stumps projecting a few feet above the ground. The trunks measured from three to five feet each in circumference. They stood a little way apart from each other, but the whole formed one distinct group.”
He brought back specimens of petrified wood, numbered and with annotations, to be catalogued at London’s Natural History Museum. Joseph Hooker, a botanist and a close friend of Darwin, was tasked with assembling the collection when employed by the British Geological Survey. However, before he could formally register the specimens, he set off on an expedition to the Himalayas. And the collection was ‘lost’, only to be rediscovered after 175 years in 2012.
I’d written about similar misadventures of dinosaur fossils, misplaced by the Geological Survey of India. It makes one wonder what kind of treasures languish in dusty drawers at these institutions!
These South American tree fossils are not explicitly mentioned in Darwin’s ‘On the Origin of Species’. Yet they were part of a larger collection that included several mammal fossils. Darwin observed that these fossil species closely resembled modern species, and while giant sloths and armadillos no longer roamed the continent, smaller, similar species did. At some point, Darwin surmised, "The most important result of these findings is the confirmation of the law that existing animals have a close relationship with extinct species.” Fossil records served as a clue of how species are not isolated in time and space but may have evolved from similar organisms.
Over the years, some findings and interpretations of the fossil record have been used to challenge aspects of his theory. However, when viewed comprehensively, the fossil record still provides strong support for the theory of evolution by natural selection, albeit with some modifications to Darwin's original conception.
Drifting Continents, Migrating Rainforests
Beyond Darwin’s theory of natural selection, another critical geological theory has been bolstered by plant fossil records — Alfred Wegener’s Continental Drift theory. For years, a particular genus of fossil woody plants, Glossopteris was identified in rock specimens across South America, Africa, India, Australia, New Zealand, and even Antarctica. The rocks were dated to the Permian and Triassic periods (roughly 300 to 200 million years ago).
The seeds of the Glossopteris seemed too large to be dispersed by the wind, nor did it seem likely that they were blown across or carried by waves across the expanse of open ocean. So, the Austrian geologist, Eduard Suess surmised that the continents were joined into a supercontinent, at some point in geological time. He coined the term, Gondwanaland, inspired by the tribal region in India, where the Glossopteris had been found. The distribution of these woody fossils was the first evidence of continental drift.
Since then, several studies of fossil plant species have drawn inferences of past climates and environments. After all, flora thrives in specific conditions. Fossilised tree rings, in particular, have been important proxies of paleoclimate and ancient habitats. However, dendrochronology — dating methods with the help of tree rings — is better applied to Cenozoic fossils, aged 65 million years or younger.
In January 2022, a group of researchers published a paper on dipterocarp diversification in Science, some of their insights stem from fossil pollen. What are dipterocarps? It is a huge family of tall trees that dominate forests in Africa, Madagascar, South America, India, and Southeast Asia, and are characteristic of the last swathes of Southeast Asia’s rainforests. Fossil pollen indicated that the trees evolved in Africa around 100 million years ago, and took root in India, before migrating slowly to the tropical climes of Southeast Asia, against a backdrop of asteroid collisions and supervolcano eruptions. This article, Fossil Pollen Reveals African Origins of Asia’s Tropical Forests, paints a vivid picture of how fossil wood and pollen can bear testimony to the migration of forest species over time.
In a sense, petrified forests and fossil pollen are time capsules. While fossil forests are not rare formations, they offer the chance to reconstruct past climates, track the migration of plant species, and understand how ecosystems respond to environmental changes over geological timescales. This information may prove critical in predicting and managing the impacts of current and future climate change on modern ecosystems.
I wish we could find ways to preserve these geological treasures as part of our geoheritage, not only for their scientific value bearing tangible evidence of Earth's incredible flora, both extinct and living but also for rich insights into past and present biogeography.
RELATED POSTS
This post made for fascinating and illuminating reading, bringing together two of my deep interests: tree/flora and geology:)
Ahhh, there's so much intrigue to deep-time stories. Yes, there's a thrill in touching ancient fossils, and trying to imagine the creatures that left these impressions. I hope you have the chance to experience that someday :D