At the base of the highest mountains, rocks are formed unlike those created under any other conditions. Twice during Earth's history, giant mountain ranges high enough to produce these rocks stretched for 8,000 kilometers (5,000 miles) across entire supercontinents, scientists report. This is equivalent to if the Himalayas stretched almost from Berlin to Bangkok. The timing of these "supermountains" approximately coincides with two of the most important outbursts of evolution, and this may not be a coincidence.
Although garnets can be formed in other ways, they are only produced in abundance under pressures exceeding 1.2 Gigapascals, something that requires the downward pressure of 45 kilometers of the Earth’s crust. That much crust translates to a mountain range with an average height of more than 6 kilometers, punctuated by peaks considerably higher still. Today, only the Himalayas meet the criteria.
In Earth’s history, other such ranges have existed, leaving a legacy in the rocks formed at their base. One of these, known as the Transgondwanan Supermountain existed some 650-500 million years ago, and stretched for 8,000 kilometers along the east coast of Africa. In Earth and Planetary Sciences Letters, a team at the Australian National University piece together evidence to show a range existed 2 billion to 1.8 billion years ago that stretched for a comparable distance, which they have named the Nuna Supermountain.
The rise and fall of these supermountain ranges may have played a crucial role in the evolution of life on Earth, they say. The first appearance of eukaryotes, complex cells that gave rise to animals and plants, was around 2 billion years ago, and the Cambrian explosion, when most major groups of animals first appeared in the fossil record occurred 541-530 million years ago.
PhD student Ziyi Zhu told IFLScience that as mountains erode some of the materials formed in their roots come to the surface. The garnets themselves are altered over time, and can be hard to date accurately. However, zircons, crystals beloved by geologists for ease of dating and holding their original form, are made under the same conditions. Zircons are very widespread, but carry a distinctive trace when they form around abundant garnets: the garnets preferentially absorb Lutetium, leaving zircons uniquely depleted in that element.
Zhu and co-authors found evidence for the legacy of high mountains from the same period in Canada, Siberia, North China, and Central India. Today these locations are widely scattered across the globe, but at the time they made a line across the then-forming supercontinent Nuna.
High mountains erode much faster than lower ones. Rapid erosion produces a surge of nutrients such as phosphorus into the oceans and is also suspected to increase atmospheric oxygen concentrations. Geologists debate whether the erosion of the Transgondwanan Supermountain led to the Cambrian explosion. The authors wonder if Nuna did the same.
"What's stunning is the entire record of mountain building through time is so clear. It shows these two huge spikes: one is linked to the emergence of animals and the other to the emergence of complex big cells,” said co-author Professor Jochen Brocks in a statement.
The connection remains speculative, Zhu told IFLScience. In the few days since the paper was published, she has received messages telling her the timing of the appearance of macroscopic cells is uncertain, making it hard to tie to Nuna. “We need more work from the paleontologists on this,” she said.
The team cannot be sure why the formation of the Nuna and Gondwana supercontinents produced such epic ranges, but Rodinia and Pangea did not, Zhu told IFLScience, adding: “We are just presenting our observations.” However, the paper suggests the size of the ocean basin swallowed up as the continents come together could be crucial.
Such supermountains would certainly have dwarfed the Himalayas in the abundance of peaks around 8 kilometers high, but Zhu told IFLScience the team does not yet know if any greatly exceeded Everest in height. “We want to investigate how high they got,” she said. Mountaineers who consider modern mountains insufficiently challenging and wish for peaks that reach the stratosphere to tackle may have simply been born too late.