Sometimes, it takes a great struggle to build towering peaks. And what more thunderous event is there than the clashing of continental plates?

Geological pileups have birthed some of the largest and tallest mountains on the planet, including Mount Everest and its Himalayan neighbors. But has there ever been a peak taller than Mount Everest, which stands 29,032 feet (8,849 meters) above sea level?

The answer is still a mystery, but there are some geological clues about where the tallest peaks would have been in the ancient past, experts told Live Science.

Earth's lost mountains

Geologists don't have the level of detail required to pinpoint an exact highest peak hundreds of millions of years ago. "We're never going to get that granularity, unfortunately," Alan Collins, a professor of geology at Adelaide University in Australia, told Live Science.

However, clues in residual sediments lodged inside Earth's crust hint at the lost grandeur of primordial snowcaps and summits. Scientists study these crusts to understand how deep they went, which can reveal how tall ancient mountains might have been.

Mountains are really just parts of the continental crust that have been thrust upward as tectonic plates slowly crash together. This crust is very heavy, and there is much more to it than what you see on the surface; even the most spectacularly tall peaks, like Everest, are anchored in deep crust. Just as crust gets thrust up to form mountain peaks, it also gets pushed deep under the surface. Both Collins and Ziyi Zhu, a geochemist at Curtin University in Australia, compared mountains to icebergs, in the sense that only a bit pokes above the surface.

"We can argue or speculate that [the biggest mountain] was in a place where there were deep roots in the crust," Zhu told Live Science.

Geologists estimate that the Himalayas, for example, have a crustal column that goes down as far as 50 miles (80 kilometers). Compare that to Everest's elevation of about 29,000 feet (9,000 m), and the world's tallest mountain at most represents only 14% of the rock slab it's part of. That means there's a limit to how tall mountains can get before they collapse under their own weight.

The Himalayas began forming between 50 million and 40 million years ago from the collision between the Indian Eurasian continental plates. As far as mountains go, they are relatively young. And that's partly why they are still so tall. Tall, jagged mountains erode quickly over time, though the Himalayas are still growing as the two plates continue to collide.

Such continental collisions have happened countless times before, though. For example, the Transgondwanan Supermountain, an ancient range in what is now East Africa, began emerging some 650 million years ago when Africa and South America crashed into Arabia, India, Australia and Antarctica to form the supercontinent Gondwana.

Gondwana split up again, but evidence of these mountains in places like southern India, Sri Lanka, Tanzania and Kenya point to them being three times the length of the Himalayas. "If you walk around there now, you can see rocks that were at the bottom of this mountain belt," Collins told Live Science. On average, the crust is deeper than the crust of the Himalayas. "We think this is at least the scale of the Himalayas if not more," Collins said.

The crusts found in these areas also hint at how tall the mountains were, with sections that descend up to 18 miles (30 km) deep, despite half a billion years of erosion. "At some point, they were likely halfway down a crustal column that was 60 kilometers [37 miles deep]," Collins said. This means that the mountains above this base could have been as tall as Everest.

In the Himalayas, the tallest mountains sit around the rim of the Tibetan Plateau, which has the highest elevation on the planet, at about 3.7 miles (6 km) above sea level, on average. The Transgondwanan range may have had a similar plateau around southern Madagascar, meaning its rim would have been in eastern Madagascar, southern India and eastern Antarctica, Collins said.

Another candidate for the tallest mountains was the Grenville orogeny, which popped up with the formation of the supercontinent Rodinia starting about 1.3 billion years ago. Its remains can be seen around southeastern Ontario and northwestern Vermont. But less is known about this extinct range, Collins said, with some scholars arguing that it wouldn't have been as tall as the Transgondwanan range or the Himalayas.

Further back, some 2 billion years ago, the formation of the supercontinent Nuna (also called Columbia) also left evidence of truly giant mountains. Zhu's paper found evidence of this supermountain by examining the composition of zircon crystals left in the crust underneath this vanished range.

Look any further back, however, and the picture gets murkier. When Earth was hotter, mountains likely couldn't get as high because warmer crusts can support less weight, Collins said. Stable continental plates and rocks didn't start to form until about 4 billion years ago, and it took hundreds of millions of additional years for large continents to form.

For Zhu, the Nuna, Transgondwana and Himalaya ranges are currently the best candidates for holding the tallest mountains that ever existed. But it's likely impossible to pinpoint the highest specific peak, at least with current scientific methods. "It's an open question," Zhu said.

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