The oldest mountains are smaller because, over time, erosion caused by weathering and tectonic movements wears them down and reduces their height.
Mountains are formed when two tectonic plates collide and push against each other, causing the rock to fold or lift to create impressive reliefs. However, right from their birth, these mountains also begin to undergo erosion immediately: wind, rain, ice, and even gravity gradually wear down their height. As long as the processes of formation dominate, the mountains continue to grow. But over millions of years, formation slows down, or even completely stops, while erosion never ceases. The result: the older the mountains are, the more worn down and smaller they become, having gradually lost their towering peaks.
As soon as a mountain forms, it immediately undergoes the relentless work of erosion: rain, wind, frost—they are the real sanders. The rain seeps into the cracks and gradually wears away the rock. The frost, on the other hand, causes the stone to crack as it expands, just like when a water bottle forgotten in the freezer bursts. And the wind? It also plays the role of a sculptor, gradually sanding down the relief with sand and dust. All this mess causes a constant transport of fragments to the valleys, slowly reducing the height of the ranges. The older a mountain is, the more time it has spent enduring these continuous attacks, so naturally, it is smaller and less impressive than in its beginnings.
Isostasy is a bit like when you put an ice cube in water: the bigger it is, the more it sinks, but it still floats because it balances with the water around it. The same goes for mountains! When they wear down and lose height, their mass decreases. As a result, the underlying Earth's crust slowly rises, like a kind of rebound. This very slow movement means that even if erosion constantly wears away from the top, the mountain doesn’t disappear instantly. It drags on, and old mountains gradually become lower and rounder, revealing their deeper, more solid, and resilient base over time. It’s a quiet process over millions of years, but it is what explains why some very old mountains look more like hills today.
The climate plays a significant role in the wear of a mountain. For example, regions with abundant rainfall promote erosion by water, which gradually wears down and weakens the rocks. Similarly, in cold climates, the freeze-thaw phenomenon greatly damages mountains: water seeps into cracks and then, upon freezing, expands and gradually breaks the rock apart. The wind also does its part: it constantly blasts sand and small debris against the rock, slowly sculpting the landscape. In short, a mountain subjected to a harsh or humid climate will quickly lose its height over time.
Mountains do not all have the same resistance to time, and this is largely due to their geological composition. Some rocks, like granite, are very hard, resistant to wear, and ensure a good longevity of the relief. In contrast, mountains that contain a lot of soft rocks like limestone or shale will erode much more quickly under the action of rain, wind, and frost. The more brittle or susceptible to dissolution the rock is, the less likely the mountain will retain its height and rugged shapes for a long time. A mountain made of durable materials can easily last several hundred million years, while a soft and fragile mountain sees its shapes soften quite rapidly over the millennia.
Did you know that the Armorican Massif in France is made up of ancient mountains that are now almost completely eroded? What seems to us like a simple hilly region is actually the remnants of a once imposing mountain range!
Ancient mountains generally have more rounded and less steep peaks, while recent mountains, such as the Alps or the Rockies, often feature sharp and very rugged summits.
Did you know that the Appalachians, one of the oldest mountain ranges in the world, were once comparable in height to the current Himalayas? Today, these mountains measure only about a quarter of their original size due to erosion.
Did you know that the phenomenon of isostatic rebound, caused by the melting of massive glaciers, can slowly lift certain ancient mountainous regions by several millimeters per year? This gradual recovery maintains their altitude despite ongoing erosion.
Isostasy is a gravitational equilibrium between the Earth's crust and the underlying mantle. When erosion removes material from the surface of a mountain range, the crust becomes lighter and tends to slowly rise. This isostatic phenomenon slows down the reduction in height caused by erosion, prolonging the lifespan of a mountainous relief.
The main agents of erosion are water, wind, ice (through glaciers), as well as gravitational movements (mudflows, landslides). The combined action of these agents gradually reduces the height and roughness of the relief.
Young mountains, such as the Alps or the Himalayas, feature steep peaks and rugged terrain, reflecting their recent formation. Older mountains, like the Appalachians, are generally softer and more rounded, with a lower average altitude, resulting from a long period of erosion.
The process of mountain erosion generally spans millions of years. The exact speed depends on the local climate, weather phenomena, the geology of the region, and the initial altitude of the mountain range.
Although the Himalayas began to form about 50 million years ago, they continue to be pushed upward by the ongoing collision of the Indian and Eurasian tectonic plates. This partly offsets erosion, keeping this mountain range at a high elevation.
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