Every year, an estimated 2 to 3 million people lace up their boots and take a hike on the Appalachian Trail. It's hard to beat the scenery. The 2,200-mile (3,540-kilometer) pathway shares its name with a mountain range whose dense forests and rocky slopes are a magnet for those who love the outdoors. Should you ever decide to climb an Appalachian summit, you'll have your work cut out for you. The 10 highest American peaks east of the Mississippi are all located in this historic mountain range. By a slim margin, the tallest among them is North Carolina's Mount Mitchell, which stands 6,684 feet (2,037 meters) tall.
It probably won't get any taller, though. From a geological standpoint, the Appalachians haven't seen much growth in quite a while. Since the dawn of the dinosaurs about 225 million years ago, this range has been getting whittled down by weathering forces. Yet elsewhere in the world, some mountains grow higher and higher on a yearly basis. So how come the Appalachians aren't following suit?
A key factor is their age. Mountains can form in a handful of different ways, but the majority come into existence when two of the Earth's tectonic plates converge. For those who don't know, tectonic plates are the moving pieces of the lithosphere, our planet's outer layer. Mind you, not all of them are alike. Continental plates are fairly light, while oceanic plates are denser in nature. During collisions between them, an oceanic plate will get pulled under a continental one. Scientists call this phenomenon "subduction." The process drives magma to the continent's surface, leading to the creation of volcanic mountains, like Mount Fuji in Japan or Mount Saint Helens in Washington State. Tectonic pressure generated at these subduction zones can also result in non-volcanic mountains like Alaska's Mount Denali, which — according to NASA — is currently getting 0.4 inches (1 millimeter) taller every year.
But what happens when a two continental plates plow into each other? Once that occurs, the crust at their boundary gets displaced and forced upwards. Thus, a new mountain range comes into being.
Both of these processes helped give birth to the Appalachians. Around 480 million years ago, an oceanic plate was getting subducted under the eastern part of North America, producing some volcanic mountains there. Then, 180 million years later, this region experienced some major uplift when the continent barreled into western Africa.
Alas, the Appalachians eventually stopped growing. Over the past 200 million years, North America and Africa have been drifting apart. The former continent's eastern seaboard is no longer pommeling into another landmass — and at present, no ocean plates are getting subducted beneath it. Tectonically, then, the Appalachian region is inactive. With no mountain-building forces currently at play there, the area's slopes haven't been able to increase their stature in 200 million years.
All mountains are constantly experiencing some form of erosion, which tries to shrink them. Tectonically active ones can overcome this with new, uplifting growth. But since their development is now arrested, the Appalachians can't offset the wear of wind or precipitation. And so they're getting smaller.
A different story is unfolding in the Himalayas. India has spent the last 50 million years pushing itself into Asia. In geological time, the Himalayas — which sit on this boundary — are rather young. Furthermore, they're still tectonically active. Thus, the range as a whole continues to grow, despite the unavoidable influence of erosion.
However, to quote author John Green, "the truth resists simplicity." Individual mountains within a given range don't always get taller or shorter in unison. Sometimes, one portion of a chain will rise while another simultaneously falls. It happened in Nepal in 2015 after a devastating 7.8 magnitude earthquake rattled the country. During the aftermath, scientists discovered that some of the Himalayas' higher peaks lost as much as 23 inches (60 centimeters) in height during the quake's first five seconds. Meanwhile, a couple of the lower mountains actually grew taller. For the record, that 2015 earthquake's impact on Mount Everest has yet to be determined. (Nepal's government is in the process of re-measuring the summit.)
We should also point out that tectonic collision is not the only way to make a mountain. Upstate New York is home to the Adirondack range. Geologists have long been fascinated with this area because, while the Appalachians are shrinking, the Adirondacks are actively growing. By some estimates, the Adirondacks are rising at a rate of 0.08 to 0.11 inches (2 to 3 millimeters). What's causing this uplift? It's thought that a hot spot of molten magma beneath the continental crust may be pressing upward on the region.
So right now, the Adirondacks are a place where uplift is outpacing erosion. But history tells us that someday, the balance between those forces will shift. On a planet whose face is constantly transforming, change is the only permanence.