Why The Appalachians Are Older Than The Rockies

The Appalachian Mountains are older than the Rocky Mountains by a wide margin. The Appalachians rose when continents crashed together between 320 and 260 million years ago, as the landmass that became Africa collided with North America and raised a chain that once stood as tall as the modern Himalayas. That same collision assembled the supercontinent Pangaea, and it happened hundreds of millions of years before the Rockies existed.

The Rocky Mountains are the newcomers, built roughly 70 to 55 million years ago during the Laramide orogeny. Instead of a head-on continental collision, an oceanic plate slid beneath North America at a shallow angle and forced the crust upward hundreds of miles inland. The age gap is still visible today. The Appalachians have had far longer to erode, which is why they are lower and more rounded, while the younger Rockies remain tall and sharp.

How the Appalachians formed

The formation of the Appalachian Mountains began when two supercontinents collided with each other. The continent of Laurussia, also known as Euramerica, included both North America and Europe, while Gondwanaland contained Africa, India, South America, Australia, and Antarctica. The original rocks that would make up the valleys and ridges of the Appalachians were in Laurussia.

Starting between 320 and 260 million years ago, Gondwanaland collided with Laurussia, as modern-day Africa met North America. This collision of continental plates formed both the massive continent of Pangaea and the Appalachian Mountains. The mountains formed during several orogenies (mountain-building episodes), beginning with the Taconic around 470 million years ago and the Acadian around 380 million years ago, and ending with the Alleghanian, driven by heat and pressure from the colliding tectonic plates. This pressure also formed the large deposits of anthracite found today in Pennsylvania.

However, this mountain chain did not just include the Appalachians; it also stretched through modern-day Scotland and Morocco. Today, the Scottish Highlands, Iberian massif, Scandinavian Mountains, Watkins Range in Greenland, and Anti-Atlas Mountains in Morocco are what remains of that original Central Pangean Mountain range.

It is unknown exactly how high these prehistoric mountains were, but what is known is that they were much taller than their current heights, possibly as tall as the modern-day Himalayan Mountains.

The parallel ridges that are found throughout the Appalachian Mountains were formed during a folding process called the Appalachian Revolution from 298 to 252.2 million years ago. The weight of the mountains on the rocks beneath the surface caused a crumpling, which in turn uplifted some areas and caused others to fall downward.

The weathering of the Appalachian Mountains

The name Pangaea itself derives from a Greek word meaning all the Earth. However, the Earth would not stay in such a state for long. Starting 200 million years ago, tectonic plates began to move apart, gradually forcing the continents to break up.

The first split opened between North America and northwestern Africa, which in turn formed the central Atlantic Ocean. Europe would remain attached to North America until about 80 million years ago, when they were separated as well.

Due to this separation, the Appalachian Mountains are no longer being uplifted by tectonic activity. They instead have been eroded by rain, rivers, wind, and glacial activity. In addition, some of the older rocks are now exposed on the mountains themselves. For example, when Pangaea was created, metamorphic rocks were pushed under the surface, but as the continent broke apart, the crust thinned, and the metamorphic rocks rebounded upwards.

Erosion of the mountains, in turn, has exposed those metamorphic rocks today. It is estimated that up to 15 miles of rock have been eroded away from the Appalachian Mountains' surface.

Geology of the Rocky Mountains

The modern Rocky Mountains are much younger by comparison than the Appalachian Mountains, but they were built over the top of an older mountain range. Some refer to them as the ancient Rocky Mountains, or the Ancestral Rocky Mountains, which began forming between 320 and 270 million years ago.

This mountain range was much smaller than the Rockies, reaching heights of no more than 2,000 feet in elevation. Scientists believe this range could have formed due to stresses put on the plates by the collision of Laurussia and Gondwanaland. They stood across Colorado between Boulder and Steamboat Springs, and they would erode away into sediments, which would be flooded by the Western Interior Seaway.

The Rocky Mountains in their current form had their origins in the Laramide orogeny, when the Farallon Plate from the Pacific Ocean slid underneath the North American Plate, creating an uplift in the continental crust.

However, the Rocky Mountains are far enough away from the coastline that many scientists have struggled to understand how they have been able to grow so tall. The current dominant theory is that the Farallon plate is sliding down at a low angle, known as a flat slab subduction, which causes the uplift to take place further inland in the compressed mantle, as opposed to right at the coast. Subduction itself takes place when a heavier plate goes underneath a lighter plate.

The height of the Rockies vs the Appalachians

One piece of evidence that proves the Rockies' earlier age as compared to the Appalachian Mountains is their size. They have not been extant long enough to erode as much as the Appalachian Mountains.

This can be seen with the tallest peak in the Rocky Mountains: Mount Elbert, which sits at 14,440 feet in elevation in Colorado. It is one of several mountains in Colorado that surpass 14,000 feet in elevation, known by locals and mountaineers as 14ers.

The tallest point in the Appalachian Mountains, meanwhile, is Mount Mitchell at 6,684 feet, less than half the size of Mount Elbert. Situated in North Carolina, Mount Mitchell is the highest peak east of the Mississippi River.

Over time, the Rocky Mountains will shrink as well, due to the impact of erosion. The effect of ice, water, and the elements is shrinking them faster than uplifts from tectonic plates.

A tale of two mountain ranges

The Appalachian and Rocky Mountains are the most defining geological landmarks in America, marking two periods of exploration, as settlers crossed the Cumberland Gap in the 18th century in the Appalachian Mountains and prospectors moved west in the 19th century seeking gold. They also mark two very different periods in geological history.

The Appalachians were formed long before North America became a separate continent by the movement of massive supercontinents, whereas the Rockies came about over the ruins of an Ancestral Mountain Range through a low-angle subduction of one plate under another. The Appalachian Mountains are so old that they have twin mountain ranges in land masses long since separated by the Atlantic Ocean, whereas the Rocky Mountains are newcomers, rising seemingly out of nowhere far from the Pacific Coast. Although the Rocky Mountains can claim higher peaks than the older Appalachian Mountains, over time, they too will erode to much humbler heights.