The Appalachians rise

The Appalachians look like old, rounded hills. They are the remains of a range that once rivalled the Himalaya, built when Africa collided with North America.

Three collisions, not one

Mountain ranges are rarely made in a single event. The Appalachians are the product of three separate impacts across 250 million years, each leaving its own set of rocks and faults.

Timeline

470 million years ago

the Taconic orogeny: a volcanic island arc is thrust onto the continent's eastern edge

380 million

the Acadian orogeny: the microcontinent Avalonia arrives

325 million

the Alleghanian orogeny begins: Africa comes

300 million

Pangaea is assembled; the mountains stand at their highest

260 million

the collision ends, and erosion takes over

200 million

Pangaea splits; the Atlantic begins to open

today

under 2,000 metres left of what was probably over 6,000

How a mountain becomes a hill

When two continents are pressed together, the layers between them fold and are thrust upward along near-horizontal faults. The fold belt of Pennsylvania and Virginia is those thrust sheets seen from above — whole stacks of sediment pushed westward like cloth on a table.

Then the wearing down begins. Mountains are heavy, and they sink slowly into the yielding layer beneath while their tops are stripped away. What erodes ends up at the foot as sediment, and the thick layers under Ohio and Pennsylvania are exactly that: the mountain, relocated.

The coal

The collision also created a rain-shadow and lowland system where humid swamp forests could stand for millions of years. Those forests became the coal of Appalachia, and so the basis of American industry 300 million years later.

The chain that was divided

The range was physically continuous with the Anti-Atlas in Morocco and with the Caledonides of Scotland, Ireland and Norway. When the Atlantic opened it was torn into three. The rocks of Newfoundland and of Norway are siblings, and that was among the arguments that finally convinced geologists the continents move.