Northeast - Andrew Collins

© 2017 Andrew Collins/Porters Landing Graphics

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Pluton

Katahdin and the mountains around it are remnants of a pluton--a giant body of magma that formed below the surface and crystallized; the surface that covered the magma when it first intruded has since eroded away--that formed 400 million years ago.

The pluton formed when the ancient microcontinent of Avalonia slammed (as much as rocks traveling a few cm/year can slam) into North America during the Acadian Orogeny. Later, what would become the mid-Atlantic ridge of volcanoes split what was left of the microcontinent, so that the fragments of Avalonia were scattered on both sides of the ocean and now comprise parts of New England, Canada, Ireland, the UK, France, Belgium, Germany, and Poland.