Easy Science: how the Appalachian Mountains formed in four steps

The history of the Appalachians spans over a billion years, with four collisions forming the Appalachian Mountains. The last collision occurred around 300 million years ago. Originally, the chain would have rivaled the Himalayas at over 5 km tall, with some mountains perhaps reaching as high as 9 km. Three hundred million years of erosion slowly whittled the mountains away; today, they average 1 km, with the highest point being 2 km at Mount Mitchell, North Carolina.

0. Greenville Orogeny: Mesoproterozoic  (1,600-1,000 mya)
   1. Amazonia (South America) slams into Laurentia (North America) during formation of Rodinia, creating mountain chain
   2. Most of this chain erodes away, but a few exposures of Grenville rock exist today
1. Taconic Orogeny: Birth of the Blue Ridge: Ordovician (485-443 mya)
   1. Taconic Volcanic Island Arc collides with eastern North America, creating mountains
   2. Compression causes a foreland basin to form on North America
   3. Basin fills with marine sediments, clastic sediments from erosion
   4. Queenston Delta with fluvial sediments develops over former basin, with Taconic Highlands and Iapetus Ocean to the east
   5. Erosion begins to level raised terrane
2. Acadian Orogeny: Raising the Ridges: Silurian-Mississippian (443-318 mya)
   1. Avalonian Island Arc hits northeastern North America
   2. Previous rocks deformed and forced up: fold and thrust belt to west of the Taconic Terrane; suture zone to the east where Avalonia accretes onto Taconic Terrane
   3. Erosion begins to level raised terrane
3. Alleghanian Orogeny: Mississippian-Pennsylvanian (359-299 mya)
   1. Gondwana (Africa) slams into eastern North America during formation of Pangaea as Iapetus Ocean closes
   2. Severe deformation; rocks thrust up
   3. Gondwana begins breaking away in Triassic
   4. Sediments pile up to east and form coastal plains throughout the Jurassic and Cretaceous as the mountains erode and lose several km of elevation

Cross-Section of Appalachian Geology

Sedimentary Appalachians (sedimentary rocks)

• Ridge & Valley: sedimentary deposits shoved west when Avalonia collided; ridges more resistant to erosion than valleys

Crystalline Appalachians (igneous and metamorphic rocks)

• Blue Ridge Mountains: pre-Cambrian rocks folded when Taconic accreted; highly metamorphosized; greenschist, slate, schist, gneiss, granite; highest elevations
• Piedmont: lower than Blue Ridge because more erodible; rolling hills and gentle slopes; weathered granite, gneiss, schist, slate, moderately metamorphosized

More Resources

• Geologic Events Affecting Eastern North America -- easy, concise overview (similar to this page, but a little more in depth)
• Wikipedia's Geology of the Apps -- good overview, maps, links
• Birth of the Mountains -- great brochure by the USGS
• Northern Appalachians -- lots of diagrams
• Appalachian Region -- discusses rocks found in the different sections
• The Mountains That Froze the World -- as if the Apps weren't "cool" enough