A Rocking Review
Peaceful Easy Obduction
The Eagles rock music group sang about the peaceful, easy feeling they got while going into the desert, but they didn’t tell us whether they planned to do a lot of geology on the way. Whatever; in this parody, you can get a peaceful, easy feeling about a whole mountain range, the beautiful Appalachians. The proto-Atlantic Ocean did close before re-opening as the modern Atlantic Ocean, acting like a very slow accordion. While the proto-Atlantic was closing, three "collisions" happened, raising mountains near the modern east coast of the US, and erosion of the mountains produced sand that now is the sandstone of the ridges of central Pennsylvania and other places along the Appalachians. The first two collisions—small obduction events—involved North America hitting island arcs containing explosive volcanoes formed at former subduction zones. The third collision was the major obduction event as the proto-Atlantic disappeared, with Africa and Europe meeting the Americas as the great supercontinent of Pangaea was assembled.
Geologists long ago decided that the place where subduction occurs is a "subduction zone” but have been slow to adopt an "obduction zone" for the place where obduction occurs. We just couldn't make the rhyme scheme work in our parody with "the mountain range formed where an obduction event happened", so we call it an obduction zone. No one is quite sure whether the mountains in our "obduction zone" were as high as the Himalayas, or how many glaciers eroded the top of our obduction zone, but the mountains probably were quite high (more or less the height of the Andes today) and at least some of the early Appalachians may have been glaciated. The diagrams in this rock video have been simplified a bit, to make it easier for you.
So, relax, get out your Irish flute, and let's go obducting.