Geology of the National Parks

A Rocking Review


Peaceful Easy Obduction

The Eagles got a peaceful, easy feeling 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 really did close before re-opening as the modern Atlantic, 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 "obduction zone" for the place where obduction occurs. We just couldn't make the rhyme scheme work 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 really as high as the Himalaya, or how many glaciers eroded the top of our obduction zone, but the mountains probably were quite high and at least some of them may have been glaciated. The diagrams in this rock video have been simplified a bit, to make it easier for you. You can see slightly more-complicated versions of the second and third collisions, and see why we simplified them.

So, relax, get out your Irish flute, and let’s go obducting.

Peaceful Easy Obduction
Click Here for Transcript for Peaceful Easy Obduction Video

[MUSIC PLAYING] RICHARD ALLEY: (SINGING) I like the way your folded mountains lay, waving smoothly across the land. I want to hike across your thrust faults today, with your ancient rocks on either hand. I get a peaceful, easy feeling, where so much history is shown, while I'm living, and I'm loving in your old obduction zone.

Photo-Atlantic slowly closed long ago, Old World drawing near the New. Subduction zones fed many a volcano, island arcs eruptions blew. See their ashes in your roadcuts, while small collisions sent down sand. And the mighty obduction was drawing close at hand.

I found out a long time ago, continents cannot sink down. But in collisions, they will fold and break. So they make deep roots and a mountain crown. Your roots were heated, metamorphic, while glaciers carved the peaks you'd grown. The Himalaya became the Smokies of your old obduction zone.

hen as the rivers washed the heights away, rocks down deep bobbed up to light. So I can feel them on your ridges today, and hold them in your valleys tonight, because I get a peaceful, easy feeling, so many secrets ours alone, while I'm living, and I'm loving in your old obduction zone.


Credit: Dr. Richard Alley