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Wednesday, April 20, 2011

A Brief Geologic History of the Hill that Broke My Wrist

Well we’ve been on hiatus for the last few months, but it’s time to get back to the blog-writing groove. Hopefully you can stay with me on this somewhat technical venture. Back in February, I started a journal entry about the geology of Grenada that I intended to turn into a blog post. A few days later, however, I was out collecting laundry in the rain and slipped. With one hand holding the laundry, my free hand reached back just in time to slam into the hill. I didn’t hear the crack, but my wrist screamed in pain and a rushing, cold sweat sent my body in shock. I rolled over and laid there for a minute- My God, I must be getting old?! Slowly I picked myself up, cursed the hill, and hobbled back up to the house, muddy laundry in my good hand.

Luckily, it was just a hairline fracture. The day I got the cast off, Stephanie and I took a vacation up to Carriacou, and caught a boat up through the southern Grenadines for a day. On the boat, backdropped by the dramatic Caribbean landscape, I thought again about that journal entry on geology. It dawned on me that, while my broken wrist may have been a freak accident, the fact that it happened on a hill certainly was not. Grenada is all hills. In fact, all these islands (except Barbados) are incredibly hilly. But, as it turns out, they aren’t just hills, they’re volcanoes (or, at least, remnants of volcanoes).


And so here goes my amateur understanding of Caribbean geologic history. If you need a picture for this, here’s a zip of some maps to follow. Beginning around 160 million years ago (late Jurassic), a series of synchronized, volcanic hotspots around the borders of the North and South American Plates hemorrhaged into a new, Caribbean Plate. The predominant theory holds that this actually happened in the Pacific. By about 110 mya, the plate began slipping between the continents, gliding over the widening Atlantic (the Panama isthmus, interestingly, didn’t form until much later). This movement caused a string of rifts to erupt laterally, close to the northern boundary, into what became the Greater Antilles (e.g. Puerto Rico, Hispaniola, Cuba, etc.), and also longitudinally, along a mostly submerged ridge to the south called the Aves. Then, around 60 million years ago (during the Paleocene- the epoch spurred by volcanic activity from the Chicxulub Meteor), another, more volatile ridge opened up further to the east of the Aves Ridge, giving rise to Grenada and most of the Lesser Antilles. Thus, for those trying to wrap your heads around this, these islands are really young- you won't find any dinosaurs here. And, of course, there are still many active volcanoes along the island arc: Kick ‘em Jenny, the only remaining submerged and active volcano, lies just to the north, between Grenada and Carriacou.

As these pyroclastic islands cooled, they became rich in life-sustaining nutrients. Soon the familiar landscape of sun-drench, palm-lined beaches and lush green mountains began to form. Life literally swept in on hurricanes, trade winds, and outwash from the Rio Orinoco (which carried the Arawaks about 4000 years ago; trade winds carried Columbus in 1498). Luckily, island life isn’t nearly as competitive as places like Central America, so jaguars, poisonous snakes, and other threats don’t exist here. Just big, slippery hills. And if you’re careful enough, you can learn to live on them quite well. Apparently, though, I’m still adjusting.
-JH

References:
Most of my references are linked, but one that formed the foundation for my understanding is P. Bouysse's piece in Biju-Duval and Moore's Initial Reports (#78-A) of the Deep Sea Drilling Project, an NSF and JOI geophysical survey