Why do ophiolites obduce upon Continental Crust?

Why Ophiolites End Up on Continents: When the Ocean Climbs Ashore

Ophiolites. Ever heard of them? They’re basically chunks of oceanic crust and upper mantle that, against all odds, have ended up plastered onto continents. It’s like a giant jigsaw puzzle piece from the bottom of the ocean decided to take a hike uphill. But how does this happen? I mean, shouldn’t the heavier oceanic plate always sink under the lighter continental plate? That’s the usual story in plate tectonics, right? Well, not always.

Think of subduction – the process where one plate slides under another – as the Earth’s recycling program. The dense oceanic plate gets shoved back into the mantle. Simple. Except when it isn’t. Obduction, that’s when things go topsy-turvy and the oceanic plate gets pushed over the continental plate. It’s a rare event, a tectonic plot twist, and it needs some pretty special circumstances to pull off.

So, what gives? Why would something that’s supposed to sink, suddenly decide to climb? A big part of it comes down to density, or rather, a temporary defiance of density. Sure, most of the time, oceanic crust is denser than continental crust. But young oceanic crust, fresh from the volcanic oven of a mid-ocean ridge, is still hot and buoyant. It’s like trying to dunk a freshly baked loaf of bread – it floats! Add a thick layer of sediment, like a heavy blanket, and you’ve got an oceanic plate that’s even less keen on diving down into the Earth.

Now, imagine a giant underwater plateau, like the Ontong Java Plateau. These things are huge – basically underwater continents. When one of these bad boys rolls up to a subduction zone, it’s like throwing a wrench into the gears. The subduction zone gets “clogged,” and something has to give. Sometimes, that “something” is the continental plate getting shoved up and over the oceanic plate.

And then there are accretionary wedges. Think of them as gigantic piles of scraped-off sediment and oceanic crust that build up in front of the overriding plate. As this wedge grows, it can put a serious squeeze on the subduction zone, like a geological traffic jam. This back pressure can be enough to lift the oceanic crust and plop it onto the continent.

Mantle plumes, those upwellings of superheated rock from deep inside the Earth, can also stir the pot. They can create buoyant oceanic plateaus, as we talked about, but they can also weaken the oceanic lithosphere, making it easier to break off and get obducted. It’s like weakening a fence post before trying to pull it out of the ground.

Don’t forget about old wounds. Pre-existing faults and fracture zones in the oceanic crust can act like highways for magma and fluids, further weakening the rock and making it more prone to obduction. Think of it as exploiting a weakness in the armor.

Of course, we can’t talk about ophiolites without mentioning mountain building. When continents collide, the oceanic crust caught in the middle gets crunched and squeezed. This intense pressure can detach slices of oceanic crust and shove them onto the edges of the continents. It’s like squeezing a tube of toothpaste – the contents squirt out in unexpected places.

The Oman Ophiolite is the rock star of obduction examples. Scientists believe its journey onto the Arabian Peninsula involved a mix of buoyant oceanic crust and a hefty accretionary wedge. Studying it is like reading the ultimate textbook on how to build a mountain range, one oceanic slice at a time.

So, there you have it. Ophiolite obduction is a rare but fascinating example of how plate tectonics can sometimes throw us a curveball. It’s a reminder that the Earth is a dynamic, ever-changing planet, full of surprises hidden in plain sight – if you know where to look, and what to look for. And who knows, maybe one day you’ll be hiking in the mountains and stumble across a piece of the ocean floor, miles from the nearest beach. Now that’s a story to tell.