26 May 2011

Weird Geology: Accretionary Wedge #34

Posted by Jessica Ball

This month’s Accretionary Wedge, hosted at En Tequila es Veridad, wants us to talk about “Weird Geology”:

Let’s face facts, people.  Geology can be strange.  Outrageous.  Bizarre.  I’m sure you’ve all run into formations and landscapes and concepts that have left you scratching your head.  Maybe they got less weird later.  Maybe they stayed strange.  But however transient or permanent that weirdness was, it got weird.

So tell us about it.  Hit us with the strangest stuff you’ve got.

One location where I definitely encountered some weird geology is a field trip stop I was at several years ago in Big Bend National Park, Texas. Here are the photos from the stop (near Cerro Castelan):

On first sight, these appear to be chunks of petrified wood – whole trees, even, with knotholes and bark and wood grain. Like these in the Petrified Forest National Park in Arizona:

So why is this weird? Well, because the first two photos are not petrified trees – in fact, they aren’t fossils at all. They’re lava necks (another term for a volcanic plug), a subsurface conduit for magma in which the magma hardened, and was later exposed by erosion. One of the most famous – and much bigger –  examples of this is Ship Rock, New Mexico.

The Big Bend lava necks protrude from Chisos Formation volcanic tuffs in the Castolon Graben (according to Big Bend Vistas, an excellent overview of the geology of Big Bend written by William MacLeod). The “wood grain” is actually striations formed during flow through a conduit (‘flow structure’, if you like), and the “knotholes” are probably locations where xenoliths weathered out of the neck.

Geology is full of these little trompe l’oeil encounters – with mineral and fossil pseudomorphs, oddly shaped rocks (even ones which look like food), even entire landforms that are not what they appear to be. And I love things that make me go “Weird!” – because it means I get a chance to pick up more clues for distinguishing confusing geologic features (like these lava necks) from features that they are unrelated to (like the petrified tree trunks).