30 November 2008

A Faulted Dike

In geology, a bedding plane is where sediments settled over a really long time and turned into layers of rock, with the oldest at the bottom and youngest at the top. Sometimes after this happens, the land shifts around so that the bedding plane isn't horizontal anymore, but might be tilted at an angle, or folded in curvy patterns.

A dike or sill is when a finger of molten rock penetrates through regular cooled, hard rock. If it's going in the same direction as the bedding plane, it's a sill. If it cuts across several of them, it's a dike.

A fault is when there's a really big crack that turns a bedding plane into two pieces, and one piece shifts position relative to the other piece.

And this:

is the sort of sight that would make your average roadside geologist swerve off the road while staring in fascination. That's a dike (or sill) that got cracked up into fragments. It's next to the Hoover Dam in Nevada/Arizona, where the land looks like it went through so much violent upheaval that there's no bedding plane at all anymore, it's all been crinkled into rubble. I imagine it's great fun trying to piece together what happened, and how, and in what order.

Note: I was on foot. No motorists or pedestrians were harmed in the taking of this picture. Also: click on the picture to get the full effect of its scale; those metal things at the top are electricity towers.


Jeri said...

Yep, I would have been swerving. LOL

I'm glad I clicked through to the big version - when you see the towers at the top, you get a sense of scale. Very cool! It's fairly thought provoking to think of the immense pressures over time that cause that kind of faulting.

MWT said...

Yep, that thing is gigantic. ;) And very much a "woah! what the hell happened there?" moment. Alas it's on the side of a cliff so I couldn't get any closer. I'm not even sure what kind of rock it's in. There are some vertical orange stripes at the far left, though, which might be either bedding plane or metamorphic foliation.

I also remember the first time I saw a dike in real life. It was about three feet wide, going through limestone, and had turned half a foot of the rock on either side into marble. Textbook descriptions just don't do justice to the sense of immensity involved in geological features.

Eric said...

It really is a great photo and a spectacular view. Jeri's right, you have to click on it to get that "Whoa!" moment.

Gorgeous shot.

John the Scientist said...

That is some spectacular country out there, I know right where you were.