Thursday, January 1, 2009

Horizontal columnar jointing, Wolf Creek Pass

Callan's post about columnar jointing and rust blisters reminded me that I've got a photo that I've been meaning to show since the 2007 GSA meeting.

Driving from Durango to Denver means crossing at least three passes. My favorite one is Wolf Creek Pass, over the Continental Divide in the San Juan Mountains. After several years of roadwork, there are some fresh roadcuts on the western side that show mostly a lot of young sediments, but some volcanic rocks, as well. This one caught my eye as I was driving up to the pass:

It's a dike cutting through pretty loose sediments - not surprising for the rocks in the eastern San Juans. But what caught my eye was the fracture pattern:

Those are horizontal columnar joints. As Callan showed in his photos, most columnar joints are nearly vertical. But that doesn't have anything to do with gravity - it's the result of cooling of a near-horizontal layer, like a lava flow. The joints form perpendicular to the cool surface. And in the case of this dike, the cool surface is vertical.

This is a neat roadcut for other reasons, too. There's baked sediment at the contact with the dike. And the sedimentary structures are nice, too. Even a squashed-rock person like me can make out some old channels, and the clasts in the conglomerate are volcanic rocks. I don't know if either the clasts or the dike have been dated, but I bet there's a pretty short time between deposition of the sediment and intrusion of the dike.

Happy New Year, everyone!


Anonymous said...

With curved cooling surfaces you can get some strange effects. This photo from Iceland shows columnar joints radiating from lava tubes.

Kim said...

Oh, great photo from Iceland! Wow.

Silver Fox said...

That's a great example of horizontal columnar jointing in a dike. (And a cool photo from Iceland!) I wish I could have had time to go over Wolf Creek Pass on my recent trip - but the roadcuts may have been under snow, anyway!

GeologyJoe said...

great photo.

Unknown said...

Nice example of horizontal and radiating columns. I'm jealous, time and like moved me out of the Rockies, I miss them.

Gorgeous neighbourhood you live in. I manged to make one trip through the area last time I abondoned the environmetal business and was living in El Paso and working in the Green River basin.

Dr.Pramod Hanamgond, GSS College, Belgaum, India said...

I was searching an example of horizontal columnar joints and there was an example from you. Yeah! I too saw a dyke in Aurangabad, India at a road cutting. At one side there is a clear vertical dyke with horizontal columns. However, on the other side of the road, there is a displacement. I would have shown the photos but do not know how to attach photos here.