Monday, September 27, 2010

Mays Peak Stop 2: Columnar Basalt

Just in time for the latest meme/theme circulating in the geoblogosphere, here is a nice outcrop of columnar basalt.  You'll need to click the pics for full size and clarity.
Columnar jointing is actually fairly common in unstressed shallow igneous rocks- by which I mean ones that, as they start to cool, aren't stressed by flow and viscosity or other forces.  Thus the feature shows up in mafic and intermediate flows- which are liquid- and in pyroclastic felsic rocks- which are fragmented, but often "sticky" enough to set into a coherent solid.  But they most often do not form in felsic flows or domes.  As the hot rock cools, it contracts and pulls away from itself. Fractures work in from the surfaces of cooling toward the later-cooling middle of the mass, so the joints are perpendicular to the surface of cooling.
One of the things I like about this outcrop is the way the columns seem to roll over, so you can see them in their traditional columnar form, and from the basal view- along axis- as well.  I have to admit, I haven't looked in detail at the contact under this outcrop, but the underlying material looks much more weathered, and may have a channel cut into it.
Here's the Flash Earth location and image... there's a nice paved pullout on the downhill side of the road for this one.  I'm not sure if the pullout is there for the obvious geology or for the view, but the view has been overgrown by all the green biological stuff over the years.
As I mentioned at the beginning, over the last day, a whole slew of geobloggers have been posting pictures of columns, and I'm tickled this came up in such convenient proximity to the theme. Others include: Geotripper, here, here and here, Sam at Geology Blues Phillip, also at Geology Blues Silver Fox, and another columnar post here. Glacial Till Life in Plane Light: Squashed columns! Aaron at Got The Time These are just the recent ones; if more come up in the near future, I'll try to keep this updated for a few days.  For example, I'm sure Dana got more photos... I didn't even get out of the car. And for more background on the interpretation of the Siletz River Volcanics, see my post on our first Marys Peak Stop. Dana has also posted some very nice photos from that stop.


Dana Hunter said...

Dana did indeed get some photos, and even managed to post them. They're at the bottom, under all the other columns.

As far as impromptu memes go, this is a great one!

glacialtill said...

I think I might have few more pics of columnar basalt. One of which is from Kimberbly, OR I believe. I'll get to posting those soon.

Anonymous said...

I'm a lurker but was much taken by the pictures of columnar joints - thought I'd link to a couple of photos of jointed dykes in South Africa - The Howick one is rather fun as you can see the dyke, on the left of the photo cutting up through the sills only by virtue of the column orientations.
Nearly forgot the link!

Lockwood said...

@ Anonymous- Very nice!

Oakden Wolf said...

This columnar islet is in the Kuril chain.

Oakden Wolf said...

I'll try that again to see if I can make it a link:

Lockwood said...

@Oakden: Zowie! That is spectacular!