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 and another!
Life in Plane Light: Squashed columns!
Aaron at Got The Time
Geology Rocks
Dana at En Tequila Es Verdad
Cujo 359 (see comment on Dana's post for description)
Wayne at Earthly Musings has a gorgeous photo of columns below the rapids at Lava Falls in Grand Canyon.
MB Griggs at The Rocks Know has photos of what may well be the most perfect columns in the world
Jessica, AKA Tuff Cookie, showcases a variety in different rock types.
Hypocentre finds columns in a very unlikely place, as well as a spectacular photo of radiating columns.
Dave Tucker at Northwest Geology Field Trips displays precisely one slew of columnar displays in Washington State.
Dave Bressan at History of Geology shares the first printed image of columnar basalts, from 1565.
A couple more variations from Dana's and my driving about W. Oregon.
Dr. Jerque has some spectacular examples from the bottom of the Grand Canyon.
Silver Fox Has another (better than mine) photo of horizontal columns in a set of dikes, and points out a couple more links to columny goodness (not to be confused with calumny, which is not good)
Dan McShane offers some more Washington State columns.
Garry Hayes, who deserves credit for starting this meme (see first links in the list, above), adds yet another set of photos from the opening of the Atlantic Ocean, and a lovely guest photo by Ivan Ivanyvienen, of columnar jointing in rhyolite at the San Juan Precordillera.
Update, October 4: Eric Klemetti- who did his Doctoral work just down the street from where I'm sitting- has joined the fray. (Also, check out the links readers have left in the comments)
Helena Heliotrope at Liberty, Equality and Geology shows off some more Washington columns.
Chris and Anne at Highly Allochthonous each toss in a photo- Tokatee Falls looks awesome!
Some more Cape Perpetua jointed dike photos from Cujo359, and Devil's Churn- again, numerous dikes with horizontal columns.

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.