Wednesday, September 29, 2010

Columns: Variations on a Meme

Jumping around a bit on the exposition of my field tripping last week, this was our next stop, but I'm not going to fully write it up until I've put together the Accretionary Wedge- I'm planning on starting that as soon as I finish this post, but the columnar meme is still going strong (see end of this post), and I want to contribute a couple more variations.
The above is, I think, the nicest outcrop of pillow basalts I've ever seen- map and further discussion to follow in a future post.  So what does this have to do with columnar basalt?  As mentioned earlier, columnar jointing forms perpendicular to the surface of cooling.  So what if that surface is roughly spherical?  Simple:
Radial jointing.  I suppose you could think of  this feature as "radial columns," though on the scale of pillows, I personally don't.  I learned the feature as "radial jointing," and that's the way I think of it. (All pics will get bigger and sharper if clicked upon) Note hand sledge in first picture for scale... I think that's a 4 lb. head, though it has been pounded many, many more than four times. On the other hand, at larger scales, such as this (suspected) filled-in lava tube, I do tend to think of them as "radial columns."

The above is a photo from OregonLive that I blogged about a bit more than a year ago, along the Molalla River, east of Portland. 
Others have also pointed out that columns are not necessarily "columns," i.e. up and down.  Here are some horizonal columns in a basaltic dike cutting through basaltic breccia. Again, the consistent theme is that the fractures form perpendicular to the surface of cooling.  This was from Cape Perpetua, between Yachats (pronounced "yah-hots") and Florence on the Oregon Coast.  Though I'll pin down scale more accurately later on, I'm thinking the dike is ~14-16 inches across.

1 comment:

Roser St said...

I am very interested in such type of phenomena and so far I have found two examples of it: one in the Pyrenees of Huesca and the other in the Penyal d'Ifach (Calp, Spain). I would like to send the images but don't know how, I'm sorry.
Your photographs are very good indeed. Congratuliations!