Friday, February 6, 2009

Friday Fragment: Welded Tuff

Today's fragment is a rock from eastern Oregon: a very nice welded tuff. I've just spent a little time trying to "disambiguate" tuff from ignimbrite; I've always sort of assumed that the ignimbrite was the event, the process so to speak, and tuff was the deposit, the product. According to wikipedia at least, both are deposits. And I'm left not too clear on exactly what distinguishes the two words. (Late addition: actually, I think tuff implies that the deposit is lithified; the deposit is an ignimbrite, whether it's lithified or not) But the general outlines, without worrying too much about terminology, are pretty easy. A large, explosive volcanic eruption ejects a sizable amount of pyroclastics (pyro=fire, clast=broken) which then travel across the countryside- I've described it to kids as a very powerful volcanic sandstorm. When this fragmentary material settles out to the ground different processes may cause it to solidfy as a rock. Whatever the process, the result is known as a tuff.

With a welded tuff, the fragmentary material is still hot and "sticky" or plastic enough to cement together as it settles. So this is not just a "volcanic sandstorm," it's a molten volcanic sandstorm. Imagine a "sandstorm" strong enough to carry rocks the size of a golf ball, and hot enough to still be a little molten (probably in the range of 700 degrees C, or about 1300 F) Now imagine this "storm" covering hundreds to thousands of square miles. It would be a very bad day. As I used to say when I was doing some volunteer teaching, it would be a fascinating event to witness. From low orbit.The major components of this rock are glass, sanidine, lithic fragments and more glass. I've looked at it in thin section, but I don't remember anything really being identifiable other than sanidine (a high-temperature form of potassium feldspar) and very dirty glass. The lithic (rock) fragments are presumably fragments of the country rock that were ripped out by the explosion and transported along with the ash flow. They're a mixed bag, though clearly of volcanic origin themselves. The thing that's really cool about this particular flow is the abundance of fiamme.
I think in all my previous rock and mineral posts, the full pictures have been reduced to 15% of original along both horizontal and vertical, and the "crops" have been bits of the full-sized picture, without reduction. The above is a crop, but reduced by 50%. I also have now figured out how to set the picture size in the the html, rather than settling for Blogger's default. This should be reflected with better resolution in my pictures. (As an aside, any geoblogger who'd like a full-sized copy of any of these pictures is welcome to contact me. I'd be happy to email the .jpg files; they're generally around 1.5 Mb) At any rate the above picture shows a nice pair of fiamme. As I mentioned in last week's Friday Fragment pumice can be thought of as the volcanic equivalent of popcorn; an enormous portion of the rock is pore space. If a pumice fragment is buried under a significant burden, and it's still hot enough to deform, it's easy to picture the pumice simply being mashed flat. The gas is pressed out of it, the walls collapse, and the glass bubbles weld together to form a more or less solid mass of glass. This isn't the only way fiamme form, but it's the common mode in welded tuffs.The above crop (full size) shows a couple of interesting features. First, you can begin to get an idea of just how abundant the sanidine is in this rock, though you can see that pretty well in the previous picture too. This picture also shows a couple of the lithic fragments along the middle of the right side. The large one is quite angular (as you would expect from an explosion fragment), and with the long axis about 60 degrees off from the bedding plane (which is basically horizontal). A fiamme would be concordant with bedding plane.I picked up this sample in spring of '84, and have never had an opportunity to get back to the spot, or more accurately to look for it and stop. I'm sure I would spot it on a daytime drive-by; I've got a clear mental image of the outcrop. But I don't know exactly where the spot is, other than it's not too far E-SE from Burns on route 78. In the above GE map, Burns is the town on the left (west) side; the road coming in from the SW and passing out to the NE is US 20. The road leaving Burns directly eastward then turning SE is 78.

This whole quadrant of Oregon is lousy with welded tuffs. When people think of Oregon volcanoes, most think of Hood, Mazama (Crater Lake), basically the big composite cones of the Cascades. People with a little more background will certainly know about the Columbia River Basalts. But it takes a while trotting around the state before you realize how ubiquitous, and varied- in composition, age and form- Oregon's volcanoes are. I think I would be quite accurate in saying that the only state more uniformly volcanic in nature is Hawaii.

1 comment:

Silver Fox said...

I love ash-flow tuffs!

An ignimbrite is a deposit formed from a nuee ardente, which translates as "incandescent cloud" or "glowing avalanche" or "glowing ash cloud". It has long been my thinking, and the thinking of many others, that if you don't see the process of "glowingness" you can't hardly call it an ignimbrite! I think that's why many go with ash-flow tuff, with varying degrees of welding. An ash-flow tuff was deposited by an ash flow -- who knows if it was "glowing" at the time of flowing?