Friday Fragment: Pumice and Tufa

Today's fragment is a piece of pumice covered with a crust of tufa, from near Mono Lake, California. Pumice is a light, frothy, rock created when silica-rich lava (rhyolite or similar) erupts from a volcano. Molten rock often has dissolved gas contents- mostly water- in the range of 1 to 3 percent, which doesn't sound like much. The mass of a cubic meter of silica rich rock is going to be around 2500 kilograms, so the mass of dissolved gasses in that will be around 25 to 75 kg (55 to 165 lb) or (roughly) 7 to 20 gallons of water. Water expands by (again, roughly) a factor of 1000 as it moves into a gaseous state, so picture two- five gallon water jugs suddenly expanding into 2000 five gallon water jugs... while buried within a cubic yard of molten rock. Suddenly 1-3% dissolved gasses sounds more substantial.

Essentially, pumice is volcanic rock that has "popped" like popcorn. It's known to countless numbers of kids as the rock that floats. Tufa can be thought of as non-marine, chemically-precipitated limestone. It's sometimes described as fresh-water limestone, but the bodies of water that it precipitates from are often salty. The above photo shows both the pumice and the odd texture of the white encrusting tufa. Below you can get a better sense of the size of the tufa crust.In the above picture, my finger is resting on the tufa crust, my thumb on the pumice. Below is the rock face my finger is on. If I was to find this with no broken surfaces, that is, with the whole rock coated in several millimeters of tufa, how would I know there was pumice inside? The simple answer is density, or perhaps more accurately, heft. I can't measure density directly by picking up a rock; I need a volume and a mass. But I do have a sense of how heavy a "normal" rock ought to be. In other words, I have a sense of how big (volume) a rock is as I reach out toward it, and my mind automatically makes an assumption regarding how much effort I'll have to put into picking the rock up (weight). If there is a mismatch between my assumption and my actual experience when I pick it up, I know to look for something "unusual."

Non-geologists tend to focus on color above all else, and can be convinced that other features are worth looking at with a little effort. But the range of observations, many almost unconcious, that one learns to make while looking at tens of thousands of rock, are more varied than any texts can ever explain. Below are a pair of crops from the first and second photos. Another interesting question this rock raises is "Is this igneous or sedimentary?" I love answering "either/or" questions with "Yes," and I will do so here. The pumice component is clearly igneous, the tufa component clearly sedimentary, so the rock as a whole is best described as having aspects of both. There are innumerable examples of rocks like this, that have two or more of the "rock-type trinity," igneous, sedimentry and metamorphic. Rather than fussing over whether a rock is one or the other, a geologist will spend more time fussing over the processes that created particular aspects or components of the rock. This is a picture my brother took with his cell phone when we were there last spring.
And the GoogleEarth view. I think I may have moved the pin since I copied the lat-long numbers (37.978308° -119.129022°), but not enough to make any difference. The major road running along the western edge of the lake is US 395, which has my vote for the most spectacular interstate route in the US.