While walking around Barcelona eating great food and contemplating the risks/rewards of piloting Vespas through the city traffic, I found myself the guide to a swift but exhilarating geologic tour.
Outside the Museo de Geologia (“Museum of Geology” for those of you who no habla espanol) are some displays of many rock types. Once we got to the exhibit my friends found themselves unwilling participants of an impromptu geology lecture. Here I am describing the contraction of basaltic lava while cooling that leads to the columnar jointing on display behind me. Look at those 120 degree angles! Also note the word “BASALTO” written on the side.
Speaking of lava flow, check out these samples of both pahoehoe and aa (great word for Scrabble). On my right is an example of pahoehoe lava flow which is recognized for its ropey texture caused from a very fluid lava underneath the thin cooling outer shell. On my left is the aa flow (pronounced “ah-ah”) which has a more blocky textured look. This particular flow carries fragments of lava called clinkers with it as it travels.
The aa sample is a local boy from Sant Feliu de Pallerols, Spain about an hour north from Barcelona.
Another great example of volcanic activity was seen in this volcanic bomb (“bomba volcanica” for those of you who DO habla espanol). These rocks are expelled high into the air during violent eruptions and fall to the earth as incandescent lava. I am pointing in the direction that the semi-molten bomb fell giving it this aerodynamic shape.
In this picture Kris, a native of Stockholm, Sweden, cozies up against a large piece of granodiorite. What’s the attraction? While it may be from a different part of Europe, this is the same type of rock seen all over Kris’ home.
Here is the same picture with some arrows to show the directions of maximum and minimum stress. The mineral grains in the rock which are subjected to stress during periods of metamorphism align in a direction perpendicular to the highest level of force (represented by sigma 1). Eventually the grains will all be aligned along a plane of foliation (probably seen on a counter top near you).
We had a unfortunate habit of showing up to every museum after it closed on this trip, so, this is where the tour ended. Here is one last picture of the rocks. Check out the basalt column.