Islands or Continents

I have been helping out an the elementary school where my wife teaches. Last week I had a smart little 4th grader ask what the difference is between an island and a continent. Of course I answered the continent was bigger. So, they ask why is Australia a continent and Greenland an island? I really had to think... Most of the geography answers really didn't help. But, the geology answers I came across seem to make sense, although the students did get that iced over look in their eyes as I rambled on about continental crust vs. oceanic crust, plates and geologically stable cratons. However, it was when I started to discuss the ideas of island arc accretion that I realized I had lost them. Perhaps next year.

volcanic ash makes great building materials

Spring break was spent in Guatemala visiting my daughter and her husband who are Peace Corps volunteers in the Western Highlands. On the bus ride between Guatemala City and her town near Xela, we passed innumerable small strip mines where some industrious locals have mixed the ash with lime to create cinder blocks which are used to build everything in modern Guatemala. In many cases, large piles of drying blocks are set out near the road.

The image above shows an ash mine with a cinder block building in the foreground. It turns out that this ash combined with a bit of lime is an excellent replacement for portland cement which is pretty pricey in the Guatemalan hinterlands.

Most of the Western Highlands are volcanic in nature with ash deposits, in many cases, 10's of meters thick and easily eroded making some interesting erosional features and some difficult terrain to get around. Most of the surface ash deposits are pozzolanic ash a fine, sandy volcanic ash originally discovered in the region around Vesuvius. This sandy ash is perfect for the production of cinder blocks and mortar as the deposits break apart easily. Unfortunately, I didn't do the homework I should have done before the trip and I found that poking around on Guatemalan private property was not a good idea. In Colorado I usually tell people I am studying the geology in their backyard...but my Spanish is not that good and I didn't need to start an international incident or make an ash of myself by examining the geology.