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From appearance to processes: What volcanic rock textures tell us about their formation

Jean-Marie Prival

Abstract

In this activity, a collection of various volcanic rocks is exhibited to the public. During the first part, people are given a few minutes to look at the rocks, and also to manipulate them (to see how light or heavy they are, for example). They are instructed to take notes -- mental or paper -- about each rock's texture: its shape, color, and density; the presence or absence of crystals or vesicles; etc. During the second part, the public is asked to guess which processes may have given its appearance to each rock. A small slideshow (one slide per rock) is presented with the solution. The collection contains a dozen different rocks, with very different looks and formation histories: a fusiform bomb; a bread-crust bomb; bombs with enclaves of peridotite or basement rock; various types of obsidian and pumices; a section of pillow lava... coming from a variety of volcanoes in France, Iceland, Italy, Indonesia. Formation histories include: What does the shape of a fusiform bomb tell us about its cooling trajectory?; How rocks that formed at the bottom of the ocean can be found today at an elevation of 3,000 m above sea level?; and many more. This demonstration has been done on multiple occasions with children from primary and secondary schools, but is suitable for all ages. It requires a table to exhibit the rocks, and a projector to present the slideshow. It can be given in French or English.