The Story Of The Rocks

1995 1996 1997 1998 1999 2000 + The Volcano & Belham

Crystals form in the magma as it rises through the crust and begins to cool. As they crystallise, the tiny crystals continuously record information about the magma, like its composition or temperature. This means that the crystals and textures of the rock provide frozen snapshots of the processes that have happened deep inside the volcano. Once erupted, we can study them in the field and in the laboratory to unravel some of these processes. This way, we can piece together the story of why and how the magma erupted.

Different types of magma interact deep in the crust and they are often brought up together. The black round pieces in the otherwise white rock in this photograph are called 'mafic enclaves' - they come from a hot magma deep in the crust, which encountered another magma body (white) on its way to the surface. Credit: Jenni Barclay

A magnified view of the contact between the black mafic enclaves (left) and the white host rock (right), captured by a Scanning Electron Microscope (SEM). Microscopic interactions between the different magmas and their crystal cargo can tell us about the depths and temperatures where the two magmas interacted with each other. Credit: Jenni Barclay

One of the most common types of crystals in the Soufrière Hills rocks are feldspars. They form white or transparent laths up to a few millimeter length. Under the microscope they often look stripy, as shown in this illustration. Feldspars are particularly useful to reconstruct magma processes. Credit: Bridie Davies / mineralogy & petrology laboratory / flickr / CC BY-ND 2.0

Another common type of crystal in the Soufrière Hills rocks is amphibole. It is usually easy to spot because of its prismatic shape. In the rocks, it will look black, but under the microscope they are often yellow or green. Note: The crystals don't really have eyes and mouths. Credit: Bridie Davies / wikimedia