An important goal for volcanologists is to understand the triggering mechanism for volcanic eruptions. When a volcano erupts, magma that is stored at shallow level in the crust must be persuaded to rise to the surface; for this to happen, it must become buoyant with respect to the surrounding country rock and its viscosity must be low enough to allow it to flow. Many eruption triggering mechanisms have been proposed and it is likely that most eruptions result from a combination of several factors. This project focuses on the role of magma mingling in triggering eruptions.
Figure 1: Mingled magmas at Chaos Crags, California
Magma that is stored in the crust undergoes many changes as it cools and crystallizes. Importantly, the density of the melt decreases, its viscosity increases and the proportion of dissolved gases increases. If a new batch of fresh, hot, dense magma intrudes into the same storage system it can heat the evolved magma, reducing its viscosity and triggering bubble formation - these changes favour eruption. Mingling of fresh and evolved magmas is thus commonly invoked as a triggering mechanism for eruption.
In this project, we are investigating natural samples of mingled magmas from Chaos Crags, California (see figure 1) and from Santorini. In these samples, blobs ('enclaves') of less evolved magma are found within the more evolved host magma. These samples therefore represent a frozen snapshot of the magmas, caught in the process of mingling. Both of the magmas contain crystals, which record changes to their environment as they grow. We see that crystals have been exchanged between the magmas which means that they should have recorded valuable information about the process of mingling in the runup to eruption. We are using a variety of geochemical and petrological tools to extract this story.
This project is funded by a NERC standard grant (NE/G002401/1), which employs Sarah Collins as a postdoctoral researcher. You can read more about Sarah's work on her project pages.