Abstract

Volcano behaviour is controlled by the composition of the erupted magma – for example, magmas containing a lot of water tend to erupt explosively while drier magmas are erupted in a more effusive (and less hazardous) manner. The composition of erupted magma is determined by processes that occur in the plumbing system underneath the volcano. The most important of these processes is the separation of crystals from the remaining liquid, driven essentially by gravity. Removing early-formed crystals changes the composition of the remaining liquid, known as fractionation. While we can examine the end result of fractionation by looking at the erupted magmas, the best approach to understanding the spatial relationships between crystals and liquid in fractionating systems is to look at the deeply-eroded remains of ancient sub-volcanic magma chambers.

The Skaergaard Intrusion of East Greenland formed about 60 million years ago. It is a body of basaltic magma 10 km across and 4 km deep that filled a space in the Earth’s crust formed during extension related to the opening of the North Atlantic. Unusually, this batch of magma was then left entirely undisturbed, allowing it to crystallise progressively inwards from the walls, floor and roof. The fully-solidified intrusion is now very well-exposed by erosion, with continuous outcrop through the magmatic stratigraphy. Since its discovery in 1933 it has become one of the world’s best natural laboratories for the study of magma fractionation. Its importance has increased recently with the discovery of commercially viable quantities of gold and platinum-group elements. Field observations coupled with laboratory analysis of selected rock samples allow us to trace the progress of fractionation, work out the behaviour of a crystal mushy layer forming on its vertical walls and begin to understand the complexities of a km-scale dynamic crystallising system.