Geology Reference
In-Depth Information
9.5 Magma Mingling in Subvolcanic
and Volcanic Environments
Subvolcanic complexes commonly preserve an enduring record of magma
mingling, indeed the concept as a general principle gained widespread
acceptance based on seminal field observations of quenched blobs of basalt
in silicic ring dykes from central complexes in Slieve Gullion, Ireland and
St Kilda, Scotland. The conduits responsible for channelling contrasting magma
types simultaneously through the crust are known as composite dykes (refer
to Dyke Chapter 6). Composite dykes occur in two forms, normal, where the
dyke margins are mafic and the core acidic, or more rarely, reversed, where
mafic magma forms the central part with acidic margins. As with normal dykes
in the field, dip, strike and overall width should be measured, but in addition
the widths of the mafic (or acidic) margins must be recorded, along with any
visible evidence for magma mingling or chilling with interior contacts.
There is good evidence that magma mingling and mixing occurs during vol-
canic eruptions. Indeed magma mingling and mixing arising from injection of
hot basalt into partially crystallised intermediate to acidic magma has been cited
as a major trigger for the explosive eruptions of Krakatoa in 1883, Pinatubo in
1991 and Montserrat in 1995. There similar evidence for magma mingling at
other centres of explosive volcanism such as Long Valley in California and
Figure 9.11
Streaky pumice from Mt Lassen, USA.
