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The Hidden Colours Inside Volcanoes

16 September 2015
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Geological Society Events
Burlington House, London
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The petrographic microscope is an important tool for geologists. Many generations of us have grappled with thin sections and polarisers in student labs. With luck and perseverance we can learn how to read the record of geological processes that is written in the shapes and sizes of crystals in rocks.

At the very least, most of us quickly appreciate the colourful beauty of thin sections of silicate rocks observed under crossed polars.

Recent rapid advances in the technology of chemical microanalysis and imaging have returned petrography to the leading edge of geological research. These techniques have enormous potential to solve a number of problems in igneous petrology in particular. Quantitative estimates of the spatial characteristics of compositional variation within rocks can be used to test and refine physical models of sub-volcanic processes.

One exciting aspect of this problem is our newfound ability to link the timescales of chemical zonation within individual magmatic crystals to timescales of magmatic evolution prior to large eruptions. The timescales are surprisingly short, bringing the prospect of linking petrological information to monitoring records from active volcanoes. 

In John's talk, he will try to show how beautiful images of igneous rocks under the microscope can help to understand the inner workings of volcanoes. 


John Maclennan

University of Cambridge

John Maclennan is a Reader in Earth Sciences at the University of Cambridge. His research combines fieldwork, state-of-the-art microanalysis and computational modelling to understand magmatic processes.

Most of his work has been focussed on recent basaltic volcanism in Iceland, but he has also worked on submerged mid-ocean ridges, the Oman ophiolite, Hawaii, Reunion and the Moon. He demonstrated the link between volcanism and glaciation on Iceland and has used basalt chemistry to constrain mantle temperature variations and flow patterns.

He is currently using electron microscopy and microanalysis to establish the timescales of magmatic gas release prior to large eruptions such as the AD 1783 Laki event. He received the President’s award of the Geological Society of London in 2006 and the Bigsby Medal in 2014. He was a Mineralogical Society Distinguished Lecturer for 2011-12. 

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