Rights: The University of Waikato Published 9 April 2010 Download

Dr Phil Shane describes how geologists study magma. How do you know where it came from? How do you find out when it formed? What does it tell you about volcanoes today?


Magmas, which are molten rock, are not all the same. Fundamentally, they are different due to what tectonic setting they occur at – what sort of major tectonic processes produce the molten rock – and so there are a range of magmas that have a range of chemical compositions.

Different eruptions will have undergone different conditions on their way to the surface. They will have stayed at different pressures and at different temperatures for different lengths of time. As a result, they have slight variations in the minerals that have formed within them, and that results in slight differences in their chemical compositions.

So as well as being able to tell whether the ash came from say Taupō or Auckland, even amongst the Taupō volcanoes – a number of them might be rhyolites – but we can tell the different volcanoes apart and we can sometimes even tell the different eruptions apart, because not every magma in every eruption underwent the same conditions before it erupted.