Keith Klepeis on How Plutons Form
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Plutons are bodies of igneous rock that crystallize from magma at depth below the Earth’s surface. But even though the magma never makes it to the surface, it still has to travel many kilometers up from its source near the base of the crust to the upper crust where plutons form. In the podcast, Keith Klepeis explains how it makes that journey and describes the shape of the resulting structures. Many of his findings come from one region in particular that provides an exceptional window into the origin, evolution, and structure of plutons – the Southern Fjordland region of New Zealand’s South Island.
Klepeis is a Professor at the College of Arts and Sciences at the University of Vermont.
Podcast Illustrations
All images courtesy of Keith Klepeis unless otherwise noted.
The Southern Fjordland region of New Zealand’s South Island. The region exposes a large batholith, called the Median Batholith, but the only exposures of rock in this rugged temperate rain forest are along the fjord coastlines and above the treeline near the peaks. In the podcast, Klepeis explains how they relied on boats and helicopters to do their fieldwork.
Subduction Zone
Most plutons form above a subducting oceanic plate. Once it reaches a depth of about 150 km, the subducting oceanic lithosphere undergoes dehydration reactions that liberate water. The water rises and, when it impinges on the overriding continental plate, causes the melting temperature to drop, with the result that magma is created. While some magma reaches the surface and forms a Cordilleran volcanic arc, the remainder cools at depth, forming the plutons and their associated structures that Klepeis describes in the podcast.
Adapted from Lillie, R.J. (2005), Parks and Plates: The Geology of our National Parks, Monuments, and Seashores
Diagrammatic cross-section through the continental crust across a subduction zone showing a volcanic system with its underlying feeder dikes and batholith. Basaltic magmas pool at the base of the crust where they differentiate and melt crustal rocks. More evolved andesitic magmas then rise to the middle crust forming batholith complexes. Magmatic fluids continue to ascend through the crust and may form a shallow pre-eruptive chamber.
Richards, 2011
The Median Batholith
Klepeis and his team focused their recent research on the Median Batholith in the South Fjordland region of New Zealand. Today, the batholith is offset by the Alpine Fault. The inset at bottom right shows a reconstruction of the batholith prior to 37 million years ago.