Abstract
Columnar joints form by cracking during cooling-induced contraction of lava, allowing hydrothermal fluid circulation. A lack of direct observations of their formation has led to ambiguity about the temperature window of jointing and its impact on fluid flow. Here we develop a novel thermo-mechanical experiment to disclose the temperature of columnar jointing in lavas. Using basalts from Eyjafjallajökull volcano (Iceland) we show that contraction during cooling induces stress build-up below the solidus temperature (980 °C), resulting in localised macroscopic failure between 890 and 840 °C. This temperature window for incipient columnar jointing is supported by modelling informed by mechanical testing and thermal expansivity measurements. We demonstrate that columnar jointing takes place well within the solid state of volcanic rocks, and is followed by a nonlinear increase in system permeability of <9 orders of magnitude during cooling. Columnar jointing may promote advective cooling in magmatic-hydrothermal environments and fluid loss during geothermal drilling and thermal stimulation.
Highlights
Columnar joints form by cracking during cooling-induced contraction of lava, allowing hydrothermal fluid circulation
Our results are consistent with incipient columnar joint formation at a relatively high temperature, yet within the range at which the magma body is fully elastic
Given that no stress accumulates at temperatures above the solidus (Fig. 3a, inset) and that an undercooling of at least 90 °C is required to induce tensile fracturing in this basalt, we advance that columnar joints must form and propagate in the solid state, at high-tomoderate temperatures
Summary
Columnar joints form by cracking during cooling-induced contraction of lava, allowing hydrothermal fluid circulation. Using basalts from Eyjafjallajökull volcano (Iceland) we show that contraction during cooling induces stress build-up below the solidus temperature (980 °C), resulting in localised macroscopic failure between 890 and 840 °C This temperature window for incipient columnar jointing is supported by modelling informed by mechanical testing and thermal expansivity measurements. Columnar joints form by cracking due to cooling-driven contraction of igneous rocks[1], which results in tensile failure[2]. Their presence in the rock record has long represented one of the most awe-inspiring geological features[3] and their regular geometry has challenged our understanding of pattern ordering during thermal contraction[4].
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