Disclosing the temperature of columnar jointing in lavas

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.

Increasing the Permeability of Hydrothermally Altered Andesite by Transitory Heating

“…Nonetheless, we observe that the physical properties of moderately and highly altered Rotokawa andesite change after exposure to temperatures as low as 350 °C and that temperature increase consistently results in an increase in porosity (Table ). We attribute the observed porosity and permeability increase to a combination of mineralogical breakdown of clinochlore, quartz, and calcite and to microcracking resulting from inevitable small stresses induced by thermal gradients and reaction driven‐volume changes during both heating and cooling (Browning et al, ; Eggertsson et al, ; Ryan & Sammis, ; Heap, Lavallée, et al, ; Lamur et al, ; Vinciguerra et al, ). In addition, the altered samples are heterogeneous and crystalline, and the assumptions used to calculate the appropriate heating and cooling rate may, therefore, lead to overestimation.…”

“…Geochemistry, Geophysics, Geosystems gradients and reaction driven-volume changes during both heating and cooling (Browning et al, 2016;Eggertsson et al, 2018;Ryan & Sammis, 1978;Heap, Lavallée, et al, 2014;Lamur et al, 2018;Vinciguerra et al, 2005). In addition, the altered samples are heterogeneous and crystalline, and the assumptions used to calculate the appropriate heating and cooling rate may, therefore, lead to overestimation.…”

“…The effects of temperature on the physical, chemical, and mineralogical properties of volcanic rocks has been the subject of a range of studies (e.g., Björnsson, ; Browning et al, ; Kitao et al, ; Siratovich, Villeneuve, et al, ,Siratovich, von Aulock, et al, ), but the range of scenarios (e.g., heating vs. cooling), conditions (e.g., magnitude of temperature fluctuations), and reservoir rock types (e.g., unaltered versus altered) has prevented the generalization of the impact on the resultant reservoir rock permeability. For instance, cooling results in contraction, which may generate macroscopic thermal cracks to promote fluid flow (Lamur et al, ), yet it may also precipitate secondary mineralization that can, through time, block otherwise permeable pathways (Heap et al, ). In contrast, the impact of temperature increase (e.g., from magma intrusion) on the resultant rock permeability remains poorly constrained (Gaunt et al, ; Kushnir et al, ) as it has received noticeably less attention than the influence of changes in pressures on permeability (e.g., Cant et al, ; Eggertsson et al, ; Gudmundsson, , ; Heap, Baud, et al, ; Heap et al, ; Lamur et al, ).…”

A Model for Crack Initiation in Solidifying Lava

“…Lamur et al () published high‐temperature mechanical data on basalt from Eyjafjallajökull volcano (Iceland). We use these data for choosing the input parameters of our model.…”

“…However, we expect that the higher strength slows down fracture advance in the lower layer. Lamur et al (2018) published high-temperature mechanical data on basalt from Eyjafjallajökull volcano (Iceland). We use these data for choosing the input parameters of our model.…”