Near-surface basaltic dykes may become unstable when buoyancy and overpressure are not maintained and solidification not fully accomplished. Additionally temporal variation of the dyke geometry such as widening may further compromise stability and induce draining. A 5-10 m wide, variably dipping basaltic-andesite dyke in the wall of Red Crater, New Zealand, records textural evidence for temporal evolution of geometry and magma flow direction. (1) A marginal zone with groundmass shear fabrics and pervasive flow foliation provides evidence for continued upward magma migration. Shear is resolved against an in-stepping front of crystallization. (2) Open dyke-length vertical fractures in the baked zone of the host scoria deposits show that the dyke inflated during continued up-flow. (3) A central zone hosts asymmetrical down-verging flow folds and a parabolic strained vesicle profile. In concert with an empty upper cavity in the middle of the dyke, these field data show convincing evidence for subsequent draining during magma withdrawal to shallow levels. We interpret these textures to establish a conceptual scenario of emplacement, widening and draining. Using simple thermodynamic considerations of the timescales of conductive cooling and a range of magma thermal diffusivities, we estimate that the emplacement phase duration was 54 – 100 hours. Furthermore, we use the geometry of the dyke to estimate that draining under gravity for a magma viscosity range of 10^2 – 10^4 Pas could occur over very short timescales. We compare theoretical draining times with the thermodynamics of solidification to scrutinize our numerical interpretation in the context of the field data. Draining of relatively low-viscosity conduits can occur when widening modifies the stability and this is a possible mechanism by which small-volume Strombolian eruptions stop.

Researchers should cite this work as follows:

• Fabian Ben Wadsworth (2013), "Timescales of dyke widening and draining during an eruption at Red Crater, New Zealand (COV 7, 2012)," https://theghub.org/resources/2380.

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1. Dyke
2. scoria cones