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1. Introduction
Large-scale basaltic volcanism is a major contributor to environmental stress and change. The disastrous environmental impact of the historical (1783–84), 15 km3 Laki basaltic eruption in Iceland (Guilbaud et al. 2005) is well known. The prehistoric continental flood basalt (CFB) provinces were constructed from large numbers of such eruptions, each of which may have produced hundreds to thousands of cubic kilometres of lava with significant volatile release over only short periods of time (years to decades, e.g. Self et al. 1997, 2014). It is therefore unsurprising that many CFB events correlate with biological mass extinctions (e.g. Wignall, 2001; Ernst & Youbi, 2017).
Flood basalt lava flows have been described as simple or compound (Walker, 1971). A simple lava flow is a thick and laterally extensive flow not divisible into internal units; a compound lava flow is composed of numerous individual flow-units (Nichols, 1936). Each flow-unit or lobe (Self et al. 1997) is a separate cooling and vesiculation unit bounded by a chilled glassy crust. Lobes range in size from toes, up to 50 cm thick and 100 cm in the long dimension, to sheet lobes that are tens of metres thick and kilometres to tens or even hundreds of kilometres long (Self et al. 1997; Thordarson & Self, 1998), and correspond to Walker’s (1971) simple flows.
In sheet lobes that are essentially stationary due to lack of new lava supply, and are solidifying, columnar joints form due to thermal contraction of the lava (Fig. 1). By linking up laterally, and propagating downwards from the top and upwards from the base, they divide the lobe into columns with five, six or seven sides. Each column grows by small increments of brittle fracturing and crack propagation at the transition between solidified and liquid lava, as the lobe cools progressively inwards. This leaves horizontal striations on the face of a joint column (Fig. 1a), called chisel marks (James, 1920) or striations (Ryan & Sammis, 1978; DeGraff & Aydin, 1987; DeGraff et al. 1989). A thick sheet lobe can develop several subhorizontal tiers with distinct jointing characteristics (Fig. 1b–d). A tier characterized by a regular arrangement of relatively thick, well-formed columns is called a colonnade, whereas a tier characterized by thin (<10 cm), chaotically arranged...