Pāhoehoe lava emplacement in Lon Reudle, Mull.

Abstract

The base of the Mull Lava Sequence, part of the Palaeogene British Igneous Province, contains a variety of volcanic features, e.g., small-scale filled lava tubes and toes that can be interpreted as large-scale inflated sheets from pāhoehoe lava flows. Here, we re-examine a previously reported unique outcrop found along the coastline of Lon Reudle, Mull, which can be better understood under that conceptual model. The outcrop is composed by multiple layers, with repeating porphyritic, poorly columnar jointed and vesicle-free layers, alternating with vesicle-rich layers with individual vesicles measuring up to 1 m. We reinterpret the outcrop as an individual inflated pāhoehoe lobe with an aggregated total thickness of at least 25 m. The estimated thickness of the upper crust of this lobe is 15 m, as the upper crust would account for 40-60% of the total volume of the flow, while the time required for this upper crust to form is ∼4 years. The alternation between vesicle-rich and vesicle-free layers is likely to be caused by instabilities of the suspended bubbles in the basaltic melt, forming a diapir with abundant evidence of bubble coalescence. Typical Hawaiian pāhoehoe lava flows have thickness of 1-3 metres, while in other flood basalt provinces thickness can reach up to 75-80 metres, suggesting that pāhoehoe structures are scale-independent. This is a key factor to take into account at the time of re-interpreting other lava structures that have been found in the Palaeogene British Igneous Province.