Lava squeeze-ups and volcanic resurfacing: a review

Squeeze-ups are common features of basaltic lava flow-fields on Earth. Squeeze-ups, rootless autointrusions of lava, drive intraflow resurfacing of active flow-fields, and the term is also used for linear or bulbous extrusions (syn. “breakouts”, “outflows”) from cracks in the solid crust, or from tensional clefts on tumuli. Despite the abundance of squeeze-ups in the active or Recent flow-fields of Hawaii, Etna or Iceland, and in the prehistoric continental flood basalt (CFB) provinces, there has been no dedicated study of them for about a century. Here we present a well-illustrated study of squeeze-ups in subaerial basaltic lavas, with a focus on the Deccan Traps CFB province of India, and show that squeeze-ups have a great range of sizes, shapes, orientations, and host flow-field morphologies including pāhoehoe, slabby pāhoehoe, rubbly pāhoehoe, slab-crusted lavas, and ‘a'ā. Squeeze-ups, as the name suggests, originate from overpressurisation of lava flow interiors, caused by mechanisms such as an increased lava flux, progressive downward solidification of the upper crust, crystallisation leading to volatile exsolution (second boiling), viscous fingering, and constricted or blocked lava tubes. The overpressurised lava may create a hydrofracture in the crustal roof in the manner of a dyke, or open an incompletely sealed contact between adjacent flow-units in a compound flow. Similar-looking features formed by passive tapping of interior lava by fractures forming in the cooling, contracting and deforming crust can be termed “lava crack-seals”. We offer a formal discussion of the theoretical and outcrop similarities and differences between dykes and squeeze-ups, features usually easy to distinguish but not always so. We also discuss the distinctions between feeder dykes and pseudodykes, and between squeeze-ups and lava fingers, features often liable to mutual confusion in outcrop. Finally, we review the existing definitions of squeeze-ups, and provide a new formal definition that includes their surface and subsurface realms, the product, and the process: “Squeeze-ups are rootless autointrusions that cause endogenic growth and intraflow resurfacing in lava flow-fields, or rootless extrusions produced by such resurfacing, involving brittle deformation of their host rock.” It is certain that, with future high-resolution remote sensing imagery, squeeze-ups will be recognised on the surfaces of the terrestrial planets other than Earth, given their ubiquity and essential role in the physical development of basaltic lava flow-fields on Earth.