Chapter 17 - Lava Flows and Rheology

Abstract

This chapter deals with the morphology and rheology of terrestrial basaltic and silicic lava flows. Here we take a facies-based approach, describing each lava type in terms of its typical geometry, texture, structures, movement patterns and incorporation of alien objects. Using such an approach we distinguish the three classically defined types of lava: ‘a‘ā, pāhoehoe and block, as well as their sub-types. Inflation is a widespread phenomenon at both ‘a‘ā and pāhoehoe flows. Indeed there is no part of the extensive pāhoehoe flow fields of Kīlauea that is not inflated. We thus also classify and describe the main inflation features, as well as the components of the distribution systems that feed lava flows, namely channels and tubes. We finish by considering the control lava rheology, cooling and cooling-driven groundmass crystallisation has on flow dynamics. We base our review on the classical studies, classification schemes and definitions whose modern roots span the 1970s through 1980s, following formalisation of the ‘a‘ā, pāhoehoe and block lava distinction in 1953. These basic, fundamental schemes were based on careful observations and descriptions which still hold today. It is our opinion that modern volcanology now needs to focus on improved measurement of the emplacement dynamics and mechanics associated with each flow type.