Morphology and Architecture of Flood Basalt Lava Flows and Sequences

Abstract

The morphological types of flood basalt lava flows (pāhoehoe, ‘a’ā, etc.) are guides to their emplacement dynamics, with the relationships between flow conditions and lava flow morphology being established through observations of modern basaltic eruptions at places like Hawaii or Reunion Island. This is an excellent example of the application of the fundamental geological axiom, “The present is the key to the past”. Of course, the volumes of even single prehistoric flood basalt eruptions (commonly reaching 500–1000 km3, e.g., Self et al. 1997) are hundreds or thousands of times larger than any modern basaltic eruptions (~1 km3), though historical eruptions such as the 1783–1784 Laki eruption in Iceland (15 km3, Guilbaud et al. 2005) provide an intermediate step through the size spectrum. The older flood basalts that formed vast lava plateaus are now mainly exposed in vertical section. The flow morphology and stacking geometry of their eruptive units, as observed in vertical section, reveal the gradual build-up of the stratigraphy by lava flows sourced from different eruptive vents, and how eruption rates and flow dynamics varied through time.