Some physical requirements for the emplacement of long basaltic lava flows

Abstract

Long basaltic lava flows (over 100 km in length) require specific emplacement conditions to prevent the lava from freezing as it is transported to the flow front. The minimum dimensions of the lava transport systems (tubes, channels, or sheets) require that the flow have a volume greater than several cubic kilometers. Long lava flows are emplaced on slopes less than 10% (∼5°) and the lava being transported must cool at a rate less than 0.5°C/km. We show that there are two modes by which thermally efficient, long distance lava transport can be achieved: (1) “rapid” emplacement in which the lava flows so quickly that it does not cool excessively despite large heat losses and (2) “insulated” emplacement in which heat loss is minimized. We here estimate cooling in the rapid mode using a modified version of a previously published thermal model for aa flows and find that, for a range of inputs appropriate for subaerial terrestrial condition, effusion rates of at least 3100 to 11000 m 3/s, channel flow velocities in excess of 4–12 m/s, and minimum channel depths of 3–17 m are required for basaltic flows >100 km in length. For emplacement in the insulated mode, we construct a very simple heat balance model for roofed sheet flows which shows that extremely long sheet‐fed flows are possible with velocities as low as 0.2–1.4 m/s, flow thickness of 6–23 m, and minimum effusion rates of the order of 50–7100 m 3/s. Also, earlier work has suggested that tube‐fed flows more than 100 km long can be produced at effusion rates as low as several tens of m 3/s and with tube diameters of a few tens of meters. We argue that flows emplaced in the rapid mode should be morphologically similar to channel‐fed aa flows while those emplaced in the insulated mode should be similar to tube‐fed or sheet‐like inflated pahoehoe flows. This leads to several field criteria for distinguishing these two modes of emplacement in ancient lava sequences. Additional constraints on the emplacement of long lava flows are expected from the continued study of the formation and evolution of lava channels, tubes, and sheets.