Morphological variability of the top of large inflated pillow mounds: indication about eruptive processes

Abstract:

Submarine eruptions produce pillow, lobate, or sheet flows, and develop a wide range of flow morphologies from individual pillow mounds to large smooth flows. High-resolution bathymetry enables classification of submarine flows and provides information about eruptive processes. Recently, 1-m resolution AUV bathymetry has revealed numerous large inflated pillow mounds along mid-ocean ridge axes as well as on other volcanic systems such as the flank of Loihi in Hawaii. Those inflated pillow mounds are generally a few hundred meters wide and a few tens of meters tall but can exceed a kilometer in width and a hundred meters in height. Their tops are flat plateaus with slopes generally less than 2^°.

Summit plateaus are often modified by depressions, inflated sheet flows, or collapsed lava lakes, that may coexist in the same flow. Those different structures and relations between them reveal information about the eruptive chronology. Depressions at the summits indicate a drainback of the lava at the end of the eruption, whereas sheet flows indicate a pressure increase in the molten core of the inflated pillow mound. In some case, they also indicate a second magmatic pulse after the drainback or a second eruption using the same tube and area of weakness, with, for example, the presence of a pillow mound inside the depression.

We analyze in detail the morphology of those flows, using high-resolution bathymetry and submersible dive observations from Axial Seamount. This study links variations of depression and inflated sheet flow morphology to variations of the thickness of the flow crust. Thicker crust tends to be preserved and tilted during the drainback, whereas thinner crust tends to sag or break and form talus. The thickening of the crust depends of different parameters including the length of time before the lava drained and the amount of inflation, which also in turn depends on lava rheology, extrusion rate, and slope. Thus, the history of the eruption is complex to determine. Crust thickness, flow volume and top morphology are compared to monitored 2015 Axial flows which lasted for several weeks as indicated by seismo-acoustic signals. This morphological analysis might help to estimate rates of emplacement for previous unmonitored similar flows.