THE CONSTRUCTION OF LAVA CHANNEL ISLANDS WITHIN BASALTIC FLOWS

Sinuous channels are observed on Earth as well as the Moon, Mercury, Venus, Mars, Io, and Titan. Identifying the style of formation of a channel on the Earth is easily done through field work. Similarly, samples returned by Apollo showed that fluvial processes did not form channels on the Moon. Subsequent exploration has unambiguously demonstrated the volcanic origin of sinuous channels on Venus, Mercury, and Io. Continued acquisition of martian data seems to heighten the debate as to whether sinuous and sometimes branching channel networks formed via fluvial or volcanic processes. The source of this debate is the possibility of both processes leading to similar landforms, especially if masked by millions of years or more of subsequent processes. Here we focus on islands, a landform that is typically associated with fluvial processes but which can also form during the volcanic construction of channel networks, and are therefore not considered to be diagnostic of past fluvial activity.

Here we present observations from locations in Hawai’i and New Mexico. Large lava channel islands can form at a distance from the vent where multiple channel branches are active to feed the advancing flow front. In this case, channel segments that have diverged can reconnect, surrounding a kipuka of older terrain with new lava. These lava “islands” might remain as a negative topographic feature that preserves older surfaces, or can eventually be buried by overflows from either or both channel segments. Other types of lava islands form within a channel when an obstacle (levee wall, rafted cone sections, lava balls, etc.) becomes permanently lodged within or welded onto the channel floor. In this scenario, continued flow begins to build a lava island around the obstacle, typically in a blunt manner upflow, towards the vent and in a tapered manner downflow, towards the advancing front. Lava islands built around obstacles in some cases rise to heights that exceed the adjacent channel levees due to lava splashing up and onto the growing island. Whereas some lava islands clearly appear unique from streamlined fluvial islands, they can also possess characteristics that are difficult to distinguish from the two processes. As such, the use of islands in channelized terrains on Mars as diagnostic evidence of a fluvial origin should be used with caution.