Volcaniclastic deposits on the mid-ocean ridge provide evidence for explosive magma fragmentation, lava flow disaggregation, mass-wasting and pelagic transport processes

Abstract:

Glassy basalt fragments on the modern mid-ocean ridge (MOR) system and in ancient exposed sections of oceanic crust (ophiolites) have long been recognized as the by-products of active deep-marine eruptions. Debate about their mode of formation has swayed from explosive to non-explosive magma fragmentation mechanisms. Recent ROV dives in volcanic arc settings have captured video footage of active explosive eruptions indicating that depth need not preclude an explosive origin to deep-marine glassy volcanic shards. Due to the absence of comparable ROV footage on the MOR, we must rely on detailed analysis of volcaniclastic lithofacies that occur there. Detailed studies of MOR volcaniclastic deposits offer a record of deep-sea eruption processes and their dynamic interaction with the hydrosphere. MOR volcaniclast componentry, granulometry and stratigraphy indicate that a variety of magma-fragmentation and clast dispersal processes occur: explosive, effusive, mass-wasting and pelagic processes, many of which operate in tandem. Moreover, rapid volatile cooling-rates upon eruption into the hydrosphere would cause “implosive” magma fragmentation processes. The deep MOR hydrosphere also causes steam-induced lava fragmentation and deeper crustal phreatomagmatic processes at pressures lower than the critical point of seawater.

Advancements in ROV coring strategies combined with high resolution AUV mapping have advanced our knowledge base of the physical and chemical characteristics of volcaniclastic deposits in modern oceanic settings. Here we present numerous volcaniclast samples from the Juan de Fuca Ridge and northern East Pacific Rise that formed from different fragmentation, transport and depositional processes. Deposits range from being nearly pure glass shards to non-descript ashy mud or ooze. Direct association, or disassociation, between volcaniclasts and adjacent lava flows, particularly for recent eruptions on Axial Seamount and highly evolved lavas on Alarcon Rise provide “type” examples for lithofacies interpretations. Chemostratigraphy and chronostratigraphy further show that some volcaniclastic deposits can be used as reliable key-stratigraphic marker beds that record important events in the history of a seamount, MOR segment or regional ocean basin events.