Chapter 19 - Submarine Lavas and Hyaloclastite

Abstract

Submarine lavas are the most widespread surficial igneous rocks on Earth and form in a range of sizes, shapes, and compositions. Pillow lavas are the best-known submarine lava, but changes in eruption conditions also allow production of basalt sheet lavas, lava ponds, and even major lava channels, in proportions that vary greatly among eruptive centers and through time. Submarine felsic lavas, along with both extrusive and intrusive lava domes, are common in island arcs where volcaniclastic sediment lies atop transitional to continental crust favorable for the formation of the evolved felsic magmas. Hyaloclastite comprising glassy fragments derived from lava spalling and fragmentation also varies in abundance, from neglible amounts associated with the advance of basaltic pillows over flat topography, to locally predominant where they flow and break up on slopes, or the margins of felsic lavas that break apart during aqueous cooling either in situ or to be redeposited on lava dome flanks. Submarine pumice domes and flows can release fragments of low density that rise buoyantly and float away from their source. Intrusions undergoing marginal fragmentation form peperite, a rock combining lava fragments with a host of pre-existing clastic sediment that can also form by mingling of liquefied sediment with fluid magma.