Volcaniclastic Sandstone

Introduction

Volcaniclastic sandstones occupy a special position within the broader lithic arenite family. They are distinguished not by their texture or sorting but by the origin of their grains - volcanic material, either erupted directly by pyroclastic activity or released by the weathering of older volcanic rocks. Their distinctive grain assemblage marks them as products of an active volcanic arc or of a terrain underlain by extensive older volcanism.

Description and Composition

Volcaniclastic sandstones are a special kind of lithic arenite composed primarily of volcanic detritus. They may be made up largely of pyroclastic materials that have been transported and reworked, or they may contain volcanic detritus derived by weathering of older volcanic rocks. ^[1]

They are especially characterised by the presence of euhedral feldspars, pumice fragments, glass shards, and volcanic rock fragments, and they generally have a very low quartz content. ^[1]

The euhedral feldspars are a key diagnostic feature. Normal sedimentary transport rounds and fragments crystals progressively. Euhedral crystals - with their original geometric form preserved - indicate either very short transport or that the crystals were phenocrysts in a lava or pyroclast that was disaggregated and deposited rapidly. Glass shards are fragments of volcanic glass, typically produced by explosive eruptions that shatter magma into fine glassy particles. Pumice is the highly vesicular, low-density froth formed when gas-rich magma solidifies; it floats on water and can be transported great distances. All three - euhedral feldspars, glass shards, and pumice - are unstable and are destroyed quickly by chemical weathering, so their presence signals geologically young or rapidly buried volcanic detritus.

Distinction from Hybrid Sandstones

A few less abundant types of “sandstones” are known whose constituents formed largely within the depositional basin by chemical or biochemical processes. These, called hybrid sandstones by some authors, include uncommon varieties such as greensands (glauconitic sands), phosphatic sandstones, and calcarenaceous sandstones (composed of sand-size carbonate grains). These rocks are not true sandstones (siliciclastic rocks) but rather are chemical or biochemical sedimentary rocks. ^[1]

This distinction matters for classification. Volcaniclastic sandstones, despite their unusual mineral assemblage, are genuine siliciclastic rocks - their particles are derived from a pre-existing rock (volcanic rock) that was broken down and transported, just like any other lithic arenite. Hybrid sandstones, by contrast, are not: their particles nucleated in the depositional basin, not in a source terrain. Glauconite, for example, forms authigenically on the seafloor; it is not a detrital grain at all.

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