Hyaloclastites are an accumulation of angular basaltic glass fragments or their breccia. A breccia is a rock with volcanic fragments cemented in a fine-grained matrix.
These angular glass fragments form when water abruptly quenches hot lava flow extruded during a submarine or subglacial eruption.
This dramatic quenching, cooling, and contraction as water encounters hot lava form a thin, brittle, glassy rind around the lava.
The name hyaloclastite comes from the Greek word hyalos, which means glass, plus the name clast, which refers to rock fragments. These two words mean glassy fragments.
Note that hyaloclastites may be fresh (unaltered), partly or fully devitrified. Also, their size may vary from millimeter to inch scale.
Appearance
Hyaloclastites are poorly sorted, angular, entirely shattered glassy fragments of different sizes. They may be loose or consolidated into a breccia.
Also, they may be fresh or altered into palagonite. Palagonite is an orangish, brown, yellowish-brown, or green-brownish material. It forms from low-temperature basaltic glass alteration.
Composition
Most of the hyaloclastites have a basaltic composition and are mostly sideromelane fragments.
Sideromelane is a transparent yellow or yellowish-brown basaltic glass. It forms when magma rapidly cools from interaction with water.
Basalt is a mafic rock rich in dark-colored minerals and high in iron and magnesium.
However, some of the basaltic glass may be tachylite. This has tiny magnetite crystals, making it opaque. It forms when cooling isn’t so rapid.
Lastly, some ancient seafloor rock successions have intermediate to felsic or silica-rich hyaloclastites. Examples are andesitic or rhyolitic. Such will have coarser, non-vesicular to pumiceous glassy fragments.
Formation
Hyaloclastites form when lava or magma interacts with water, ice, or wet sediments. Also, they form when lava flows into rivers, lakes, seas, or other water bodies.
In a broader sense, the interaction of water or ice will cause explosive and non-explosive shattering that will form clasts.
Explosion and shattering may occur when hot magma and water interact. This happens when water forms steam, which, together with volatiles in magma, will cause an explosion and magma fragmentation. This creates breccias and hyaloclastites.
On the other hand, hyaloclastites can form from non-explosive spalling or shattering that occurs when hot lava or magma encounters water. The lava or magma surface will rapidly cool, forming a glassy rind, which will spall or shatter as it shrinks.
The spalling and shattering occur due to thermal stress resulting from contraction, forming angular glassy fragments of various sizes.
Brecciation and palagonitization
Freshly formed hyaloclastites are usually loose or unconsolidated. They typically have unaltered volcanic glass, usually basaltic.
However, this glass will quickly undergo alteration into palagonite.
Also, weathering and diagenesis (conversion of loose fragments to rock) will form a hyaloclastite breccia. One process that aids breccia formation is glass alteration or palagonitization.
Occurrence
Hyaloclastites are found in shallow or deep seas, including emergent islands, midocean ridges, island arcs, or seamounts.
Also, some occur under glaciers. British Colombia, Canada, and Iceland have landscapes of those formed during the last glacial period, which happened between approximately 115,000 and 11,700 years ago.
Usually, hyaloclastites occur as thin deposits on gaps or crevices and spandrels between pillow lavas and sheet flows.
However, some are thicker, especially those on steeper topography or associated with explosive eruptions of volatile-rich lavas.
References
- Batiza, R. & White, J. D. L. (2000). Submarine lavas and hyaloclastite. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes (1st ed. pp. 371-378) San Diego: Academic Press.