Hyaloclastite is a type of volcanic rock formed when a volcano erupts underwater; it is a breccia, a rock composed of the fragments of other rocks. It may consist of a number of different minerals, and is distinguished by its highly particulate structure, containing a large amount of volcanic glass fragments.

Normally, when a volcano erupts underwater it will be deep enough that the water pressure helps stabilize the interactions twixt steam and magma, resulting in pillow lava -- bulbs of lava that form calmly, without chaos and noise, and without turning their contents to purée.

We're not certain what conditions are needed to disrupt deepwater lava flow and cause them to form hyaloclastite rather than pillow lava. (Perhaps strong vibrations? Lava fragments thrown free of the main flow?) But we do know why hyaloclastite forms in shallow* water; when the hot lava hits the water, the water expands explosively into steam, violently fragmenting and cooling the lava; this quick cooling may in turn lead to further fragmentation.

These fragments are not undistinguished rubble, but a mish-mash of various minerals. The quick cooling of the lava causes obsidian to form, although this is usually obsidian formed from basalt, meaning that is comparatively low in silica; because of this it must be cooled faster, and from a higher temperature, than the rock we usually think of as obsidian. Specifically, hyaloclastite contains tachylyte, a black glass, along with sideromelane, a yellow-brown glass lacking the iron oxide that gives tachylyte its dark color. When the sideromelane interacts with water certain metals are leeched out (iron, magnesium and calcium), and are replaced by aluminum, forming palagonite. The process of palagonitization helps to cement the hyaloclastite together more firmly.

The flecks of glass imbedded in hyaloclastite may be from 1mm to several cm in size, and often aren't apparent until you look for them under a microscope.

Hyaloclastite ridges, AKA tindars, are ridges or mounds formed by a volcano erupting under a glacier. The volcano will first form pillow lava, and then as the lava melts though the ice and reaches shallower water, will form a layer of hyaloclastite on top. These formations are common in Iceland, although they aren't as well known, or as well studied, as are the less common tuyas, volcanoes that built up over the water level, forming flat-topped mountains.


* Where exactly 'deep water' ends and 'shallow water' begins is a matter of some debate, but it is at least a hundred meters down, and perhaps 3-400 meters. It may also depend on other factors, i.e., the temperature of the lava, extra pressure caused by steam and water being trapped under a heavy layer of ice, etc.

References:
http://www.eos.ubc.ca/~krussell/subglacial/pages/deposit_chars.html
http://en.wikipedia.org/wiki/Hyaloclastite
http://www.ees1.lanl.gov/Wohletz/PhysSub.pdf
http://cat.inist.fr/?aModele=afficheN&cpsidt=16171350
http://eprints.lancs.ac.uk/270/01/Schopka_et_al_2006.pdf

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