pyroclastic and volcaniclastic rocks

pyroclastic and volcaniclastic rocks Pyroclastic and volcaniclastic rocks are rocks that are formed of rock fragments (‘clast’, from the Greek klastos, broken in pieces), that are flung out of volcanoes. In practice pyroclastic and volcaniclastic rocks include all the products of extrusive volcanic activity except lava flows. A volcaniclastic rock is one containing volcanic material in whatever proportion and without regard to its origin or environment of deposition. The term ‘pyroclastic’ is used for classic rock material formed by volcanic explosion or aerial expulsion from a volcanic vent. The clasts in volcaniclastic rocks may range from fine ash to bombs and enormous blocks. The substances known as Pelée's hair and pumice are also grouped with pyroclastic and volcaniclastic rocks. Pelée's hair, a natural product of some volcanoes, is a very fine volcanic glass spun out like candy-floss. Pumice is a foam of volcanic gas bubbles volcanic glass. Some pumice is so filled with gas that it will float on water.

Large ejecta (materials thrown out of a volcano) are usually deposited close to the vent; progressively smaller particles travel farther. The finest-grained clasts are ash. Much ash falls immediately to the ground but often part of it heats the air around it, making it buoyant. When this happens ash clouds can rise up to 10 km or more before they reach neutral buoyancy and spread out to form a thin sheet in the atmosphere. Ashes from such sheets can be blown great distances by wind over long periods of time and over huge areas; indeed, ash occasionally travels round the world. Large quantities of volcanic ash in the atmosphere cause spectacular sunsets. Sometimes hot ash emerging from a volcanic vent rushes in a hot cloud called a nuée ardente down the side of the volcano. A hot ash cloud like this erupted from Mt. Pelée on May 1902. The burning cloud was hot enough to melt bottles and to twist metalwork, and it destroyed the town of St Pierre. The heat of a nuée ardente welds and streaks out the ash particles, which are then called flammé, to form an ignimbrite. A welded ash without flammé is called a welded tuff. Ash that has been deposited without being welded is simply called a tuff.

Small particles of lava (1 cm across) are called lapillae or scoria. Scoria is the most common material in most volcanic cones. A typical volcanic cone also contains lava flows and larger pieces of volcanic material called blocks and bombs. Blocks are lumps of solidified lava that are blown out of the volcano; bombs are portions of lava that are thrown out still molten. Bombs are divided into several types according to their form. Bread-crust bombs are those whose outer surface solidified while the inside was still emitting gas and expanding as a foam. As the inside expanded it cracked the outer shell, making it look like the crust of a loaf of bread. Splatter bombs are those that did not have time to solidify before they hit the ground; they resemble cow-pats.

Volcanic activity is often so violent that it entrains (picks up) pieces of ‘country rock’, which are not of volcanic origin. These accidental additions to volcanic rocks are called xenoliths (from the Greek, ‘strange stones’).

Judith M. Bunbury

Bibliography

Bullard, F. M. (1962) Volcanoes: in history, in theory, in eruption. Thomas Nelson, Edinburgh.
Fisher, R. V. and and Schminke, H.- U. (1984) Pyroclastic rocks. Springer-Verlag, Berlin.
Francis, P. (1993) Volcanoes: a planetary perspective. Clarendon Press, Oxford.