Rocks and Their Origins

by Grenville A. J. Cole

Fig. 14. Side of a Volcanic Cone. Ash-layer of 1906 on the west flank of Vesuvius. Cliffs of the exploded crater of Monte Somma behind.

The fine-grained layers of volcanic dust, commonly spoken of as ash, and the coarser tuffs, in which lumps of scoriaceous lava are clearly visible, bridge the gap between sedimentary and igneous rocks. The dust, during a great eruption, is distributed by wind over hundreds of square miles of country. The tuffs, deposited nearer the orifice of the volcano, vary in coarseness from day to day, and exhibit marked stratification. Ash-beds and tuffs may be laid out in lakes or in the sea, and their layers may then include organic remains. Waves may round their particles on the shore, and may sift them till only a coarse volcanic sand remains.

After an eruption, the newly deposited ash and tuff usually form obvious layers on the surface of the country. Landslips on the side of the volcanic cone may reveal sections of the new coating and of previously stratified material ([Fig. 14]). In certain districts, sedimentary and other rocks torn off from below form a large part of the fragmental deposits of volcanic action. The characteristic volcanic cone is itself due to the greater accumulation of tuffs and ashes near the vent ([Fig. 15]).

The loose tuffs formed of scoriæ allow water to percolate easily through them, and a cone of fairly coarse material resists the weather well. The remarkable freshness of the extinct "cinder-cones" of Auvergne was thus long ago explained by Lyell. Surfaces of ash, on the other hand, are easily washed down by rain in the form of dangerous mud-flows, which spread across the lowlands, and give rise to compact clays, shrinking as they dry.

Fig. 15. Tuff-Cone with Tuff-Beds at the base. Puy de la Vache, Puy-de-Dôme, France.

Lava-flows are masses of molten rock that have welled out from the vent, without being torn to pieces by the explosion of the gases that they contained. The rapidity of their flow depends on their chemical composition, on the amount of gases present, and on the temperature at which they are extruded. The more highly siliceous lavas, for a given temperature, are more viscous than those towards the basaltic end of the series, which contain only about 48 per cent. of silica. A lava of considerable fluidity will consolidate in somewhat thin sheets with smooth and ropy surfaces. A less fluid type will become markedly scoriaceous, where the vapours endeavour to escape from it; the rugged crust formed on its upper cooling surface will be broken up by the continued movement of the more liquid mass below, and the blocks thus formed may become rolled over the advancing front of the flow and entombed in the portion that has not yet consolidated.

The surface of ordinary lava-flows remains rough for centuries, and only slowly crumbles down before weathering to form a soil. While tuff-beds provide light and fertile lands, the lava-streams remain marked out among them, as sinuous bands of rock, given over to an irregular growth of woodland. By repeated outflows, lavas tend to fill up the interspaces between the earlier streams, just as these have filled up the hollows in the country over which they spread. A uniform surface thus arises, and lava-plains eventually bury a varied land of hill and dale. Where a number of small vents have opened, perhaps along parallel fissures in the earth, the flooding of the country with igneous rock may lead to an appearance of stratification in masses extending over hundreds of square miles. Sections in the igneous series, however, show that the individual flows dove-tail into and overlap one another, more rapidly than is the case with the lenticular masses that constitute an ordinary sedimentary series.

After the constituents of the lava have begun to crystallise, and when the rock may be considered solid, cracks due to contraction are set up. The upper part of the flow, radiating its heat and parting with its gases into the air above, solidifies comparatively rapidly, and cracks arise without much regularity. Now and then, columnar structure, like that of dried starch, appears on a small scale, the columns starting from various oblique surfaces of cooling, and lying in consequence in various directions in the rock.

J. P. Iddings shows that curvature of the columns will result if one portion of the surface loses heat more rapidly than another. As the contraction-cracks bounding the columns spread inwards, the layer reached by them at any time in the lava will be farther in from a part of the surface where cooling is rapid than it will be from a part where it is slow. Hence the layer in the lava where contractional stresses are producing cracks, i.e. the layer reached at any time by the inner ends of the contraction-columns, will be a curved one, and its curvature will increase as it occupies positions more and more removed from the surface of the lava-flow. The axes of the contraction-columns, as they spread, are perpendicular to this layer, and the columns will thus curve as their development proceeds.