Volcanoes shape landforms from "summary" of Principles of Geology, Volume 1 by Charles Lyell,Sir Charles Lyell
The geological agency of volcanoes in modifying the surface of the earth is extremely powerful. The action of fire and heat, combined with that of water, has been singularly efficient in producing changes on the crust of our planet. The lava which flows from a volcanic vent may either spread over the country, or accumulate near the orifice from which it issued. In the former case, it may form a wide and extensive plain, like that which covers the Deccan in India, or the Patagonian steppes. In the latter case, it may rise up into a conical hill or mountain, varying in height and bulk according to the quantity of lava poured out, and the length of time during which eruptions continue with more or less vigor. The surface of the cone is usually covered with loose scoriæ, cinders, and volcanic ashes, which have been ejected during eruptions. These materials being highly porous, and the lava itself often decomposing so as to become porous, rain-water filters readily through them, and may give rise to the issue of a stream of water from the foot of the cone. Such a stream is called a lava stream, and, being impregnated with various mineral ingredients, may become highly charged with differe...
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