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 different substances. The water may also dissolve a large quantity of carbonic acid gas, and may carry this gas up to the surface in such abundance as to produce violent eruptions, in the form of jets of water, which are thrown up with great force and noise, and which may cause much damage to the surrounding country. But the most important action of volcanoes in modifying the surface of the earth is by the ejection of vast quantities of solid matter, consisting of scoriæ, cinders, and ashes. These materials fall back to the earth, and accumulate in a more or less regular manner around the orifice of the volcano. By this means, a conical hill or mountain is gradually built up, which may rise to a great height, and which may have a more or less regular outline. Such a hill or mountain is called a volcanic cone, and may be regarded as a most interesting and instructive example of the action of internal forces in modifying the surface of the earth. Thus, it is evident that volcanoes play a crucial role in shaping the landforms we see today. Their eruptions, lava flows, and accumulation of materials contribute to the formation of various geological features such as plains, hills, and mountains
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