ABSTRACT
Volcanoes do not remain intact for very long after they stop erupting because atmospheric agents of erosion soon attack them and alter their appearance. These agents, chiefly rivers, weathering, glaciers, landslides and the sea, are all very selective in their action. They attack weaker rocks faster than stronger rocks, so that the weaker are carved into valleys and the stronger survive as hills and ridges. Volcanic rocks are particularly prone to selective erosion because they are so varied. In general, the compact rocks such as lavas occurring in flows, domes, or vent plugs are the most resistant to atmospheric agents, whereas loose fragments that are not bound together with a firm cement, such as ash, tuffs, lapilli, cinders and pumice, are always weak. One exposed layer of loose rock can weaken a whole mass so that it is quickly degraded, but, on the other hand, a lava-flow can also form a protective cap-rock which may delay erosion for millennia. Thus, a volcano composed of many lavaflows is much more resistant to atmospheric degradation than one that has been built up of unconsolidated fragments. Lava is the armour-plating of volcanoes.
