When water is heated by volcanic activity, strange and spectacular landscapes are created. Known as hydrothermal areas, they can feature_ steaming hot springs, gurgling pools of mud and jets of water spouting hundreds of feet into the air.
Volcanoes mark vents where molten rock achieves the Earth’s surface — often in violent fashion. From subtle fissures to skyscraping peaks, these landforms are both destructive and constructive: They can smother terrain and ecosystems with lava, mudflows and ash, but also nourish biological communities with fertile soil and — significantly — create new topographic features.
Volcanoes, of course, are themselves landforms: sometimes subtle, sometimes unmistakable and dramatic. The steeply conical silhouette of a composite or stratovolcano — the classic image of a volcano in most minds — derives from intermixed layers of viscous lava, ash and other “pyroclastic” materials accumulated over many eruptions and emissions. In sharp contrast, a shield volcano — such as enormous Mauna Loa and Mauna Kea in Hawaii — assumes a much gentler slope from easily flowing basaltic lava. Volcanoes may also assume the shape of cinder cones and lava domes. Where weathering and erosion have stripped outer layers from extinct volcanoes, all that may be left on the landscape are resistant remnants of their “throats” and conduits in the form of volcanic necks (or plugs) and dikes. A world-famous example of the former is Shiprock in New Mexico. In the oceans, volcanic seamounts and island arcs are major features marking volatile tectonic margins.
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