Anatomy of a volcano

Pay attention to the following terms in bold. (R.P. – 5.3)

Volcanoes take two main forms. Fissure or linear volcanoes chiefly emit basic lava from a crack in the Earth’s crust. Central volcanoes yield lava, ash and other products from a single hole. These products build a shield- or cone-shaped mound: the typical volcano shape. Central volcanoes can grow high and fast. In western Mexico in 1943 Paricutin grew 490 ft. (150 m) high in a week and reached 1500 ft. (450 m) in a year. In western Argentina extinct Aconcagua towers 22,834 ft. (6 960 m) above sea level; this is the highest mountain in the western hemisphere.

A cross section through an active central volcano would reveal these features. Miles below the surface lays the magma chamber - a reservoir of gas-rich molten rock under pressure. This pressurized magma may “ balloon outward ” against the surrounding solid rock until it can relieve the pressure by escaping through a weakness in the crust above. From the chamber, magma then rises through a central conduit. As magma rises the pressure on it is reduced and its dissolved gases are freed as expanding bubbles. Finally the force of gases blasts opens a circular vent on the Earth’s surface. From this outlet ash, cinders and flows of lava build the main volcano shield or cone. Vent explosions shape its top as an inverted cone or crater. Meanwhile, side vents on the flanks of the volcano release ash or lava that may build subsidiary cones.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)