60 



GEOMORPHOLOGY: 



Figure 4.22 Some types of land forms created by volcanoes and tfieir activity. 



masses, called volcanic bombs; of gravel-sized cin- 

 ders; and of fine volcanic ash. When volcanic activity 

 is quieter, nearly flat lava domes are created, vi/ith no 

 debris. These flat cones have a wide vent opening, or 

 crater, a feature in distinct contrast to the small crater 

 of the high cinder cones. When activity is continu- 

 ously and exceptionally quiet, the flat volcano may be 

 little more than a lava pool, or shield volcano, on a 

 plain. Any nonexplosive activity results in lava flows 

 and/or mud flows. 



Most volcanoes are composite cones. The word "com- 

 posite" is applied because the cones are formed by a 

 combination of quiet and explosive cycles of activity. 

 For this reason, most volcanoes are cone-shaped and 

 are associated with a surrounding terrain that re- 

 flects both quiet and violent activity in the history 

 of the cone. 



Various features may be found in a volcanic land- 

 scape. In addition to the already mentioned land 

 forms, including intrusive ones, there are such things 

 as calderas, parasitic cones, spatter (or driblet) cones, 

 nested craters, and lava bridges (or tunnels). Calderas 

 are created by the explosive rejuvenation of an inac- 

 tive volcano. In this rejuvenation, the top of the old 

 volcano is freed and then collapses within the molten 

 magma. Because the top is melted, it disappears. 

 When the caldera becomes inactive, it possesses the 

 broad opening of collapse and an extensive flat plain 

 at the bottom of the opening. Crater Lake, Oregon, 

 is one of the famous calderas. (It is misnamed, be- 



cause its opening is a caldera, not a volcanic crater.) 

 Wizard Island, a volcano rising from the floor of 

 Crater Lake, represents a later stage of caldera de- 

 velopment, namely rejuvenation in the form of a new 

 cone upon the flat plain. 



Parasitic cones can form on the side of an inactive 

 volcano when activity is resumed. This land form and 

 the caldera indicate the very resistant nature of 

 previously formed igneous rocks. Because rocks fill 

 old vents, renewed activity often is not from the exact 

 site of the previous vent, so the new activity often 

 creates a parasitic cone or, rarely, a caldera. 



Spatter cones are formed if gases and lava sputter 

 out of a crack in the side of a cone. .Vested craters are 

 produced when each new member of a series of re- 

 juvenated cones is formed within the last inactive 

 cone. In other words, nested craters are the conse- 

 quence of a progressive process of cone-within-cone 

 formation. Lava bridges, or lava tunnels, are remnants 

 of lava passage under a pre-existing lava field. The 

 new lava is so hot that it melts the old lava and "cuts" 

 a path down and through the field. The tunnel forms 

 because the final downhill flows, those that solidify 

 upon the tunnel bottom, are less extensive and the 

 "freezing" of the final flow involves contraction. 



DISTRIBUTION OF VOLCANOES 



Much of the world is affected by current and past 

 vulcanism. At the present time there are about 430 



