Volcanoes in Action (1935)

terrifying spectacle of a volcano in action once regarded with superstitious all for many centuries today we realize that even such demons of destruction are merely manifestations of great natural forces within the Earth's crust this crust is relatively thin beneath it the hot interior has a density much greater than that of surface rocks due to the tremendous pressure from above most of this interior is regarded as solid in spite of its great heat but when the pressure on an area near the surface is reduced by any great crustal movement large quantities of the inner rock melt and form a liquid called magma this hot magma may solidify before reaching the surface and form a great mass known as a battle is if later the overlying straighter are eroded away rugged mountains such as these may be carved out of the exposed granite battle is or the hot magma make cut across existing rock strata cooling and hardening there to form another type of igneous rock such formations called dykes may later be laid bare by erosion commonly they are found standing as irregular walls above the adjacent and less resistant rock the hot magma may also force its way in between rock strata and raising the overlying beds may spread out over a wide area the resulting formation is called as sill the dark band along this mountain face is the age of the sill 75 feet 6 this once molten rock is responsible for the baked zone in the adjacent strata in some fills the magma on cooling has formed columnar joints occasionally the molten magma arches the overlying straighter and takes the shape of a dome called alec olas erosion has laid bare such a formation here in eldon mountain arizona this eroded lako list is about three miles in diameter when hot magma intrudes into limestone the baking effect of the great heat may change the fine-grained limestone into coarse-grained marble if the magma contains compounds of lead zinc silver or other metals their ores may be deposited in the limestone such ores are commonly mind by sinking shafts into these ancient igneous contact zones magma or lava on cooling at some depth beneath the surface may become granite if the cooling is relatively rapid the Rockies fine grained but if the cooling is very slow of coarse grained granite results instead of solidifying a great depth magma may work its way toward the surface here if the molten rock contains enough highly compressed gases a violent explosion results steam and other gases burst from the crater with tremendous force solid fragments of hardened lava varying in size from fine dust two huge blocks are thrown up with the great heights scattered far and wide or molten lava may issue from the crater and spread out over the surrounding land as this lava creeps slowly forward over the countryside it might very fertile seized and destroyed buildings and even entire cities it is this lava and solid debris that build up volcanic cones commonly the cone is build of alternate layers of lava and ash lava causes the cone to widen out solid debris causes the cone to build up steeply a nearly perfect example of such a composite cone is mayon volcano in the Philippines within our own country lofty Mount Shasta now a dormant volcano also belongs to this time occasionally cones resulting from explosive eruptions may be built almost entirely of volcanic ash the larger fragments fall nearest event but the smaller particles are thrown farther out on the flanks this small cone in Arizona known as Sunset Crater shows the characteristic seat cinder cone slope a remarkable example of such a cinder cone is wizard island which rises out of Crater Lake Oregon this lake occupies the crater of a much larger and older volcano it measures about 5 miles in diameter and offers a clue as to the great size of the old cone the top of the old volcano shown here in reconstruction is gone either because it collapsed or was blown to bits or because it was destroyed by a combination of these two phenomena then a new eruption built a cinder cone with in this huge basin or caldera rain and snow filled the basin with water thus was formed the present crater lake with wizard island rising above it we have now seen how cones are formed by composite products or by cinders alone a cone may also be built up almost wholly of lava if the lava is somewhat thick and viscous there will be a fairly pronounced slope although not nearly so steep as those formed from finish this is the peak of mana law a lava cone which makes up most of the island of hawaii it has built up its great pile of lava 30,000 feet above the Pacific's floor on the slope of this great cone and 4000 feet below its summit is located Kilauea with its almost constantly active tips of Halla moma lava in an extremely liquid condition clothes like heavy oil and thus may spread out over a very wide area instead of issuing from a single hole la valise flow out of a long fishery in the geologic past molten rock flowing through such fissures very large portions of the states of Washington Oregon and Idaho six sheets of loud the two distinct major flows separated here in animation are easily discernible in the field just south of this area is lassen peak California the only active volcano within the boundaries of the United States langston eruption in 1914 its present activity is confined to mud geysers and fumarole but as shown on this map there are many recently extinct volcanoes especially in our northwestern state the world's active volcanoes lie in regions of recent mountain building thus we find several hundred act events forming a veritable ring of fire around the unstable margins of the Pacific in the Dutch East Indies which are a part of this rain lies the volcanic island of Krakatoa it was the scene of one of the greatest eruptions that man has known in 1880 the island looked like this on the 27th of August 1883 there occurred three violent explosions the last of these virtually blew the island to pieces merely a broken shell remains the dotted line shows the outline of the island before the explosion this and other violent volcanic eruptions almost constantly occurring somewhere in the world are spectacular evidence that our apparently stable earth is in reality dynamic and ever-changing