The Montgomery advertiser., 1902-05-18

HnaI TOT the u1OSI part dTG u0Utf cIiOi TIT ous pressure. Owing to the irregularities of the Assures. the molten rock pressed up from below does not commonly rise through them freely. @but at the most more fra cures intersect. There have been times in the history of the earth when outweilings of molten rock have occurred throughout the extent Of many long Assures. and vast inundations of lava have resulted, as. for example. when the lava plains crossed by the Co., lumbia River were formed. The plains. some 100000 square miles in area. are underlain bv sheets of once molten rock which have an average thickness of something like 4,o00 feet Between out pourings of veritable floods of lava. as in the instance cited, and the eruption plete gradation. showing that all vol canoes have essentially the same mode of origin.
 Local eruptions. or volcanoes, in dis tinction from "fissure eruptions, present two leading .characteristics-in one class the lava is poured out quietly, frequently in vast quantity, but without explosions; and in the other class the matter ex- in amount but accompanied by explo- sions. frequently of great violence.
 Eruptions of Quiet Type.
 During eruptions' of this quiet type. the lava comes to the surface in a highly liquid condition-that is. it is thoroughly
 
 of water. It spreads widely even on nearly level plain, and may form com paratively thin sheet several hundred square miles dn area. as has been ob- Snake River plains. in Southern Idaho, there are sheets of once molten rock which were poured out In the manner just stated, some four hundred square miles in area and not over seventy-five feet in average thickness. When an CT uption of highly liquid lava occurs in a mountainous region. the molten rock may cascade down steep slopes and flow more before becoming chilled sufficiently to arrest its progress. Instances are abundant where quiet eruptions have oc- cured in the midst of a plain. and built Immensely expanding bases Illustra- stions are furnished in Southern Idaho, in which cones formed are only three hundred or four hundred feet high, but have a breadth at the base of eight or ten miles. In the class of eruption illus trated by these examples. there is an absence of fragmental material. such as explo~Ive volcanoes hurl into the air, and a person may stand within a few yards of a rushing stream of molten rock, or examine closely the opening from which it Is being poured out. with out danger of serious inconvenience. When Eruptions Become Violent. The quiet volcanic eruptions are at tended by the escape of steam and gases from the molten rock, but lava being in highly liquid state. the steam and
 gases dissolved in it escape quietly and mofen rock is ess d in viscous condition, the vapors and gases contained in it find difficulty in escaping, and may be retained until. be- coming concentrated in large volume, they break their way to the surface, pro- sducing violent explosions. Volcanoes dn which the lava extruded is viscous, and the escape of steam and gases is re- tarded until the pent-up energy bursts all bounds, are of the explosive type One characteristic example Is Vesuvius
 When steam escapes from the summit Of volcanic conduit-which, in plain terms is a tall vessel filled with intense ly hot and more or less viscous liquid masses of the liquid rock are blown into the air, and on falling build up a rim or crater about the place of discharge. Commonly the lava in the summit DOT ton of a conduit becomes chilled and perhaps hardened. and when a steam ex- plosion occurs this crust is shattered and the fragments hurled into the air and contributed to the building of the walls of the inclosing crater. The products of explosive volcanoes are vapor and gases, molten rock and solld rock. The vapor is steam, and the gases, in cer- tain instances, at least, largely free n.y. drogen and free oxygen The explosions, although commonly ascribed to the vio- lent escape Of steam, may in part he due to the union of the oxygen and hy- drogen present. The liquid rock escapes as streams or lava-f1ows. and in part is blown into the air. The liquid or plastic protectiles produced in this manner may become sufficiently solid to retain their form on striking, and are termed vol canic bombs They frequently have spindle shape. with twisted ends. show Ing that they rotated while in the air about an axis; but in other instances they fatten on striking, and may even be so nearly liquid that the material composing them spreads out as a thin cake when it comes to rest. Bombs are volcano that elected them. and fiattened and even thin lava cakes are sometimes abundant at distance of one. or two miles from the place where they began their aerial journey.
 The solid rock blown out by volcanoes consists usually of highly veslcular ma terial which hardened on the surface of the column of lava within a conduit and was shattered by explosions beneath it. These fragments vary in size from dust particles up to masses several feet in diameter. and during violent eruptions are hurled miles high The larger frag- ments commonly fall near the place of origin and usually furnish the principal part of the material of which craters are built. but the gravel-like kernels. lapilli, may be carried laterally several miles if a wind Is blowing. while the dust is frequently showered down on thousands of square miles of land or sea. The solid and usually angular fragments manufac- tured in this manner vary in tempera- ture, and may still be red hot on falling
 Volcanoes of the explosive type not un- commonly discharge streams of lava. which may flow many miles. In certain instances these outwellings of liquid rock occur after severe earthquakes and vio- lent explosions, and may have all the characteristics of quiet eruptions. There is thus no fundamental difference be- tween the two types into which it is con- Yenisnt to divide volcanoes.
 Mountains Blow Their Heads Of
 In extreme examples of explosive vol canoes. the summit portion of a crater. and several thousand feet high. is blown away Such an occurrence is recorded in the case of the volcano Caseguina, Mica ragua in 1885 Or, an entire mountain may disappear, being reduced to tlapillI and dust and blown into the air. as in the case of Krakatoa, in the Strait of Sunda, in 1Ss3.
 The essential feature of a volcano. IS stated above is a tube or conduit. lead ing from the highly heated sub-crust portion of the earth to the surface. through which molten rock Is forced up- ward to the surface. The most marked variations in the process depend on the quantity of molten rock extruded, and on the freedom of escape of the steam and gases contained in the lava.
 The cause of the rise of the molten rock in a volcano Is still matter Of discussion. Certain geolog1sts contend that steam Is the sole, motive power; while others consider that the lava Is forced to the surface owing to pressure on the reservoir from which it comes.