EARTHQUAKES AND VOLCANOES

Wairarapa Age, Volume XXIX, Issue 8225, 31 August 1906, Page 3

EARTHQUAKES AND VOLCANOES

LATEST THEORIES AS "_U TiiElß NATURE.

Discussing "Recent Earthquakes •in the Geographical Journal, K.U.0., says that volcanic eruptions and .great Earthquakes differ radically in their immediate cause:— 'Small earthquakes may be due to any oause which uuu start an elastic wave-inotiuii iu I'tio earth, but the really great earthquakes, bo far as is Unown are always started by the sudden fracture of the solid • crust- of the earth. In, some way not iuily known, though probably it is more or less directly connected with ttie gradual coolina of the earth, the eurtb's crudt is thrown into a state of strain which ultimately grows too great to do borne, and fracture takes place. 'Where the power of resistance is small, the aoautnulated strain is also small,"and so, too, tue resulting disturbance; but whore the power of ■ resistance is great the strain also becomes great before yielding at last taneß place, and thou the disturbance, often accompanied by permanent displacement of the ground, carries destruction in its j wake. Tue occurrence of p really great earthquake means not only a greatness of force, but also of resistance: it is due to the disruption of solid rock.' The great difference in the productive cause uf au earthquake and a volcanic eruption is that in the -former the rooks concerned are solid, while in the latter case they • are in a fluid sta>e. it might appear, therefore, lint there is no oonnection bjtween the two phe.aonieua:— 'The absence of direct oonnection. of the nature of cause and effeut .between these two classes of phenomena dues not, huwevei, pre eluded the possibility of Loth being consequences of a oomnjou cause, which-has bten deacrioed as the reaction of the jut.rior on the exterior .of the eartn, a phrase which will be found in many textbooks "f geology, and of whioh we may say that it represents a reality whose precise nature and hmiiat.on we are unable to define. 'Whatever the character of this ■. reaction, its manifestation is certainly liabla to periodio variation; the last twelvemonth Las been one of more than average frequency of great earthquakes, and has witnessed more than un average, though not aii exoeptioual, development ot seismio and volcauio activity. 'lbe Kangra ■ earthquakes of April 4, 1905, the two Central Asian earthquakes, of July 9 aud 23, the Colombian earthquake of January 31 last, ana the San Francisco euitbquake of April 18 were all disturbances of fhe grea'est magnitude, but the total amount of seismic activity wa* probably no greator than in 1897 98, and there is no indication of any spreading out of great earthquake • centres from the regiony whence they commonly originate." The distribution of earthquake 'Centres is localised, and those of the greatest earthquakes are limited, so far as experience goes, to ceratin areas which do not cover more than, at most, fire per cent, of the earth's surface. Great Britain lies welt outside any of these regions, and, so far as that country 19 opn"Oerned , there is n0 reason to anticipate the slightest probability of an earthquake euob ab led to the destruction of San Francisco. 1 A. P. Coleman, Professor ofGeology at 4 Toronto Uuiversity deals with the connection between earthquakes and volcanoes in un excellent article in the Canadian Maga- • zine:—■ 'Earthquakes and volcanoes are only incidents in the processes that elevate continents, build mountain chains, and deepen aea (bottoms; and the real cause of all U 638 mighty changes, is to be found in the shrinking of the earth's interior aud the wrinkling ot its orusfc to adopt itself to the lessened surface. 'Often the origin of the shook is due to a shifting cf the sea hot torn, • as proved by the breaking of cables. In such earthquakes the sea first retreats from the shore and then rushes back as a wave 50ft high, sometimes sweeping away towns and villages, and drowning thousands of people. The coasts of Japan and of western South America have been devastated more than once by such 'tidal waves.' The centre of disturbance in the late earthquake in California must have been inland, however, since no serious ocean nave was noticed on the coast. 'Eathquakes may Iflst a few seconds or minutes or may continue for months or even years, as in Calabria from 1783 to 1786; and they are apt to occur again and sgain in the same region, with intervals of years or of a generation between destructive shocks. The line of faulting allows slip after slip as the adjoining areas slowly adjust their level to the changing conditions.' It is gratifying [to read that no voloanio eruptions or destructive earthquakes have been recorded in Canada, though n violent and longcontinued earthquake occurred iu Quebec, in 1663, when the Indiana declared that 'lbe trees sere drunk.' Processor E. is. Mathews, of the Join Hopkins University, takes volcanoes for his subject in the World To-day:--'Voloanoes show a marked tendency toward a periodicity iu their activities. Thus all the known and carefully studied voloanoes show periods of marked activity and explosive force separatod by lender or shorter intervals of quiescouoe. The Jeugch of the intervals may vary from a few minutes as at Stromboli, to a few years, as at Vesuvius, •or to great intervals of time, as in Santorin, a volcano in the Aegean Sea.'

He suggests that tbo origin of volcanoes is not right clown tine bowels of the eartb, but; comparatively uour the surface: — 'The s»rcat Karakatnuan eruption of 1884, which reddened the sunset of the world for a year or more, witb its ashea driven miles iuto the upper atmosphere, has been computed as starting not over 15 miles below the surface. The focus of the little volcano of Monte Nnove is thought to have been less than 4,000 ft below the surface. 'Again, cracks and fissures cannot extend as far into the interior. 'lt baa been shown by Van; 'ilise and others that open oraofes or

' Sssures cannot extend over six miles | . beneath the surface, as they would ! be closed by the pressure of the j rocks. Even this estimate is re- ' garderl as representing more than I the average depth of the greatest ! fissures.' !