322 PHYSICAL GEOLOGY 



the snow and ice on and near the summits of the volcanoes. During 

 an eruption of Cotopaxi in 1877 enormous torrents of water and mud 

 produced by the melting of the snow and ice on the cone, together 

 with great blocks of ice from the glaciers, rushed down the mountain, 

 burying fields and villages beneath mud, lava, and ice for a distance 

 of 10 miles. In contrast to the above is the existence of a great sheet 

 of ice on Mt. Etna, which for nearly one hundred years has been 

 protected from evaporation and thaw by a sheet of lava which over- 

 flowed it without the heat being sufficient to melt it. 



As torrents of water rush down the side of a volcano, they not only 

 erode it deeply but are also soon converted into streams of mud, 

 when dust and ash are abundant on the cone. Herculaneum (p. 303) 

 was buried in this manner, and in Java in 1881 torrents of mud and 

 water from Galoon-goon flooded the rivers to such an extent that 

 every village and plantation in this populous region was entirely de- 

 stroyed for a distance of 24 miles. 



During a comparatively recent eruption of Vesuvius so much hy- 

 drochloric acid was dissolved in the rain water which fell through the 

 clouds of volcanic gases that the vegetation for miles around was 

 injured by it. 



A subsidence of the land sometimes follows an eruption, as has been 

 noted in the case of the Temple of Jupiter near Naples (p. 229). The 

 sinking is probably brought about either by the withdrawal of molten 

 rock from beneath the affected areas or by the weighting of the ad- 

 jacent land by the ejected material, or by a combination of both. 



Volcanoes and Climate. 1 — It has been shown that volcanic dust in the high atmos- 

 phere decreases the intensity of solar radiation in the lower atmosphere. Therefore 

 the average temperature of the earth is decreased when dust is present. From these 

 observations some investigators have concluded that volcanic dust must have been a 

 factor, possibly an important one, in the production of many climatic changes of the 

 past. It has not, however, been shown that the periods of glaciation coincided with 

 prolonged volcanic outbursts. 



Subordinate Volcanic Phenomena 



There are a number of phenomena which are the direct result of 

 heat and are usually connected with present or comparatively recent 

 volcanism. 



1 Bull. Mt. Weather Observatory, Vol. 6, Pt. I, 1913; Smithsonian Misc. Coll., Vol. 59, 



, 1013. 



