AN UNUSUAL FORM OF VOLCANIC EJECTA 1 



WALLACE E. PRATT 

 Chief Division of Mines, Bureau of Science, Manila, Philippine Islands 



In the course of a study of the eruption of Taal Volcano, in 

 southwestern Luzon. Philippine Islands, during the month of 

 February, 191 1, I noted the presence of small concretion-like 

 bodies in the finest-grained portion of the blanket of fragmental 

 ejecta which the eruption spread over the surrounding country. 

 It will be recalled that the eruption in question was characterized, 

 by the expulsion of great volumes of water-vapor, charged with 

 ash or sand, together with a small proportion of coarser fragmental 

 material. The eruption destroyed completely a dozen small 

 villages, with attendant damage to crops and live stock, and killed 

 1,335 people. A thin layer of mud and dust was spread over an 

 area of about 1 ,000 square kilometers, extending principally to the 

 north and west of the crater. I commented upon the presence of 

 the spherical bodies in the ash-fall at the time as follows: 



An interesting feature of the fall of the ejecta is the formation of drops 

 or balls of mud. These were observed most abundantly on the island itself, 

 but were seen at Talisay and Bariadero also. They range in size from large 

 shot to hazelnuts, and when broken sometimes show concentric markings. 

 Apparently they fell late during the activity, being found just below the sur- 

 face of the deposit. These mud balls cannot be classed as lapilli in the strict 

 sense of that term, since they are built up, probably through the condensa- 

 tion of steam into drops of water. The accompanying vertical section of the 

 fall of mud or ash [text Fig. 2] was taken on the southwest slope of the volcano. 2 



Text Fig. 2, referred to in the quotation, is reproduced here- 

 with as text Fig. 1. 



Taal Volcano forms an island near the center of a lake from 

 15 to 20 kilometers in diameter. Thus the mud balls, which were 

 found both on the slopes of the volcano and at the villages of Talisay 



1 Published by permission of the Director, Bureau of Science, Manila, Philippine 

 Islands. 



- Wallace E. Pratt, Phil. Jour. Sci., Sec. A (1911), VI, 71. 



45° 



