FACTORS IN COLUMNAR STRUCTURE IN LAVAS 467 



Fife. 12. — Section showing lava cooling from 

 two surfaces, and columns penetrating the 

 solid lava as cooling proceeds. Columns are 

 perpendicular to the isothermal planes and 

 where two cooling surfaces are inclined to one 

 another a major joint plane separates the two 

 sets of columns. 



6. Columns formed from inclined cooling surfaces. — The dotted 

 lines on Figure 13 represent the edges of the isothermal surfaces. 

 Columns form at right angles to these surfaces. The isotherms 

 shown in Figures 12 and 13 give data for the rate of formation 

 of the columns, their size, and their direction. The closer the 

 isotherms the larger are 

 the columns because the 

 lava is able to resist a 

 greater tension without 

 cracking when that force is 

 gradually applied. Col- 

 umns from the two inchned 

 surfaces approach one 

 another and meet along a 

 major joint plane which 

 would be enlarged by 

 drainage. Note that along 



this major joint the columns curve toward the higher temper- 

 atures. 



7. Bent columns. — These (Fig. 14) are formed in cases where 

 the isotherms are not parallel to one another. For reasons such 

 as those enumerated below, the cooling in one direction is more 



rapid than in another. 

 ^^^^^mi^^^m^^^mm^'^^ ^J The columns bend in the 



direction of greatest cool- 

 ing (Fig. 15). Dotted lines 

 represent edges of iso- 

 thermal surfaces. The fol- 

 lowing are suggested causes 

 for change in direction of 

 cooling: (i) presence of 

 scoria, etc., in close proximity to the lava; (2) irregularities in the 

 valley floor; (3) the warping of the lava's upper surface; (4) a 

 scoriaceous and vesicular surface; (5) included gas, water, etc., 

 in the lava; (6) convection currents in the air above the lava; 

 (7) convection currents in the lava; (8) penetration of water into 

 the lava and scoria; (9) rain from the volcano; (10) movement 



Fig. 13. — Junction of the slender upper 

 columns and the massive basal columns of a 

 lava flow. The junction somewhat resembles 

 the junction between two lava flows. 



