The amounts of potassium, strontium, and rubidium in volcanic rocks of the Indonesian active volcanic arc, extending from Sumatra through Java, Sumbawa, and Flores to Sulawesi, Sangihe, and Halmahera, have been related to depth of the underlying Benioff zone by multiple linear regression analysis. The following equations for the arc from Sumatra to Sumbawa have correlation coefficients better than 0.80 (level of significance <0.001):

Depth to the Benioff zone (km)

  = 397 - (5.26 x % of SiO2 ) + (35.04 x % of K2O)

  = 373 - (4.36 x % of SiO2 ) + ( 0.73 x ppm of Rb)

  = 110 - (0.03 x % of SiO2 ) + ( 0.14 x ppm of Sr).

The standard error of depth estimate is about 30 km. The correlation coefficient for the first equation is slightly better than for the other two.

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