Figure 4: Plot of 10Be/9Be for very young Central American basalts and andesites, plotted as a function of longitude (see Figure 12a for location). Because 10Be is produced cosmogenically in the upper atmosphere, high ratios of 10Be/9Be in fresh young lavas from Nicaragua indicate that virtually all the sediments carried into this trench are subducted to where magmas are generated, more than 100 km deep. Low 10Be/9Be in young Costa Rican lavas indicates that such sediments are not being delivered to the deep mantle beneath this arc segment, or are being diluted by eroded material.