Silicate weathering is often invoked as the dominant mechanism for sequestering CO2 over geologic timescales, where extrusive lithologies are assumed to disproportionately contribute to global silicate weathering fluxes due to their higher reactivity relative to intrusive substrate. While Iceland is largely considered monolithologic (i.e. basaltic), compiled bedrock and solute measurements herein suggest that river water is enriched in Ca and Na relative to a bedrock array which ranges from basaltic to rhyolitic. To explore this relationship, we evaluate environmental factors that could explain the solute trends including clay precipitation (type-I incongruent weathering), variable dissolution rates of primary minerals (type-II incongruent weathering), and rhyolite dissolution/ groundwater contributions. Through forward models we conclude that the precipitation of high-Mg smectite exerts the most control on solute movement relative to type-II incongruency or dissolution of rhyolite. However, more direct measurements of clay mineralogy and rhyolitic ash dissolution are necessary to better constrain this dataset.