Calcium is stored in geologic reservoirs, most commonly in the form of calcium carbonate or as calcium silicate. Calcium-containing rocks include calcite, dolomite, phosphate, and gypsum. Rocks slowly dissolve by physical and chemical processes, carrying calcium ions into rivers and oceans. Calcium ions (Ca) and magnesium ions (Mg) have the same charge (+2) and similar sizes, so they react similarly and are able to substitute for each other in some minerals, such as carbonates. Ca-containing minerals are often more easily weathered than Mg minerals, so Ca is often more enriched in waterways than Mg. Rivers containing more dissolved Ca are generally considered more alkaline. 
Calcium is one of the most common elements found in seawater. Inputs of dissolved calcium (Ca) to the ocean include the weathering of calcium sulfate, calcium silicate, and calcium carbonate, basalt-seawater reaction, and dolomitization.