The surface of Mars exhibits strong evidence for a widespread and long‐lived cryosphere. Observations of the surface have identified phases produced by water‐rock interactions, but the contribution of glaciers to the observed alteration mineralogy is unclear. To characterize the chemical alteration expected on an icy early Mars, we collected water and rock samples from terrestrial glaciated volcanics. We related geochemical measurements of meltwater to the mineralogy and chemistry of proglacial rock coatings. In these terrains, water is dominated by dissolved silica relative to other dissolved cations, particularly at mafic sites. Rock coatings associated with glacial striations on mafic boulders include a silica‐rich component, indicating that silica precipitation is occurring in the subglacial environment. We propose that glacial alteration of volcanic bedrock is dominated by a combination of high rates of silica dissolution and precipitation of opaline silica. On Mars, cryosphere‐driven chemical weathering could be the origin of observed silica‐enriched phases. Plain Language Summary The planet Mars has glaciers and ice sheets on its surface and probably did in the past. Minerals on the planet's surface form in the presence of water, but it is unclear which minerals may have formed due to liquid water under warm climates versus those formed under much colder climates. In order to study this problem, we collected rocks and water from Mars‐like analog sites: glaciated volcanoes. We measured the chemistry of the water and the mineralogy and chemistry of rock coatings found near the glaciers. Both the water and the rock coatings were high in silica. We propose that glaciers alter volcanic bedrock by dissolving and precipitating noncrystalline silica. Silica detected on the surface of Mars could have formed due to similar processes. Key Points Chemical alteration of glaciated volcanic bedrock is dominated by silica dissolution and precipitation Dissolved silica in glacial meltwater is greater at more mafic study sites and results in deposition of opaline silica rock coatings Past glacial chemical weathering may be responsible for some amorphous silica deposits on the highly mafic surface of Mars