The major component of the Earth's upper mantle is silica (SiO), which makes it the primary source of silica in hydrothermal fluids. SiO is a stable component. It often appears as quartz in volcanic rocks. Some quartz that is derived from pre-existing rocks, appear in the form of sand and detrital quartz that interact with seawater to produce siliceous fluids. In some cases, silica in siliceous rocks are subjected to hydrothermal alteration and react with seawater at certain temperatures, forming an acidic solution for silicification of nearby materials. In the rock cycle, the chemical weathering of rocks also releases silica in the form of silicic acid as by-products. Silica from weathered rocks is washed into waters and deposit into shallow-marine environments.