Hydrothermal veins are mineral-rich deposits formed when hot, mineral-laden water moves through cracks in rocks. As the water cools, it deposits minerals, creating veins that can contain valuable resources like gold, silver, and copper. These veins often occur in areas with volcanic activity, where heat from magma heats groundwater. The formation of hydrothermal veins is a key process in geology, contributing to the concentration of minerals in the Earth's crust. They can vary in size and composition, depending on the temperature, pressure, and chemistry of the fluids involved. Understanding these veins helps in mining and resource exploration.