A hydrothermal system with recharge and discharge channels is referred to as open system, ones with only discharge channels are called closed systems. Water to rock ratios in hydrothermal systems vary from very low (0.1) to very high (150/1). This leads to two kinds of hydrothermal systems-rock dominated (water-rock ratio 40 or 50/1). The distinction is based on the reactivity of seawater magnesium during reactions with basalt-ie. all the magnesium is extracted (removed from seawater) during the reactions (low ratios), or not (high ratios).