The total flux of metals from these eruptions is significant. Submarine volcanism is even more significant than that at continental margins. It has been conservatively estimated that there are at least 3,000 vent fields on the mid-ocean ridges. The planet earth is thus the ultimate source of all metals. Metals are ubiquitous in the lithosphere, where they are inhomogenously distributed and occur in different chemical forms.
Ore deposits are merely natural concentrations which are commercially exploitable.
While such anomalous accumulations are the focus of mineral exploration the background concentrations of metals which occur in common rocks, sediments and soils are of greater significance to the total metal loading in the environment. Indeed, all known elements are present at some level of concentration throughout the natural environment. They are present in minerals, vegetables and animals, and their beneficial and harmful effects have been present since evolution began. An understanding of the nature and magnitude of these geological sources is a prerequisite for developing approaches in assessing the risk posed by metals in the environment. It is important to be able to distinguish between natural and anthropogenic contributions to metal loadings.