Puu OO fumes in the foreground (note the dense white fume from a vent in the east wall) and Halemaumau fumes in the distance. (Photo courtesy of Hawaiian Volcano Observatory)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Most accounts of Captain Cook’s first arrival in Hawaii describe the Hawaiians’ fascination with metal objects.  

The crew soon found that an iron nail could be traded for any number of goods or services. The fascination with metal stemmed from its novelty.  Although Hawaiians had encountered metal before Cook arrived — either from other European ships or from debris washed ashore — it was, at best, exceedingly rare.   

Naturally occurring or “native” metals, such as gold, silver and copper are rare anywhere on earth, and are beyond rare in Hawaii — they don’t exist.  Nickel-iron meteorites are also rare everywhere, but Hawaii has an equal opportunity to receive them. So it’s possible that meteoric iron was known to the ancient Hawaiians.   

Though native metal is rare, metal combined with other elements is as common as rock itself. For example, about one-third of Hawaiian basalt is iron oxide, aluminum oxide, and calcium oxide, in roughly equal proportions.   

To obtain metal in quantity, however, it must be smelted–separated from the other elements with which it is combined. To be suitable for smelting, a rock must contain a much higher proportion of a single metal-bearing mineral than would be found in common rock. Rocks suitable for smelting are called ores.   

Many different mineral-concentrating processes are known to produce ore deposits. Placer deposits form when weathering releases mineral grains from rocks and flowing water segregates the minerals according to their densities.  

Minerals with high densities — most ore minerals — will tend to stay put while less dense minerals are carried away by the current. Placer deposits thus form along streams, rivers or the sea shore. The mineral olivine, heavy but not an ore mineral, was concentrated in this way to form the Green Sand Beach. 

Some ores are produced in association with large intrusions of basalt–great blobs of magma that are injected into the earth’s crust but that don’t reach the surface.   

Instead they slowly cool deep underground. The slow cooling allows mineral grains to grow and to rise if they are less dense than the magmatic liquid or to sink if they are more dense than the liquid.   

The dense ore minerals collect in layers on the bottom of the intrusion. Essentially, all of the world’s chromium ores and some platinum, nickel, and copper ores formed in this fashion.   

Other ores are produced around the roots of volcanoes, where magma gives off heat, chemical-rich solutions, and vapors to surrounding rocks.  These products mix with the groundwater — the water found in fractures and cavities in the rock virtually everywhere.  

The hot mixture is less dense than normal ground water so it migrates upward through fractures. The hot solutions can selectively dissolve low-concentration minerals contained in the country rock.  As this witch’s brew rises it cools and experiences a pressure reduction. The temperature and pressure reduction reduces the solubility of the various dissolved substances until they eventually crystallize on the fracture surfaces.

In this way, metallic minerals that were in low concentrations in the country rock can become highly concentrated in the fractures to form veins. Vein ores yield copper, lead, zinc, gold, silver, tin and other metals. 

Given that ores are commonly associated with igneous processes — that is, with volcanism and magmatic intrusions — you might expect Hawaii to have some metallic ores.   

But in fact, ore deposits are unknown in Hawaii. This may be because they never formed, because they are just not exposed at the surface, or because they have not yet had time to form. Deep erosion is required to bring to the surface ores formed at great depths. So if metallic ore exists within Hawaiian volcanoes, erosion has not been deep enough to bring it to the surface.   

The Hawaiians had learned to exploit wood, fiber, stone, and bone to the fullest, so why hadn’t they developed the ability to mine, smelt, and work metals?  

Well, you can’t exploit a non-existent resource. “Paradise” didn’t come equipped with the raw material to produce metals.