September 24, 2009
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Ancient Hawaiians: Plenty of Oars but No Ores
Most accounts of Captain Cook's first arrival in Hawai`i 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 Hawai`i—they don't exist. Nickel-iron meteorites are also rare everywhere, but Hawai`i 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 Hawai`i to have some metallic ores. But in fact, ore deposits are unknown in Hawai`i. 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.
Kīlauea Activity Update
Lava continues to erupt from the TEB vent, on Kīlauea's east rift zone and flow through tubes to the ocean at Waikupanaha. A small lava breakout on the rootless shield complex was active for a few days this week, but has now stagnated. A lava breakout from the tube near the top of the Royal Gardens subdivision has been active throughout the past week. The flows are mostly staying close to the breakout point and are building a broad shield-shaped mound of lava.
Faint glow above the vent at Kīlauea's summit has been visible at night. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.
Two earthquakes beneath Hawai`i Island were reported felt this past week. A magnitude-2.5 earthquake occurred at 5:56 a.m., H.s.t., on Monday, September 21, 2009, and was located 9 km (6 miles) northeast of Waiki`i at a depth of 26 km (16 miles). A magnitude-2.3 earthquake occurred at 7:28 a.m. on the same day and was located in the same area at a depth of 25 km (16 miles).
Visit our Web site (http://hvo.wr.usgs.gov) for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to askHVO@usgs.gov.
Updated: October 1, 2009 (pnf)