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October 19, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Lava rocks come in many colors

Driving along the Queen Ka`ahumanu highway from Kailua-Kona to Waikoloa, one passes a vast expanse of well-exposed lava rock. These flows are from Hualalai and Mauna Loa Volcanoes and are of various ages. If you slow down and look carefully, you are able to recognize individual flows by their distinctive surface texture, color, or luster.

We have addressed the different textural surfaces between pahoehoe and `a`a lava flows in earlier articles. Now we would like to explain what gives lava rock different colors and lusters. The flow surfaces range from shiny jet black to waxy battleship gray to dull yellow-brown. The freshness of the iridescent glassy surface and the cooling time of the rock determine the luster and color.

When molten lava chills quickly, a thin, glassy rind forms on the exposed surfaces because crystallization of the minerals in the melt could not take place in such a short time. This layer of glass provides the iridescent luster to the rock. When crystallization occurs, the first minerals to crystallize are mafic minerals, which are dark-colored and contain abundant iron and magnesium. Rocks that cool quickly, especially the outer layers of a flow, are primarily composed of glass particles and tiny mafic minerals. This is why the outer surface of a flow is black.

If you look at a road cut where the interior of a flow is exposed, you will see that the rocks are mainly gray and have a waxy luster. The interior of a flow cools slower, so the light-colored felsic minerals are also able to crystallize. The most abundant felsic mineral in lava rock is plagioclase feldspar, which gives the surfaces a waxy luster. The combination of dark-colored mafic and light-colored felsic minerals creates a gray rock.

Often, the very dense interior of a thick flow is exposed. This is where there are no gas vesicles or "holes" in the rock, and the grain sizes of the minerals are slightly larger. The gray rock appears to have a tint of blue to it, and this is often called "blue rock" in Hawai`i. Contractors have other names for it, but they are not printable in a family newspaper. "Blue rock" is the bane of contractors, especially road builders and pipeline installers, because it is difficult to break. The largest bulldozers and backhoes are regularly humbled by this dense rock, causing contractors to revert to expensive drilling and blasting techniques.

While encountering "blue rock" is a contractor's worst nightmare, it is also the most sought-after rock type by commercial quarry operators. The best grade construction aggregates are from "blue rock." The concrete made with a mixture of crushed "blue rock" and cement has the structural integrity to pass necessary strength tests.

Another type of "blue rock" is a blue-glassy pahoehoe only found in small, thin flows at the base of tumuli such as in the current Kilauea eruption flow field. The shade of blue of this rock is closer to gun-metal and is probably caused by the refractive index of the glass. Eventually, with exposure to the elements, the color of the rock turns to black.

As older lava flows weather, the minerals in the rocks oxidize and often turn to clay minerals. The once shiny rock becomes dull and cloudy, or the glassy surface breaks and falls away to expose the dull interior. One of the most common minerals found in the decomposed rock is hematite, an iron oxide that is usually yellow-brown or rust colored. The abundance of this mineral in the exposed weathered surfaces gives the older flows their distinctive yellow-brown appearance.

The color changes of lava from its molten to its solid state are similar to that which you can observe in the tungsten coils of an electric stove. The higher the temperature of the lava or the stove coils, the brighter the shade of red attained.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing southeast through a tube system down to the flats below Pulama pali and beyond to the ocean. Lava is inflating the area near the coast and also entering the ocean along a broad front at Kamokuna located 1.6 km (1 mi) west-southwest of Waha`ula. Calving of the new lava delta was observed on Friday, October 13. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

One earthquake was reported felt during the week ending on October 19. Residents from every district on the island of Hawai`i except Ka`u reported feeling an earthquake at 9:53 p.m. on Friday, October 13. The magnitude-4.3 earthquake was located 16 km (9.6 mi) north of Honoka`a at a depth of 39 km (23.4 mi).

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Updated: October 23, 2000