Basalt is the predominant igneous rock on the surface of the earth, the moon and probably other bodies of the solar system. It is hard and dark and its presence evokes feelings of strength and power. In nature, basalt’s high vertical cliffs stand tall for millennia; in sculpture, it is a symbol of eternity and the simplicity and fundamentalism of nature. We are blessed in the Pacific Northwest to be in the midst of one of the larger outpourings of basalt in the world.
Basalt is a predominantly extrusive igneous rock characterized by small grain size (less than 0.2 inches or 5 mm) and approximately equal proportions of calcium-rich plagioclase feldspar and calcium-rich pyroxene (augite and hypersthene). Other minerals commonly found in basalt are olivine, magnetite, hematite, ilmenite, apatite, quartz and glass.
This rock is most commonly black or dark gray; however, where it has been weathered, basalt can be brown or red. Owing to its high iron and magnesium content, its density is relatively high, ranging from 160 to 190 pounds per cubic foot. Where the rock is filled with holes (vesicles), the unit weight is on the lower side of the range. Basalt is very strong rock; its unconfined compressional strength ranging from 5,000 to 25,000 pounds per square inch. Although there is not an official hardness for basalt because of its variable mineral composition, it is generally about 5 to 6 on the hardness scale.
Basalt is lava that moves to the earth’s surface in two basic ways. It can flow from the vent of a volcanic cone or from a fissure in the ground. Where its vent is below the water surface, it forms pillow lava, large rounded blobs of basalt that contain many joints and cracks. From the crater or side-slope fissure of a volcano, lava can flow long distances and form either ropy (pahoehoe) or blocky (aa) basalt. These types of basalt are not commonly used for sculptural purposes, because they are filled with voids and joints and may be very brittle. The most commonly used sculpturing basalt is from flood basalts - lava that emerged from fissures and flowed for many miles, accumulating to thicknesses of hundreds of feet. Two of the larger basalt flows in the world are on the Columbia River plateau in Washington State and the Deccan plateau in India. A typical cross-section of a single flow is a massive interior overlaid by rubbly zones. It is in the massive interior that the best sculpture stone is found; it is no coincidence that this zone is also where columnar basalt is found.
The columns are 5 to 20 feet long in a vertical direction and their horizontal shape is hexagonal and ranges from 10 to 36 inches. The hexagonal pattern develops from internal shrinkage stresses during cooling of the lava. Although some vertical or subvertical joints develop, the primary joints (other than the continuous vertical shrinkage cracks) are horizontal, perpendicular to the column planes.
One interesting feature of some basalts is the presence of voids or vesicles throughout the rock. They represent the existence of gasses in the rock that could not escape before the solidification of the molten rock. This is called vesicular basalt; where the voids dominate the rock, it is called scoria. Other areas of extensive surficial basalt flows are in the Snake River plain, the Lake Superior area, Scotland, Iceland, Greenland and Brazil.
The biggest use of basalt in the world is for road surfacing and railroad ballast, because of its high compressional strength and its resistance to weathering. It is also employed as a building stone, riprap and for roofing granules and flagstones. In the Pacific Northwest, the largest use of basalt is currently for rockeries, 2- to 4-foot diameter rocks that are stacked 5 to 15 feet high along road and driveway excavation slopes to protect the slopes from erosion.
Ordinary basalt is blasted to obtain road building and rockery materials; however, basalt intended for sculpture is obtained from rock that has accumulated in talus piles or from carefully mined columns in natural rock exposures. Sometimes the columns can be pried free with just a steel bar, but usually a large backhoe or crane is used to loosen the column. If blasting is necessary, horizontal holes are drilled along the base of the columns and then a light charge is detonated. The columns are place on a low-boy, truck-trailer rig for transportation to the stone yard or display site.
Working the Stone
The hardness and brittleness of this stone precludes the use of hand tools and, in most cases, carbide power tools. Diamonds are required. For through-cuts, a diamond wire or saw will suffice. For incisions into the stone, a diamond wheel is the best, but silicon carbide can be used. Multiple cuts can be made into the stone, and then the intervening ridges knocked out with a carbide-tipped chisel. Use of heat and flame from an oxy-propane torch is also very effective for removing material.
In general, quarried or cut stone will work better than stone that has been sitting in a talus pile at the base of a cliff for hundreds of years. As with most other stones, a surficial hardening occurs that renders the outside of the stone more difficult to remove than the fresher stone on the interior.
For detail work, it is necessary to use diamond bits and blades and to finish with diamond polishing pads. A high polish can be obtained, owing to the fine grain of the crystals in the stone.
The selection of stone and the planning of a piece should emphasize the raw, natural character of the stone. Iron oxide (red) surficial rinds can enhance the artistic presentation, as well as the presence of lichen (green or brown) in irregular forms on the stone. The hardness and resistance to weathering lend basalt very well to outdoor display.
The chief faults in this stone are joints, as described above. The stone should be carefully inspected, both visually and with a hammer. Inclusions may include zenoliths (foreign bodies) that were emplaced during extrusion of the lava, and may therefore be different hardness than the rest of the stone, and minerals such as quartz and zeolites.
Because of the monumental nature of most basalt sculptural pieces, the biggest safety issue is that of the moving of this heavy stone. It is deceptively heavy. Undetected joints can grow in width during the handling or transportation of the stone, and cause the stone to fall apart unexpectedly.
Basalt contains 45 to 54 percent silicon dioxide, The dust from which can be harmful to your lungs, so keep your mask on as you work it.
Thanks to Richard Hestekind for his knowledge of selection and carving basalt. Richard’s work in this enigmatic stone is inspiring.