Stone Selection

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Hawaiian Lava Rocks(`A`A)

`A`a (pronounced "ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers. The incredibly spiny surface of a solidified `a`a flow makes walking very difficult and slow. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an `a`a flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an `a`a flow.

There are three categories of parent rock material. The first category is igneous rock. Igneous rock is cooled molten lava that originated from within the earth. There are two types of igneous rock: intrusive and extrusive igneous rock. Intrusive rocks are large crystals that have slowly cooled within the crust of the earth. An example is granite, which is used as a common building material. While this type of parent rock is common on the continental United States, it is not common in Hawaii. Instead, the most common parent rock material in Hawaii is extrusive igneous rock, which is often associated with volcanic activity. It is comprised of small crystals that cooled rapidly on the earth’s surface. An example of extrusive igneous rock is basalt.

Now, let’s consider the two general rules of soil formation that were stated early. Since extrusive rock materials, such as basalt, are finely textured, the soils weathered from this parent rock tend to be finely-textured. And so, this is a reason why much of Hawaii soils are finely-textured. The next category of rock material is sedimentary rock. After long periods of compression, weathered products from older rocks will become cemented. As a result, another type of rock forms. Examples of this type are sandstone, shale, and limestone.
The final category of parent rock is metamorphic rock. Metamorphic rocks have formed as the result of extreme heat or pressure. As a result, a change in the form of igneous or sedimentary rock occurs. For example, under extreme heat or pressure, igneous rocks will form schist gneiss, limestone will form marble, and shale will form slate.

Crushed Stone vs. Gravel
To a geologist, “crushed stone” and “gravel” are two distinctly different materials. “Crushed stone” is a commercial product made by mining rock and crushing it into angular pieces. “Gravel” is a natural material that consists of water-transported particles of rock that are larger than two millimeters in diameter and usually have a rounded shape as a result of their water transport. The shape of the grains and man’s role in producing them are the differences that separate crushed stone from gravel.
The average person in the United States rarely uses the term “crushed stone.” Instead, the word “gravel” is used generically for almost any type of rock material with a particle size over a few millimeters. Their "gravel" includes both the crushed stone and the gravel of geologists.