January 2002
It's a Matter of Time
It’s a Matter of time
Author’s Note: For the past few years this column has taken on issues of concern to the aggregate industry and has described how geology relates to those issues. While most folks who read this column probably have some knowledge of geology, it is a science with a history worth sharing. These next few articles will provide brief descriptions of some of the theories that have evolved into the science of geology.
Geology quite appropriately gets its name from Gaia, of Greek mythology, and logy, a suffix meaning “knowledge of.” Gaia was Mother Earth, an ancient primeval goddess who emerged at the creation of the universe, second only to Khaos (Air). She was shown half risen from the earth, unable to completely separate herself from her element. In pre-Hellenic myths, Gaia brought forth all life from darkness and chaos, bringing form and matter to the earth.
The form and matter that Gaia, or Mother Nature, created is only one momentary view of an ever-changing kaleidoscope of widely varied landscapes. The movement of minerals by wind, water, gravity and ice (glaciers); the growth and movement of plants and animals; and the shifting about of gigantic plates on the earth’s surface; all bring about changes. These changes include the formation of new rocks from old, new lands and seas, new mountains and plains, and new climate and weather.
While ancient Greeks may have accepted the fact that Gaia created changes to the Earth rather rapidly, modern geology is based on a recognition that it is the passage of vast lengths of time that allow many changes to take place, changes that have been going on for something like 4.6 billion years. This concept can be demonstrated by the mythology of another culture. Korean mythology asks you to imagine a mountain that is one mile high and made of solid granite. Once every thousand years an angel flies down from heaven and brushes the summit of the mountain with her wings. Given long enough, the angel will erode the mountain down to sea level. That, as described in Korean mythology, is indeed a very long period of time.
James Hutton (1726-1797) first expressed the modern concept of geologic time in his “Theory of the Earth”, which he communicated to the Royal Society of Edinburgh in 1785. The basic principal of all sciences is that nature is orderly, not chaotic, and Hutton applied the orderliness of nature to geology. He showed that sediment continually being removed from the land by rivers is eventually deposited as sand and mud on the sea floor, and that existing sedimentary rocks have all the characteristics of the unconsolidated sediments that currently are being deposited. He also proposed that the geologic processes and natural laws presently operating to modify the earth’s crust have acted in the same regular manner with essentially the same intensity throughout geologic time, and that “the past history of our globe must be explained by what can be seen to be happening now.” Simply put—the present is the key to the past. (Charles Lyell eventually gave this concept the term “uniformitarianism.”) Most importantly Hutton realized, as no one had done before, that the vast thicknesses of the older sedimentary rocks implied the operation of erosion and sedimentation took place over a period of time that could only be described as inconceivably long.
Today, most of the rocks that are quarried for aggregate and dimension stone formed and have been modified over “inconceivably” long periods of time—geologic time. As a general rule, and with all other things being equal, the older unweathered sedimentary rock is, the harder it is, and the better aggregate it will make. For example, young, unweathered limestones generally are softer than older unweathered limestones. Of course, some young limestones are formed under certain favorable conditions that make them harder than older limestones formed under less favorable conditions.
The same observation about age may be made for certain types of igneous and metamorphic rocks. For example, some “young” igneous rocks contain unstable varieties of quartz that can be reactive when used in portland cement concrete. Over geologic time, those unstable varieties of quartz are altered into stable, non-reactive varieties of quartz.
Sand and gravel deposits that are used as aggregate formed during geologic time, although they are relative youngsters in a geologic sense—only a few tens to hundreds of thousands of years old. During those few (in a geologic sense) years, unconsolidated materials are exposed to surface weathering, and the longer they are exposed to weathering, the weaker they become. Therefore, as a general rule, and again with all other things being equal, the younger unconsolidated deposits tend to be better aggregate than the older ones.
In the aggregate industry, it is important to remember that some things happen over very short periods of geologic time, but occur over long periods of time in a human sense. Recognizing this is especially important when collecting data for site characterization and permitting because data cannot always be collected on a schedule that is convenient to you. For example, many species of plants can only be identified when they are flowering, and some animal species may only be present during certain seasons. Also, most sites are affected by events that vary from year to year, such as rainfall, temperature, stream flow and ground-water levels. The proper analysis of these events requires data collection over several years, which in geologic time is just one blink of Gaia’s eye or one beat of an angel’s wing.
William H. Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey.