Vincent E. McKelvey

Director

U.S. Geological Survey

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In his invitation to participate in the symposium honoring Cecil and Ida Green, Frank Press proposed that I focus on the general subject of natural resources and suggested that "it would be appropriate for you to express some new idea that you have recently entertained, some new policy directions, some recommendation for government or industry or universities, or your predictions of things to come." When I accepted, I thought I would probably pass the first of these options; as an administrator of an organization that is rapidly expanding to meet some of the critical resource and environmental problems facing this country, my thoughts these days are mainly directed to program and management problems that are not of general academic interest. Several weeks thereafter, however, I did embark on what was for me a new train of thought concerning the earth's use of solar energy topic that I believe is appropriate for this symposium on "The New Wave of Exploration in the Earth Sciences.” The stimulus for this inquiry was a few paragraphs I was reading on plate tectonics, describing the movement of an oceanic plate beneath a continent and the igneous activity and mountain-building that result. Suddenly, I thought: What an enormous amount of energy is involved in the crustal and mantle processes that are responsible for the movement of crustal plates, the drifting of continents, mountain-building and crustal uplift and downwarp, igneous intrusions, and volcanic eruptions. How much is it and where does it come from? This line of thought led naturally to a list of the sources and, presently, to some notes on their magnitude and significance. First, there would be primordial energy, consisting of the earth's rotational energy, some amount of residual heat, and most importantly, the gravitational energy represented by the earth's mass. Then there would be the heat generated by the decay of uranium, thorium, potassium, and other radioactive elements. At that point I wondered if there might be some input of captured and stored solar energy.

This is the topic I found so interesting, for — although it appears that captured and stored solar energy represents only a small fraction of the energy in the earth and probably contributes to processes that operate only in the shallower part of the crust and at the surface solar energy has been captured in more diverse ways and has contributed more to geologic processes than I had realized.

The Sun as a Dynamic Agent in Earth History Approximately 1.5 quadrillion (1.55 × 101) megawatt hours of solar energy reach the earth's outer atmosphere

each year. According to Peter Weyi (Oceanography: An Introduction to the Marine Environment, New York: John Wiley and Sons, 1970), about 35 per cent of this amount is reflected back into space, 18 per cent is absorbed by the atmosphere, and 47 per cent is received by the earth's surface. For comparison, the amount reaching the earth's surface is about 13,000 times man's current annual consumption of energy in commercial forms, excluding food. Probably the amounts of solar energy reflected back into space and absorbed by the earth and its atmosphere have not been constant over geologic time, but however they may have varied, the total amount received over the earth's 4.5 billion year history staggers the imagination. Most of this has been radiated back to space, but some of the solar radiation absorbed has been used to heat the earth's surface or to supply the energy for photosynthesis, evapotranspiration, and other earth processes. Some of these processes form products that still retain energy, such as that in plant matter. The fossil fuels represent such captured and stored solar energy, of course, but what are other examples?

Because very large numbers are involved in describing energy used in earth processes, in the discussion that follows I use the amount of solar energy reaching the outer edge of the earth's atmosphere each year the figure! have already mentioned of 1.5 quadrillion megawatt hours as an energy unit, and report other amounts of energy as rounded multiples or fractions of that unit, which I will call a SERPY, abbreviated from “solar energy received per year." Keep in mind that a SERPY is a very large amount of energy — about 28,000 times as much as man now consumes each year.

Returning to the subject of captured and stored solar energy, one of the major contributions of solar energy has been the warming of the surface and near-surface part of the earth's crust. To appreciate the grand contribution of solar energy to earth processes, imagine what the earth would have been like without it. Bemardo Grossling of the U.S.G.S. estimates that without solar heat, the tem perature of the earth's surface would be about -238°C or about 253° C. colder than it is now. The heat stored in the upper part of the crust that is of solar origin resulting from thermal conduction in a solid model, throughout the life of the earth, Grossling estimates to be about 32,000 SERPYS. If one were to allow for the convective motion involved in sea floor spreading, the amount car ried into the earth would be larger still. Without that heat that is, at surface temperatures of -238° C. -nitrogen would condense and water would exist only as ice. Nothing resembling the oceans, even in the form of

ice, would exist, and the processes of erosion and sedimentation which so influence the configuration of the earth's surface would never have come about. Inasmuch as the organic building blocks from which life was formed are thought to have resulted from ultraviolet radiation of compounds such as methane, life would not have begun, nor could there be photosynthesis or any other of the processes driven by solar energy. So, solar energy has indeed been of fundamental significance in earth history.

When I began to think of how solar energy may have been captured by the earth, one of the first things that came to mind was the oxygen atmosphere, which is a product of photosynthesis and, according to the late Gerard P. Kuiper (The Sun, Chicago: University of Chicago Press, 1953), to photodissociation of water vapor to oxygen and hydrogen. Atmospheric oxygen represents an enormous store of chemical energy responsible in large part for the weathering and decomposition of both organic and inorganic materials at the earth's surface. Motokai Sato of the U.S.G.S. estimates that existing atmospheric oxygen corresponds to the energy storage of nearly 3 SERPYS (4.4 x 1015 Mwh.). As atmospheric oxygen is rejuvenated every 3,000 years, the total energy captured since Mississippian time (350 million years ago) amounts to 350,000 SERPYS (5.1 x 100 Mwh.), of which Sato estimates about 44 SERPYS (6.6 x 1016 Mwh.) have been fossilized. Most of this fossilized oxygen has been used for the oxidation of ferrous to ferric iron and sulfide to sulfates in sediments and natural waters. It is interesting to note that, whereas a large portion of the stored solar energy is dissipated as heat of oxidation in these processes, the reduction of hematite to magnetite or of sulfate to sulfide by organic matter is exothermic, so fossil oxygen still stores some solar energy.

The kinetic energy in the atmosphere in the form of wind has been estimated to be about 1/60th of a SERPY (2.6 × 1013 Mwh.) per year; it, too, is stored energy, but it is small in comparison to the solar energy received.

Although the ocean- and the atmosphere, too, for that matter- receives some heat from the earth's interior, the bulk of the ocean's heat is acquired from the sun, and it represents a very large amount of stored energy-some 146 SERPYS (2.2 × 1017 Mwh.). In addition to heat, the ocean also contains energy in its currents

which arise partly from the Coriolis force, partly from the wind, and partly from gravitational tidal forces. The ocean also contains energy stored in its salinity, for (as Richard Norman pointed out in Science for October 25,

1974) the addition of fresh water to saline water is an exothermic reaction — one which yields heat. Ocean salinity is another example of a product — resulting from work performed by solar energy (in this case evaporation) which has captured and stored some of the energy input. Sato points out that another way to look at this reaction is that the energy is stored in the fresh water resulting from the desalination of sea water by solar energy.

Solar energy, coupled with gravitational and the Coriolis forces (due to the earth's rotation), provides much of the drive for atmospheric and oceanic circulation and for the hydrologic cycle. Hence, solar energy is mainly the power also for the processes of weathering, erosion, and sediment transport. The annual waterpower energy totals only 1.7 millionths of a SERPY (2.5 × 10 Mwh.), but over geologic time the work expended by hydrologic processes has been enormous mountains and continents have been eroded and base-leveled and sediments have been formed from weathered rocks and transported to the seas and oceans. In one way or another, solar energy is also responsible for the deposition not only of sediments made up of fragments of preexisting rocks but of the sediments of organic origin and the great bulk of the chemical sediments, the precipitation of most of which depends on temperature, salinity, CO, content, or some other variable that is basically affected by solar energy.

Processes of erosion and sedimentation have involved a substantial redistribution of materials at the earth's surface. They have added to the load on the earth's crust beneath the thick accumulations in sedimentary basins and have lightened it in other places where once-high mountain ranges have been eroded away.

To what extent have this loading and unloading contributed to crustal dynamics? Ten or 15 years ago, when the geosynclinal theory was still in relatively good standing, some would have said that loading was in fact the drive for much of the geosynclinal process, which was thought to begin with the downwarping of the crust, to continue with subsidence resulting from the accumulation of a thick pile of sediments, and to result finally (for reasons not fully understood) in mountain building and uplift. Now, with the new concepts of plate tectonics, folded mountains are thought to be a part of the grander processes of plate movement and subduction. But the theory of isostasy that crustal loads are supported buoyantly, as are icebergs in the ocean, with heavier loads riding more deeply is still in good standing. A clear example of its operation—and with a solar energy drive —

was the depression of the crust in northern North America and Europe under the weight of the ice cap during the Pleistocene glaciation and the rebound that is still underway. I cannot answer my question about the extent to which crustal loading and unloading contribute energy to crustal dynamics, but my intuition tells me that it is substantial, whether or not it has been sufficient to change the course of processes driven by energy from within the earth.

While the amounts of solar energy stored in various forms are not impressive in terms of the 4.5 billion SERPYS received by the earth over its history, the amount of energy captured to perform work in weathering, erosion, sedimentation, evaporation, crustal loading and unloading, photosynthesis, and so on has been substantial. The sun has indeed been a dynamic agent in earth history.

Man's Use of Earth's Energy: A Trivial Fraction Incidentally, Harold Jeffreys and other geophysicists long ago answered my initial question of how much energy is in the earth. The rotational energy of the earth is equal to 40,000 SERPYS (6.00 x 1019 Mwh.) and the original gravitational energy of the earth is equal to 41 million SERPYS (6.25 x 10" Mwh.), much of which has been converted to heat and elastic energy. The heat generated from the decay of radioactive materials is about one tenthousandth of a SERPY (1.6 × 1011 Mwh.) a year, and of course there is a huge potential in the nuclear energy yet to be generated. The total heat content of the earth is of the order of 5 million SERPYS, and the heat brought to the earth's surface from all sources and dissipated into the atmosphere amounts to about one seven-thousandth of a SERPY (2.3 x 1011 Mwh.) a year-only a tiny fraction of the total heat received from the sun. The relatively small heat flow to the surface is a misleading indication of the total energy being expended within the earth, for it does not reflect the enormous amount of work performed within the earth by physical and chemical processes.

Knowing of my involvement in energy source problems, I suppose many may suppose that my interest in earth energy arises from the possibility of recovering some of it for man's use. I will admit to having thought about that briefly; but the result did not generate any excitement about the prospects, especially for the near term. I am impressed, however, by the enormous amounts of energy involved in the sun-earth system the amount reaching the earth from the sun, the amount of solar energy captured and stored in various forms, and the amount in the earth. Of all this, the energy being used by

man is trivial, and the amount of heat being used from the solid earth is miniscule, for we have barely begun to recover either geothermal energy or energy from the atom. Certainly we will increase our capture and use of solar, geothermal, and if we can overcome the technological and safety problems involved — nuclear energy as well and there are exciting potentials in all of these areas. Possibly other means of capturing energy from the sun-earth system will suggest themselves in the future, but first we need to enlarge our understanding of the system itself. This applies not only to energy per se but to geologic processes driven by energy. Brian Mason (Principles of Geochemistry, New York: John Wiley and Sons, 1960) observed some years ago, in an excellent review of these processes, that whereas we usually consider the geochemical cycle in terms of the material changes that take place, the energy changes during the geochemical cycle are equally significant even if less well understood. "Geochemical processes," he said, “operate only because of a flow of energy from a higher to a lower potential or intensity; hence energy is no less important than matter in the geochemical cycle." The same can be said about physical processes.

Referring again to Frank Press' invitation to discuss the general subject of natural resources, I trust I have met his request on at least the aspect of generality. And I believe, too, that I have kept to the subject of natural resources, for it is, of course, the sun and the earth and their materials and energy that are our basic natural resources.

Several years ago, Kenneth Boulding, the economist, used the concept of Spaceship Earth to remind us that the earth is finite and that we are confined to it. Fair enough; but one might add that it is some ship, that it has an auxilliary external power source, and that the first challenge to us in learning to use it wisely is to understand it. This is what earth and planetary science is all about, and the new wave of exploration could not have come at a more propitious time. As much as any institution and more than most, we can count on the Ida and Cecil Green Center for Earth Sciences to help propagate this wave into new and fruitful ground.

Vincent E. McKelvey joined the U.S. Geological Survey in 1941, shortly after completing undergraduate (1937) and Master's (1939) degrees in geology at Syracuse University and the University of Wisconsin, and he later completed his Ph.D. at Wisconsin, (1947). A distinguished economic geologist, he has been Director of the Survey since 1971. Dr. McKelvey acknowledges with appreciation the assistance of Bernardo Grossling and Motokai Sato of the U.S.G.S. in supplying many of the calculations he presents.