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Geochemistry

Geochemistry is the application of chemistry to geologic problems. Its primary focus is the distribution and migration of the elements (particularly the rarer ones) in Earth materials. It most often refers to a method of prospecting for buried ore deposits by analyzing soil, water, and vegetation. Geochemistry also touches upon mineralogy, petrology, oceanography, and economic geology.

Geochemistry began as a science in the early 19th century in Germany and Sweden with the accumulation of analytical data on rocks and minerals. Later, the chemistry of ore deposits, particularly as related to hot springs and volcanic emanations, attracted much attention in France. Only toward the end of the 19th century was geochemistry recognized as a discipline in its own right and extended to the study of chemical changes that are part of geologic processes. Especially notable in the early 20th century were the work of chemical analysts, particularly F. W. Clarke and H. S. Washington of the United States Geological Survey, the synthesis of planetary chemistry by V. I. Vernadsky in Russia, and the beginning of systematic efforts to duplicate natural chemical changes under laboratory conditions at the Geophysical Laboratory of the Carnegie Institution of Washington. After World War I, the Norwegian geochemist V. M. Goldschmidt used the newly developed techniques of X-Ray diffraction and rapid spectrographic analysis (spectroscopy) to determine the rules that govern the occurrence of rare elements in rocks and minerals. Only since World War II, however, has the field attracted many scientists and has the term geochemistry been widely used.

DIFFERENTIATION OF EARTH MATERIALS

On a planetary scale the most striking chemical feature of the Earth is that its material is separated into three distinct layers: a core extending out from the center about halfway to the surface, primarily composed of metallic iron; a MANTLE making up most of the rest of the Earth, composed in large part of compounds containing the four elements iron, magnesium, silicon, and oxygen; and a thin outer crust, variegated in composition but far richer in compounds of sodium, potassium, aluminum, silicon, and oxygen than the materials beneath it. Outside the solid crust are the oceanic liquid water and the atmospheric gases. How the planet's original material came to be segregated or differentiated in this way is unknown, but the process probably involved melting of at least part of the Earth, followed by heavy materials settling toward the center and light materials rising toward the surface. Recent studies indicate that the Moon and Mars underwent a similar geological history. Although much of the Earth may have been molten at some time in the distant past, at present the mantle and crust are largely solid, and only the outer part of the core is liquid.

Chemical changes have occurred in the crust and upper mantle all through the Earth's 4 1/2-billion-year history and continue today. New rock is formed and added to the crust by molten lava that comes to the surface along the mid-oceanic ridges and solidifies there. The lava originates as magma in a part of the upper mantle where temperatures are close to the melting point. Locally, large bodies of this liquid rock ascend through fissures and pour out on the sea bottom, forming basalt. This dark-gray rock, richer in iron and magnesium than most surface rocks, composes most of the ocean floor. Carried by currents (convection cells) in the hot, plastic mantle beneath, it is being slowly rafted away from the mid-ocean rises by seafloor spreading. At some continental margins (subduction zones) the basaltic layer, together with the thin veneer of sediments on its surface, is carried down into the mantle, its temperature rising as it descends. The temperature ultimately becomes high enough for basalt and sediment to begin melting. The first melt, rich in silicon, sodium, and potassium, is lighter than the rocks around it and rises through fissures, some to emerge at the surface and build volcanoes, and some to solidify under the surface and form batholiths of granite.

This process, which constantly adds new material to the existing continents, is probably the one that has operated gradually throughout geologic time to build the continents. In its chemical aspects, the process involves removing elements typical of granite (sodium, potassium, aluminum, silicon) from basalt and returning the basalt's iron and magnesium back to the mantle. The reactions occurring today represent a late stage in the long history of differentiation of the Earth's original materials. The continents are islands of silicon-rich scum, or slag, accumulated on top of a huge circulating system of heavier materials rich in iron and magnesium. The process's chemistry is similar to the operation of a blast furnace, in which silicon-rich slag forms on top of molten iron, although more complex.

THE ROCK CYCLE

Granite is formed by the solidification of molten rock at great depths, but slow movements in the solid crust eventually bring much of it to the surface, where, exposed to air and water, it undergoes slow chemical changes called weathering. Aluminum minerals are very slowly changed to clays, making the rock soft and vulnerable to disintegration. Iron in the granite's minor, dark-colored minerals is oxidized, giving the yellow and brown iron-oxide stains common on rock surfaces. Much of the sodium, potassium, and calcium is dissolved in rainwater and carried into streams and ultimately into the ocean. Only the very resistant minerals, especially quartz (silicon dioxide), are left as the weathered rock disintegrates; these, moved by wind and water, are worn down to form grains of sand. Part of the clay and sand accumulate on the granite surface to form soil, part are carried by streams and deposited as layers of sediment on plains or in the ocean. Such layers eventually harden into sedimentary rocks.

Through erosion and sedimentation granite's original chemical constituents become separated and then redeposited to form new kinds of rock. As sedimentary layers (strata) pile up on one another in a geosyncline, the lowest part of the pile may sink to great depths and become chemically altered to metamorphic rock by high pressure and temperature. Ultimately, some of this material may begin to melt. Metamorphism and partial melting take place on a large scale in sedimentary piles (eugeosynclines) near certain continental margins, where subduction, the movement of the seabed under a continent, crumples the sedimentary layers and drags them down, with basalt, into the mantle. The sediment and basalt form new granite melt, completing what is known as the rock cycle: granite is originally formed from partly melted basalt and sediment; the granite is raised to the surface and weathered; sedimentary rock is formed from the weathered material; then metamorphism of the sedimentary rocks occurs and new granite is generated by partial melting. The cycle is not quite closed because light minerals (compounds of silicon, sodium, potassium, and aluminum) are added to the continents, and heavy minerals rich in iron and magnesium are added to the mantle.

SEAS, AIR, AND LIFE

Geochemistry is concerned not only with these transformations in the Earth's solid materials but also with the envelopes of water and atmospheric gases that surround the planet. The water and gases have come primarily from the Earth's interior, probably mostly in the early part of geologic history, and have undergone continual chemical changes. Dissolved material from the land has poured into the ocean, forming seawater salt, and much of this material has precipitated out of the water as marine sediment. Long ago a balance was reached, the addition of each element to the ocean being compensated by its deposition in sediment, so that for much of geologic time the composition of seawater has remained nearly constant. The composition of air has changed profoundly: initially large amounts of carbon dioxide were present, but only traces of oxygen; now oxygen is a major gas and carbon dioxide is a minor constituent. This change is largely caused by the development of living organisms.

The origin of life on the Earth's surface is one of the most fascinating questions for geochemists. If the original atmosphere consisted mainly of carbon dioxide, nitrogen, and water vapor, complex organic molecules could have been formed from these gases by lightning discharges, ultraviolet light from the Sun, or simply the heat of molten rock from volcanoes. If such molecules dissolved in the warm water of shallow seas, a type of molecule probably would form that was capable of duplicating itself. Many steps in this progression from simple gases to living organisms remain to be traced, but the general progression is evident.

Once life had established itself, the geochemistry of the Earth's surface changed slowly but profoundly. Of particular importance was photosynthesis in green plants, by which carbon dioxide was converted into organic compounds and free oxygen released to the air. Operating over a vast period of time, two billion years, this activity could have changed the atmosphere from one dominated by carbon dioxide to one with oxygen as a major constituent. The Earth's present oxygen-rich atmosphere is unique in the solar system. It has permitted development of the higher forms of animal life and makes oxidation a major part of the weathering of rocks and minerals.

Living creatures also have a strong influence on the chemistry of sedimentary rocks. Many marine organisms use the dissolved calcium carbonate of seawater to build calcite and aragonite shells, and others use silica. Remains of these organisms have accumulated in enormous quantities to form beds of limestone, which is mostly calcium carbonate, and chert, mostly silica. When the soft parts of dead organisms are deposited in restricted environments out of contact with air, the organic matter slowly changes to simpler compounds. coal, petroleum, and natural gas accumulate from such changes. Organisms influence other geochemical processes less directly, for example, in the formation of some iron and manganese ores.

APPLICATIONS OF GEOCHEMISTRY

During all chemical changes involving the major elements of the Earth's crust and upper mantle, the minor elements partly follow the major elements and partly separate from them. Geochemists study the processes of separation in order to understand how some of the rare elements are concentrated into ore deposits. During the cooling of a granite pluton, for example, the last liquid remaining as magma crystallizes is a water-rich solution, part of the water coming from the molten granite and part from surrounding rocks. In this hot water are dissolved elements such as copper and gold, the atoms of which do not fit easily into the crystal structures of the major granite minerals. Movement of the water into surrounding cooler rocks can form veins or disseminated ores of these valuable metals. Again, dilute solutions of rare metals are sometimes added to seawater, either by streams or by submarine volcanic activity, and if chemical conditions and ocean currents permit, the metals may precipitate to form high concentrations in sediments.

Not only the distribution of elements, but the distribution of isotopes as well, has become a major subject of geochemical research. Isotopes of a few light elements are slightly separated in ordinary geochemical processes, and the subtle variations in their distribution often shed light on the origin and history of rocks and minerals. The isotopes of elements produced by radioactive decay have a special interest because analyses for these isotopes make possible the radiometric age-dating of geologic materials.

One branch of geochemistry has proved useful to the mining industry. This is a technique of exploration for ore deposits that are hidden by soil, vegetation, or thick sediments laid down by streams. For example, if a copper deposit exists beneath a layer of soil, the soil may look no different from soil elsewhere, and the deposit would be missed by ordinary prospecting methods. The soil above the deposit may, however, contain copper from weathering of the ore, and sensitive analysis can detect it. The hidden deposit may also become evident through abnormal copper concentrations in nearby vegetation, or in the water and sediments of streams that drain the area. Geochemical prospecting, the technique of finding such chemical clues, is a major tool in mineral exploration, along with geologic mapping and geophysical surveys.

Geochemistry has become a part of environmental health as more is learned about how human and animal well-being depends on trace elements in plants, soil, and water. Some of the rarer elements--mercury is an example--are toxic in even minute quantities. Others, like selenium and fluorine, are essential for life in trace amounts but toxic in larger amounts. Deficiencies or excesses of a number of elements must be remedied, and geochemical knowledge of their distribution and mobility is necessary.

Konrad B. Krauskopf

Bibliography:

  • Berner, Elizabeth and Robert, The Global Water Cycle (1986);
  • Bowie, S., and Thornton, I., eds., Environmental Geochemistry and Health (1985);
  • Brown, G. C., and Mussett, A. E., The Inaccessible Earth (1981);
  • Cannon, Helen L., Geochemistry and the Environment (1974);
  • Goldschmidt, Victor M., Geochemistry (1954);
  • Krauskopf, Konrad B., Introduction to Geochemistry, 2d ed. (1979);
  • Levinson, A. A., Introduction to Exploration Geochemistry (1974);
  • Saxena, S. K., ed., Chemistry and Physics of Terrestrial Planets (1986);
  • Siegel, Frederick R., Applied Geochemistry (1974);
  • Thornton, I., ed., Applied Environmental Geochemistry (1984).
 
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