Geochemistry

Introduction

The tools of chemistry can be applied to the study of the structure and composition of Earth and its neighboring planets in the solar system. Geochemistry has revealed that Earth has a layered solid structure. The thin outer crust, along with its water, and the gaseous atmosphere are capable of supporting life. Earth is a dynamic object, with circulation currents in its atmosphere and hydrosphere being driven by solar energy and the energy beneath its surface.

Geochemistry is concerned with the exploitation of Earth’s resources, such as metals and petroleum. It has also helped in the understanding of some of today’s most important environmental problems, such as global warming, ozone depletion, and soil and water pollution. There is still a great deal to be learned about the chemistry of Earth, the dynamic processes that have shaped it and how human activities may change it.

Historical Background and Scientific Foundations

Geochemistry is the study of the structure and composition of Earth. Its foundations were laid by Swiss mineralogist Victor Goldschmidt (1888–1947) in the early twentieth century. An important advance took place in the 1960s, when new oceanographic evidence backed the controversial theory of continental drift that had been put forward by German scientist Alfred Wegener (1880–1930) in 1912, in which he suggested that the continents had moved away from one another over geologic time. The underlying reason for continental drift is the slow movement of so-called tectonic plates, which are large blocks of Earth’s crust, driven by forces deep inside Earth.

The solid part of Earth has a three-layered structure. The central core extends from the center of Earth for about 2,100 mi (3,380 km) and accounts for one third of its 5.976 × 10²⁴ kilograms total mass. It is composed of iron and nickel. The inner part of the core is liquid. The core is surrounded by a layer called the mantle, which accounts for most of the rest of Earth’s mass and is around 2,000 mi (3,220 km) thick. The outermost area, the one people are most familiar with as earth, is the crust which comprises only 0.5% of the mass of Earth. Earth’s crust is much thinner than the other two layers, being only 60 mi (96 km) at its maximum depth and, in most places, it is much thinner than this.

The major elements in Earth’s mantle and crust are, in order of abundance: oxygen (44%), magnesium (23%), silicon (21%), iron (8%), calcium (2.5%), and aluminum (2.4%). Iron and aluminum are of great economic importance, as are elements present in lesser amounts such as copper, silver, and, of course, gold. The mantle and crust are composed of minerals, which are inorganic crystalline compounds, which make up a hard substance called rock. Aluminum and magnesium silicates are the most common mineral groups found in rock, but there are also carbonates, sulfates, sulfides, and oxides. Over 3,500 different minerals have been found on Earth and characterized by chemical analysis and structural techniques like x-ray crystallography.

Each type of rock has its own characteristic mixture of minerals, occurring in different percentages and making up different grain sizes. For example, granite is a mixture of the minerals mica, feldspar, and quartz. Rocks might seem solid and stable but, in fact, they are constantly being broken down and reformed in a phenomenon known as the rock cycle. Rocks are classified according to how they are formed. Igneous rocks, like basalt, are formed from the cooling of molten rock emerging from volcanic vents. Sedimentary rocks are formed from sediment, which is an accumulation of rock

deposits and particles and precipitated matter from solution over a period of time. Examples include gypsum, sandstone, and limestone. Metamorphic rocks come from the action of pressure or heat on a rock after its original formation. For example, marble is a metamorphic rock made out of limestone.

WORDS TO KNOW

CHLOROFLUOROCARBONS (CFCS): A family of chemical compounds consisting of carbon, fluorine, and chlorine that were once used widely as propellants in commercial sprays but regulated in the United States since 1987 because of their harmful environmental effects.

CORE: The central region of a star, where thermonuclear fusion reactions take place to produce the energy necessary for the star to support itself against its own gravity.

CRUST: The hard, outer shell of Earth that floats upon the softer, denser mantle.

MANTLE: The thick, dense layer of rock that underlies Earth’s crust and overlies the core.

OZONE LAYER: The layer of ozone that begins approximately 15 km above Earth and thins to an almost negligible amount at about 50 km, and which shields Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 25 km above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcano eruptions and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern.

TECTONIC PLATES: Thick crustal blocks that move relative to one another on the outer surface of Earth, causing continents to shift.

Rocks are broken down over a period of time by a process called weathering. Mechanical weathering is a physical disintegration of rocks into smaller particles by the action of waves, glaciers, rain, and wind. Chemical weathering actually changes the chemical composition of a rock by subjecting it to chemical reactions. Common forms of chemical weathering include oxidation, which is a reaction with oxygen in the atmosphere, and hydrolysis, which is a reaction with moisture.

Besides the solid part of Earth, geochemistry also covers the hydrosphere, which is the water in the oceans, seas, lakes, and smaller bodies, and the atmosphere. Together, Earth’s crust, hydrosphere, and lower atmosphere make up the biosphere, which is the zone at Earth’s surface that is occupied by plant, animal, and microbial life. The thin layer of soil on top of Earth’s crust is a mixture of minerals and organic matter that supports plant and invertebrate life.

The Earth is a dynamic system. Solar energy drives ocean and atmospheric circulation and the hydrologic cycle. Forces within Earth arising from its liquid component cause movements of the tectonic plates that not only move continents and oceans on the geologic timescale, but also cause natural catastrophes such as volcanic eruptions and earthquakes.

Impacts and Issues

Earth’s crust contains many materials of economic value, such as metals, coal, and petroleum. The most significant metals are aluminum, nickel, iron, copper, zinc, mercury, tin, and lead. Gold, silver, and diamond also have a high value. Other important mineral resources include sand, gravel, limestone, and clay, which are used in building and construction work. Rock salt is important for culinary use and for water softening. Meanwhile, sulfur is mined for the manufacture of sulfuric acid, which is one of the most important raw materials of the chemical industry.

Mining and extracting these valuable geochemical substances can have severe environmental and social consequences. Mining disfigures the landscape in a very obvious way and is often a dangerous occupation. It also releases pollutants into the air and water supply. This is why recycling and reducing our consumption of mineral resources, coal, and petroleum is so important.

Geochemistry also plays an important role in helping to understand and control environmental problems. Analysis of atmospheric ozone levels over Antarctica in the early 1980s led to the discovery of an unexpected hole in the stratospheric ozone layer and its link to emissions of chlorofluorocarbons (CFCs). The findings helped shape the world’s first environmental treaty, the Montreal Protocol, in 1987, which requires the phasing out of CFCs so that ozone depletion can be slowed and eventually halted. Geochemical measurements in the atmosphere and oceans are also monitoring the effects of global warming. Increasingly sensitive instrumentation also allows for ongoing monitoring of pollution levels in air, water, and on land so that environmental legislation on emissions can be supported and enforced by high quality data.

See Also Laboratory Methods in Environmental Science; Mining and Quarrying Impacts; Resource Extraction; Soil Chemistry

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Web Sites

International Mine Water Association. “Geochemistry by William M. White, Cornell University.” August 25, 2005. http://www.imwa.info/geochemistry/ (accessed March 25, 2008).