Transformations of materials aided by energy from the sun and the Earth's core are at the basis of all phenomena. The conditions on Earth over the last four and half billion years, the energy flow from the sun, and the relative abundance of different elements form the basis of all phenomena on Earth, including life. In this unit, we examine some of the most basic systems of materials--elements and compounds.

Most materials in nature, especially those in the biosphere (the part of the Earth which holds life) undergo chemical and physical changes constantly. Some materials, such as carbon and water, contribute to the cycle through various physical and chemical stages. Many of the environmental problems surrounding material use are actually disruptions of the cycles which arise from taking or putting in too much or too little material too fast or too slow into one or more of the phases in the material cycles.

Solar energy in the form of electromagnetic radiation streams in through the atmosphere onto the surface of the Earth providing energy at the rate of about 1 kilowatt per square meter at places of peak intensity. In one hour, we receive more solar energy spread over the land area of the United States than we get from the fossil fuels we burn in one year! Except for this vast and continuous input of energy, most of the material on the Earth remains at a constant amount, changing forms in some cases, going through cycles that keep many of these in forms that are accessible to life. So, as far as most materials are concerned, the Earth is a "closed system." However, solar energy is the vital part - the input into this closed system - that maintains the material system suitable for life.

Most of the matter on Earth generally remains on Earth, due to a continuous recycling of materials. Figure 1 shows the natural, large-scale processes that recycle materials.

Figure 1: Natural Recycling of Materials on Earth.

Through processes of transport and transformation in the atmosphere, absorption and settling in the oceans, and subduction and volcanism in the lithosphere, materials are recycled in nature through both physical and chemical changes. Residual heat in the core of the Earth and heat from radioactive processes provide energy from within the Earth. Solar energy drives the water cycle and the atmospheric currents, aided by the gases in the atmosphere. Water, carbon dioxide, nitrogen, chlorine, and sulfur are the main materials that cycle through atmosphere, oceans, and sediments.

In this unit, we examine several material cycles including cycles of water, carbon, and nitrogen. As materials cycle through, we note that the total quantity (mass) of matter remains the same; energy that is put in changes to work, often to rearrange forms of matter; and is eventually lost to the surroundings. Human intervention has disrupted natural environmental processes. Later on in this unit, we look at some of these disruptions and the impacts, as well as the new paradigm of industrial ecology that seeks to recycle rather than discard materials as part of the industrial processes.

History of Materials on Earth

The planets of the solar system are believed to have been formed from materials that broke off from the sun about 4.5 billion years ago. The Earth is presumed to have been in a completely gaseous state, cooling rapidly and gathering dust and smaller pieces of material, growing in size initially. Remains of pieces not coalescing initially with the planets remain as large rocky asteroids in orbits between Mars and Jupiter. One estimate is that a mass the size of the Earth originally at 6000° K (temperature of the sun's outer layer or photosphere) should have cooled to about 1500° in about 15,000 years. In about 25,000 years the temperature of the surface would have reached very nearly that of the Earth at present.

At least sixty-six of the ninety-eight elements on Earth have been detected on the sun by means of spectroscopy. As the Earth cooled, the lighter atoms would tend to move away from the center more rapidly than the heavier ones, leading to a certain degree of layering. In the early period, significant amounts of hydrogen and helium--the main constituents of the solar nebula--remained on the planets. Some of the lightest atoms (hydrogen and helium for example) would escape into space, unless they combined chemically with other elements. The lack of ability to combine could be why we find little helium on Earth while the lighter hydrogen--which due to its weight should be able to escape with greater ease--has been captured in the form of water and other compounds. To escape from the Earth's present gravitational pull, a molecule must have the "escape velocity" 11.3 km/second moving perpendicular to the Earth's surface.

Table 1 shows the most abundant elements on Earth and a comparison of their estimated concentration in the sun.

Table 1: Proportion of Elements in Earth and Sun.

Much of the Earth's material is in combination, as molecules. Even when the gases oxygen and nitrogen occur as elements in the atmosphere, they occur as molecular compounds O₂ and N₂, rather than in the atomic form.

Seismology has given us much of our knowledge of the interior of the Earth. The core is approximately 3,500 km in radius with an average density 10.72 g/cc. The mean radius of the Earth is 6,371 km. The mantle, which is therefore about 2900 km thick has an average relative density of about 2.7 near the surface. The temperature of the core is between 2000 and 4000° K. The core therefore consists of molten heavy metals such as iron (Fe), nickel (Ni), and uranium (U), and minerals containing these metals as well as compounds of silicon (Si), aluminum (Al), and magnesium (Mg) with oxygen, carbon, and sulfur.

Table 1 shows that the 3 most abundant elements on Earth are oxygen, iron, and silicon. However carbon, which is only 1 in every 1000 atoms, is the basic molecule of all life. The chemistry of carbon and its capacity to form numerous components are described in the unit on Biological Systems. It is interesting to note that Silicon, in the same chemical family as carbon, abounds on Earth in the form of sand (SiO₂) and other rocks. While carbon chemistry has given us live intelligence, we have used silicon chemistry for artificial intelligence--as silicon is the basic material for computers.

It is the coincidence of the strong hydrogen - oxygen bond and carbon chemistry coupled with the abundance of these three elements, and the Earth's gravity, distance from the sun (ensuring a particular temperature range) and speed of rotation (ensuring day and night) that gave us a water planet that could evolve our life forms!

One of the basic tenets of nature is a recycling of materials that plays a role in ecosystems--water, carbon, nitrogen, oxygen, and to a smaller extent, materials such sulfur and phosphorus.

There are numerous other materials--elements and compounds--that we otherwise mine or extract and use in a variety of ways. They range from carbon-based materials that may be like oil, gas, and coal derived from carbon that has been sequestered by plants, or metals such as Aluminum, iron and uranium. Depending on the use, these materials may be dispersed into the atmosphere or Earth during processes like the burning of coal, or built into structures that slowly erode, such as buildings or monuments.

The early atmosphere of the Earth contained water and other compounds including nitrogen, carbon dioxide (CO₂), methane (CH₄), and ammonia (NH₃). The oceans and the water cycle were established early. The Earth is the only planet in our solar system with surface oceans. Mars and Venus, the once "identical" planets, so called because of similarity in size and composition to Earth, have dry surfaces. The gases in our atmosphere - H₂O, CO₂, and N₂ - have different primary reservoirs. Water is mostly in the ocean reservoir; CO₂ in sedimentary rocks as carbonates, and N₂ is in the atmosphere.

In this unit we first describe the natural cycle of materials that pass through biological and geological cycles. Then we describe the use of materials in industrial processes, and how, over the last few decades, an examination of environmental impacts have led to some recycling of materials in the industrial setting.

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