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EARTH MATERIALS: MINERALS AND ROCKS

After the grand vistas of cosmic and planetary evolution and the outline of the global processes (what we affectionately call arm waving geology) this up-coming section describes the actual materials the earth is made of, and how these materials are constantly modified, altered sometimes at the surface and sometimes at depth, and how they become the source of new materials.

Before plate tectonics, geologists tended to consider only crustal materials, under the assumption that the surface of the earth was, with rare exceptions, sealed off from the deeper layers. With plate tectonics, we have come to realize that the crustal materials are really but one part of this giant puzzle that is the earth. We now know that this lithosphere which is so important to us, is but a small portion of an ever cycling earth and that any understanding about materials, their origins and interactions must be rooted in plate tectonic processes and in events that stretch back nearly five billion years in time.

MINERALS

We know that everything in the world is composed of atoms, which are further organized into elements (one kind of atoms) and compounds (two or more kinds of atoms bonded together). To most geologists, the particular materials which make up the earth have special importance. Endlessly re-created, recombined and recycled in the great plate tectonic, weathering and erosional cycles, these earth materials have been basic human resources since time immemorial. Stone tools and shelters not only enabled our ancestors to survive, but also to become the dominant species of this world. As civilizations arose, minerals were used to adorn the bodies of women and men. For millennia, when rulers died, they were buried with their gold and jewels in sarcophagi of porphyry and basalt, in tombs of rock (cf. the pyramids). We are still fascinated by Stonehenge, and the Elgin marble friezes of the Parthenon are one of the crowning achievements of western art.

Today, these materials still adorn us, shelter us, and grace our lives, and just as importantly, they are the raw materials and ores of our highly technological civilization. Their absence or presence has started and ended wars, created and sundered alliances. Cultures and empires have risen, flourished and fallen over their control. Their exploitation fueled greed and slavery. On a more scientific and intellectual level, their very make-up helps us understand the origin and evolution of the earth and the processes that have shaped it.

Mineral definition

In geology, these fundamental combinations of atoms are called minerals. A mineral is defined as any solid, inorganic, naturally occurring element or compound with a fairly fixed chemical composition (atoms linked together electronically in definite ratios) and a crystalline structure (atoms are linked in distinct repeated internal patterns).