Once the earth formed, earth time began. The largest unit of time is the aeon. In the earth's history there have been five aeons, including the Hadean (4500 to 3900 mya), Archean (3900 to 2600 mya), Proterophytic (2600 to 1500 mya), Proterozoic (1500 to 570 mya), and Phaneorozoic (570 to present). Each aeon is divided into eras. For instance, in the Phaneorozoic aeon, there are three eras: Paleozoic (570 to 225 mya), Mesozoic (225 to 65 mya), and Cenozoic (65 mya to present). The eras in turn are divided into periods. In this part and the following one, aeons are discussed. But as we approach the present aeon, we switch to periods.

Chapter five deals with the basic structure of the earth formed in the Hadean aeon. It stresses those concepts necessary for understanding the ecology of our world today. The chapter emphasizes the existence of an early environment that was largely hostile to life. Chapter six discusses the actual origin of life in the Archaean aeon. It stresses organic chemistry as the basis for the creation of life. It is necessary to understand this basic fact, for although the forms of life become more complicated over time, they are still variations of basic chemistry. Chapter seven begins the descriptions of the five kingdoms of life, Prokaryotae, Protoctista, Fungi, Animalia, and Plantae. This particular chapter deals with the origin of the prokaryotes, which occurred in the Proterophytic aeon. Chapter eight deals with the origin of the next three kingdoms in the Proterozoic aeon. Most of our present kinds of animals evolved during this aeon. Therefore, chapter nine deals with taxonomy, specifically focusing on the evolution of animals. The next part of the book will discuss the modern aeon, the Phanerozoic.


The Earth formed from silicates and metals. As the planet cooled, the outer surface formed the crust, which makes up only about 0.3 percent of the mass of the Earth. The oceanic crust, which covers 55 percent of the surface, is typically about eight kilometers thick and is composed of basalts. The continental crust, which covers 45 percent of the surface, is from twenty to seventy kilometers thick and is predominantly granitic.

The major part of the earth is called the mantle, which stretches from the base of the crust down to a depth of 2900 km. Its composition is believed to be igneous silicate rocks. Samples of upper mantle material are occasionally ejected from volcanoes, permitting a detailed analysis of its chemistry.

Below the mantle is the core of the earth, a high-density region with a radius of 3500 km. The outer part of the core acts like a liquid, for it does not transmit certain kinds of seismic waves. The innermost part of the core (about 1200 km in radius) is extremely dense and probably solid. The primary constituent of both parts of the core is believed to be iron, along with substantial quantities of nickel, sulfur, and other cosmically abundant elements.

The primary heat source in the earth's interior is the decay of radioactive elements in the mantle and crust. In the mantle, this heat is carried upward by convection, the slow movement of currents of hotter material. Heat also escapes upward through the crust by conduction, which is the transfer of heat between parts of a system due to differences in temperature between the parts. This heat is often released in the form of molten lava from volcanic eruptions.

Plate Tectonics

The primary reason for discussing the nature of the earth's composition is to explain the nature of the movements of the earth's crust. Early scientists thought the mantle and crust of the earth were solid. Early in the twentieth century, however, German meteorologist Alfred L. Wegener (1880-1930) proposed that the earth's continents had drifted apart. He based his argument on detailed geological similarities between the east and west shores of the Atlantic Ocean. But he had no plausible mechanism by which the continents could move. As a result, the scientific community not only rejected Wegener's ideas, but ridiculed them as well.

The direct evidence for moving plates did not arise until the 1960s when detailed studies of the Atlantic Ocean became available. Geologists demonstrated that fresh lava was being injected along the mid-Atlantic ridge, a line of volcanic mountains running approximately north and south along the center of the ocean basin. They showed that the ocean floor was gradually separating at a speed of a few centimeters per year. Now the basis for the movement of continents had been found. The new theory came to be called plate tectonics.

The crust and upper mantle (to a depth of about 60 km) are divided into about a dozen major plates that fit together like the pieces of a jigsaw puzzle. This mobile part of the earth is called the lithosphere. The analysis of the interactions of moving plates provides the basis for understanding most of the large-scale geological activity on earth.

Four basic kinds of interactions between plates are possible. In rift zones, the plates can be pushed apart, as is the case with the mid-Atlantic ridge. In fault zones, the plates slide alongside each other. California is one place where a large slippage of the plates is long overdue. The plates can also jam together in a process that builds mountains. Such is the case for the Himalayan Mountains, created when the subcontinent of India thrust into the Asian continent. And, lastly, there are the subduction zones in which one plate burrows under another. Such is also the case along the California coast where the Pacific plate burrowed under the North American plate, thereby creating the Sierra Nevada chain.

The plates have moved considerable distances over time. About 280 million years ago the various continents merged into one supercontinent called Pangea. This broke up later into a southern part, called Gondwannaland, and a northern part, called Laurasia. Besides providing an explanation for many of the continents' topographic features, continental drift is important because it explains why certain similar plants and animals are found on widely separated continents. Examples are the Aracauria genus (including the Monkey Puzzle tree in Chile and the Bunya Bunya tree in Australia) and the marsupial animals (including the opossum in Chile and the kangaroo and many other marsupials in Australia). With the formation of the Earth we can now turn our attentions to the development of life itself.


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