An Overview of Geologic Time

So far, we have discussed Earth’s size and shape, its internal layering and composition, and the operation of its three major geosystems. How did Earth get its layered structure in the first place? How have the global geosystems evolved through geologic time? To begin to answer these questions, we present a brief overview of geologic time from the birth of the planet to the present. Later chapters will fill in the details.

Comprehending the immensity of geologic time is a challenge. John McPhee notes that geologists look into the “deep time” of Earth’s early history (measured in billions of years), just as astronomers look into the “deep space” of the outer universe (measured in billions of light-years). Figure 1.19 presents the “arrow of geologic time,” marked with some major events and transitions.

Figure 1.19: This geologic time line shows some of the major events observed in the geologic record, beginning with the formation of the planets. (Ma, million years ago.)

The Origin of Earth and Its Global Geosystems

Using evidence from meteorites, geologists have been able to show that Earth and the other planets of the solar system formed about 4.56 billion years ago through the rapid condensation of a dust cloud that circulated around the young Sun. This violent process, which involved the aggregation and collision of progressively larger clumps of matter, will be described in more detail in Chapter 9. Within just 100 million years (a relatively short time, geologically speaking), the Moon had formed and Earth’s core had separated from its mantle. Exactly what happened during the next several hundred million years is hard to know. Very little of the rock record survived intense bombardment by the large meteorites that were constantly smashing into Earth. This early period of Earth’s history is appropriately called the geologic “dark ages.”

The oldest rocks now found on Earth’s surface are over 4 billion years old. Rocks as ancient as 3.8 billion years show evidence of erosion by water, indicating the existence of a hydrosphere and the operation of a climate system not too different from that of the present. Rocks only slightly younger, 3.5 billion years old, record a magnetic field about as strong as the one we see today, showing that the geodynamo was operating by that time. By 2.5 billion years ago, enough low-density crust had collected at Earth’s surface to form large continental masses. The geologic processes that then modified those continents were very similar to those we see operating today.

The Evolution of Life

Life also began very early in Earth’s history, as we can tell from the study of fossils, traces of organisms preserved in the geologic record. Fossils of primitive bacteria have been found in rocks dated at 3.5 billion years ago. A key event was the evolution of organisms that release oxygen into the atmosphere and oceans. The buildup of oxygen in the atmosphere was under way by 2.7 billion years ago. Atmospheric oxygen concentrations probably rose to modern levels in a series of steps over a period as long as 2 billion years.

Life on early Earth was simple, consisting mostly of small, single-celled organisms that floated near the surface of the oceans or lived on the seafloor. Between 1 billion and 2 billion years ago, more complex life-forms such as algae and seaweeds evolved. The first animals appeared about 600 million years ago, evolving in a series of waves. In a period starting 542 million years ago and probably lasting less than 10 million years, eight entirely new branches of the animal kingdom were established, including the ancestors of nearly all animals inhabiting Earth today. It was during this evolutionary explosion, sometimes called biology’s “Big Bang,” that animals with shells first left their shelly fossils in the geologic record.

Although biological evolution is often viewed as a very slow process, it is punctuated by brief periods of rapid change. Spectacular examples are mass extinctions, during which many kinds of organisms suddenly disappeared from the geologic record. Five of these huge turnovers are marked on the geologic time line in Figure 1.19. The most recent one was caused by a large meteorite impact 65 million years ago. The meteorite, not much larger than 10 km in diameter, caused the extinction of half of Earth’s species, including all the dinosaurs.

The causes of the other mass extinctions are still being debated. In addition to meteorite impacts, scientists have proposed other kinds of extreme events, such as rapid climate changes brought on by glaciations and massive eruptions of volcanic material. The evidence is often ambiguous or inconsistent, however. For example, the largest mass extinction of all time took place about 251 million years ago, wiping out nearly 95 percent of all species. A meteorite impact has been proposed by some investigators as the cause, but the geologic record shows that ice sheets expanded and seawater chemistry changed at this time—a finding that is consistent with a major climate change. At the same time, an enormous volcanic eruption covered an area in Siberia almost half the size of the United States with 2 million to 3 million cubic kilometers of lava. This mass extinction has been dubbed “Murder on the Orient Express” because there are so many suspects!

Mass extinctions reduce the number of species competing for space in the biosphere. By “thinning out the crowd,” these extreme events can promote the evolution of new species. After the demise of the dinosaurs 65 million years ago, mammals became the dominant class of animals. The rapid evolution of mammals into species with bigger brains and more dexterity led to the emergence of humanlike species (hominids) about 5 million years ago and our own species, Homo sapiens (Latin for “knowing human”), about 200,000 years ago. As newcomers to the biosphere, we are just beginning to leave our mark on the geologic record. Indeed, our short history as a species can be appreciated by noting that it spans less than a line’s width on the geologic time line (see Figure 1.19).