The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

city and regions where the rocks were found; this is why division names frequently vary on geologic time scale charts from different countries.

      What are the major time units used in the geologic time scale?

      There are five major time units on the geologic time scale. The units are—in order of descending size—eons, eras, periods, epochs, and stages (although some list this division as ages and subages). The eon represents the longest geologic unit on the scale; an era is a division of time smaller than the eon, and is normally subdivided into two or more periods. An epoch is a subdivision of a period; a stage is a subdivision of an epoch.

      What do the divisions on the geologic time scale represent?

      The geologic time scale is not an arbitrary listing of Earth’s natural history, nor are the divisions merely fanciful. Each boundary between divisions represents a change or an event that delineates it from the other divisions. In most cases, a boundary is drawn to represent a time when a major catastrophe or evolutionary change in animals or plants (including the evolution of specific species) occurred.

      Natural erosion clearly reveals the layers of Earth’s crust, such as seen here in Badlands National Park in South Dakota. Observing these layers is like taking a trip back in time, with each lower level representing a different time period in the planet’s history (iStock).

      What is relative time in relationship to geologic time?

      Relative time is a way to establish the relative age of rocks and fossils. It is based on the location of a rock layer in comparison to the location of other rock layers; that is, it is only relative, not absolute, time. In many cases, rock layers are laid down in order, the older layers being below the younger layers. For example, a fossil found in a higher rock layer is usually younger than a fossil found in a rock layer below it. During the nineteenth century, scientists used this method to date rock layers relative to each other and to establish and construct the first geologic time scale.

      What is absolute time in relationship to geologic time?

      Absolute geologic time is the (approximate) true age of the rock; that is, the absolute time that the rock layer formed. Typically, radiometric techniques, which measure the amount of radioactive decay in rocks, are used to determine absolute time.

      When were radiometric dating techniques discovered?

      The basic principles and techniques of radiometric dating were not discovered until the turn of the twentieth century. In 1896, French physicist Antoine-Henri Becquerel (1852–1908) accidentally discovered radioactivity when a photographic plate left next to some uranium-containing mineral salts blackened, proving that uranium gave off its own energy. In 1902, British physicist Lord Ernest Rutherford (1871–1937) collaborated with British chemist Frederic Soddy (1877–1966) to discover that the atoms of radioactive elements are unstable, giving off particles and decaying to more stable forms. These findings led United States chemist Bertram Borden Boltwood (1870–1927) to argue that, by knowing the decay rate of uranium and thorium into lead, the dating of rock would be possible. In 1905, Boltwood and John William Strutt dated various rocks, obtaining ages of 400 to 2,000 million years for various rock samples and proving such dating could be done.

       Why do some dates differ on the various geologic time scale charts?

      Determining the true age divisions of the past 4.6 billion years for the geologic time scale is not a perfect science. (Determining the date of a rock layer is not as precise as knowing your own age.) In addition, there is often disagreement as to the extent of certain time periods, since rocks and fossils found on different continents vary. Even radiometric dating does not reveal the true age of a rock or mineral because there is always a certain amount of estimation involved.

      Who first developed an absolute geologic time scale using radiometric dating?

      In 1911, British geologist Arthur Holmes (1890–1965) began to formulate a geologic time scale based on absolute time, using the uranium-lead dating method to determine the age of rocks. In 1913, he published The Age of Earth, in which he outlined how radioactive decay methods, in conjunction with geological data, could be used to construct an absolute geologic time scale. In 1927, Holmes estimated that the age of Earth’s crust, based on his radiometric techniques, is approximately 3.6 billion years old.

TIME PERIODS

      What is the Pre-Cambrian era?

      The Pre-Cambrian era represents the time of Earth’s beginning to just before the big explosion of life in the oceans—from about 4.54 billion to about 543 million years ago. During this time, Earth was cooling, developing its oceans, and building the continental crust; in addition, scientists believe that life began during the early part of the Pre-Cambrian. The following lists one interpretation of three Pre-Cambrian divisions, the approximate dates, and major evolutionary events during these times:

      Hadean—4.5 to 3.8 billion years ago, the time when Earth was forming in the early solar system.

      Archaean—3.8 billion years to 2.5 billion years ago, the oldest bacteria evolved.

      Proterozoic Era—2.5 billion to 543 million years ago, a time in which multicelled eukaryotic (a cell with a definitive nucleus) evolved—in other words, animals.

      Why do scientists believe that several ice ages occurred during the late Pre-Cambrian era?

      Chemical and isotopic analysis of rocks found in Africa show that Earth may have gone through at least four ice ages between 750 and 570 million years ago. These were very deep ice ages, essentially turning Earth into a “snowball planet.” From the evidence to date, some scientists think the oceans were covered with ice almost 300 feet (91 meters) deep, and the land was completely dry and barren of life.

      Some scientists believe the Pre-Cambrian ice ages may have been caused by Earth’s tilt toward the Sun. The planet may have been tilted at a much larger angle—upwards of 55 degrees—than today’s angle of 23.5 degrees. This large degree of tilt meant that the polar areas received most of the Sun’s warmth, keeping them ice-free. But the areas around the equator would have been colder, allowing glaciers to form. If this was true, the buildup and melting of the glaciers around the equator during the Pre-Cambrian era may have created enough force to move the planet’s axis to its modern position. Some scientists have equated this process to repeatedly pushing on a swing at just the right moment in its movement, adding energy to make it go higher. The influence of the alternating advance and retreat of the glaciers could have caused the axis to straighten to its present angle.

      Some scientists believe the “heroes” that thawed the snowball planet and paved the way for an explosion of life were none other than volcanoes. As these surface blisters erupted toward the end of the Pre-Cambrian era, they sent massive amounts of carbon dioxide into the atmosphere, an increase of approximately 350 times its present concentration. This increase trapped re-radiating solar energy, warming the planet as it created a super greenhouse effect. The temperatures rose enough to melt the ice-covered oceans and end the ice age.

      Why was the “Cambrian Explosion,” also called the “evolutionary big bang”?

      Just after the end of the Pre-Cambrian era, about 543 million years ago (during the Cambrian period), a great burst of evolutionary activity began in the world’s oceans. Based on the fossil record of the Cambrian period, scientists estimate that the number of orders of animals doubled roughly every 12 million years. At this time, too, most of the modern phyla of animals began to appear in the fossil