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# 5.17 The Geological Timescale

When geologists in the early 1800s began to inspect the layers of rock in Europe, they realized that each layer had different fossils. Young rock layers, or strata, near the surface had fossils and bones of contemporary animals and plants. Older strata at deeper depths had unfamiliar animals and plants, now extinct. The variation in the fossils meant that a succession of different species of animals and plants had lived in that area. Using the information contained within these successive layers of rocks and fossils, scientists have been able to describe the history of not only the Earth, but of the whole Earth-Moon system. This history is usually expressed in terms of strata formed at different times, and it is called the geological time scale.

Earth's Geological Timescale with Milestones. Click here for original source URL

The turbulent geologic history of the Earth makes it difficult to sort out the different rock layers. The strata below our feet are not always arranged neatly, like the layers of a cake. In two different locations, a layer of a particular age might be made of two different types of rock. It might be near the surface in one location and be buried far below the surface in another location. A violent movement of the earth might even invert the order of some layers, so younger rocks are below older rocks.

Earth's Geological Timescale. Click here for original source URL.

In addition, the fossil record is far from complete. Specific conditions are required for an intact living organism to die and be turned into stone. The extreme heat and pressure under the Earth’s surface can easily obliterate any traces of life, leaving only about 1 in 10,000 of the earliest life forms represented in the fossil record. Decoding this information involves painstaking detective work.

Geologists and paleontologists gradually realized that the oldest layers had only simple animals and plants, such as fishes and primitive organisms that lived on the sea floor. They concluded that the unique fossils in different rock layers could be used to arrange the layers in order of age. Based on the dominant fossil organisms in different layers, geologists divided the known history of the Earth into several geological eras, and the eras were divided into smaller subdivisions called geologic periods. The periods are tens of millions of years long, and certain characteristic species dominate each period.

When radioactive dating became available, geologists discovered that the oldest layers with abundant, simple fossils did not date to the beginning of the Earth’s history at 4.5 or even 4.0 billion years ago, but only to 600 million years. Prominent animals and plants on the land surfaces go back only about 400 million years. In other words, we have fossil records of prominent species existing in the sea for only about 13% of the age of Earth, and on land, only 9%!

The record of well-developed fossils covers only a small fraction of Earth’s history, and the earliest rocks are missing altogether. Therefore, little was known of Earth-Moon system’s earliest history until 1969. In that year, Apollo astronauts and three Soviet robotic probes began collecting rock and soil samples from the Moon. The rocks dated from 3 to 4.4 billion years ago, which gave us information about early time periods that are poorly represented on Earth. They revealed the cosmic environment during those early eras. Counting lunar craters offered some important clues about how planets might have formed from the accumulation of smaller bodies. The lunar samples also showed that the Moon was volcanically active for the first third of its history, after which it cooled off and became more or less dormant.

The combination of terrestrial and lunar data has allowed us to piece together a fairly complete geological history of the Earth-Moon system. Following the history from beginning to end, we see the formation of both the Earth and Moon, the dates of the oldest rocks, the emergence of oxygen, sea life, land plants, and so on. Early Earth would have seemed like an alien world to us. Soil chemistry and other data indicate that oxygen was first produced by microbes starting about 2 billion years ago, but it was quickly absorbed in the oceans and went into rocks where it &quo;rusted&quo; iron deposits. Oxygen did not become substantial in our atmosphere until plants emerged and began converting CO2 into O2, around half a billion years ago. Tiny, single-celled organisms probably first emerged in the seas 3.5 to 4 billion years ago. If you had a time machine and could visit our planet during the first 85% of its history, you would find a barren and alien planet with an unfamiliar and unbreathable atmosphere, and no creatures larger than your thumb.