Earth Through Time

The age of Earth is calculated to be approximately 4.6 billion years old. Scientists can learn about the history of the Earth by studying rocks and fossils.

Uniformitarianism

The Earth has evolved, or changed, over time. Uniformitarianism is a geological principle stating that processes shaping the Earth today operate the same way and at the same rates as they did in the past. Another way to state uniformitarianism is that the present is the key to the past.

For example, geologists assume that volcanoes erupted in Earth's ancient past much the same way they do today. This assumption is supported by the fact that lava flows and volcanic ash layers from Earth's past share many similarities with those forming today. Similar connections exist in the rock record for many other geological processes occurring today, such as plate tectonics, rock metamorphosis, and erosion.

Most of the changes that the Earth has undergone have been caused by natural processes. Humans have existed only a very short time relative to the Earth's 4.5-billion year history

Geologic Time Scale

Scientists learn about Earth's history by studying the rock and fossil record. Based on this record, scientists have learned how Earth and its atmosphere have changed over time, and they have divided Earth's history into distinct intervals of time on the geologic time scale. The geologic time scale, which is shown below, arranges time intervals from oldest (bottom) to most recent (top). The age units to the right are given as "Ma," which is a unit equal to 1 million years. Although the geologic time scale begins with the formation of the Earth around 4.54 billion (4540 Ma) years ago, 3.8 billion (3800 Ma) years ago is the approximate time that Earth's crust had become widespread and plate tectonics likely had begun.

The geologic time scale uses a hierarchy of time intervals. The broadest intervals of time (eons) are on the left side of the scale. As you move to the right, time intervals are divided into more specific intervals—eons are divided into eras, eras are divided into periods, and periods are divided into epochs.

A common way to organize geologic time is to break it down into four main intervals. The first interval is Precambrian time, which accounts for all of Earth's history before the Paleozoic era. After Precambrian time, Earth's history is divided into three eras, beginning with the Paleozoic era, then the Mesozoic era, and finally the Cenozoic era.

Each of the three eras can be divided into periods. The first period of the Paleozoic era is the Cambrian period. The most recent period of the Cenozoic era, which is still going on today, is the Quaternary period.

Each period can then be divided into epochs. The most recent epoch of the Quaternary period, which is still going on today, is the Holocene epoch.

Ice Core Data

Ice core data is gathered by climate scientists to compare the composition of the atmosphere today to its composition in the past. When snow or ice falls on certain regions on Earth, such as on Greenland, much of it does not melt. It is instead preserved for many thousands of years in layers of ice, with each layer representing one year. Such snow contains information about the atmosphere that it formed in. For example, from the ice cores, scientists can learn the concentrations of different gases in the atmosphere at different times in Earth's history.