The age of the Earth is about 4.6 billion years old or 4,600,000,000. Because of this very big number, the measurement used when referring to the age of the Earth is called Geologic Time. Geologic Time is the chronological time related to the events that have occurred during Earth's history. It is used by geologists (study Earth's matter), paleontologists (study Earth's organisms), and other Earth scientists to describe the timing and relationships of these events.
The Geologic Time of the Earth has been determined by using methods which can determine the age of rocks, fossils, and other materials that are part of the Earth. Geologists use radioactivity to discover the age of substances found throughout the Earth. They measure the amount of certain radioactive elements in a rock such as uranium or lead. The radioactive elements change over time, and this helps geologists determine the approximate age of the rock. If the rock includes fossils, this helps identify what appeared during various time-periods of the Earth's existence.
The Geologic Time System (GTS) is used to show the development of the Earth during its 4.6-billion-year existence. The scale is broken down into eons, eras, periods, epochs, and events. There are three main eras used by geologists: Cenozoic, Mesozoic, and Paleozoic and the Precambrian Eon, which contains three other shorter eras. The eras are then broken down further into periods, and some of the periods are split into epochs. The use of the GTS is helpful to show the different events that have taken place on the Earth beginning with the formation of the Earth's crust to human civilization today.
Early life during the Precambrian Era is found in the seas only. The algae and other organisms during this time-period released oxygen into the air, and continue to do so today. It is why the ocean's organisms are vital for the existence of future life on the Earth.
During the Paleozoic Era and future periods, many organisms became extinct and then new organisms came back many years later. During the Earth's development, this was not unusual. Remember, all of this is taking place over millions and millions of years. Extinction occurs over an extended period, especially when it is due to the cooling of the Earth, and then it takes many more years for organisms to appear again. Some organisms, though, do survive during extinction cycles.
The Mesozoic Era is called the 'age of Reptiles' and includes the Triassic, Jurassic, and the Cretaceous Periods. This was the era when dinosaurs walked the Earth. Scientists cannot be 100% sure of the causes for the dinosaurs' extinction, but the theories include meteorites or asteroids colliding with Earth, glaciation which causes Earth to be covered in ice, or other reasons.
The Earth is currently in the Cenozoic Period, which so far, has lasted for about 65 million years. The era is known as the 'Age of Mammals', and the Quaternary Period is known as the 'Age of Man.'
To further understand this incredible Geologic length of time, beginning 4.6 billion years ago to the present, if 1 second equals 1 year, then dinosaurs became extinct just two years ago and humans have existed for only 1/100 of a second. The Earth is very old.
In summary, Geologic Time of the Earth is broken down into eons, eras, periods, and epochs. The system is used by scientists to help them identify the events that have taken place on Earth, which includes the first appearance of single-celled organisms, or the first appearance of mammals and humans.
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