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From its formation until today, planet Earth has undergone many changes. This is his story divided into Geological Eras.
The first stages, from the beginning of the solidification of the incandescent mass until the appearance of a permanent crust, left no evidence of its passage, since the rocks that were generated, melted again or were simply "swallowed" by A new eruption.
These primitive stages are still a mystery to science. In addition, the passage of time, erosion, different changes ... have been erasing the signals, so, the older the period to be analyzed, the more difficult we will find to study it. The Earth, let's not forget it, continues to evolve and change.
The geological history and the biological history of the Earth study the terrestrial past according to a time scale agreed between the scientists. This study is based on the analysis of the upper layers of our planet and the fossil remains found in them. The science that is responsible for this study is called paleontology and has a branch, the peleobiology, which specializes in the organisms of the terrestrial past.
Eons, Eras, Periods and Geological Epochs
He Eon It is the largest unit of geological time. It is divided into various geological ages. Every was It includes some periods, divided into periods. The more recent a geological period, the more data we can have and, consequently, it is necessary to divide it into smaller groups.
The following table shows the most important divisions, as discussed in this chapter. For more exhaustive information you can consult the Geological temporal scale according to Wikipedia.
The proportion between the previous divisions is better understood by observing this diagram It shows geological time like a day clock. It has represented the homininis, almost imperceptibly, at the end of the day. Actually, it refers to the genus Homo. If the history of the earth is represented in 24 hours, Homo appears 38 seconds before midnight. Our species, Homo sapiens sapiens, exists for less than 4 seconds.
In the investigation to determine and understand the geological history of the Earth records of four main rock classes are obtained, each produced in a different type of cortical activity:
1.- Erosion and transport which enables subsequent sedimentation and which, by compaction and lithification, produces successive layers of sedimentary rocks.
2.- Expulsion of molten rock from deep magma chambers, which cools on the surface of the crust, giving rise to volcanic rocks.
3.- Geological structures formed in pre-existing rocks that suffered various deformations.
4.- Plutonic or magmatic activity inside the Earth.
Dating, the dates of the past
The divisions of the geological timescale in the history of the Earth are based, to a large extent, on the variations of the forms fossils and other materials found in successive strata. However, the first 4,470 to 540 million years of the earth's crust are recorded on rocks that contain almost no fossils, that is, there are only suitable fossils of the last 540 million years.
For this reason, scientists divide the huge geological history of the Earth into two major time divisions: the Precambrian Eon, which includes the Azoic, Archaic and Proterozoic eras, and the Phanerozoic Eon, that of the fossils, which begins in the Period Cambrian and reaches the current era.
The discovery of radioactivity allowed geologists and paleontologists of the twentieth century to devise new dating methods, thus being able to assign absolute ages, in millions of years, to the divisions of the timescale.
• New chronostratigraphic table in Spanish
• Rock dating, Mexican Geological Service
• Paleontology News
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