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For example, 201.4 ± 0.2 Ma, the lower boundary of the Jurassic Period, is defined as 201,400,000 years old with an uncertainty of 200,000 years. Other SI prefix units commonly used by geologists are Ga (gigaannum, billion years), and ka (kiloannum, thousand years), with the latter often represented in calibrated units ( before present ).
The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, [7] [8] [9] during the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia.
A variation of this analogy instead compresses Earth's 4.6 billion year-old history into a single day: While the Earth still forms at midnight, and the present day is also represented by midnight, the first life on Earth would appear at 4:00 am, dinosaurs would appear at 10:00 pm, the first flowers 10:30 pm, the first primates 11:30 pm, and ...
Basin Groups Era begins on Earth. c. 4,450 Ma – 100 million years after the Moon formed, the first lunar crust, formed of lunar anorthosite, differentiates from lower magmas. The earliest Earth crust probably forms similarly out of similar material. On Earth the pluvial period starts, in which the Earth's crust cools enough to let oceans form.
The last estimate Kelvin gave, in 1897, was: "that it was more than 20 and less than 40 million year old, and probably much nearer 20 than 40". [27] In 1899 and 1900, John Joly calculated the rate at which the oceans should have accumulated salt from erosion processes and determined that the oceans were about 80 to 100 million years old. [25]
The Earth's magnetic field was established 3.5 billion years ago. The solar wind flux was about 100 times the value of the modern Sun , so the presence of the magnetic field helped prevent the planet's atmosphere from being stripped away, which is what probably happened to the atmosphere of Mars .
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See calendar and list of calendars for other groupings of years. See history, history by period, and periodization for different organizations of historical events. For earlier time periods, see Timeline of the Big Bang, Geologic time scale, Timeline of evolution, and Logarithmic timeline