There are multiple methods we use to determine the age of our planet, but we didn’t always know these methods. Methods such as studying layers of sediment to find how rocks or mountains are, but we didn’t really explore this method until 1969. This being the year Nicolas Steno published ?his work on examining the strata (layers of rock that form earth’s outer surface). Steno reached the conclusion, that the layers that were closer to the surface had to be newer, than the layers that were further away from the surface. Meaning that the fossils that’re found in the lower layers, had to be older than the ones found in the upper layers.
There are four types of sedimentary layers: Primary, Secondary, Tertiary, Quaternary. These layers were labeled by Giovanni Ardunio. He gathered these results by studying the Italian Alps, and organized them by their depth and composition. Primary is the lowest layer, and is comprised metamorphic and volcanic rocks. Secondary is second from the bottom, and is made up of hard sedimentary rocks. The highest layers are the Tertiary and Quaternary, which are made up of a soft material. But there was one main flaw, rock layers don’t appear in the same order in all mountains. That was until William Smith proposed that fossils were the answer to this problem.
Scientists used the work of Steno, Smith, and Ardunio to create the Geologic Time Scale. And while the G.T.S. (Geologic Time Scale) continues to be updated to this day, the current one uses five categories. Eons being the largest measurement of time, contain eras.
The Hadean Eon is the first on the timescale, and this time is described as a fiery landscape, but some life was able to form in these harsh environments. The Archean Eon is when life really started to form on the earth. This Eon’s air contained mostly carbon dioxide, but thanks to bacteria that soon changed in the Proterozoic Eon. While this new air killed most of the pre-existing life on the earth, it made way for new forms of life.
?Era’s are smaller chunks of history, especially compared to the large time gaps in the Eons, and because of this we know more in depth details about them. For instance, we know that the Cambrian Explosion happened in the Paleozoic Era, and this was when we started to see visible life in earth’s oceans. But within the same Era the world experienced, “The Great Dying”, a mass extinction that whipped out 70% of creatures on land, and 96% of the creatures in the ocean. Scientists don’t exactly know what caused the extinction, but it almost definitely caused by multiple factors. While it took a while for life to bounce back, of course it eventually did, and it did so in the Mesozoic Era.
This era brought new forms of life, a.k.a. reptiles, a.k.a. dinosaurs. But just like the the Paleozoic Era, most of the animals of this time were killed by another mass extinction. This one being labeled the, “cretaceous-Paleogene extinction”, but this time scientists are certain that an asteroid caused this extinction. Though birds did survive this extinction, and are believed to be dinosaurs ancestors.
This brings us to our final era, and the one we’re currently in. The Cenozoic Era, the era of the mammals. Mammals flourished in this new warm environment that was created after the cretaceous-Paleogene extinction. And this era was when ice caps started to form, because of even more drastic climate changes. This new climate also created a new environment, the grasslands. Later on those ice caps stretched out even further from the poles, this caused what we know as the “ice age”. But like the eras that came before it, many forms of life began to die, but this time due to the climate once again changing, to better reflect the current one.
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