It is now usually divided into the Archean and Proterozoic eons. The Archean begins with the formation of the Earth's crust 4 billion years ago and extends to 2. Proterozoic Eon From Britannica concise encyclopedia Younger of the two divisions of Precambrian time, from 2. Carboniferous Period: Topic Fifth period of the Paleozoic era of geologic time, from to million years ago. It lasted for approximately 80 million years, ending 65 million years ago.
Obviously, it doesn't make sense to talk about everything on the same time scale. That's why we've broken up our time. Years are made up of months, months are made up of weeks, weeks are made up of days, and so on. Geologists use the very same strategy to talk about the history of the earth. They break up geologic time into larger and smaller chunks, so that major events are easier to talk about. Okay, now let's go ahead and check out the major divisions of the geologic time scale.
Eons and Eras The first principal subdivision is called the eon. An eon, the largest division of the geologic time scale, spans hundreds to thousands of millions of years.
Geologists generally agree that there are two major eons: the Precambrian eon and the Phanerozoic eon. The Precambrian goes from the formation of the earth to the time when multicellular organisms first appeared - that's a really long time - from 4, million years ago to just about million years ago.
Then begins the Phanerozoic eon, which continues up to today. Eons are made up of eras, divisions that span time periods of tens to hundreds of millions of years. The three major eras are the Paleozoic, the Mesozoic, and the Cenozoic.
The Cenozoic era is the one we are in today. Use Advanced Search to search by activities, standards, and more. The planet Earth is approximately 4. The time span of 4. These four eons are further subdivided into eras Table 7. These horizontal layers are called rock strata and can be seen in canyons and rocky cliffs all over the world Fig. The Law of Superposition in geology states that layers of sediment and rock are deposited over time in sequence with the oldest layers on the bottom and the youngest layers on top.
For example, a fossil found in a lowest rock stratum would likely be much older than fossils found in higher strata. Because the different geological time units are named for specific rock strata patterns and prominent features, they vary in time spans. For example, the Silurian and Ordovician are both geological periods in the Paleozoic Era, but the Silurian Period spanned million years while the Ordovician Period lasted only 45 million years.
Scientists use evidence both from Earth as well as from other objects in our solar system to understand the beginnings of Earth.
The best data currently available has led scientists to estimate that our solar system, including the earth, formed approximately 4. The earth formed when meteorites collided and stuck to one another, gradually forming a planet.
The oldest known Earth mineral has been dated to 4. The oldest moon rock is approximately 4. The moon is less geologically active than earth, and thus has rocks that are older. The ages of several meteorites, which have the least geologic activity, have been calculated to be between 4. To understand the ocean floor scientists examine both geological and biological history.
One of the methods used by scientists to observe the change of Earth over time is the study of life in the fossil record. The fossil record shows that life on Earth has grown increasingly complex over time. Studying this increase in complexity in the fossil record gives clues about the timing of the changes we can observe in the ocean floor. Although scientists do not know exactly how life first arose on Earth, there is evidence that primitive single-celled organisms called prokaryotes first appeared between 3.
Modern-day bacteria are related to these ancient prokaryotic microbes. For more detail on prokaryotes, see What is Alive?
For more detail on the origins of life on Earth, see Building Blocks of Life. Around 3. Cyanobacteria are microbial organisms that can produce their own food through photosynthesis, in a process roughly similar to modern plants. Most of them were aquatic; the most famous examples are trilobites, small arthropods creatures with exoskeletons whose distinct fossils are still being discovered today. During the Ordovician period, fish, cephalopods, and corals first appeared; over time, these creatures eventually evolved into amphibians and dinosaurs.
During the Mesozoic era, which began about million years ago, dinosaurs ruled the planet. These creatures were the largest to ever walk the Earth. Titanosaur, for example, grew up to feet long, five times as long as an African elephant. The dinosaurs were eventually wiped out during the K-2 Extinction, an event that killed about 75 percent of the life on Earth. Following the Mesozoic era was the Cenozoic, which began about 66 million years ago. This period is also known as the "Age of Mammals," as large mammals, following the extinction of the dinosaurs, became the dominant creatures on the planet.
In the process, mammals diversified into the many species still present on the Earth today. Early humans, including Homo habilis , first appeared about 2. These enormous changes to life on Earth have taken place over a period of time that, compared to geologic history, is relatively small.
Human activity has transformed the planet; some scientists have proposed a new epoch, the "anthropocene," to describe this new period of life on Earth. Actively scan device characteristics for identification. Use precise geolocation data. Select personalised content. Create a personalised content profile. Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads.
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