Geologic time scale1-- (how earth scientists read the record of time)

Just as human beings go through different stages in life, from infancy to adulthood, so has the earth gone through many stages, each one, beginning with some important event or change. These stages in the Earth's history were calculated in geologic time which stretched over enormously long periods of time.
One would wonder how scientists are able to find out about events that took place millions or even billions of years ago. Well, the rocks of the hold all the evidence, scientists simply learned how to read the record of time preserved in the rocks. These methods are:
-* Rock layering: Layers of sedimentary rock provide scientists with important evidence of past events. As we know layers of sedimentary rock form one after the other, stacked according to age like a pile of old magazines. Older ones are found under the younger ones, therefore digging into deep layers of rock is like going back in time. This principle holds true as long as the rock layers were not turned upside down by intense folding. Hence, the layering of sedimentary rocks allows earth scientists to determine the relative age of these rocks.

-* Unconformities: A break in the layering of rock can be informative. For instance where a rock with many layerings is titled at an angle and covered by horizontal layers, an earth scientists knows that the tilted rock layers are much older than the horizontal layers. The line between them, called an Unconformity, represents a break in time.

-* Absolute age: Geologists have methods to determine the approximate age of rocks in years. This method tell them the absolute age of rocks and it is done through radio metrics dating. This same method is used to determine the relative age of a fossil. Many rocks contain small amount of radioactive isotopes during their lives. After a rock forms or an organism dies, these isotopes start to decay at a regular rate and each radioactive atom turns into an atom of another element. So, scientists calculate the ratio of the amount of radioactive isotopes present in a rock and the amount of decay product to determine the amount of time that has passed since the formation of the rock.

This last part may seem complicated but I do hope you all learnt a thing or two. On my next, I'll go into more details of the different geologic time scale and its characterization.

***Reference: Scott Foresman (2010) "science insight"