Geologic time by Matt L.

Geologic Time

Have you wondered what happened before we arrived on earth?  Earth is about 4.6 billion years old, and humans have only appeared in the last 100,000 years or so.  In the billions of years to form, a lot has happened.  We call that period geologic time because it is the time of the formation of earth.  In other words, geologic time is the time of earth’s development to the present day.  Events that have happened on earth can ordered on a timeline.

In geological time there has been 6 major events:  the formation of the earth and moon 4.6 billion years ago; earliest life 3.9 billion years ago; early land plants about 420 million years ago; largest mass extinction 248 million years ago; dinosaur extinction 65 million years ago; and “Lucy” Early Hominid 4 million years ago.

Source:  http://www.ucmp.berkeley.edu/education/explorations/tours/geotime/gtpage2a.html

You can think of earth’s past as a book.  If you were to write earth’s history and each page represented one year, the book would be 4.6 billion pages long.   This book would be 145 miles thick.  An average person reads about one page every two minutes.  It would take that person 17,503 years to finish the book if they read non stop.


Source  http://www.petrostrategies.org/Learning_Center/exploration.htm

Scientists have figured out events in the earth’s past through the study of rocks.  When fossils are found, geologists can date them by looking at the layers of rocks on earth.  Geologists have figured out that the earth is covered with layers of rocks, with the older rocks on the bottom and the newest at the top.  Basically, this means layers of rocks build on top of one another as time passes.  Geologists refer to this as the Law of Superposition.

Source:  http://lifesciencedaily.com/wp-content/uploads/2011/05/Fossils.jpg

With each layer of rock representing a period of time, scientists looking at the sequence of the layers of rocks can tell the order of when events on earth happened.  Scientist can use all of the sequence of layers to also figure out the relative order of when events happened on earth.  Evidence about the order of events in earth’s history are also found in fossils buried within the rock layers.  This ordering is known as relative time.  
I think that it is pretty awesome that we can figure out what happened billions of years ago when we have lived for about hundred thousand years.