Geologists are obsessed with age.  They want to know how old rocks are and which rock is older than which.  They talk about thousands and millions of years like they were mere decades and they even have their own odd words like Cambrian, Carboniferous, and Holocene to describe geological time periods.

Geologists estimate Earth’s age to be around 4.6 billion years old[1].  For perspective; if I were to live 100 years, one could fit 46,000,000 of my lifetimes into the current estimate for Earth’s age.  Though helpful, this example still does not do justice to the depth of time that I am trying to describe.  To better appreciate the magnitude of what I am saying, here is another analogy:  if you think of the entire 4.6 billion years of Earth’s age as a single 24-hour day starting and ending at midnight;

• Free oxygen became present in the atmosphere at 10:42 am.  Up to that time, Earth was a very nasty place.
• Early life began at 8:58 pm
• The first land plants evolved at 9:40 pm
• The first birds and mammals appeared at 10:56 pm
• Dinosaurs evolved at 10:42 pm and died out at 11:40 pm
• The last ice age was 3.2 seconds before midnight
• I was born 0.01 milliseconds before midnight[2].

Here is a morbid thought:  major geological time periods are most often defined by mass extinction events.  This is because these events are readily visible in the fossil record.  When suddenly all of the fossils of one type disappear from the rock record, a geologist can deduce that there was a significant worldwide event which caused the disappearance.  Because of this, these extinction events provide us with excellent benchmarks in time.  A popular example is the Mesozoic Era, or the “Age of Dinosaurs.”  The Era’s beginning is defined by the appearance of dinosaurs approximately 250 million years ago, and ending by their extinction 65 million years ago.  By the way, it should be noted here that the appearance of dinosaurs was immediately preceded by perhaps the largest mass extinction event at the end of the Permian.  Because extinction events do not happen in regular intervals, geological time units can vary dramatically in length.  For instance, the Quarternary Period is 2.6 million years long; whereas the Cambrian Period is 54 million years long.

The largest unit of geologic time is the Eon.  Presently there are four of these and they range from about ½ a billion years to over 2 billion years in size.  The next finer unit is the Era.  Eras are typically several hundred million years each and are made up of Periods (this is the geological time unit with which most people are familiar).  Periods are typically a few tens of millions of years in size.  The finest unit of geologic time commonly used is the Epoch, which is generally single-digit millions of years in size.[3] Presently, we live in the Phanerozoic Eon, Cenozoic Era, Quaternary Period, and Holocene Epoch.

With such vast expanses of time, it should come as no surprise that a lot has happened where you live today and the landscape there can and likely has been very different in the past.  In Earth’s history, continents have formed, broken up, re-combined, been shoved deep into Earth, and re-born.  Mountain ranges have risen, only to be eroded back down again.  Present day deserts have been past oceans, and modern forests sit atop ancient seas.  The only constant on Earth is that it changes.

Jason Burris, P.G.
Regional Geologist
Luca Technologies Inc.

[1] U.S. Geological Survey, 1997, “Age of the Earth,” retrieved from http://pubs.usgs.gov/gip/geotime/age.html on 8 March 2011.

[2] Ages in millions of years are from Press, F. and R. Siever, 1994, Understanding Earth, W.H. Freeman and Company:  New York; and were converted to clock times.

[3] The International Commission on Stratigraphy is responsible for publishing the geologic time scale.  The 2009 version can be found at http://www.stratigraphy.org/upload/ISChart2009.pdf