19.6: Understanding Geological Time

It’s one thing to know the facts about geological time—how long it is, how we measure it, how we divide it into smaller time intervals, and what we call the various intervals— but it’s quite another to really understand geological time. The problem is that our lives are short and our memories are even shorter. Our experiences span only a few decades, so we really don’t have a way of knowing what 11,700 years means. What’s more, it is hard for us to understand how 11,700 years differs from 65.5 Ma, or even from 1.8 Ga. It’s not that we can’t comprehend what the numbers mean, it’s that we can’t really appreciate how much time is involved.

You may wonder why it’s so important to understand geological time. There are some very good reasons. One is so that we can fully understand how geological processes that seem impossibly slow can produce anything of consequence. Consider driving from one major city to another, where a journey of several hours might occur at speeds of ~100 km/h. Continents move toward each other at rates of a fraction of a millimetre per day, a speed something on the order of 0.00000001 km/h (try walking at this speed!). And yet, at this impossibly slow rate, continents can move thousands of kilometres through geological time. Sediments typically accumulate at even slower rates—less than a millimetre per year—but are still thick enough to be thrust up into huge mountains or carved into breathtaking canyons.

Another reason is to understand issues like extinction of endangered species, and human influence on climate. People who don’t understand geological time are quick to say that the climate has changed in the past, and that what’s happening now is no different. And climate certainly has changed in the past: from the Eocene (50 Ma) to the present day, Earth’s climate cooled by ~12°C on average. This is a huge change that ranks as one of the most important climate changes of Earth’s past, and yet the rate of change over this time was only 0.000024 °C/century. Recent warming has occurred at a rate of ~1.1°C over the past 100 years (NASA GISS), 45,800 times faster than the rate of climate change since the Eocene.

One way to wrap your mind around geological time is to put it into the perspective of single year. At this rate, each hour of the year is equivalent to approximately 500,000 years, and each day is equivalent to 12.5 million years. If all of geological time is compressed down into a single year, Earth formed on January 1, and the first life forms evolved in late March (~3,500 Ma). The first multicellular life forms appeared on November 13 (~600 Ma), plants appeared on land on November 24, and amphibians on December 3. Reptiles evolved from amphibians during the first week of December, and dinosaurs and early mammals evolved by December 13. Non-avian dinosaurs, which survived for 160 million years, went extinct on Boxing Day (December 26). The Pleistocene glaciation began at ~6:30 p.m. on New Year’s Eve, and the last glacial ice melted from southern Canada by 11:59 p.m.

It’s worth repeating: on this time scale, the earliest complex ancestors of the animals and plants we know today didn’t appear on Earth until mid-November, the non-bird dinosaurs disappeared after Christmas, and most of Canada was periodically locked in ice from 6:30 to 11:59 p.m. on New Year’s Eve. As for people, the first to inhabit Canada arrived about one minute before midnight.

References

NASA Goddard Institute for Space Studies (n.d.). GLOBAL Station Temperature Index in 0.01 degrees Celsius base period: 1951-1980 [data file]. Retrieved from http://data.giss.nasa.gov/gistemp/ta..._v3/GLB.Ts.txt