Geological Time

Geological time is measured in millions of years and is almost impossible to imagine. If you assume that the average life span of a human being is about 75-80 years, it would take over 12,000 lifetimes to experience one million years. Think how long it has been since the end of the First World War - that is just over one average human lifetime ago.

Now think back to 1066 when William the Conqueror invaded Britain, that's about 12 average lifetimes ago - both of these seem a long way in the past. To begin to appreciate geological time needs you to multiply that by hundreds and thousands of times. The oldest rocks that have been found at the surface of the Earth formed nearly 4,000 million years ago, getting on for 50,000 human lifetimes ago!

Life seems to have started on Earth about 3,500 million years ago and was in the form of single-celled microbes. Around 1,500 million years ago, simple multicelled organisms had evolved. These were soft-bodied and finding traces of them is difficult. All this happened during the precambrian which is by far and away the longest period of geological time, it accounts for 88% of the time that has passed since the planet Earth formed. All the time, weathering and erosion has been taking place and tectonic activity has gradually provided us with the rocks that form our landscapes today. If you look at old pictures of mountains, taken 100 years ago, apart from changes in vegetation, they look the same as they do today. They change on the geological time scale, not the human time scale.

About 540 million years ago, life really took off - this has been called the cambrian explosion. Since then, life has continued to evolve, with mass extinctions and other significant events marking the end of each period and era. The largest divisions of geological time are called eons. There are only four eons recognized since the Earth was formed. Eons are broken down into eras. The eras are broken down into periods and that is as far as you need to know for high school geography. For your general knowledge, periods are broken down into epochs and epochs are broken down into ages, stages and substages!