Reed Hopkin, age 12, of Huntsville, Ala., for his question:
How long is an Era in geology?
Our word era was borrowed from a Latin word for the brass counters used to keep accounts. Accounts suggest bookkeeping, which suggests a period of time. An era came to mean a period of time starting on a certain date, and in modern language its length may be brief or immensely long.
There is plenty of reason for us to become confused about this small word. Since Roman days, when it was a brass money counter, it has taken on many meanings. As a rule it is a period of time, but the duration of time may vary. The Christian era is some 2000 years old. The era of a king may be a generation. The era of the auto began in the days of your grandparents, and sometimes era and day or days have the same meaning.
The eras of geological history are immense periods of time covering hundreds of millions of years. But they are eras nonetheless and as eras are related indirectly to the word day. They could be those stupendous days of the Earth's creation, for each covers a stage in the miraculous development from its bleak and chaotic beginnings to the luxurious, teeming planet of the present era.
Geologists divided the age old history of our planet into five eras and named them for the progressive stages of the story of life here on earth. We start some 1+ billion years ago with the Archeozoic Era, the days of a barren, newborn planet that lasted about 650 million years. Its name means early beginnings of life, and simple living things may have been created before this era closed.
The Proterozoic Era mans earlier life, and it was the day of much simpler, much earlier creatures than those that were to follow. It lasted almost 900 million years. The following Paleozoic Era was the day of the old life. The bare Earth began to clothe herself in green plants and populate the land with insects and creeping, stubby legged amphibians. It lasted some 320 million years and saw the first reptiles. The following Mesozoic Era tells the story of middle life. It was the day of the dinosaurs. Though it lasted only 110 million years, it saw the early and earlier forms of life transformed into the types of animal that populate our familiar planet.
The Cenozoic Era of recent life Opened less than a million years ago. The Earth was all ready, adorned with plants and tenanted with hosts of fascinating animals. The day came for the creation of man and the human family arrived at last to inhabit his luxurious planet home.
Since the span of a geological era runs to hundreds of millions of years, the recent Cenoozoic Era in which we live has barfly begun. We can expect it to stretch at least 100 million years into the future and perhaps much, much farther. The newly arrived human family has struggled and coped with many of the problems in its new home. We have enjoyed the place, thank you, and done quite well for ourselves. Who knows what wonders of nature control, planet control and self control we shall reach before this present Cenozoic Era of Earth's history closes its last page.