Volcanic eruptions, glaciation, tectonic collisions and mountain building have all occurred many times in various places on earth. What information do we have today that tells us of these events? The rocks themselves!
Fossils of organisms are evidence of these occurrences by studying the characteristics of layered rocks and changes in life through time, geologists have been able to unravel earth’s history.
Geologic Time Scale
A hike down the kaibab trail in grand canyon reveals the multi-coloured layers of rocks that make up the canyon walls. These layers or strata, are made of different fossils in them. At the bottom of the grand canyon, is the Colorado river, which has been cutting downward through the rocks of the canyon for millions of years. Also at the bottom are rocks that date back 400 million years or more.
These rocks record the many advances and retreats of the oceans and the development of plants and animals. By studying the characteristics of rocks and fossils within them, geologists can interpret the environments the rocks were deposited in, reconstruct earth’s history and possibly predict events or conditions of the future.
Going Through Rock Records
To help in the analysis of earth’s rocks, geologists have divided the history of earth into time based upon the fossils contained within the rocks.
These time units are part of the geologic timescale, a record of the earth’s history from it’s origin 4.6 billion years ago to the present.
Since the naming of the first geologic time period ” The Jurassic”, in 1797, development of the timescale has continued to the present. The names of the periods do not change, but the years marking the beginning and end of each unit of time are continually been refined.
The geologic timescale enables scientists from around the world to correlate the geologic events, environmental changes and developments of life forms that are preserved in the rock record.
A Peck Into Geologic Time
The oldest division is at the bottom of the geologic timescale. Moving upward on the scale, each division is younger, just as the rock layers in the rock record grow generally younger as you move upward.
The timescale is divided into units called eons, eras, periods and epochs. An eon is the longest time unit and is measured in billions of years.
The Archean, Proterozoic, and Phanerozoic are eons. An era is the following longest span of time after the eons, it is measured in hundreds of millions to billions of years.
Eras are defined by the differences in lifeform found in rocks; the names of eras are based on relative ages of these life-forms for example in Greek, paleo means “old”, meso means “middle”, and ceno means “recent”. Zoic means “of life” in Greek, and thus mesozoic means “middle life”.
Precambrian Time, which makes up approximately 90 percent of the geologic time is divided into the Archean and Proterozoic Eons. The Proterozoic is more recent of the two, and the end of it is marked by the first appearance of organisms with hard parts.
All life-forms up until then had soft bodies and no shells or skeletons. Some of these resembled organisms that exists today, such as sponges, snails, and worms, while other cannot be accurately assigned to any known animal plant group.