The geologic time scale is the record of the history of earth. Layers of sediment accumulating over the eons provide us with a way to look back through time on earth and learn about what our planet was like thousands, millions, and even billions of years ago.
Plastiglomerate (Photo by Jell Elstone).
Generally, geologists have divided the history of earth into sections based on major events that have left a mark on the very sediments of the earth. The largest commonly used section is an Eon; Eons are divided into Eras; Eras are divided into Periods; Periods are divided into Epochs; and Epochs are divided into Ages. So, for example, the Mesozoic Era is characterized by the Permian extinction at its earlier end, and the K-T extinction at its more recent end. In this range of time, about 186 million years, is the age of the dinosaurs (or the age of conifers for those with a botanical slant)! Within this Era are three Periods (Triassic, Jurassic, and Cretaceous) which are each defined by major shifts in climate, tectonics (for example, Pangaea breaking apart), and biota.
The more recent history of earth is designated the Cenozoic Era, which contains several Periods. The most recent of which is the Quaternary, which contains several Epochs. The most recent of which is the Holocene and that is where some new developments are occurring.
The Holocene has been characterized as an interglacial period. It encompasses the increase in human impacts and our species has spread across the globe. These impacts include the effects of agriculture and also changes in atmospheric and ocean chemistry.
Plastiglomerate (Photo by Jeff Elstone).
And now an additional human impact is being incorporated into the geologic record. A new layer is starting to form and be preserved in the geologic record, and it is directly linked to humans. This new layer is comprised of plastiglomerates. Plastiglomerates are a new type of rock that is created when plastics get hot and melt onto rock. These chunks of combined rock and plastic fuse together, and in some places such as beaches in Hawaii, for fairly continuous layers. As more sediments are deposited on top of these plastiglomerate layers they are starting to form recognizable bands in sedimentary formations. This band of plastiglomerate is, and will persist as, a permanent marker in the history of the earth for future geologists to find. It will stand as a record of where a human created substance (plastic) changed the very structure of the rocks that make up the skeleton of the earth.
If you would like to read more about plastiglomerates, here is a paper about them.
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