Understanding geologic time
IN LAST week’s edition of Earth Matters, we referred to some elements of the Geologic Time Scale (GTS), such as the Pre-Cambrian Super Eon, the Paleozoic and the Mesozoic eras. A number of readers contacted Sunday Pepperpot about the article, and from one of them came a suggestion that an explanation of the GTS would be helpful in the context of the series.
Accordingly, we hereby try to facilitate an understanding of the GTS.

The GTS is a chronological frame that is used by geologists, paleontologists, and earth scientists worldwide to describe the timing and relationships between events that have occurred during the history of the Earth. The various time periods are defined by age dating of rocks (magmatic); by fossils in rocks; and by the stratigraphy, or layers of the rocks.
The largest defined unit of time is the supereon, composed of eons (half a billion years or more). Eons are divided into eras (several hundred million years), which are in turn divided into periods, epochs (tens of millions of years) and ages (millions of years).
It should be noted that each era on the scale is separated by a major and/or changing event, such as mass extinctions. For example, the boundary between the Permian and the Triassic geologic periods, 252.28 million years ago, is marked by the Permian–Triassic extinction event, informally known as the Great Dying, during which 96% of all marine species, 70% of terrestrial vertebrate species, and all insects  became extinct.
Similarly, the boundary between the Cretaceous period and the Paleogene period, 65 million years ago, is defined by the Cretaceous–Tertiary extinction event, which marked the demise of the dinosaurs and many marine species.
The following are the various units of time on the universally accepted GTS, from the beginning to the present.
The Hadean Era within the PreCambrian Supereon): Lasting from 4,500 millions years ago to 4,000 million years ago.

The Archean (Pre-Cambrian Supereon): From 4,000 million years ago to 2,800 million years ago.

The Proterozoic (Pre-Cambrian Supereon): From 2,800 million years ago to 570 million years ago.

The Cambrian: Part of the Paleozoic Era, began 570 million years ago, and lasted for about 70 million years.

The Silurian/Ordovician: Time periods which form parts of the Paleozoic from 500 million years ago.

The Devonian: The Devonian period (Paleozoic) started 410 million years ago and
ended about 375 million years ago.

The Carboniferous: This period (Paleozoic Era) went from 360 million years ago to 286 million years ago.
The Permian: The Permian (Paleozoic Era) started 286 million years ago and went to
248 million years ago.

The Triassic
This period is the beginning of the Mesozoic Era. It started 248 million
years ago and ended 213 million year ago.

The Jurassic
The Jurassic period, part of the Mesozoic Era, started 213 million
years ago and lasted until 240 million years ago.

The Cretaceous
This period started 144 million years ago and lasted to 65 million years ago.

The Paleogene
Part of the Cenozic period, it started 65 million years ago and lasted up to 28
million years ago.

The Neogene
Also part of the Cenozoic, lasted from twenty-five million years ago to
two million years ago.

The Quaternary
This period follows the end of the Neogene period in the Cenozoic era,
and spans two and a half million years ago to the present.
Guyana is a two-billion-year-old resident of the heartland of South America’s oldest and hardest earth crust, the Amazon Craton.
Craton is a Greek word meaning strength, or stability.
Cratons are also called Shields, and are found in the interior part of Tectonic plates which have survived cycles of hundreds of millions of years of merging and rifting of continents.