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Geological time

A scene in Ontario, Canada, where some of the most ancient rocks on Earth are found at the surface. The igneous rocks, which form...Read More >>A scene in Ontario, Canada, where some of the most ancient rocks on Earth are found at the surface. The igneous rocks, which form the Canadian Shield, date back to the Archaean Aeon in the Precambrian: between 3800 and 2500 million years ago. They have been repeatedly uplifted and eroded ever since. During the latest Ice Ages, ice sheets scraped away the region's soil and scooped out thousands of lakes. The Earth is 4.6 billion years old. This extremely long stretch of time is difficult to imagine, so events in the Earth’s history are therefore measured in geological time, spans of millions of years. A “recent” event in geological time might be, for example, the current Ice Ages that peaked about 20,000 years ago when ice caps extended over the continents of the Northern Hemisphere.



A chart showing geological periods

Aeons and periods

Geologists divide time into three aeons: the Archaean (“ancient”), from the origin of the Earth to about 2.5 billion years ago, the Proterozoic (“first life”) to 541 million years ago, and the Phanerozoic (“visible life”) to the present. The Archaean and Proterozoic are often referred to together as the Precambrian. The Phanerozoic is subdivided into eras: the Palaeozoic (541–252 million years ago), the Mesozoic (252–66 million years ago) and the Cenozoic (66 million years ago–present). The eras are split into periods, which are shown here. The Palaeogene, Neogene and Quaternary periods are themselves divided into epochs.
Life, scientists believe, began about 3.8 billion years ago. Since then, many different kinds of plants and animals have evolved and died out. The Earth, itself, has changed, too. Mountains have been pushed up and worn down. Even the continents, the Earth’s great land masses, have drifted, very slowly, around the globe.

Life, scientists believe, began about 3.8 billion years ago.

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