By Richard I. Gibson
Here's the podcast:
Transcript: All this time we’ve been progressing through the Precambrian – obviously the time in earth history before the Cambrian. We’ll get to the Cambrian in February but for now, let’s just acknowledge that geologists give names to time periods simply to have an easy way to talk about them, just as we give names to the months so it’s easy to know when we mean.
Geologists also subdivide the big blocks of time – the years and months – into smaller portions, the equivalent of weeks, days, and hours – and they give names to all those segments as well. Everything before the Cambrian is divided into two great packages, the Archean and the Proterozoic Eons. Archean means “ancient time” and Proterozoic means “early life”, and Eon is the top-level division of time in earth history.
The Archean started with the origin of the earth, back on January 1, and continued until about 2.5 billion years ago, when the Proterozoic started. The divisions of earth time are not arbitrary – usually, there’s evidence in the rocks of some kind of major change at the boundary, and that’s how early geologists defined the packages of rock and time, long before actual age dates were available by analysis of radioactive decay.
Generally, rocks of the Archean Eon are intensely metamorphosed, changed by heat and tectonic events such as continental collisions. Younger Proterozoic rocks are typically less deformed, but there are plenty of exceptions to these rules. Archean rocks also tend to occupy the central cores of continents – cratons or shields, which we’ll talk about in a few days.
Early geologists called most of the Precambrian Azoic, meaning “without life,” but since fossils older than 3 billion years ago exist, this is a misnomer and is no longer used. Today, the very oldest time period, before the oldest known rocks, before 3.8 billion years ago, is called the Hadean Eon. That name refers to Hades, the god of the underworld, and the hellish conditions that ruled the earth at that time.
We’ll spend the rest of January in the Proterozoic Eon of the Precambrian. The Proterozoic came to an end about 570,000,000 years ago when the Phanerozoic Eon – that means visible life – began.
Here's the podcast:
Transcript: All this time we’ve been progressing through the Precambrian – obviously the time in earth history before the Cambrian. We’ll get to the Cambrian in February but for now, let’s just acknowledge that geologists give names to time periods simply to have an easy way to talk about them, just as we give names to the months so it’s easy to know when we mean.
Geologists also subdivide the big blocks of time – the years and months – into smaller portions, the equivalent of weeks, days, and hours – and they give names to all those segments as well. Everything before the Cambrian is divided into two great packages, the Archean and the Proterozoic Eons. Archean means “ancient time” and Proterozoic means “early life”, and Eon is the top-level division of time in earth history.The Archean started with the origin of the earth, back on January 1, and continued until about 2.5 billion years ago, when the Proterozoic started. The divisions of earth time are not arbitrary – usually, there’s evidence in the rocks of some kind of major change at the boundary, and that’s how early geologists defined the packages of rock and time, long before actual age dates were available by analysis of radioactive decay.
Generally, rocks of the Archean Eon are intensely metamorphosed, changed by heat and tectonic events such as continental collisions. Younger Proterozoic rocks are typically less deformed, but there are plenty of exceptions to these rules. Archean rocks also tend to occupy the central cores of continents – cratons or shields, which we’ll talk about in a few days.
Early geologists called most of the Precambrian Azoic, meaning “without life,” but since fossils older than 3 billion years ago exist, this is a misnomer and is no longer used. Today, the very oldest time period, before the oldest known rocks, before 3.8 billion years ago, is called the Hadean Eon. That name refers to Hades, the god of the underworld, and the hellish conditions that ruled the earth at that time.
We’ll spend the rest of January in the Proterozoic Eon of the Precambrian. The Proterozoic came to an end about 570,000,000 years ago when the Phanerozoic Eon – that means visible life – began.
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