October 25, 2021
Ages of the Earth - The Hadean Eon
I am going to run a series of posts on the ages of the Earth, beginning with the earliest unit of geological time, the Hadean Eon.
First, let's look at a good representation the Geological Time Scale that shows the relative lengths of time for the various divisions of the Time Scale.
Figure 1 -
Geological Time Scale Clock
Credit: Woudloper, public
domain
As you can see, the Hadean Eon encompasses the first 600 million years of the Earth's existence, a time period longer than the current age of complex life, the Phanerozoic Eon. Much of what we know about the earliest history of the Earth is based upon observation of stellar nebulae and the behavior of that material. Also, much of the standard scientific story on the origins of the Earth and indeed the entire Solar System is speculative in nature and awaits further research to flesh it out or recast it in the light of new evidence.
The early history of the Earth can be divided into four main chapters:
The Early Accretionary Erawhen the material that made up the Sun, the Earth and the other planets coalesced into their early forms.
The Separation Era where the heavier elements making up the Earth, such as iron, separated out to form the Earth's core. This is the origin of the Earth's magnetic field.
The Birth of the Moon, where a collision with another planetary body tore out a huge chunk of material that coalesced into the Moon.
The Late Heavy Bombardment Erawhere a wave of meteors, asteroids, and comets pelted the Earth, adding important materials such as water.
Let's look at these events
Early Accretionary Era
Figure 2 -
Comets Kick up Dust in the Helix Nebula
Credit: NASA,public
domain
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Around 5 billion years ago, things began to happen within the stellar nebula that would later become our Solar System. First, much of the matter in that nebula, especially the hydrogen and helium, coalesced into the star we call the Sun. Then, the early planets began to form. The core accretion hypothesis describes this process.
This early period was quite chaotic, Jupiter's orbit was not firmly established and, in its wanderings, a planet that had formed between Mars and Jupiter was broken up into the asteroid belt. As well, there may have been another, smaller, planet in a chaotic orbit that brought it in close proximity to the early Earth. We'll discuss the fate of that body later.
The accretion of material into each of the planets involved what was probably a steady bombardment of material. Most of the material that made up the Earth accumulated during this early accretionary era.
As each particle or chunk of rock struck the growing Earth, it imparted a bit of kinetic energy that became heat. Eventually enough heat built up to melt the accumulated material. This set the stage for next Era.
Separation Era
Figure 3 -
Layers of the Earth
Credit: Kelvinsong, Creative Commons Attribution-Share Alike 3.0 Unported license
Once the Earth became a molten ball of rock, heavy elements, such as iron and nickel, began to separate out of the molten mass. Under the influence of gravity, the heavy metal material dropped to the centre of the earth and the lighter minerals floated out to outside, forming a series of layers inside the Earth.
Once the iron-nickel core of the Earth formed, the interplay of the solid core, liquid care and surrounding mantle created a geo-dynamo that in turn led to the creation a magnetic field around the Earth.
Birth of the Moon
Figure 4 -
The Moon
Credit: Lakshyajit2002,Creative Commons Attribution-Share Alike 4.0 International license.
The current most common hypotheses for the birth of the Earth's moon involves the near collision of the Earth with a wandering planet roughly the size of Mars. This near miss pulled out a string of material that eventually coalesced into the moon. This near miss also tilted the Earth on its axis, eventually giving rise to the Seasons. Another effect of the creation of the Moon was to stabilize the Earth's wobble. Also, the Moon is largely responsible for tides in the ocean. There is some thought that the Moon was absolutely necessary for development of life on the Earth.
Late Heavy Bombardment Era
Figure 5 -
Late Heavy Bombardment
Credit:
Earth Blog, September 27, 2016
The final Era in the Hadean was the Late Heavy Bombardment Era. While subsequent erosion and plate movement have removed almost all traces of this Era from the surfaces of Venus and the Earth, its effects can be seen on Mercury, Mars and the Moon.
The cause of the Late Heavy Bombardment appears to be in the chaotic conditions in the Solar System at the time. As described in the Nice Model of the early solar system, the orbits of the outer planets, Jupiter, Saturn, Uranus and Neptune had not yet stabilized at that time. As their orbits moved around, they disturbed asteroids in the zone between Jupiter and Mars as wells as comets in the Oort Cloud. The new orbits of these disturbed meteors, asteroids, comets and minor planets brought them into the Inner Solar System, where they crashed into Mercury, Venus, the Earth, the Moon and Mars as well as impacting the outer planets and their moons. Some of these bodies continue to come into the inner part of the Solar System to this day.
There is a lot of speculation about the effect of the Late Heavy Bombardment Era in bringing certain critical materials to the Earth. Some think that it was during this time, comets brought water to the Earth. Others believe that these comets brought the building blocks of life or even the earliest forms of life itself, as in the Panspermia Hypotheses. Professor Chandra Wickramasinghe is an advocate of this hypothesis and he discusses it in a lecture here.
Speaking of YouTube, there is a good discussion of the Late Heavy Bombardment on The Entire History of the Earth Channel: The Mystery of the Late Heavy Bombardment.
Standard Caveat
The purpose of my weblog postings is to spark people's curiosity in geology. Don't entirely believe me until you've done your own research and checked the evidence. If I have sparked your curiosity in the subject of this posting, follow up with some of the links provided here. If you want to, go out into the field and examine some rocks on your own with the help of a good field guide. Follow the evidence and make up your own mind.