Event 6

4.5 billion years ago

Formation of Earth's Continents
Early Earth and Moon. This is an artist’s conception of early earth, as continents and oceans are being formed with the cooling of earth’s molten surface.
Credit: Illustration by Chesley Bonestell, courtesy of Bonestell LLC. This image is used with permission. All rights reserved.

It is likely that after a hard rocky crust formed on the earth it was periodically re-melted from impacts with other large planetary bodies in the early solar system. Finally, the earth hardened into rocky plates that continued to move through the course of the earth’s history. The core of these plates, called cratons, became the cores of the continents, composed of lighter granites riding over the heavier basalt oceanic crust. Much of the water that made up the oceans is believed to have come from comets impacting the early earth as well as water vapor produced by early volcanoes. It is believed that the earth’s early atmosphere contained almost no free oxygen (compared to about 20 percent today), but was composed primarily of water vapor (about 80 percent), carbon dioxide, sulphur dioxide, carbon monoxide, sulphur, chlorine, nitrogen, hydrogen, ammonia, and methane. (For comparison, our modern atmosphere is composed primarily of nitrogen (78 percent) and oxygen (21 percent).

Early earth's magma ocean
This is an image of the early Earth surface. It depicts the Earth as oceans of magma with fragments of thin crust, probably composed of elements similar to today’s igneous rocks.
Credit: Image courtesy of NASA/Nicholas M. Short, Sr. (RST-Remote Sensing Tutorial)

It is hard to conceive of how inhospitable the young Earth was for the first half billion years of its existence. It lacked oxygen in its atmosphere, it roiled with toxic gases outgassing from cooling rock and volcanic eruptions, it was extremely hot, and it endured massive bombardments by asteroids and other celestial objects over the course of hundreds of millions of years. Water built up gradually on the surface from condensation and from ice and water that came to earth along with asteroids, comets, and icy materials from the Kuiper belt and beyond, and built up the early oceans.

Our Moon has made significant contributions to our Earth. The seasonal changes we experience on Earth today are due to the axial tilt of the Earth (23.45 degrees) relative to plane in which it orbits, and it is believed this tilt is a direct result of the oblique blow made by the Mars-sized object that collided with the Earth and led to the formation of the moon. The experience of a warmer seasonal ‘summer’ period happens in the northern hemisphere when that portion of our sphere is tilted toward the sun, and then six months later in the southern hemisphere when the Earth’s orbit presents that part of our planet toward the sun. (Interestingly, in the Northern Hemisphere, the Earth is actually closer to the sun during winter than it is in the summer). Overall such seasonality helps create a range of environments in which eventual life forms could adapt and diversify. If the Earth’s rotation axis were not tilted, climate would not vary seasonally the way it does on Earth today, and there likely would be differing climatic bands ringing the Earth according to latitude. The influence of our Moon (which is quite large relative to the size of our planet) on amplifying ocean tides also has significance for the ultimate evolution and diversification of life, as oscillating tide levels provide regularly shifting and varied environments in which life forms evolved and adapted.

It has also been found that the Earth has an unusually high amount of certain metals incorporated into its mantle, which likely represent the inner core of this moon-producing colliding body left behind when the lighter mantle materials careened off the Earth. These metals have been incorporated into the Earth’s mantle and shaped by earth processes over time into the ore deposits we mine and utilize for our technological innovations. It is interesting to speculate what kinds of technologies would be possible without these readily available metallic ores to work with

The history of the Earth’s continents it tied to an intricate sequence of movements of seven or eight tectonic plates upon which the lithosphere (the Earth’s crust and mantle) rides. The movement of the parts of the lithosphere on these plates has produced an ongoing, elaborate tableau of land masses colliding and uniting (converging), as well as dividing and drifting apart (diverging) over the course of billions of years. Many people have heard of Pangaea, the supercontinent that started breaking up about 200 million years ago to form Laurasia (which then broke up to form North America and Eurasia) and Gondwanaland (which then broke up to form the southern continents). But prior to the formation of Pangaea there were a number of previous supercontinents. The major ones include Columbia, which formed by 2 billion years ago and broke up between 1.5 and 1.3 billion years ago; Rodinia, which formed by 1 billion years ago and started breaking up by 750 million years ago; and Pangaea, which formed by 250 million years ago and gradually broke apart between 175 and 55 million years ago, resulting eventually in the continents in their present configuration. Throughout this shifting of the tectonic plates and merging and dividing of the continental masses, these continents were also moving relative to the poles, shifting from polar regions to tropical regions and back again, and the oceans were likewise changing in their size, shape, currents and connections with other bodies of water.



Modern volcanic eruptions and the gasses they produce give us clues to what the earth’s primordial atmosphere was like. Gas inclusions in early rocks have trapped small samples of earlier atmospheres as well.


The Earth is our home, the only home we know. We and all the other living organisms on our planet have evolved to adapt to the Earth’s changing nature – its evolving atmosphere and continents and seas. We don’t know for sure if there is another planet in the universe where we and other living creatures on earth could survive – it is possible, but we have evolved to survive within certain limits of solar radiation, heat and cold, and availability of water.




News articles on finds of the earliest rocks on earth:

A New Contender for Earth’s Oldest Rock:

The Oldest Rock in the World:

A documentary clip from a National Geographic film, telling of the discovery of water molecules trapped in ancient minerals:

Traces of Earth’s oldest rocks found in Australia:

Evolution of Earth’s atmosphere:

The Hadean period of Earth’s history:

Tracking down a mistak, revised theory of the Hadean period of Earth:



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