Ocean, Atmosphere, Life
Age of Earth
Last time we looked at the formation of the universe and got to the creation of
galaxies and our solar system.
Recall that a disk of spinning interstellar dust
turned into our solar system about 4.6 billion years ago.
Most of it concentrated
in the center, making the sun, and most of the rest became the planets.
dust, most is hydrogen, deuterium, and helium, made during the Big Bang and
the rest is all of the other elements, made in the explosions of massive stars.
Well, one of the planets became our Earth, so let’s concentrate on this planet
As some of the dust coalesced, forming Earth, three parts developed.
center is the
, outside that is the
, and there is a thin skin, called the
We’ll get to the details later. As Earth was forming, the heavier elements,
mostly iron and nickel, concentrated in the center, forming the core.
elements formed the mantle, mostly molten in the early days.
A thin scum at the
surface of the mantle cooled and became the crust.
As this crust went through
cycles of melting and cooling, the lighter elements in it rose to the surface and
congealed into the continental crust, which now is very different than the crust of
the ocean basins.
While Earth was very young, a small planet collided with it.
This jolt caused the
mantle of the smaller one to shoot out, possibly taking some of Earth’s mantle
Its core joined up with Earth, ending up as part of Earth’s core.
mantle that shot off formed our moon, and continues to rotate around our planet.
(This is the leading theory for the formation of the moon, but it is far from certain.)
Ocean, Atmosphere & Life
Early on, the ocean and atmosphere started forming.
The early mantle may have
been 10 to 20% water, which escaped as the mantle material melted.
have been a true ocean by four billion years ago.
The first atmosphere was
made up of nitrogen, hydrogen sulfide and carbon dioxide, which also were
supplied by the mantle.
Atmospheric oxygen was not present at first, but showed
up later: some from the breakdown of water and most from photosynthesis after it
Now, the atmosphere is about 78% nitrogen, 21% oxygen and 1%
other gases—very different than in the beginning.