The Early Earth—A Short History .....................................................

LECTURE

But what I would really like you to notice is how natural the crossing

can be. There are no border patrols, and there are no geologists

saying: “You’ve been traveling in astronomy. You need a visa to

enter geology.”

T

he previous lecture described the origins of our Sun and solar system
about 4.5 billion years ago. Now we cross from astronomy to geology
as we focus on our own planet, the Earth. We’ll look at the Earth in
its early days. What was it like? How friendly an environment was it for life?
And how different was it from today’s Earth?

The early Earth would have seemed an unlikely home for life. The earliest
era (strictly “eon”) of Earth history is known as the Hadean (or “hellish”)
eon. It lasted from about 4.5 to about 3.8 billion years ago. It is the ¿ rst
of four eons in the Earth’s history. (The others are the Archaean eon, from
3.8 to 2.5 billion years ago; the Proterozoic eon, from 2.5 billion years ago
to 550 million years ago; and the Phanerozoic eon, or “eon of multi-celled
organisms,” from 550 million years ago to today.)

As the young Earth formed through accretion, it heated up and melted. It was
heated by three different forces. The ¿ rst was repeated violent collisions.
The early solar system was much like a cosmic demolition derby. The second
force was radioactivity, because the early solar system contained many
radioactive materials created in supernovae. The third force was pressure. As
each planet grew in size through accretion, pressure and temperature built up
at its center.

Then what took place was a sorting of elements by their density in a process
known as “differentiation.” Heavy materials sank to the center, and lighter
materials rose to the surface. Differentiation gave the Earth the internal
structure it has to the present day. Metals (mainly iron and nickel) sank
to the center to form the core, with a solid center and a liquid outer layer.
The core generated the Earth’s magnetic ¿ eld. Lighter materials formed