oxygen was continually being freed by some process occurring on or near
the Earth’s surface. That process, as we will see later, was photosynthesis:
Free oxygen was produced by plant-like organisms, a ¿ rst hint of how living
organisms could transform the Earth’s surface.
How do we know about the early history of the Earth? The Earth has
changed so much that we have little direct evidence from the Hadean eon.
The oldest Earth rocks date to about 4 billion years ago. Our understanding
of the Hadean eon depends on a
combination of theoretical inference
and the indirect evidence described
in the previous lecture.
At present, we cannot drill deeper
than about 7 miles into the Earth,
which is just 0.2% of the distance
to the center (4,000 miles). To
understand what’s inside the Earth
we have to use indirect methods. One
of the most important techniques
depends on seismology, the study of earthquakes. Different types of seismic
waves travel in different ways and at different speeds through different types
of rock, so careful comparisons of their movements, using seismographs
placed at many different parts of the Earth, can tell us much about what is
inside the Earth. Seismographs have played a similar role in study of the
Earth’s interior to spectroscopes in studies of the stars.
How can we know what was in the ancient atmosphere billions of years ago?
Capturing the gases released by volcanoes today can tell us much about the
constituents of the Earth’s early atmosphere, as we know that volcanoes
generated much of that atmosphere. We can observe and date the buildup of
free oxygen in the atmosphere from the appearance of increasing amounts of
rusted iron in the Archaean eon.
This lecture has described the early Earth and some of the more important
changes it underwent during its early years through accretion, differentiation,
and early changes in its atmosphere. We saw that the early Earth was very
Capturing the gases released
by volcanoes today can
tell us much about the
constituents of the Earth’s
early atmosphere, as we know
that volcanoes generated
much of that atmosphere.