308 The Poetry of Physics and The Physics of Poetry
The Age of the Earth
By measuring the age of rocks on the surface of the Earth we are able to
determine that the Earth is at least 3.75 billion years old. The oldest
rocks found were discovered along the coast of Greenland where they
formed some 3.75 billion years ago. Their age was determined using the
techniques of radioactive dating. By determining what percent of
radioactive krypton has decayed into argon or radioactive uranium into
lead the age of the rocks were determined. These results only give a
lower limit on the age of the Earth. They essentially tell us how long ago
the crust of the Earth formed. It is most certainly likely that the Earth is
older than its crust. Measuring the true age of the Earth by looking at
features other than the crusts, presents a problem since it is not possible
to explore very far below the surface of the Earth’s crust.
The way the age of the Earth was determined, oddly enough, was by
measuring the lifetimes of meteorites. Making use of the rhubidium-
strontium clock one discovers that all meteors were formed at more or
less the same time. It was also discovered the ratio of the various
isotopes of lead showed a pattern similar to the ratio found in terrestrial
rock sample containing different amounts of lead. These patterns
indicate that the meteors and the Earth were formed at the same time,
approximately 4.6 billion years ago. This age is consistent with the 3.75
billion year age of the crystal rocks.
The Earth formed 4.6 billion years ago but it took almost a billion
years for the Earth to cool down to the point where rocks formed on the
surface. Other events that took place on Earth can be dated by using
radioactive clocks to determine the ages of fossils or the records
inscribed in the rocks of mountain formations, or the movement of
glaciers during various ice ages. The most notable event of all was the
first signs of bacterial life found in rocks over 3 billion years old. The
first fossil remains of shell fish and more complicated forms of life are
not more than 600 million years old. The first mammals appeared 225
million years ago. The Rocky Mountains formed 65 million years ago,
just at the time the dinosaurs were becoming extinct and the first
primates were appearing. Two and a half million years ago the latest
series of ice ages occurred in which polar and high altitude glaciers
advanced and retreated. There have been three other major series of ice
ages in the history of the Earth, the first of which dates back to just over
2 billion years ago. We are probably still in the middle of the period