Geoscience Reference
In-Depth Information
fifty thousand years, however, because then the half-life of carbon-14
(C-14) is only about 5,700 years. In objects older than about fifty
thousand years, there is usually not enough C-14 remaining to obtain
accurate measurements for calculating the age. The half-lives of the
uranium (U-238) and potassium (K-40) are much longer. Half of
the U-238 atoms decay to lead (Pb-206) atoms in about 4.5 billion
years, and half of the K-40 atoms decay to argon (Ar-40) atoms in
about 1.3 billion years, which makes these techniques suitable for
estimating the age of much older objects but less useful for dating
more recent objects.
We hoped that the greenish sands at Auca Mahuevo might contain
minerals that were erupted out of volcanoes when the greenish sands
were formed, so we collected many samples for analysis back in the
laboratory. Unfortunately, none of the rock samples that we brought
back from these greenish layers surrounding the fossil eggs contained
mineral crystals that had been erupted out of volcanoes. Instead, they
represented tiny pieces from a large pool of molten rock that had
cooled and crystallized deep underground, so we could not be sure that
these crystals formed at the same time as the rock layers that con-
tained our fossils. Hence, we could not estimate the age of the rock
layers and eggs through radioactive decay. It was frustrating to fail in
this initial attempt, but that is the nature of scientific research. We
knew that we would have to expand our search for layers of ancient
volcanic ash into other adjacent regions.
In addition to collecting rock samples for possible dating through
radioactivity, we had collected rock samples for dating through mag-
netic analysis.
The earth is like a giant bar magnet with a north pole and a south
pole. Throughout the known history of the earth, our planet's mag-
netic poles have commonly reversed directions, such that the mag-
netic end of a compass needle that points north today would have
pointed south then. Recent geologic research has suggested that such
reversals of the magnetic poles can happen over a period of a few hun-
dred years. This may seem like a long time to humans, but it is only a
brief instant in comparison to the vast expanse of geologic time. In the
last 65 million years, for example, the magnetic poles have switched
positions about thirty times, and the last time the poles switched was
about 750,000 years ago. But how do geologists know this?
