Geoscience Reference
In-Depth Information
Why the earth's magnetic field occasionally reverses direction is
not well understood, but the sequence of these reversals has been
recorded in the earth's rocks. By combining these sequences of rever-
sals with ages from rock layers that can be dated through radioactive
decay, geologists have reconstructed a calendar of when the magnetic
poles were oriented as they are today and when they were reversed.
Using samples of rock from Auca Mahuevo, from the layers contain-
ing the eggs and embryos, we might be able to estimate the age of the
rock by comparing our data to this global magnetic calendar.
We had carefully collected rock samples from eight different layers
associated with the fossil eggs. First, using a small pick and a rock
hammer, we had dug into the hillside to expose a two-foot-square
area of unweathered rock. (Weathered rock could easily yield unreli-
able magnetic information.) Next, we used a hand rasp to plane a
horizontal surface between three and five inches across on a chunk of
rock. Fortunately, the mudstone and siltstone layers in the sequence
were fairly soft, so the rasp worked effectively. To check that the
planed surface was level, we used a special geologic compass, called a
Brunton compass, which has a leveling bubble inside. Once a truly
level surface had been planed off, we used the Brunton compass to
mark an arrow pointing toward the north magnetic pole, thus show-
ing how the chunk of rock was oriented in the ground in relation to
the earth's present magnetic field. Once the arrow was marked, we
had to carefully dig the chunk out of the ground without breaking it.
To keep it in one piece during the trip back to the lab, the chunk was
wrapped in aluminum foil and secured with masking tape. Finally, the
sample was labeled with a number, and the position of the sample
was recorded on the drawing of the stratigraphic section, so that we
would know where it had come from once the magnetic analysis was
conducted. The magnetic analyses, which might determine whether
the rocks had been formed during a "normal" or "reversed" period in
the earth's magnetic history, had to wait until we got the samples
back to the lab.
The rock samples that we collected for magnetic analysis were
somewhat more helpful in establishing the time of the embryos'
death than were the samples collected for radiometric analysis. Carl
Swisher and Gary Scott of the Berkeley Geochronology Center deter-
mined that the rocks containing the sauropod embryos at Auca
