The actual divisions of geologic time are not arbitrary, or uniform. The larger divi-
sions are based on major events that occurred sporadically over the Earth’s long histo-
ry. For example, the end of the Permian Period, about 240 million years ago, was
marked by a major catastrophe. Some scientists estimate that close to 90 percent of all
species on the Earth died at that time, resulting in a major extinction event that may
have been caused by huge volcanic eruptions or even a space object striking the Earth.
The smaller divisions are usually based on specific local structures or fossils found
within the rock. Most often they are named after local towns, people, and sundry other
nearby associations.
What is the longest span of timemeasured on the geologic time scale?
The longest span of time measured on the geologic time scale is the Precambrian Era
(also called the Precambrian Eon). It represents the time between 4.55 billion years to
about 544 million years ago, or about seven-eighths of the Earth’s history. This time peri-
od includes the beginning of the Earth’s formation, its cool-down, its crust’s formation,
and, within the last billion years of the time period, the evolution of the first single-celled
to multi-celled organisms. The demarcation of 544 million years ago represents a burst in
the evolution of multi-celled organisms, including the first plant and animal species.
How do geologists use anglesto understand rock layers?
Mathematics—especially geometry—is instrumental in understanding rock layers. In
a branch of geology called stratigraphy, scientists measure angles and planes in rock
in order to know the location of certain rock layers and the possible geologic events
that affected the layers over time. In particular, geologists measure strike and dip.
Strikeis the angle between true north and a horizontal line contained in any planar
feature, such as a fault (usually caused by an earthquake) or inclined bed (often caused
by the uplift of hot molten rock around a volcano). Dipis the angle at which a bed or
rock vein is inclined to the horizontal; it is measured perpendicular to the strike and
in the vertical plane (as opposed to the strike’s horizontal line). 297
MATH IN THE NATURAL SCIENCES
What is the difference between the Earth’s sidereal and solar days?
T
he difference between the Earth’s sidereal and solar days has to do with angles
and the Earth’s rotation. The mean solar day is equal to 24 hours, or the aver-
age of all the solar days in an orbital year. The mean sidereal day is 23 hours, 56
minutes, and 04.09053 seconds. It is not exactly equal to a solar day because by
the time the Earth has rotated once, it has moved a little in its orbit around the
Sun. Thus, it rotates for about another four minutes before the Sun is considered
to be back in exactly the same place in the sky as it was the day before.