THE HiSToRy oF LiFE 433
move over the denser, more plastic asthenosphere below. Because the heat of
Earth’s core sets up convection currents within the asthenosphere, magma from
the asthenosphere rises to the surface, cools, and spreads out to form new crust,
pushing the existing plates to either side. The plates move at velocities of 5–10 cm
per year. Where two plates come together, the leading edge of one may be forced to
plunge under the other, rejoining the asthenosphere (subduction). The pressure of
these collisions is a major cause of mountain building. When a plate moves over a
“hot spot” where magma is rising from the asthenosphere, volcanoes may be born,
or a continent may be rifted apart. The Great Lakes of eastern Africa lie in such a
rift valley; the Hawaiian Islands are a chain of volcanoes that have been formed by
the movement of the Pacific plate over a hot spot (see Chapter 18).
The absolute ages of geological events can often be determined by radiometric
dating, which measures the decay of certain radioactive elements in minerals that
form in igneous rock. (Carbon-14 is used for dating biological materials, such as
wood or bone, that are no older than about 75,000 years.) The probability that a
radioactive parent atom (e.g., uranium-235) will decay into a stable daughter atom
(lead-207) is constant over time. As a result, each element has a specific half-life.
The half-life of U-235, for example, is about 0.7 billion years, meaning that in each
0.7-billion-year period, half the U-235 atoms present at the beginning of the period
will decay into Pb-207. The ratio of parent to daughter atoms in a rock sample thus
provides an estimate of the rock’s age. Only igneous rocks can be dated radio-
metrically, so the age of a fossil-bearing sedimentary rock must be estimated by
dating igneous formations above or below it.
Long before radioactivity was discovered—indeed, before Darwin’s time—
geologists had established the relative ages (i.e., earlier vs. later) of sedimentary
rock formations by applying the principle that younger sediments are deposited on
top of older ones. Layers of sediment deposited at different times are called strata.
Different strata have different characteristics, and they often contain distinctive
fossils of species that persisted for a short time and are thus the signatures of the
age in which they lived. Using such evidence, geologists can match contemporane-
ous strata in different localities. In many locations, sediment deposition has not
been continuous, and sedimentary rocks have eroded; thus any one area usually
has a very intermittent geological record, and some time intervals are well repre-
sented at only a few localities on Earth. In general, the older the geological age, the
less well it is represented in the fossil record because erosion and metamorphism
have had more opportunity to take their toll.
Most of the eras and periods of the geological time scale (TAB LE 17. 1) were named
and ordered before Darwin’s time. These geological eras and periods were dis-
tinguished, and are still most readily recognized in practice, by distinctive fossil
taxa. The absolute times of these boundaries are subject to slight revision as more
information accumulates.
Phanerozoic time (whose beginning is marked by the first appearance of diverse
animals) is divided into three eras, each of which is divided into periods. We will
frequently refer to these divisions, and to the epochs into which the Cenozoic peri-
ods are divided. It is useful to learn the sequence of the eras and periods, as well
as a few key dates, such as the beginning of the Paleozoic era (and the Cambrian
period, 541 million years ago, or 541 Mya), the Mesozoic era (and Triassic period,
252 Mya), the Cenozoic era (and Paleogene or Tertiary period, 66 Mya), and the
Pleistocene epoch (2.58 Mya).^1(^1) Commonly used abbreviations for geological time include Gy (billion years) and Gya (billion
years ago), My (million years) and Mya (million years ago), Ky (thousand years) and Kya (thou-
sand years ago).
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