Page 415 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

AQuantitativeSolution Radioactivity: Earth’s Time Clock
The age of Earth and the age of the earliest known crustal rock are outstanding, for we think in terms of Earth’s trips around the Sun and the pace of our own lives. We need something greater than human time to measure the vastness of geologic time. Nature has provided a way: radiometric dating. It is based on the steady, exponential decay of certain atoms.
An atom contains protons and neutrons in its nucleus. Certain forms of atoms called isotopes have unstable nuclei; that is, the protons and neutrons do not remain together indefinitely. As particles break away and the nucleus disintegrates, radiation
is emitted and the atom decays into a different element—this process is radioactivity.
Radioactivity provides the steady time clock needed to mea- sure the age of ancient rocks. It works because the decay rates for different elements are determined precisely, and they do not vary beyond established uncertainties. The decay rate of an element is commonly expressed as its half-life, the time required for one-half of the unstable “parent” atoms in a sample to decay into stable “daughter” atoms.
Some examples of unstable elements that become stable elements (with their half-lives in parentheses) are thorium-232
to lead-280 (14.1 billion years); uranium-238 to lead-206
(4.5 billion years); potassium-40 to argon-40 (1.3 billion years); and, in organic materials, carbon-14 to nitrogen-14 (5730 years).
The presence of a decaying element and its stable end prod- uct in a sample of sediment or rock allows scientists to apply the
known decay rate and read the radiometric “clock.” The formula for the age of a sample is:
1D t = ln q1 + r
lP
where:
t is the age of the sample,
l is the decay constant for the parent element,
ln is the natural logarithm (that is, the logarithm to base e),
D is the number of atoms in the daughter isotope product today, and
P is the number of atoms in the parent isotope today.
D and P in a sample are measured with a mass spectrometer. l
is related to the half-life (t1/2) of the parent element with:
l = ln(2) = 0.693 t1/2 t1/2
For example, the 5730 year half-life of carbon-14 equates to a decay constant of 1.21 × 10−4 · yr−1.
Errors can occur if the sample was disturbed or subjected to natural weathering processes that might alter its radioactivity. To increase accuracy, investigators may check the age of a sample using more than one radiometric measurement, or by cross- checking a result against other dating methods such as tree-ring or fossil analysis.
concepts review
Key leArning
■ Distinguish between the endogenic and exogenic systems that shape Earth, and name the driving force for each.
The Earth–atmosphere interface is where the endogenic system (internal), powered by heat energy from within the planet, interacts with the exogenic system (external), powered by insolation and influenced by gravity. These systems work together to produce Earth’s diverse land- scape. Geomorphology is the subfield within physical geography that studies the development and spatial dis- tribution of landforms. Knowledge of Earth’s endogenic processes helps us understand these surface features.
endogenic system (p. 348) exogenic system (p. 348) geomorphology (p. 348)
1. Define the endogenic and the exogenic systems. De- scribe the driving forces that energize these systems.
■ Explain the principle of uniformitarianism, and discuss the time spans into which Earth’s geologic history is divided.
The most fundamental principle of Earth science is uniformitarianism, which assumes that the same physical processes active in the environment today have been op- erating throughout geologic time. Dramatic, catastrophic events such as massive landslides or volcanic eruptions
can interrupt the long-term processes that slowly shape Earth’s surface. The geologic time scale is an effective de- vice for organizing the vast span of geologic time. Geolo- gists assign relative age based on the age of one feature relative to another in sequence, or numerical age acquired from isotopic or other dating techniques. Stratigraphy is the study of layered rock strata, including its sequence (superposition), thickness, and spatial distribution, which yield clues to the age and origin of the rocks.
uniformitarianism (p. 348) geologic time scale (p. 348) stratigraphy (p. 348)
2. Explain the principle of uniformitarianism in the Earth sciences.
3. How is the geologic time scale organized? What era, period, and epoch are we living in today? What is the difference between the relative and numerical ages of rocks?
■ Depict Earth’s interior in cross section, and describe each distinct layer.
We have learned about Earth’s interior indirectly, from the way its various layers transmit seismic waves. The core is differentiated into an inner core and an outer core, divided by a transition zone. Above Earth’s core lies the mantle, differentiated into lower mantle and upper mantle. It experiences a gradual temperature increase with depth and flows slowly over time at depth, where it is hot and pressure is greatest. The boundary between
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