What is Geology?
The Origin of the Solar System
Early Concepts of Geologic Time
What is Geology?
Geology, from the Greek geo
and logos, is defined as the study of the Earth. It is generally
divided into two broad areas-physical geology and historical geology. Physical
geology is the study of Earth materials, such as minerals and rocks, as well as the
processes operating within the Earth and upon its surface. Historical geology examines
the origin and evolution of the Earth, its continents, oceans, atmosphere, and life.
The Origin of the Solar System and the Differentiation of the Early Earth.
According to the currently accepted theory for the origin of
the solar system, interstellar material in a spiral arm of the Milky Way Galaxy condensed
and began collapsing. As this cloud gradually collapsed under the influence of
gravity, it flattened and began rotating counter-clockwise, with about 90% of its mass
concentrated in the central part of the cloud. As the rotation and concentration of
material continued, an embryonic Sun formed, surrounded by a turbulent, rotating cloud of
material called a solar nebula.
The turbulence in this solar nebula formed localized eddies
where gas and solid particles condensed. As this condensation proceeded, gaseous,
liquid, and solid particles began accreting into ever-larger masses called planetesimals
that eventually became true planetary bodies. While the planets were accreting,
material that had been pulled into the center of the nebula also condensed, collapsed, and
was heated to several million degrees by gravitational compression. The result was
the birth of a star, our Sun.
Some 4.6 billion years ago, enough material eventually
gathered together in one of the turbulent eddies that swirled around the early Sun to form
the planet Earth. Scientists think this early Earth was rather cool, so that the
accreting elements and nebular rock fragments were solids rather than gases or liquids.
This early Earth was thought to be of generally uniform composition and density
throughout. It was composed mostly of compounds of silicon, iron, magnesium, oxygen,
aluminum, and smaller amounts of all the other chemical elements. Subsequently, when
the Earth underwent heating, this homogeneous composition disappeared, and the result was
a differentiated planet, consisting of a series of concentric layers of differing
composition and density. This differentiation into a layered planet is probably the
most significant event in the history of the Earth. Not only did it lead to the
formation of a crust and eventually to continents, but it was also probably responsible
for the emission of gases from the interior that eventually led to the formation of the
oceans and the atmosphere.
Early Concepts of Geologic Time and the Age of the Earth.
The concept of geologic time and its measurements have changed
through human history. Many early Christian scholars and clerics tried to establish
the date of creation by analyzing historical records and the genealogies found in
Scripture. Based on their analyses, they generally believed that the Earth and all
of its features were no more than about 6,000 years old. The idea of a very young
Earth provided the basis for most Western chronologies of Earth history prior to the
eighteenth century.
During the eighteenth and nineteenth centuries, several
attempts were made to determine the age of the Earth on the basis of scientific evidence
rather than revelation. The French zoologist Georges Louis de Buffon (1707-1788)
assumed that the Earth gradually cooled to its present condition from a molten beginning.
To simulate this history, he melted iron balls of various diameters and allowed
them to cool to the surrounding temperature. By extrapolating their cooling rate to
a ball the size of the Earth, he determined that the Earth was at least 75,000 years old.
While this age was much older than that derived from Scripture, it was still vastly
younger than we now know the Earth to be.
Other scholars were equally ingenious in attempting to
calculate the Earth's age. For example, geologist reasoned that by determining
deposition rates for various sediments, they could calculate how long it would take to
deposit any rock layer. Furthermore, they could then extrapolate how old the Earth
was from the total thickness of sedimentary rock in the Earth's crust. Rates of
deposition vary, however, even for the same type of rock. Furthermore, it is
impossible to estimate how much rock has been removed by erosion, or how much a rock
sequence has been reduced by compaction. As a result of these variables, estimates
ranged from less than a million years to more than a billion years.
Another attempts to determine the Earth's age involved ocean
salinity. Scholars assumed that the Earth's ocean waters were originally fresh and
that their present salinity was the result of dissolved salt being carried into the ocean
basins by streams. John Joly, a nineteenth-century Irish geologist, knew the volume
of ocean water and its salinity, and measured the amount of salt currently in the world's
streams. He then calculated that it would have taken at least 90 million years for
the ocean to reach their present salinity level. This was still much younger than
the now accepted age of 4.6 billion years for the Earth, mainly because Joly had no way of
calculating how much salt had been recycled or the amount of salt stored in continental
salt deposits and sea-floor clay deposits.
In addition to these efforts, the naturalists of the
eighteenth and nineteenth centuries were also formulating some of the fundamental geologic
principles that are used in deciphering Earth history. From the evidence preserved
in the geologic record, it was clear to them that the Earth is very old and that geologic
processes have operated over long periods of time.