Encyclopedia of Evolution.pdf - Online Reading Center

0 uniformitarianism
layers up and over, leaving them upside down. Alternatively,
when two formations collide, one of them (even if it is older)
can be thrust over the other (even if it is younger). The result
of either of these processes would be older rocks on top of
younger rocks.
Unconformities are among the many things that make
the interpretation of the geological record challenging. Creationists
(see creationism) often cite unconformities as
examples of the failure of the scientific interpretation of billions
of years of Earth history and of the evolutionary science
based upon it. However, creationists are unable to explain
how unconformities could have occurred if all of the deposits
were produced during a single, worldwide Noachian
Flood. How could horizontal layers be deposited upon vertical
layers, if they had all been mud at the same time during
the Flood? In fact, it was the angular unconformity at Siccar
Point that led James Hutton (see Hutton, James) to reject
the prevailing model of catastrophism and paved the way
toward the acceptance of uniformitarianism.
uniformitarianism Uniformitarianism is an assumption
and a procedure that underlies much of modern geological
science. It began mainly with the geological studies of Sir
Charles Lyell (see Lyell, Charles) and his predecessors such
as James Hutton (see Hutton, James), although Lyell called
it simply uniformity.
Lyellian uniformitarianism can be summarized as “The
present is the key to the past.” Uniformitarianism, in part,
attempts to explain all past geological events in terms of
processes that are now in operation, rather than by special
events, whether natural or supernatural. Lyell proposed it
as an alternative to catastrophism, in which some scientists
(see Cuvier, Georges) explained the geological past
as a series of worldwide catastrophic events that separated
disjunct ages of Earth history during which very little (geologically
speaking) happened. Catastrophism did not demand
solely supernaturally caused catastrophes, although catastrophists
considered the biblical Flood of Noah to be the
most recent of the worldwide catastrophes.
Lyell assigned four meanings to uniformity, some of
which are considered unquestionable today and others of
which are now rejected by most scientists.
Uniformity of law. The laws of physics and chemistry
that underlie geological processes have remained unchanged
through time. Scientists do not question this assumption, for
to do otherwise would make the scientific investigation of the
past impossible.
Uniformity of process. Past geological events should
be interpreted as the result of processes that we can see in
operation today. All geologists accept this. Lyell favored
the interpretation that scientists should explain the past
only in terms of processes that are happening all the time,
at any given moment—processes such as volcanic activity,
earthquakes, erosion, sedimentation, compression, uplift,
and (something not known in Lyell’s time) plate tectonics.
Geologists today would include processes that happen
only rarely but which have been observed within recent his-
tory, such as the impact of extraterrestrial objects upon the
Earth. When Lyellian uniformitarianism dominated geological
science in the early to middle 20th century, such impacts
were ignored. What may have been a comet exploded near
the ground in Siberia in 1908, and large meteorites have
apparently struck the Earth in the recent past, such as the
impact that created the Barringer Crater in Arizona only
50,000 years ago (see asteroids and comets). The work
of astronomer Gene Shoemaker (now famous as codiscoverer
of the Shoemaker-Levy comet that slammed into Jupiter
in 1995) in the 1960s on what is now recognized as the
Ries Crater in Germany opened the door for geologists to
consider meteorite and other impacts upon the Earth. The
most famous extraterrestrial impact was the asteroid that
caused the Chicxulub Crater in Yucatán 65 million years
ago and contributed to the extinctions of many species,
including all dinosaurs (see Cretaceous extinction).
Lyell would not have been pleased at this. Modern geologists
continue to reject supernatural causation for past geological
events.
Uniformity of rate. This meaning, now called gradualism,
demands that geological processes operated in the past
at the same rate that they do today—in other words, that
there were no periods in the past in which volcanic eruptions
occurred on a greater scale than they do today. Geologists
now recognize that this is not strictly true; massive volcanic
eruptions, far exceeding anything to occur within historical
times, produced the Deccan Traps in what is now India, starting
just before the end of the Cretaceous period.
Uniformity of state. This is the meaning that is not
accepted in modern geology—that directional change (progress)
has not occurred during Earth history. Uniformity of
state maintains that there have been changes, but no permanent
ones. Volcanoes erupt, sea levels go up and down, glaciers
advance and retreat, but the Earth, according to this
view, always goes back to the way it was before. In geological
sciences, this is now known not to be strictly true. The
early Earth, during the Archaean eon, was very different, its
atmosphere without oxygen, and with massive tides caused
by a close moon (see Precambrian time). Lyell extended his
philosophy to include life—he believed that species may disappear
but they will return; extinction, in his view, is not
forever. This assumption was what led Lyell (at first) to reject
any form of evolution, even from his friend Charles Darwin.
It was the catastrophists such as Cuvier who insisted that the
history of life had a direction, and that changes occurred over
time, not the uniformitarians. Today, scientists reject uniformity
of state, not only because of the triumph of evolutionary
science but also because they know Earth history has
been characterized by very different assemblages of organisms
through time: lots of marine but barren landscapes in
the Cambrian, forests dominated by seedless vascular plants
in the Carboniferous, the age of dinosaurs, the age of mammals,
the ice ages, to name a few.
It is the last meaning, the extreme form of uniformitarianism,
that invoked some of the sarcasm for which writer
Mark Twain was famous. Twain pointed out that the Mis-