History of Geology

These are notes from a lecture presented in May 1996 at "Learning from the Past":

The fourth annual natural history festival for the Black Hills Museum of Natural History .

Correlating Earth's History - Lecture Notes

by Paul R. Janke

The following chronological summary highlights some of the scientists, experimenters, theologians and thinkers of the past and how their ideas have contributed to our understanding of Earth's history. ---

Herodotus (500 BC) observed that the Nile deposited silt during floods but believed most features were the result of sudden, violent processes.

Aristotle (384-322 BC) recognized river deposits and realized that fossil seashells from rocks were similar to those found on the beach, indicating the fossils were once living animals. He deduced that the positions of land and sea had changed and thought these changes occurred over long periods of time.

Theophrastus (374-287 BC) wrote a mineralogy book, Concerning Stones, which dominated thought through the middle ages. Unfortunately, he also suggested that fossils might not be the remains of once living animals.

Eratosthenes (250 BC) calculated the circumference of the Earth by measuring noontime shadows at two localities of different latitude. He extrapolated the distance from his reference points at Syene and Alexandria in Egypt and determined Earth's circumference to be ~40,000 km, a remarkably accurate estimate and major scientific accomplishment.

Augustine of Hippo (354-430 AD) stated the the crucifixion was a unique event from which all events could be measured, thus initiating the BC/AD time scale. Note: modern archaeologists sometimes use a bce/ce scale to denote "before common era" and "common era".

--- The Dark Ages (~500-1100 AD) ---

St. Thomas Aquinas (1225-1274) attempted to weave Aristotle's theories and the Bible into a single system covering all aspects of life.

--- Printing Press invented (1450) [Renaissance begins] ---

Leonardo da Vinci (1452-1519) recognized that material carried by rivers to the sea was eventually compacted into sedimentary rock and later uplifted to form mountains. He concurred with Aristotle's view that fossils were the remains of ancient life. As a by-product of his engineering work on canals and rivers, he studied the deposits of the Po River and concluded that they must be at least 200,000 years old. He surmised that the whole of geologic time must be much longer.

James Ussher (1581-1665) was the first to estimate the age of the Earth using genealogies of the Bible. Ussher stated in 1650 that the Earth was created on October 22, 4004 BC. This date was later reproduced in many editions of the Bible and was incorporated into the dogma of the Christian church. For nearly a century thereafter, it was considered heresy to assume that Earth and its puzzling geologic features were more than ~6000 years old. Thus, a very young Earth provided a basis for most chronologies until the 18th century.

Nicolaus Steno (1638-1686) articulated three basic geological principles. In 1669 he wrote the following: At the time when any given stratum was being formed, all matter resting upon it was fluid, and, therefore, at the time when the lowest stratum was being formed, none of the upper strata existed. This little bit of compelling logic is now known as the Law of Superposition: strata as originally deposited are successively younger upward. Steno went on to say that all strata are initially depositied horizontally. This is now known as the Law of Original Horizontality: undisturbed true bedding planes(not cross-bedding) are nearly horizontal. His third astute observation was perhaps the most insightful: Hence it follows that in whatever place the bared sides(edges) of the strata are seen, either a continuation of the same strata must be sought, or another solid substance [i.e. the confines of the basin] must be found which kept the matter of strata from dispersion. This is known as the Law of Concealed Stratification: every outcrop in which the edges of strata are exposed demands an explanation(erosion, folding, volcanism, earthquake, etc.)

Georges Buffon (1707-1788) tried to deduce the age of the Earth experimentally by heating a small metallic globe and measuring the rate at which it cooled. Extrapolating his results to the Earth yielded an estimate of 75,000 years old.

Georges Cuvier (1769-1832) introduced the concept of catastrophism which dominated European thought for several decades. He said that 6 major catastrophies had occurred in the past(corresponding to the six days of biblical creation), the last being Noah's flood. While not all catastro-phists accepted the 6,000 year age for the Earth, it had wide appeal among theologians and was basically consistent with a relatively young Earth.

Abraham Werner (1749-1817) wrote a book which challenged catastrophism. His encompassing theory known as "neptunism" stated that all rock sequences(including igneous rocks) were precipitated out of a huge primieval ocean. The concept of a universal ocean was somewhat palatable to theologians as it could be fitted to the biblical flood narrative.

James Hutton (1726-1797) rejected catastrophism and championed plutonism (after Pluto, god of the fiery underworld). He reasoned that internal forces driven by heat within the Earth could uplift mountains in certain areas where they would then be subjected to erosion, re-deposition and volcanism. His other major insight was the recognition that basalt and granite were once molten. Over time, plutonism displaced the neptunism view that igneous rocks were deposited from seawater. The concept of a dynamic Earth in which both internal and external processes act over very long time intervals laid the foundation for modern geology. He is most famous for The Principle of Uniformitarianism: the past history of Earth must be explained by what can be seen to be happening now. He also described the Principle of Cross-Cutting Relationships: an igneous intrusion or fault must be younger than the rocks it intrudes.

William Smith (1769-1839) determined that two layers from different sites can be regarded as similar in age if they contain the same fossils.

Charles Lyell (1779-1875) wrote Principles of Geology (1830) in which he argued that the present was the key to the past. Lyell established Hutton's uniformitarianism at the expense of catastrophism. He wrote that geological features take shape, erode and reform at a constant rate through time. By careful study of the processes taking place today and the laws that govern them, we can by inductive reasoning and analogy, gain insights into the history of Earth. He believed that each geological period lasted for many years (perhaps hundreds of millions) and thus the age of the Earth had to be several times that.

Charles Darwin (1809-1882) shook the foundations of Victorian England by contradicting the biblical version of creation in his epic work: On the Origin of Species. Darwin favored the very old Earth idea because he believed that evolutionary processes required large spans of time.

Thomas Huxley (1825-1895) earned the nickname of "Darwins Bulldog" for his staunch and passionate defense of Darwin's theories. Darwin himself was mild mannered and would not generally argue his controversial ideas in public.

William Thomson( aka Lord Kelvin) (1824-1907) performed a seemingly flawless calculation which indicated that the Earth had not existed for eternity but had cooled from a molten state over ~100 million years.

John Joly (1899) devised a new geologic technique for determining the Earth's age. He maintained that all the salt in the oceans had come from mineral deposits that had eroded and dissolved. He further assumed(erroneously) that the amount of salt in the ocean could not decline. By multiplying the salinity of the ocean by its volume and dividing by the annual increase, he determined that the brackish sea had developed over 80-90 million years.

Pierre and Marie Curie (1867-1934) discovered radium (1898) which led to a new tool (measuring radioactive decay) for absolute dating of certain rocks. In 1903 they discovered that radioactive decay produces heat as a by-product. This invalidated Lord Kelvin's calculations since it introduced a new heat source that Kelvin had not accounted for. Geologists no longer had to assume that the Earth had steadily cooled from a molten state. The fuel of plutonism had been clearly identified.

Bertram Boltwood (1907) dated the Earth's age as somewhere between 400 million and 2.2 billion years using the radioactive decay method.

Alfred Wegener (1912) proposed continental drift theory based on geological, paleontological and climatological evidence.

Joseph Barrell (1917) reinterpreted geologic history to conform with the latest results of radioactive dating. Even though these results indicated an age of a few billion years, many geologists still preferred the 100 million year old Earth.

George Ashley (1923) published A Geologic Time Scale while serving as state geologist of Pennsylvania. At this point, Earth's geologic history was already divided into Paleozoic(400 million years duration), Mesozoic(150 million years) and Cenozoic(61 million years). [Current revisions are approximately 360 million, 185 million and 65 million years respectively.]

Arthur Holmes (1926) was the primary author of a report for the National Academy of Sciences in which the committee agreed unanimously that radioactive dating was the only reliable geologic timescale. By this time, the constants of radioactivity were firmly established and other sources of problems such as specimen selection and lead isotopes were understood. The scale has been further refined over the last 70 years. Currently, the record for the oldest known rocks on Earth is 3.96 billion years.

Harry Hess (1960) placed sea-floor spreading on firm theoretical and empirical footing. The modern study of plate tectonics is born.

Alvarez et. al. (1980) proposed a major asteroid or comet impact at the K/T boundary which was probably the major causative agent of the K/T mass extinction. The Alvarez hypothesis sparked interest in searching for similar evidence at other extinction horizons.

Eugene Shoemaker (1983) tabulated lists of Earth crossing asteroids and comets and calculated cratering rates for Earth.

Harland et. al. (1989) published A Geologic Time Scale which is considered an authoritative work on on the geologic timescale and is widely used by geologists and paleontologists.

D. McLaren and W. Goodfellow (1990) presented a model for the environmental effects of large asteroid and comet impacts and some evidence for large impact events at most of the major extinction horizons.

Pan Terra, Inc. (1994,1998, 1999) published A Correlated History of Earth which correlates the latest data on a full-color wallchart including geologic events, orogenies, volcanic episodes, ice ages, tectonic maps, geostratigraphy, impact craters, classic fossil localities, major extinction events and evolutionary history of living organisms. This wallchart was designed as an educational tool and can be found in classrooms, museums and homes in the USA and around the world. See the WMNH Giftshop for more information.

The natural history festival is an annual event held in May in Hill City, South Dakota.