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| What is Stratigraphy | Last Updated • March 31, 2015 | |||||||||||||||||||||
As a general concept, Stratigraphy is "the study of rock successions and the correlation of geological events and processes in time and space" [1]. What does this mean?
In the end, Stratigraphy can be described as the study of layered rocks and of their temporal implications.
Stratigraphy allows the reconstruction of the sequence of the events in Earth history and of the evolution of life on Earth.
Today's meaning and significance of stratigraphy is much more accurate and precise than it was in the past. But when stratigraphy is interpreted as the study of the natural environment and its phenomena, we realize that it is a very old science, which originated when ancient philosophers and wise men started to observe, describe, and think Earth's natural phenomena. | Stratigraphy in Ancient Times | Last Updated • March 31, 2015 |
While overall ancient philosophers elaborated each their own theory and speculations about Earth, the perception of the natural world was, and in part still is, different throughout the various cultures that arose in different parts of the world:
For instance, in far Eastern cultures Earth was eternal and immutable while in Indian cultures Earth changed according to infinite cycles of creation. The ancient Greeks had a more pragmatic, realistic approach: for them, Earth was subject to continuous change over time because of natural laws, without the necessity of an external divinity to modify or alter the picture.
In ancient times the approach to fossils was also variegated, and they were object of curiosity too. The word "fossil" comes from the Latin fossilis which means, literally, "that can be dug out" (modern Italian, an alteration of Latin, still uses the words "fossa" to indicate a dig, a hole, and "fosso" to indicate a ditch). Such a wide description, a fossil is anything dug out of the ground, tells us that in the past a fossil could have actually also indicated a rock, a mineral, a groups of layers, or even human artifacts. The modern meaning of the word "fossil" only applies to the remains of ancient organisms, or of their activity (as in the case of trace fossils).
Among others, Pythagoras (6th century BC) and Herodotus (5th century BC) interpreted marine shells found on mountains as remains of organisms that lived in the ocean: according to them, the ocean had covered the land and had retreated from it at least once during Earth history.
The Greek Xenophanes, also in the 5th century BC, argued that marine fossils found at the top of mountains, or on land in general, were a line of evidence that nothing is immutable and that physical changes actually occur on Earth, and that land and sea can "blend" together. The landscape we see is only but a moment in time of a continuously changing Earth. Aristotle (4th century BC) thought that "fossils" were produced by a natural (not divine) "formative force" that imitated extant organisms. During Roman times Pliny the Elder (1st century AD) was the major naturalist of the period (among other things, he is the author of Historia Naturalis, a 37-volumes encyclopedic work about Natural History, the first ever conceived). He died of asphyxia while observing a geological event, the catastrophic volcanic eruption of Mount Vesuvius in Pompeii, Italy, in the year 79 AD. In the following years, the spreading of a new religion, Christianity, started to condition natural philosophers and theologians in their thinking. According to Christianity, there were no "natural causes" but everything was consequence of a divine "creation". Some of these thinkers, such as, for instance, the Roman Tertullian, were somehow able to maintain the idea of the organic origin of fossils (instead of a "creation"), but they still had to fit their origin within the religious framework of the biblical deluge to avoid persecution. Later on, at first when Christianity eventually became the official religion of the Roman Empire (Edict of Constantine, 313 AD) and then when the Empire fell at the hands of invading barbaric populations from the north and the east (476 AD), the Roman culture and civilization, from which western culture derives, suffered a severe blow: the society as a whole entered a deep stagnation, and science in particular was deemed purposeless and was not cultivated any longer in the western world.
Through Avicenna (Ibn-Sina in Arabic, 980 AD - 1037 AD) the Greek Aristotelian School was eventually revived in the west and many European scholars developed again the idea that fossils could have been organic in origin, possibly representing a series of failed attempts of the divinity to originate life. In the 1200s in Swabia (in today's Southern Germany), Albertus Magnus (Albert the Great) speculated on the nature of fossils as the remains of once-living ancient organisms. It is eventually in the works of the Italian Leonardo Da Vinci (1452-1519) that for the first time we read about the mechanisms of sedimentary erosion and deposition in mountains and rivers, about the role that rivers play in the weathering of land and soil, and the basic principles of the Law of Superposition (which was to be demonstrated by Steno in Florence only a couple of centuries later). Leonardo also understood that different layers of sedimentary rocks, and the fossils they contain, can be traced over distances that extend beyond a single outcrop (correlation), and that these layers were formed in a sequence, at different, successive times. Unfortunately after his death, while the bulk of his major works was always well known, many of his books became scattered throughout Europe's libraries and courts, and his geological observations were ignored or forgotten for a few centuries. Later on, an enormous contribution to the field of geology came from an essay (De Re Metallica, or "Metals") by the Saxon (Saxony is today a German region) Georg Bauer, better known by his Latin name of Georgius Agricola, which was published in 1556, a few decades after Leonardo's death. Among other observations, Agricola outlined how sedimentary layers were deposited in a consistent, sequential order. Because most of his studies were specifically devoted to ore deposits and mining industry, Agricola can be accounted, together with Steno (see next section below), as one of the founders of modern geology. It is noteworthy to mention that, once earth-related economical interests became important in western society (the need for mineral and metal resources) enormous advances were made. Geology really became important when, in the 19th century, the industrial revolution required enormous amounts of coal as a source of energy. Something similar happened in the 20th century with the development of the oil and gas industry, whose workings provide us with a myriad information that would not be otherwise available. And it is happening today with, for instance, the hunt for "rare earths", a series of elements (Scandium, Yttrium, and the Lanthanides) that are extremely important components of our computer-based society.
In the year 1603 the Bologna, Italy naturalist Ulisse Aldrovandi, who was hailed by the Swedish Carolus (Carl) Linnaeus and by the French Comte de Buffon as the father of natural history, was the first scientist to ever use the word "geology" in its proper, scientific meaning[2]. All these advances prepared the ground for the next major step, the layout of Steno's principles.
| Stratigraphy from the Renaissance to modern times | Last Updated • March 31, 2015 |
Stratigraphy, or the art of being able to put order in a sometimes chaotic jumble of rocks, soon was at the core of every serious geologic study that took place during the Renaissance. Steno was the one who first outlined stratigraphy's principles.
Steno Steno is famous because he has been the first to clearly outline the three basic principles of stratigraphy: superposition, original horizontality and lateral continuity:
James Hutton After Steno, one of the most significant figures for the development of geology as a science was the Scottish naturalist James Hutton (1726-1797): James Hutton was the first to speak about Uniformitarianism, or the Uniformitarian Principle. He grasped the significance and the importance of the concept of "Uniformity" and he was the first to understand how "deep" the geologic time actually was. The principle of Uniformitarianism states that the geologic forces and processes that shaped our planet in the geologic past were the same that are at work on Earth today, and that these forces and processes are part of our every day experience and can be observed directly by anyone. In 1785 Hutton published his book Theory of the Earth, where he proved that the hills and mountains of today are being slowly eroded, and that the sediment produced during these erosional processes is being transported and eventually deposited as sand and mud on the sea floor to form sedimentary rocks[3]. Hutton argued that the same processes of erosion and deposition observed today must have always been at work in Earth's past. He also realized that the vast thickness of sedimentary rocks sequences that can be observed in nature implied the fact that these same processes of erosion and deposition had to have occurred throughout an inconceivably long period of time (that he called "deep time"). Such a contribution was very distinct from the mainstream ideas of the time, which were still very heavily influenced by religious beliefs. At that time, a popular theory was that Earth had formed (was created) during the month of October of 4004 BC, a date calculated by an Irish bishop using biblical genealogies.
This is known as the Principle of Faunal and Floral Succession: fossils faunas and flora are found in a known and predictable order. Smith did not know the reason for this (it is a consequence of the evolution in time of living organisms) but he was right in his observations. Thanks to his observations, discoveries, and collection of information Smith was able to publish the first ever large-scale geologic map ever conceived between the years 1814 and 1815.  William Smith's "Strata of England and Wales" geologic map, preserved at the headquarters of the Geological Society of London, England
Despite his significant geological contributions, Darwin is mostly remembered for his Theory of Evolution through gradual variation and natural selection,
exposed and discussed in his aptly titled book On the Origin of Species by Means of Natural Selection.
The continuous, rapid establishment of biostratigraphic zones that followed these discoveries led to the development of a more and more refined relative time scale, which eventually allowed the construction of what we know today as the Geologic Time Scale with its subdivisions. | Stratigraphic units | Last Updated • March 31, 2015 |
What all these scientists have been working on is what we in the end call stratigraphy.
Stratigraphic Units include strata, or groups of adjacent strata, that are distinguished from each other by mean of physical, chemical or paleontological properties; stratigraphic units also include time units that are based on the ages of such strata. Correlation is the establishment of equivalency between geographically separated stratigraphic units, or parts of stratigraphic units.
We can correlate in two distinct ways: while we can have lithologic correlations (that is, we say that two different outcrops are actually the same rock, or Formation) most of the times, when we speak of correlation we mean temporal correlation
[1] Koutsoukos, E. A. M., 2005. Stratigraphy: Evolution of a Concept. In: E. A. M. Koutsoukos (ed.), Applied Stratigraphy, 3-19, Springer. [2] Vai, G. B., 2003. Aldrovandi's will: introducing the term "geology" in 1603. In: G. B. Vai, and W. Cavazza (eds.), Four centuries of the word geology: Ulisse Aldrovandi 1603 in Bologna: Minerva, Bologna, 64-111. [3] Doyle, P., Bennett, M. R., and Baxter, A. N., 2001. The Key to Earth History: An Introduction to Stratigraphy (2nd edition). John Wiley and Sons, New York NY. In parts 2 and 3, we will be looking at time units and time-rock units. | |||||||||||||