• Ashwini

GS15: Principles of Stratigraphy

Updated: Jul 25, 2020

It was hard for our ancestors to believe that the history of our planet extends billions of years into the past. This information changed the perception of the humanity of the Time and the Universe as profoundly as the astronomical discovery that the expanse of our pervasive space extends much beyond the edge of our Solar Systems.

Unconformity, Hutton discovery
The unconformity at Siccar Point, Scottland

Not until the challenges made by James Hutton, a Scottish farmer, and doctor, for the persisting views that the geologic time began about the same time that human history began and we believed matter and life originated together. Hutton’s idea of the Principle of Uniformitarianism enunciated that the present is the key to the past.

Hutton concluded this idea while wandering in the highlands (historic region of Scottland) where he noted that many features in the sedimentary rocks (such as ripple marks and cross-beds; will discuss them soon) resembled features that he could easily observe in modern depositional environments. According to his principle, physical processes that operate in the modern world also operated in the past, at roughly the same rates and these processes were responsible for forming geologic features preserved in outcrops which we observe today. He challenged the former widely accepted idea because the development of geologic features took a long time, and not all of them formed at the same time, so the Earth must have a history that surpasses the history of humans.

As detectives strive in solving the enigmatic sequence of events in a crime, likewise geologists attempt to establish the correct chronology of the events that produced the geological feature and, when possible, the date on which each event happens. Either, we can distinguish the age of one feature with respect to another in a sequence of relative age i.e. one feature is older than the one lying above it, provided the region has been tectonically stable since the inception of the features or, the age of a feature given in years as its numerical age (or ‘absolute age’).

Geologists learned how to determine relative age long before the methods to determine absolute age were known to them. All the relative age determination was based on the set of principles which are followed ubiquitously.

Relative age tells us about ‘younger or older’ whereas the absolute age tells us ‘how old?’.

The works of the early naturalist and geologist laid down a set of formal, usable geologic principle which provides us a basic framework within which geologists read the record of Earth history and determine relative ages. Some of the notable contributors to these principles are Hutton, Steno, Charles Lyell, etc.

The principle of uniformitarianism states that physical processes we observed operating today also operated in the past, at approx. the same pace as today. In short, the present is the key to the past (by James Hutton in his book Theory of the Earth-late 18th century)

The principle of original horizontality states that layers of sediment, when first deposited, are fairly horizontal because sediments accumulate on surfaces of low relief in the presence of the gravitational field. If sediments were deposited on a steep slope, they would likely slide downslope before they could be buried and lithified. In parallel, this principle also concludes that examples of folds and tilted beds represent the consequence of deformation after deposition i.e. post-deposition (Nicholas Steno).

Evolution on the Earth. Geology. Earth Science.
Timeline of the Earth

The principle of superposition states each layer must be younger than the one below because a layer of sediment cannot deposit until there is a substrate on which it can be collected. However, this law is obeyed only in those regions which have not undergone any tectonic deformation, as they could result in an upside-down arrangement of strata where the upper one is older than the one below it (popularized by William 'Strata' Smith, although given by Nicholas Steno-17th century).

The principle of lateral continuity states that the sediments generally accumulate in continuous sheets within a given basin. If today we encounter a sediment layer cut by a canyon, then it is for sure that the layer once spanned the area which was later eroded by the river that formed the canyon by the process of weathering and erosion (Nicholas Steno-17th century).

The principle of crosscutting relations states that if one geologic feature cut across another then the feature that has been cut is older than the one which is cutting. For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike. If a fault cuts and displaces a body of rock, then the fault must be younger than the layers. But if the same fault or dike is buried by another layer of sediments, then the sediment will be younger than the fault or dike (Danish geologist, Nicholas Steno-17th century).

The principle of baked contacts states that an igneous intrusions bakes (in geological term metamorphoses) surrounding rocks, so the rock that has been baked must be older than the intrusions (Charles Lyell).

The principle of inclusions states that a rock containing an inclusion (fragment of another rock) must be younger than the included body. For example, a conglomerate containing pebbles of basalt is younger than the basalt and a dike containing fragments of sandstone must be younger than the sandstone (Charles Lyell).

The principle of floral and faunal succession states that a particular assemblage of floral and faunal remains is restricted in certain strata, and not above or below it. And once it disappears it never appears again at shallow level in the rock succession even if a similar habitable environment is restored. This principle clearly states that assemblage of fossil in a rock sequence exhibits a certain pattern of evolution such that simpler types or the primitive ones occur in older rocks and the advanced or more complex towards the younger ones. Therefore, each rock succession actually represents a time segment from the time period of the Earth which can easily be understood by the knowledge of the distinctive assemblage of fossils. Also, with the help of the fossil assemblage, a body of rock can be easily correlated with the rocks at some other location possessing similar fossils (Charles Lyell).

The concept of catastrophe states that each major geological time was terminated by a worldwide catastrophe that extinguished most of the life forms of the Earth at a time. However, during the catastrophe new and more complex life forms are also born which adapt to the changed condition (French scientist, Georges Cuvier-early 19th century).

In the upcoming articles, we will get to know more about Geological History, Radioactivity, Isotopic dating analysis, and many more. Stay tuned. Till then check our previous articles.

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