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5.4.1. Relative dating

Relative dating

The dating or relative geochronology is sort geological events , saying what happened before and what happened after, without assigning a specific date in millions of years.

The layers are horizontal layers that are deposited sediments in the bottom of a sedimentary basin. The strata contain information about the environment, age, and circumstances that characterized the environment in which they were deposited. It could be said that they are the pages of the book "History of the Earth" that we must "read" to know it. Although we do not have all these "pages", we can base ourselves on a series of geological principles that will allow us to date, in a relative way, the main geological events.

Principle of superposition of strata

The sediment layers are deposited in a temporal sequence, in which the oldest ones are in a lower position than the most recent ones.

Therefore, if there has been no deformation, the lower layers were deposited earlier than those above, so they are older. On the other hand, if there has been deformation, the folds or faults may have caused changes that make older layers are arranged above more modern ones.

Principio de superposición de los estratos

By Kurt Rosenkrantz [CC BY-SA 3.0], via Wikimedia Commons

Interactive activity: Order the strata from oldest to most modern .

Geological Song: Principle of Overlapping Strata and How to Recognize If the Stratum is in a Normal or Inverted Position .

Principle of faunal succession

The strata that were deposited at different geological times contain different fossils, due to the continuous and irreversible nature of biological evolution.

The fossils, in principle, the same age as the strata in which they are, so according to the principle of superposition of strata, those in the lower strata will also be older than those in the upper strata.

As the strata containing the same type of fossil were deposited at the same time, if we know the relative age of the fossil, we can know the age of the stratum that contains it, even if it is elsewhere.

Principio de sucesión faunística

By Kurt Rosenkrantz [CC BY-SA 3.0], via Wikimedia Commons

Principle of superposition of events

A geological process that affects some materials is always after them, and before the materials that it does not affect and the processes that affect it.

For example, if a rock appears folded, the rock will be older than the fold. Or if it is a short stratum to another, the one that appears cut is the oldest.

Principle of uniformism or uniformitarianism

The uniformism or uniformitarianism is the principle according to which the natural processes that acted in the past are the same that act in the present and with the same intensity.

This principle states that geological processes (erosionplate movement, etc.) have remained uniform throughout geological time.

Principle of actualism

The actualismo can be summed up with the phrase "the present is the key to the past". It means that the processes that take place today (magmatism, sedimentationearthquakes, etc.) are the same that have happened throughout the history of the Earth.

Thus, by studying the present we can obtain information about what happened in the past and get an idea of ​​the changes that have occurred on Earth since its formation.

Ondulaciones en la playa

The ripples in the sand on the beach, current and fossil, are similar .


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.