Chronostratigraphy

The formation of rocks has long fascinated humans. Natural wonders, such as the Grand Canyon in Arizona, draw millions of tourists a year. From the rim of the Grand Canyon, layers of colored rock are visible in wide bands that tell the geological history of the land. Chronostratigraphy is a field of study of stage and age of rock formation. Using chronostratigraphy, geologists have estimated that the Grand Canyon formed about two billion years ago. A base layer of rocks formed first, and then sedimentary rocks were deposited on top of those in layers. Tectonic plate action uplifted the entire area, creating a flat surface, the Colorado Plateau. About five or six million years ago, the waters of the Colorado River began to carve a canyon through the surface rock of the Colorado Plateau. Erosion from water and wind further widened and deepened the canyon, making the colored striations of rock visible. Natural forces will make the canyon slightly wider and deeper with each passing year.

Background

Geology is the study of rocks and rock formations, and stratigraphy is a branch of geology that specifically studies the layers of rocks and the patterns they form, which is known as “stratification.” “Strata” refers to multiple layers of rock. Strata can be composed of packed soil, sedimentary rock, or igneous rock. Igneous rock comes from volcanic magma, while sedimentary rock is formed from organic material or minerals deposited over time.

Chronostratigraphy adds the element of time to the study of strata. This field of scientific study seeks to discover the geological history of Earth by determining the sequence of events that resulted in a specific layer being formed. This involves studying not only rocks but also the fossils within them that formed eons ago to determine the geologic record.

The geologic record refers to all the strata of rock that was laid down in a particular area. A geological record tells scientists much about the past, including what the climate was like, what type of plant or animal life existed, how the landscape looked, and how it changed over time. Because time in a geological record may be as large as billions of years, units of time that account for these large amounts use the geological timescale (GTS) to describe the timing and relationship of events. Eons are the largest span of time, generally referring to about a billion years. Eras are the second longest, usually several million years. Periods, epochs, and ages further provide time division. These time scales can be further divided into defined classifications to designate a time. “Early,” “middle,” and “late” are often added to a time unit to further qualify a timespan. For example, the Early Jurassic Epoch refers to a time on the GTS a little over two hundred million years ago.

Overview

Geochronology is the scientific study of the age of rocks that chronostratigraphy relies on to date rocks, fossils, and sedimentary material, which puts together the pieces of Earth’s geologic history. Absolute dating to determine a more definitive date can be done using radiometry to analyze radioactive isotopes to determine the age of rocks or other material. Variations are measured by isotope radio mass spectrometry, providing not only the estimated age of them but also the conditions relating to their origin.

Sometimes absolute dating is not necessary or difficult because the geological record of a specimen contains such large spans of time. Relative dating, which compares one thing to another using inference, can be used to determine a timescale that gives geologists insight into surrounding events or conditions. One basic way to do this is to look at the layers in the order in which they appear. A stratum is a basic geological unit represented in a stratigraphic column, or a rock cross-section sample that shows the layers in a rock in the order they formed, one on top of the other. A stratigraphic column diagram is used by geologists to represent and study the strata in order of formation. This is guided by the principle of superposition, which means the oldest layer of rocks is on the bottom, and the youngest layer of rocks on the top.

The strata of rocks in chronostratigraphy are named according to the timespan in geological time that they cover, which corresponds to the GTS. Eonothem covers eons, erathems cover eras, systems cover periods, series cover epochs, and stages cover ages. Terms such as “lower,” “middle,” and “upper” are added to relate to “early,” “mid,” and “late” when referring to layers in the stratigraphic column. For example, the Early Jurassic Epoch would be called the “Lower Jurassic Series” when referring to rock layers in chronostratigraphy.

How straight the rocks are in layers relates to the principle of original horizontality. Without any major geologic event to interfere, rock layers will be laid down on top of one another in a fairly uniform horizontal pattern. However, slanting or intrusion of other types of rock indicates that there was some sort of disruption to the rock layer, such as a volcanic eruption or tectonic plate activity that occurred after the rocks were originally deposited. A disruption in a stratum can provide relative dating using the cross-cutting relationships principle, which states that if a rock or other material intrudes into a rock layer, it is younger than the strata since the strata had to be already formed to be disrupted. An example of this is pressurized hot magma flowing from a volcanic eruption that penetrates rock strata, which causes a visible intrusion of basalt.

Cross-dating can also be used when analyzing strata. This involves looking at fossils preserved in the layers to provide a date according to established fossil records. Simple organisms indicate an earlier date, while more complex forms of life indicate an older rock layer. From the type and amount of plant or animal fossils found in rock stratum, much can be learned about the time and conditions that existed according to the GTS formed by the strata.

Bibliography

“Chronostratigraphic Units.” International Commission on Stratigraphy, 2020, stratigraphy.org/guide/chron. Accessed 15 Feb. 2022.

“Geologic Principles—Cross-cutting Relationships.” National Park Service, 2022, www.nps.gov/articles/geologic-principles-cross-cutting-relationships.htm. Accessed 8 Nov. 2024.

“Geology: How Grand Canyon Was Formed over Time.” National Park Service, 2022, www.nps.gov/grca/learn/nature/grca-geology.htm. Accessed 8 Nov. 2024.

Hounslow, Mark W. “Chronostratigraphy.” Research Gate, Jan. 2020, www.researchgate.net/publication/343358161‗Chronostratigraphy. Accessed 15 Feb. 2022.

“How Old Are Rocks?” Geological Survey Ireland, 2022, www.gsi.ie/en-ie/education/rocks-minerals-and-gems/Pages/How-old-are-rocks.aspx. Accessed 15 Feb. 2022

Luciano, Eugenio. "Is 'Anthropocene' a Suitable Chronostratigraphic Term?" Anthropocene Review, 24 Feb. 2022, doi.org/10.1007/s44177-022-00011-7. Accessed 8 Nov. 2024.

Richter Daniel. “Chronostratigraphy.” Encyclopedia of Geoarchaeology, edited by Allan S. Gilbert, Encyclopedia of Earth Sciences Series, Springer, Dordrecht, 12 Aug. 2016, doi.org/10.1007/978-1-4020-4409-0‗5. Accessed 15 Feb. 2022

Vai, Gian Battista. “A History of Chronostratigraphy.” Stratigraphy, vol. 4, no. 2, Jan. 2007, pp. 83–97, www.researchgate.net/publication/286993797‗A‗history‗of‗chronostratigraphy. Accessed 15 Feb. 2022.

“What Are Stratigraphic Columns?” Digital Atlas of Idaho, digitalatlas.cose.isu.edu/geo/basics/sedstrat.htm. Accessed 15 Feb. 2022.