The Geologic Column
|Global correlation of strata led to the development of the geologic column.|
As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history (figure above a, b). The column is divided into segments, each of which represents a speciﬁc interval of time. The largest subdivisions break Earth history into the Hadean, Archean, Proterozoic, and Phanerozoic Eons. (The ﬁrst three together constitute the Precambrian.) The sufﬁx zoic means life, so Phanerozoic means visible life, and Proterozoic means ﬁrst life. (It wasn’t until after the eons had been named that geologists determined that the earliest life, cells of Bacteria and Archaea, appeared in the Archean Eon.) The Phanerozoic Eon is subdivided into eras. In order from oldest to youngest, they are the Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life) Eras. We further divide each era into periods and each period into epochs.
Where do the names of the periods come from? They refer either to localities where a fairly complete stratigraphic column representing that time interval was ﬁrst identiﬁed (for example, rocks representing the Devonian Period crop out near Devon, England) or to a characteristic of the time (rocks from the Carboniferous Period contain a lot of coal). The terminology was not set up in a planned fashion that would make it easy to learn. Instead, it grew haphazardly in the years between 1760 and 1845, as geologists began to reﬁne their understanding of geologic history and fossil succession. Also, because the divisions were deﬁned before numerical ages could be determined, they are all of different durations.
|Life evolution in the context of the geologic column. The Earth formed at the beginning of the Hadean Eon.|
The succession of fossils preserved in strata of the geologic column deﬁnes the course of life’s evolution throughout Earth history (figure above). Simple bacteria and archaea appeared during the Archean Eon, but complex shell-less invertebrates did not evolve until the late Proterozoic. The appearance of invertebrates with shells deﬁnes the Precambrian-Cambrian boundary. At this time, there was a sudden diversiﬁcation in life, with many new types of organisms appearing over a relatively short interval this event is called the Cambrian explosion.
Progressively more complex organisms populated the Earth during the Paleozoic. For example, the ﬁrst ﬁsh appeared in Ordovician seas, land plants started to spread over the continents during the Silurian (prior to the Silurian, the land surface was unvegetated), and amphibians appeared during the Devonian. Though reptiles appeared during the Pennsylvanian Period, the ﬁrst dinosaurs did not stomp across the land until the Triassic. Dinosaurs continued to inhabit the Earth until their sudden extinction at the end of the Cretaceous Period. For this reason, geologists refer to the Mesozoic Era as the Age of Dinosaurs. Small mammals appeared during the Triassic Period, but the diversiﬁcation (development of many different species) of mammals to ﬁll a wide range of ecological niches did not happen until the beginning of the Cenozoic Era, so geologists call the Cenozoic the Age of Mammals. Birds also appeared during the Mesozoic (speciﬁcally, at the beginning of the Cretaceous Period), but underwent great diversiﬁcation in the Cenozoic Era.
|Correlation of strata among the national parks of Arizona and Utah.|
To conclude our discussion of the geologic column, let’s see how it comes into play when correlating strata across a region. We return to the Colorado Plateau of Arizona and Utah, in the southwestern United States (figure above a, b). Because of the lack of vegetation in this region, you can easily see bedrock exposures on the walls of cliffs and canyons; some of these exposures are so beautiful that they have become national parks. Using correlation techniques, geologists have determined that the oldest sedimentary rocks of the region crop out near the base of the Grand Canyon, whereas the youngest form the cliffs of Cedar Breaks and Bryce Canyon. Walking through these parks is thus like walking through Earth’s history each rock layer gives an indication of the climate and topography of the region at a time in the past. For example, when the Precambrian metamorphic and igneous rocks exposed in the inner gorge of the Grand Canyon ﬁrst formed, the region was a high mountain range, perhaps as dramatic as the Himalayas today. When the fossiliferous beds of the Kaibab Limestone at the rim of the canyon ﬁrst developed, the region was a Bahama-like carbonate reef and platform, bathed in a warm, shallow sea. And when the rocks making up the towering red cliffs of sandstone in Zion Canyon were deposited, the region was a Sahara-like desert, blanketed with huge sand dunes.
Credits: Stephen Marshak (Essentials of Geology)