Rock Strata and Uniformitarianism

Introduction:

In 1869, a one-armed Civil War veteran named John Wesley Powell set out with nine companions to explore the Grand Canyon, the world’s largest canyon. They drifted down the Colorado River, which flows through the canyon floor, for three months. During their journey, rock walls amazed the explorers, encouraging them to ask important questions about the Earth and its history, which the tourists visiting the canyon may still do today: how long did the canyon take to carve? When did the rocks that make up the canyon walls form?

Thinking about such questions leads to the understanding of geologic time or the period of time since the formation of the Earth.

Our modern understanding of geologic time is based on work done in the nineteenth century that established principles for describing the relative age of geologic features. We define relative age as whether one feature is older or younger than another. By keeping this in mind, we will discuss relative-age determination in this chapter. Understanding relative ages paved the way for the creation of the geologic column, a chart that divides geologic time into intervals. When geologists developed isotopic dating methods (radiometric dating) in the mid-twentieth century, it became possible to define the numerical age—the age in years—of rocks. This tool resulted in the creation of the geologic time scale and, ultimately, an estimate of when the Earth formed.

Geological Principle and Relative Age

By the 1850s, geologists had established several principles that served as a foundation for the development of the concept of geologic time.

Uniformitarianism

While exploring the Scottish hilltops, James Hutton (1726-1797) noted that many of the features he discovered in sedimentary rocks resembled features he could see forming in modern depositional environments. Some sandstone beds, for example, had ripple marks similar to those he saw on a modern beach. These observations led Hutton to speculate that ancient rocks and landscapes were created by the same natural processes that exist today.

According to uniformitarianism, “the present is the key to the past.” Hutton went on to conclude that not all of the rocks he saw, or the structures that influenced them, could have formed at the same time because no one can see the entire process of sediment turning into rock and then rising into mountains. Hutton concluded that rock formation is very slow and that the Earth’s history must predate human history. This proposal, along with a few others, became the foundation for so much geologic thought that modern geologists regard Hutton as the “Father of Geology.”

Determining Relative Ages and Geologic History

Geologists, like historians, try to piece together the sequence of events that resulted in a collection of geologic features. The age of one feature in relation to another in a sequence is referred to as its relative age.

Nicolas Steno (1636-1686), a Danish scientist, established a set of formal geologic principles, which served as the foundation for Hutton’s principle of uniformitarianism. Steno’s and Hutton’s ideas were popularized by Charles Lyell (1797-1875), a British geologist, in his book Principles of Geology, which was the first comprehensive textbook of geology. This book explains how geologic principles, such as uniformitarianism, can be used to determine relative ages.

The principle of original horizontality states that layers of sediment are roughly horizontal when they are first deposited. Why? Sediments accumulate on relatively flat surfaces, such as floodplains or the seafloor, due to gravity. If they accumulate on a steep slope, they will slide downslope before being buried and lithified. With this principle in mind, geologists conclude that folds and tilted beds represent post-depositional deformation events.

The principle of superposition states that each layer of sedimentary rock must be younger than the one beneath it because a layer of sediment cannot accumulate unless there is already a surface on which it can collect. As a result, in a sedimentary sequence, the oldest layer is at the bottom and the youngest is at the top.

According to the principle of cross-cutting relations, if one geologic feature cuts across another, the feature that was cut after is older. If an igneous dike cuts through a series of sedimentary beds, the beds must be older than the dike. If a sediment layer buries the dike, the sediment must be younger than the dike, and if a fault cuts across and displaces layers of sedimentary rock, the fault must be younger than the layers.

According to the baked contacts principle, an igneous intrusion “bakes” (metamorphoses) wall rock, so the baked rock (which contains the metamorphic aureole) must be older than the intrusion.

According to the principle of inclusions, a rock that contains an inclusion (fragment of another rock) must be younger than the inclusion. A conglomerate containing basalt pebbles, for example, is younger than the basalt, whereas a basalt containing sandstone fragments must be younger than the sandstone.

Geologists use the principles outlined above to determine the relative ages of geologic features (rocks, structures, and erosional features), each of which is the result of a distinct geologic event. Deposition, erosion, intrusion, or extrusion of igneous rocks and deformation are examples of geologic events (folding or faulting). The geologic history of the region is defined by the sequence of events in terms of relative age.

Creating the Geologic Column

Stratigraphic Formations

Geologists divide a region’s strata into distinct units known as stratigraphic formations. A stratigraphic formation (or simply formation) is a series of beds that can be traced across a relatively large area. Some formations are composed of a single rock type, whereas others are composed of interlayered beds of two or more rock types. While most formations are composed of sedimentary strata, some also contain extrusive volcanic products. A formation typically represents the products of deposition over a defined time interval. As a result, a specific geologic age or age range can be assigned to a given formation, and the characteristics of rocks in a formation provide information about the environment at the time of deposition. The exposed formations on the Grand Canyon’s walls stand out as distinct stripes.

Unconformities – Gaps in the Record

An unconformity is a contact between two rock units, the upper unit being much younger than the lower unit. Unconformities are generally buried erosional surfaces that represent a break in the geologic record that can last hundreds of millions of years or more. For example, the contact between a 400 million-year-old sandstone deposited by a rising sea and a 600 million-year-old weathered bedrock surface is an unconformity that represents a 200 million-year time gap. During that 200 million-year period, the sediment and/or rock that was deposited directly on the bedrock was eroded away, exposing the “basement” surface. Unconformities are classified into three types: disconformities, non-conformities, and angular unconformities.

Disconformity

Disconformities are typically erosional contacts that run parallel to the upper and lower rock unit bedding planes. Because disconformities are difficult to detect in a layered sedimentary rock sequence, they are frequently discovered when studying the fossils in the upper and lower rock units. A gap in the fossil record indicates a gap in the depositional record, and the amount of time represented by the disconformity can be calculated. Disconformities are usually caused by erosion, but they can also represent periods of nondeposition.

Nonconformities

The contact that separates a younger sedimentary rock unit from an igneous intrusive rock or metamorphic rock unit is referred to as a nonconformity. A nonconformity indicates that long-term uplift, weathering, and erosion exposed the older, deeper rock at the surface before it was buried by the younger rocks above it. The old erosional surface on the underlying rock is referred to as nonconformity.

Angular Unconformity

An angular unconformity is a contact between a younger, gently dipping rock unit and older, tilted, or deformed layered rock. Because the rock units are not parallel and appear crosscutting at first, the contact is more obvious than a disconformity. Because the underlying rock was usually metamorphosed, uplifted, and eroded before the upper rock unit was deposited, angular unconformities generally represent a longer time gap than disconformities.

The sequence of strata at any location provides a record of Earth’s history. However, due to unconformities, the geologic record preserved in rock layers at any given location is incomplete.