The Concept of Geologic Time

In 1869, when Major John Wesley Powell came out of the Grand Canyon with a team of geologists and scouts, after a 3-month journey through its canyons and rapids, they could not help but ponder about questions that even casual tourists pose today: Did the Colorado River sculpted this marvel? If so, how long did it take to do it? How old is the canyon? And the rocks on its wall?

Until relatively recently, people in most cultures have believed that geologic time began about the same time that human history began, and that our planet has been virtually unchanged since its birth. It was common knowledge to accept that all Earth’s natural features (mountains, valleys, oceans, rivers…) had been produced by a few great catastrophes and have remained the same since then. This idea is usually called catastrophism. It is not a hypothesis because none of its believers ever tried to test it. They just accepted it as the truth.

At the end of the 18th century, this idea was challenged by a Scot named James Hutton. While wandering in the highlands of Scotland, a region where rocks are well exposed, Hutton noted that many features found in sedimentary rocks resembled features he could see forming today in modern depositional environments (features such as ripple marks and cross beds). Based on such observations, Hutton developed an idea, which later came to be known as the principle of uniformitarianism, and published it in a 1785 book called The Theory of the Earth. The principle of uniformitarianism states that physical processes we observe today also operated in the past at roughly the same rates, and that these processes were responsible for the formation of geologic features we see in the natural world. You probably have heard this principle in a different way: the present is the key to the past.

Hutton deduced that the development of individual geologic features took a long time, and that not all features formed at the same time, as common knowledge suggested. According to him, the Earth must have a history that includes a succession of slow events. In fact, he even speculated that, when it comes to Earth, “there is no vestige of a beginning, nor prospect of an end”.

One of the pieces of evidence for Hutton’s conclusions is this famous unconformity at Siccar Point (figure below), in the east coast of Scotland. He noted that the sedimentary rocks below the unconformity (white line) were tilted, almost upright, while the sedimentary rocks above it are only slightly tilted. He deduced that the layers above were deposited long after the layers below had been tilted. Geologists have since determined that the strata above (Devonian sandstone) are about 50 million years younger then the strata below (Ordovician sandstone).

Geologic Principles and Relative Age

In the 1600s, Danish scientist Nicolas Steno (1636-1686) laid out a set of formal geologic principles which served as a foundation for Hutton's principle of uniformitarianism. As noted earlier, this principle states that physical processes we observe operating today also operated in the past, at roughly the same rates. In other words, we use “clues” from processes forming sediments/rocks today to interpret the conditions of formation or sediments/rocks in the past (the present is the key to the past).

British geologist Charles Lyell (1797-1875) popularized the works of Steno and Hutton when he published the first modern textbook of geology (Principles of Geology, in 1830). This book contained a set of stratigraphic principles for determining the relative age of geologic features. The first of these is the principle of uniformitarianism, first proposed by James Hutton.

Another very straightforward geologic principle is the principle of superposition. According to this principle, in a sequence of sedimentary rock layers, each layer must be younger than the one below, and older than the one above. Thus, the layer at the bottom of a sequence is the oldest (the first to deposit), and the layer at the top is the youngest (the last to deposit). Of course this principle is only valid if the strata is undeformed (i.e., has not experienced deformation by tectonic forces).

The figure above shows how the principle of superposition works. The white sand is deposited first (it is the oldest in the sequence and is at the bottom). The orange sand is deposited second (it is younger than the white sand – as it was deposited later – but older than the one above it). The last layer to be deposited is the youngest of all.

Another principle that allow geologists to “read” the geological history of a rock is the principle of original horizontality. This principle states that sediments on Earth settle out of a fluid (air or water) in a gravitational field (attracted to the ground by gravity). As a result, layers of sediment, when originally deposited, accumulate in a horizontal fashion.

The figure below shows a horizontal bedding in an outcrop of sandstone in Wisconsin. It was deposited at the bottom of an ancient stream (“settle out of a fluid”) and accumulated in fairly horizontal layers. If you see sedimentary layers that are not horizontal, it means that the strata was exposed to tectonic forces AFTER it was deposited.

Now let’s imagine that the ocean that submerged our outcrop dries out (or maybe tectonic forces uplift the land above sea level). The rock layers are going to be exposed again and erosion will take place. A river may carve a canyon through this strata (like in the picture below). If a geologist come to visit this area, it can use another geologic principle to “correlate” the similar layers on both sides of the canyon. They have the same age since they were formed at the same time!!! The principle that help us make this correlation is called the principle of lateral continuity.

According to this principle, sediments generally accumulates in continuous sheets within a given region. If today you find a sedimentary layer cut by a canyon, then you can assume that the layer once spanned the area that was later eroded by the river that formed the canyon.

This is what happened to the Grand Canyon. The sedimentary layers that you see in the Grand Canyon walls were deposited when the region was a large shallow ocean, about 700 million years ago. Then North America broke off from Pangaea and started moving northwest, colliding with the Pacific plate. This collision generated tectonic forces that compressed the North American craton, leading to the uplift of the Colorado Plateau. The Colorado River, then isolated, began to “carve” its way out through the rocks, forming the Grand Canyon (figure below).

Now let’s imagine a different situation. In figure 1 below, we have our sedimentary layers, undeformed (horizontal) and, as originally deposited, oldest at the bottom and youngest at the top. If these layers are cut by another feature (it could be a fault or, like in the figure 2 below, an igneous intrusion – a dike of magma). The feature that cuts the layers has to be younger than the layers in order to be able to cut through them. This is the principle of cross-cutting relations. In the example below, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike.

By the same token, if another geologic feature (in this case a fault) cuts through all the previous layers and displaces them, including the dike), the fault is the youngest geologic event (figure below).

Now, what if the layer above were to be submerged again and new deposition occurred? The new layer would be the youngest event (figure below)!

Now, lets imagine the situation below. Which is the fourth event, i.e., which one happened after the third layer was deposited? The intrusion of the pluton or the deposition of the top layer? It is hard to tell ... The information provided in the figure is not enough since the pluton and the top layer do not “cross” each other. In this case, the only thing we can say for certain, based on the principles of cross cutting relations, superposition, and original horizontality, is that the pluton AND the layer at the top are both younger than layer 3.

If we had the situation below instead, the top layer is clearly being deposited AFTER the pluton intruded. In this case, using the principle of cross-cutting relations, we can say for sure that the pluton was the fourth event and it happened BEFORE the deposition of the top layer (event #5).

In the two figures above, you can see a white “halo” around the pluton. This is called a baked contact (a metamorphic aureole produced by the rising magma that created the pluton). The principle of baked contacts states that the rock that has been “baked” (changed) MUST be older than the igneous intrusion (the pluton).

The last stratigraphic principle is the principle of inclusions. An inclusion is a piece of one material incorporated into another. This piece or fragment is called a xenolith (xeno = foreign; lith = rock). As you know, a magma moves up and the heat from the magma melts and incorporates fragments of rocks from its surroundings. Lets suppose this magma moving up cools out before melting all the rocks that it incorporated. The cool magma now becomes an igneous intrusion (a pluton) and inside it you will find this fragment of a pre-existing rock that did not melt completely. The principle of inclusions establishes that the XENOLITH is older than the rock around it (since it is a fragment from a pre-existing rock that did not melt when it was incorporated into the magma).

Another way of looking at this principle is to think of a fruit cake (figure below). Consider the fruit fragments inside the fruit cake and the cake itself and answer: which one is older? The fruit pieces, or the cake? The fruit pieces are older since they were incorporated into the cake dough before the cake was put to bake. The fruit pieces represent the “inclusions” or xenoliths.

Formation and Preservation of Fossils

Most fossils are found in sediments or sedimentary rocks. Fossils form when organisms die and become buried by sediment, or when organisms travel over or through sediment and leave imprints or debris. The degree of preservation of a fossil reﬂects the context of burial. For example, rocks formed from sediments deposited under anoxic (oxygen-free) conditions in quiet water (such as lake beds or lagoons) can preserve particularly ﬁne specimens. In contrast, rocks made from sediments deposited in high-energy environments where strong currents tumble shells and bones and break them up, contain at best only small fragments of fossils mixed with other clastic grains. Fossils sometimes occur in volcaniclastic rocks, but they are not found in intrusive igneous rocks and tend to be destroyed by metamorphism.

Paleontologists refer to the process of forming a fossil as fossilization. To see how a typical fossil develops in sedimentary rock, let’s follow the fate of an old dinosaur as it searches for food along a riverbank (figure below). On a scalding summer day, the hungry dinosaur, plodding through the muddy ground, succumbs to the heat and collapses dead into the mud. Soon after, scavengers strip the skeleton of meat and scatter the bones. But before the bones have had time to weather away, the river ﬂoods and buries the bones, along with the dinosaur’s footprints, under a layer of silt. More sediment from succeeding ﬂoods buries the bones and prints still deeper, so that the bones cannot be reworked by currents or disrupted by burrowing organisms. Later, sea level rises and a thick sequence of marine sediment buries the ﬂuvial sediment.

Eventually, the sediment containing the bones and footprints turns to rock (siltstone and shale). The footprints remain outlined by the contact between the siltstone and shale, while the bones reside within the siltstone. Minerals precipitating from groundwater passing through the siltstone gradually replace some of the chemicals constituting the bones, until the bones themselves have become rock-like. The buried bones and footprints are now fossils. One hundred million years later, uplift and erosion expose the dinosaur’s grave. Part of a fossil bone protrudes from a rock outcrop. A lucky paleontologist observes the fragment and starts excavating, gradually uncovering enough of the bones to permit reconstruction of the beast’s skeleton. Further digging uncovers the footprints. The dinosaur rises again, but this time in a museum.

Perhaps when you think of a fossil, you picture either a dinosaur bone or the imprint of a seashell in rock. In fact, paleontologists distinguish many different kinds of fossils, according to the speciﬁc way in which the organism was fossilized. The figure below looks at examples of these different categories of fossils.

Not all living organisms become fossils when they die. In fact, only a small percentage do, since it takes special circumstances to produce a fossil and allow it to survive.

Many fossil organisms resemble modern organisms, so it is relatively easy to ﬁgure out how to classify them taxonomically. For example, a fossil clam (class Bivalvia) looks like a clam and doesn’t look like, say, a snail (class Gastropoda). At the taxonomic levels below class, identiﬁcation may involve recognizing such details as the number of ridges on the surface of the shell. Not all fossils, however, resemble living organisms, and ﬁguring out their taxonomic relation to other organisms may be a challenge.

The figure below shows examples of some of the major types of invertebrate fossils. With this ﬁgure, you should be able to identify many of the fossils you’ll ﬁnd in a bed of sedimentary rock.

Using Fossil Succession to Determine Relative Age

Throughout the 18th century, scientists learned to recognize distinct layers of sedimentary rock based on their fossil assemblage, i.e., the group of fossil species that each layer contained. They found that a particular assemblage could only be found in a limited sequence of strata and not above or below. In other words, once a fossil species disappears in a sequence of strata, they never reappear higher in the sequence. Extinction is forever. The principle of fossil succession states that the species of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age.

Fossils occur for a distinct, limited interval of time. In the figure below, that distinct age range for each fossil species is indicated by the grey arrows underlying the picture of each fossil. The position of the lower arrowhead indicates the first occurrence of the fossil and the upper arrowhead indicates its last occurrence – when it went extinct. Using the overlapping age ranges of multiple fossils, it is possible to determine the relative age of the fossil species (i.e., the relative interval of time during which that fossil species occurred).

For example, there is a specific interval of time, indicated by the red box, during which both the blue ammonite and orange ammonite co-existed. If both the blue and orange ammonites are found together, the rock must have been deposited during the time interval indicated by the red box, which represents the time during which both fossil species co-existed. In this figure, the unknown fossil, a red sponge, occurs with five other fossils in fossil assemblage B. Fossil assemblage B includes the index fossils the orange ammonite and the blue ammonite, meaning that assemblage B must have been deposited during the interval of time indicated by the red box. Because, the unknown fossil, the red sponge, was found with the fossils in fossil assemblage B it also must have existed during the interval of time indicated by the red box. This is the principle of fossil succession in action!

Fossil species that are used to distinguish one layer from another are called index fossils. Index fossils occur for a limited interval of time. Usually index fossils are fossil organisms that are common, easily identified, and found across a large area. If the same index fossil is found in different areas, the strata in each area were likely deposited at the same time. Thus, the principle of fossil succession makes it possible to determine the relative age of unknown fossils and correlate fossil sites across large discontinuous areas (figure below).

Unconformities: Gaps in the Geologic Record

To ﬁnd good exposures of rock, James Hutton sometimes boated along the coast of Scotland, where waves of the stormy North Sea have stripped away soil and shrubbery. He was particularly puzzled by an outcrop at Siccar Point, where two distinct sequences of sedimentary rock lie in contact (figure below). In the lower portion of the outcrop, beds of gray sandstone and shale dip nearly vertically, whereas in the upper portion, beds of red sandstone and conglomerate display a dip of less than 20 degree.

Further, the gently dipping layers on the top seem to lie across the truncated ends of the vertical layers, like a handkerchief lying across a row of books. We can imagine that as Hutton was examining this odd geometric relationship, the tide came in and deposited a new layer of sand on top of the rocky shore. With the principle of uniformitarianism in mind, Hutton suddenly realized the signiﬁcance of what he saw. The gray sandstone-shale sequence had been deposited, turned into rock, tilted, and truncated by erosion before the red sandstone–conglomerate beds had been deposited.

Hutton deduced that the truncated surface between the gray and red rock sequences represented a time interval during which new strata had not been deposited at Siccar Point and the older strata had been eroded away. We now call such a surface an unconformity, representing a period of non-deposition and possibly erosion. The gap in the geologic record that an unconformity represents is called a hiatus, especially if erosion took place and depositional layers were removed entirely.

Geologists recognize three kinds of unconformities:

Angular unconformity - this type of unconformity separates two sedimentary layers BUT the ones at the bottom have been tilted or folded before the top layers have been deposited (just like the Siccar Point example). Because of this, usually, the unconformity cuts across the tilted layers and the orientation of the layers below the unconformity differs from the layers above it. In the example below, the angular unconformity separates HORIZONTAL layers above the unconformity from TILTED layers below the unconformity.

Video: "Angular Unconformity" (1:15)

Nonconformity - this type of unconformity separates young sedimentary rocks (on top) from much older intrusive (plutonic) igneous rocks or metamorphic rocks. In the example below, the layers of sedimentary rock (above the unconformity) were deposited on top of the much older igneous rock (granite).

Video: "Nonconformity" (1:18)

Disconformity - this type of unconformity ALSO separates two sedimentary layers but in this case, the layers ABOVE the unconformity are PARALLEL to the layers BELOW the unconformity. Imagine that a sequence of sedimentary beds has been deposited beneath a shallow sea. Then sea level drops, exposing the beds for some time. During this time, no new sediment accumulates, and some of the pre-existing sediment gets eroded away. Later, sea level rises, and a new sequence of sediment accumulates over the old. The boundary between the two sequences is a disconformity. Even though the beds above and below the disconformity are parallel, the contact between them represents an interruption in deposition. In the example below, BOTH the layers above the unconformity AND the layers below the unconformity are horizontal, they are PARALLEL to each other.

Video: "Disconformity" (0:54)

The succession of strata at a particular location provides a record of Earth history there. But because of unconformities, the record preserved in the rock layers is incomplete. It’s as if geologic history is being chronicled by a tape recorder that turns on only intermittently when it’s on (times of deposition), the rock record accumulates, but when it’s off (times of non-deposition and possibly erosion), an unconformity develops. Because of unconformities, no single location on Earth contains a complete record of Earth history.

Video: "Relative Dating of Rock Layers" (26:37)