Illustrate the immense scale of geologic timeSummarize the geologic time scaleExplain the principles of relative dating and unconformitiesApply principles of relative and absolute dating to determine the ages of rocks
Earth is 4.543 billion years old. That’s 4,543,000,000 years, an amount of time so immense that it’s challenging to grasp just how long it is. To put this into perspective, if the average human lifespan is 80 years, the Earth has been around for 57,000,000 lifetimes. Or if you have a penny for every year the Earth has been around, you would have $45.4 million! Constantly writing out millions and billions of years is time-consuming, so when geologists talk about ages, they use a few abbreviations. The symbols ka (thousands), Ma (millions), and Ga (billions) refer to points in time like a date. For example, the dinosaur extinction occurred at 66 Ma. Geologists also use other abbreviations for lengths of time, including ky, kya, kyr, and k.y. for thousands of years; my, mya, myr, and m.y. for millions of years; and by, bya, byr, and b.y. for billions of years. All four varieties of abbreviations mean the same thing in this case. Here, you would say the dinosaurs have been extinct for 66 myr. If this sounds confusing, you’re not alone because even some geologists use all of the abbreviations interchangeably. There is a debate amongst geologists, and other sciences, over the notation used for geologic time.
Fun fact: The Tyrannosaurus rex was one of the last dinosaurs to evolve about 70.6 Ma (a specific date). The first dinosaurs evolved about 230 Ma (a specific date), 159 myr (a length of time) before T-rex evolved. We are closer in time to the T-rex than the T-rex is to its earliest dinosaur ancestor! See the difference in abbreviations yet?
3.2 Geologic Time
Since 4.54 byr is a large chunk of time, geologists have divided it into more manageable chunks by creating a time scale. The commonly accepted time scale comes from the International Commission on Stratigraphy (Figure 3.1). It is continually revised as new research fine-tunes numbers between time scale divisions. The one in Figure 3.1 is the most up-to-date at the time of this writing and will be referenced throughout this manual. The divisions on the time scale are often based on significant events that have taken place tectonically, biologically, or climatically, and the numerical ages are derived from radiometric dating of rocks, minerals, and fossils.
Geologic time is first divided into eons; these are the Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons are often referred to as the Precambrian, which we’ll call a “super” eon. The eons are subdivided into eras, and eras are subdivided into periods, and periods into epochs, and epochs into ages. For the purposes of this lab manual, we will refer to this nomenclature. You’ll notice there is a second way to refer to these divisions on the time scale. The difference between the two is discussed here, but it is not important for our purposes. Throughout this lab manual, you will mainly see us referencing periods of geologic time.
Figure 3.1 – The geologic time scale subdivided by eon, era, system, series, epoch, and stage. What do you observe about each period of time? It is simple; they are color-coded. We will use these standard colors throughout this lab manual. The golden spikes at age boundaries indicate a specific place used to define the age boundary. Image credit: International Commission on Stratigraphy.
Many depictions of the geologic time scale don’t show the divisions of geologic time on the same scale. Look at the time scale in Figure 3.1, for example. The far-right column goes from 4.6 Ga to 541 Ma; that’s about 4 billion years of history in one small column! The other three columns make up the remaining 500 myrs. The reason for this is that geologists know much more about the last 500 myrs of Earth’s history than the first 4 byrs. So, let’s make a geologic time scale where all geologic time is shown at the same scale.
Using a 2.5 m long roll of paper, create your own geologic time scale using the following scale: 1 cm = 20 million years. For the purpose of this exercise, round Earth’s age to 4.6 Ga and use a tick mark spacing of every 100 myrs.Label the Precambrian and its associated eons.Label the Phanerozoic eon and its associated eras and periods.For the Cenozoic era, label the epochs.Using a cell phone or laptop, look up when these events actually occurred and label them on the other side of your time scale. Label physical events in one color and biological events in a different color. Some of the events will have a range of ages.How close were your guesses?
|Formation of Earth’s Moon||Great oxidation event|
|Dinosaur extinction||Formation of the Himalayan Mountains|
|First fossil evidence of life||First homo sapiens|
|First fish||First mammals|
|First reptiles||First amphibians|
|First major glaciation, called Snowball Earth||End of the last ice age|
|Breakup of Pangea||Formation of Rocky Mountains|
|Earliest trace of life (bacteria)||Oldest oceanic crust|
Come up with a mnemonic device to help you remember the periods of geologic time in the Phanerozoic Eon. For example, did you know that the word “scuba,” as in scuba divers, is actually a mnemonic device that stands for Self-Contained Underwater Breathing Apparatus?
Geologists use abbreviations to refer to the different portions of geologic time (Table 3.2), particularly on geologic maps. A few of these abbreviations may seem puzzling because the abbreviation isn’t always the first letter of the name. Carboniferous got the “C”, so Cambrian got a C with a slash through it “Ꞓ“. This left Cretaceous with a “K.” Tertiary had originally received the “T”, which left Triassic to be symbolized with a T that had an R subscript. In 2003, “Tertiary” was stricken from the geologic timescale and replaced by the Paleogene and Neogene. You will still see “T” on older maps for Tertiary, and some geologists still use the term today. So, be careful! There were already some M’s and P’s; so, Proterozoic eras were assigned as X, Y, and Z.
Geologists use two methods for dating events in Earth’s history. The first is called relative dating, meaning how events relate to each other in time, or more plainly, they figure out the sequence of events (what came first, second, third, etc.). Relative dating has no regard for numerical ages. The second method is absolute dating, where geologists use radioactive isotopes to figure out the numerical age of a rock or mineral.
3.3 Relative Dating and Correlation
There are several principles geologists use for relative dating. The first four principles were developed in the 17th century by an early geologist named Nicolas Steno, three of which pertain to sedimentary rocks. The first is the law of superposition, which states that in layers of horizontal sedimentary rocks, the oldest rock layer is at the bottom, and the youngest is at the top (Figure 3.2). The second rule is the principle of original horizontality, which says that layers of sediment are originally deposited horizontally (Figure 3.2). So, any tilting or folding of the rock occurred after it was deposited (Figure 3.3). The third principle states that layers of sedimentary rock are continuous, and anything that interrupts the layer (like a river or canyon) happened after the rock formed. This is called the principle of lateral continuity (Figures 3.4 and 3.5).
Figure 3.2 – Horizontal layers of red sedimentary rocks from the Moenkopi and Chinle Formations from the Triassic period in the Virgin River Valley near Zion National Park. Layers of sedimentary rock are originally deposited horizontally, with the oldest layer at the bottom. This shows both the principles of superposition and original horizontality. Image credit: James St. John, CC-BY.
Figure 3.3 – These are tilted layers of the Moenkopi Formation from the Lower Triassic, just south of Split Mountain, Dinosaur National Monument in Utah. They were originally deposited horizontally and were titled at some point in geologic time. Image credit: James St. John, CC BY.
Figure 3.4 – A depiction of the principle of lateral continuity. The layers of rock on one side of this valley were once connected to the same rocks on the other side. The valley was carved after these rocks were deposited. The dashed red lines show how the units were connected. Image credit: Wikimedia user Wouldloper, Public Domain.
Figure 3.5 – A panoramic view of meanders in the San Juan River cutting through layers of sedimentary rock in Goosenecks State Park, southwestern Utah. If you look carefully, you can trace layers of sedimentary rock that have been cut by the river across the entire image; an example of lateral continuity. Image Credit: Gernot Keller, CC BY.
Steno’s fourth principle is cross-cutting relationships. This principle is used when other geologic events cut through sedimentary rocks, like an igneous dike or a fault. This principle basically states that when a geologic event cuts across another, the event doing the cutting is younger than the one being cut (Figure 3.6). For example, if sedimentary rocks are cut by an igneous dike, the igneous dike is younger than the sedimentary rocks it’s cutting through. The same can be said of a fault that cuts through any rock; the fault has to be younger because the rocks had to exist first to be faulted.
Figure 3.6 – A mafic dike (dark rock) cutting through granitic pegmatite (light rock) in Ruggles Mine, New Hampshire. To determine which is younger, look to see which unit cross-cuts the other. Since the mafic dike is cutting through the pegmatite, the dike is younger by the principle of cross-cutting relationships. Image credit: James St. John, CC BY.
Some 200 years later, the fifth principle of relative dating was developed by Charles Lyell called the principle of inclusions. This principle explained that a clast, or a different-looking rock that is contained inside of another rock, is older than the rock that contains it (Figure 3.7). How can this happen? Originally, a mafic magma was cooling quickly, producing the finer-grained mafic rock in the middle of Figure 3.7. Then, something happened to change the chemistry of the magma to felsic and slowed the cooling rate to produce the surrounding, coarse-grained granite. The mafic rock formed first, and then the felsic rock formed around it.
Figure 3.7 – A mafic inclusion in granitic rock. Inclusions are older than the surrounding rocks. A penny shows the scale of the image. Image credit: Marli Miller, CC BY.
Simply speaking, an unconformity is a pattern that you look for in a group of rocks that tells you erosion has taken place. Rocks exposed on the Earth’s surface are affected by physical and chemical weathering processes that work to break them into smaller pieces or dissolve them in water. This material is then transported away by wind, water, or ice, a process known as erosion. Many people use weathering and erosion interchangeably, but they do mean different things: weathering is the breakdown of rocks, while erosion removes the broken down material.
There are four types of unconformities, and each forms in a slightly different way (Figure 3.8). They all involve sedimentary rocks, changes in sea level, and/or uplift from an orogeny. Each unconformity tells a unique story of the geologic history of the area they’ve been found.
Figure 3.8 – Types of unconformities. (a) Disconformity; (b) Nonconformity; (c) Angular unconformity; (d) Paraconformity. Image credit: Wikimedia user דקי, CC BY-SA.
A disconformity (Figure 3.8a) is an erosional surface where the rocks below the unconformity are much older than the rocks above. This type of unconformity typically forms when horizontal layers of sedimentary rock are deposited in a shallow marine environment; then sea level lowers to expose these rocks and allows erosion to occur; then sea level rises again, and new horizontal layers of sedimentary rock are deposited. Erosion removed some of the original rock, creating a large age gap between the rocks above and below the erosional surface. This age gap is the disconformity and is located at the contact point between the older rock and younger rock. Oftentimes the erosion process leaves behind evidence of river channels or soil development, which provide clues to geologists to locate the unconformity in what looks like a continuous succession of sedimentary layers.
A nonconformity (Figure 3.8b) forms when igneous or metamorphic bedrock is eroded, and then horizontal layers of sedimentary rock are deposited directly on top of it. The unconformity is where the bedrock meets the sedimentary rock. For example, when a mountain belt is eroded below sea level, and afterward sediments are deposited on top of the igneous or metamorphic rock, the contact is a nonconformity.
An angular unconformity (Figure 3.8c) is created when horizontal layers of sedimentary rock lie on top of tilted layers of sedimentary rock. The most famous angular unconformity is from Siccar Point in Scotland. Figure 3.9 shows the process of creating an angular unconformity. For this to occur, sedimentary rocks deposited in the marine environment are lifted above sea level by an orogeny or similar event. The orogeny causes the sedimentary rocks to become tilted or folded. Since these rocks are exposed above sea level, erosion takes place. The rocks can either be eroded below sea level, or sea level can rise, which would allow new, horizontal layers of sedimentary rock to be deposited on top of the titled ones. This creates an angle between the younger, horizontal layers on top and the older, tilted layers below.
Figure 3.9 – A depiction of how an angular unconformity forms. 1) Deposition of sedimentary rocks; 2) uplift and folding of the sedimentary layers; 3) erosion; 4) resumed deposition on top of the folded layers. Image credit: Utah Geological Survey, Public Domain.
A paraconformity (Figure 3.8d) is very similar to a disconformity, except the evidence for erosion is not present. Either no evidence of the erosion was left behind, or erosion didn’t happen, and instead, there was only a pause in sediment deposition. We know these occur because sediment above and below the paraconformity have been radiometrically dated and reveal a large gap in time.