The goals of this chapter are to:
- Illustrate the immense scale of geologic time
- Summarize the geologic timescale
- Explain the principles of relative dating and unconformities
- Apply principles of relative and absolute dating to determine ages of rocks
Earth is 4.56 billion years old. That’s 4,560,000,000 years. That amount of time is so immense that it’s truly difficult to grasp just how long it is. To try and put this into perspective, if the average human life-span is 80 years, then 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 $45.6 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; these include ky, kya, kyr, and k.y. for thousands of years, my, mya, myr, and m.y. millions of years, and by, bya, byr, and b.y. 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 also debate amongst geologists, and other sciences for that matter, over the notation used for geologic time.
Fun fact: The Tyrannosaurus rex was one of the last dinosaurs to evolve, at around 70.6 Ma. The first dinosaurs evolved about 230 Ma, 159 myr 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 4.56 byr is a large chunk of time, geologists have divided it into more manageable chunks by creating a timescale. The commonly accepted timescale comes from the International Commission on Stratigraphy (Figure 3.1). It is continually revised as new research works to tweak numbers between timescale divisions. The one in Figure 3.1 is the most up-to-date as the time of this writing and is what will be referenced throughout this manual. The divisions on the timescale 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.
Many depictions of the geologic timescale don’t show the decisions of geologic time on the same scale. Look at the timescale 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 timescale where all of geologic time is shown at the same scale.
- Using a 2.5 m long roll of paper, create your own geologic timescale 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.
- Table 3.1 is a list of some major events in Earth’s history. On one side of your timescale label when you think these events occurred using a color of your choice.
- Using a cell phone or laptop, look up when these events actually occurred and label them on the other side of your timescale. 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?
Table 3.1 – A list of major events in Earth’s history 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 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
There are a number of principles geologists use for relative dating. The first four 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 a vertical succession of flat 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).
Steno’s 4th principle is the principle of cross-cutting relationships. This one can also involve sedimentary rocks and includes other rocks and geologic events, like an igneous dike or a fault. This principle basically states that when you have one geologic event cutting across another, the event doing the cutting is younger than the one being cut (Figure 3.6). For example, in layers of sedimentary rock 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 in order to be faulted.
Some 200 years later, the 5th principle of relative dating was developed by Charles Lyell, called the principle of inclusions. This principle explained that a clast or 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? One way is in a magma chamber that begins cooling deep within the earth. Its composition is felsic and it’s cooling slowly, so the igneous rock granite is starting to form. If that magma is suddenly forced upward toward the surface, its cooling rate changes, and its chemistry can potentially change if it melts any surrounding rocks along the way. So this granite, which already formed, is brought upward where a new rock starts to solidify around it. That granite will become “included” in the new rock, and is, therefore, older than the rock that surrounds it.
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 Earth’s surface are affected by physical and chemical weathering processes that work to break them down 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, and erosion is the removal of 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 and changes in sea level and/or uplift from an orogeny and each one tells a unique story of the geologic history of the area they’ve been found.
A disconformity (Figure 3.8a) is an erosional surface where the rocks below the unconformity are much older than the rocks above it. 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 take place, and then sea level rises again and new horizontal layers are deposited. The erosion removed some of the original rock, which creates a large age gap between the rocks above and below the erosional surface. This age gap is the unconformity 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 clue geologists into where the unconformity is located in what looks like a continuous succession of sedimentary layers.
A nonconformity (Figure 3.8b) forms when igneous or metamorphic bedrock is eroded and flat, horizontal layers of sedimentary rock are deposited directly on top of it. The unconformity is where the bedrock meets the sedimentary rock. One way a nonconformity forms is when a mountain belt is eroded below sea level and then sediments are deposited on top of the igneous or metamorphic rock.
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 form, 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, flat layers on top and the older, tilted layers below.
A paraconformity (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 just a pause in sediment deposition.
The images in Table 3.2 are different types of unconformities (Figures 3.10-3.12). In the blank spaces provided next to the images, create a sketch for each unconformity, label where you think the unconformity is located, and identify what type of unconformity it is.
Not put all of the principles you’ve learned to work. Below are relative dating outcrop diagrams that represent sections of rock. Each letter represents the deposition of a different layer of sedimentary rock or geologic event.
- Use the principles of relative dating and unconformities to determine the sequence of events for each diagram in Table 3.3 (Figures 3.13-3.16).
- Identify the type of each unconformity.
The symbols used for the rocks are standard USGS symbols for common rock types. Only conglomerate, limestone, sandstone, shale, granite, and gneiss are used. The subscript letters stand for igneous dikes (D), faults (F), and unconformities (U). The colors for each unit are from the geologic time scale shown in Figure 3.1. Hint: it is easier to start with the oldest event and work your way forward through time.
Using the web program Visible Geology, you create a block model involving a complicated geologic history. An example of a diagram made with this website is in Figure 3.17. Answer the following questions:
- Place the following in order from oldest to youngest: Fault, Dike, Folding. ______________________________
- If the dike is 100 Ma, what is the age of the normal fault? ____________________
Create your own block model using the web program Visible Geology.
- Your model should include 2 to 6 sedimentary layers. You may want to vary their thickness. Each rock type is assigned a unique color. You can also include an unconformity (angular, disconformity, or nonconformity).
- You can include tilting, folds, thrusts, strike-slip faults, and normal faults.
- You may want to include some igneous rocks as sills or dikes. If you include a dike or sill, you may want to assign it a specific age such as 100 Ma. This numerical age can be used to give specific dates to events in this geologic history.
- Include erosion at some point in your model
- When you are done with this, rotate the cube and decide which side has the easiest and which side has the most difficult history to assess. Or perhaps you made a model that is easy on all sides? Explain why there is this difference.
- Create a geologic cross-section through your model from corner to corner. Draw a sketch of one of these cross-sections.
Cross-Section Sketch #1:
- Sometimes geologists only have cliff faces or road cut to investigate. Do you think you will get the same geologic history with this cross-section as one of the sides?
- Now draw a quick sketch of one of the sides. Show it to one of your classmates and see if they can interpret its geologic history. If you included an igneous feature, assign in an age. If your colleague can’t get the history, give them some clues to help.
Cross-Section Sketch #2:
One of the ways geologists investigate Earth’s history is by imaging what is below the surface of the earth, called a seismic survey. This is done by sending sound waves into the ground or ocean. As these sound waves move through different layers of rock or sediment, some of the wave is reflected back toward the surface and recorder by . Different types of sediment or rock change the characteristics of the wave, such as its velocity. Geologists set up geophones that record these reflected waves and they provide an “image” of what the subsurface looks like. The results are recorded seismic amplitude; this is a measure of the difference in rock properties between two layers. Seismic data is commonly converted to impedance (= hardness). So, the relative hardness, can be positive, negative or the same. Thus, many seismic sections will use three colors to better distinguish different strata.
The continental slope off of the north island of New Zealand is called the Hikurangi margin. Geoscientists wanted to explore this area to better understand the evolution of this accretionary plate boundary. The International Ocean Discovery Program (IODP) did a seismic survey (Figure 3.18) to assess the possibility of active thrust faults in the area they had proposed to put a drill core at Site U1519.
- What type of unconformity is the yellow line? ____________________
- What type of unconformity is the pink line? ____________________
- Do you think it would be safe to drill here? Explain your answer.
The basement of the Gulf of Alaska is riddled with seamounts as the Pacific plate traveled over numerous hot spots. These are a variety of ages and sizes and you can see in Figure 3.19. These are partially covered by a thick blanket of sediments up to 1.5 km thick.
- Identify two areas where faulting has taken place by outlining them in Figure 3.19.
- Why isn’t the contact between the crystalline basement and the sedimentary layers above an unconformity?
Sedimentary Rock Correlation
The principles of relative dating allow geologists to compare seemingly similar groups of rocks that are separated by some distance. On a small scale it’s as simple as saying that rocks on either side of a canyon are in fact the same and were once connected before the canyon formed, like Figure 3.5. On a larger scale of kilometers to hundreds of kilometers, it can be comparing a similar succession of sedimentary rocks to each other.
Geologists can correlate sedimentary rock units over great distances by matching patterns in the outcrops. Figure 3.20 below contains two outcrop drawings of sedimentary rock outcrops that are separated by 100 km.
- Draw lines that correlate the rocks units across these two areas. You can color the units to help you.
- One of these areas contains an unconformity. Mark the location of the unconformity in Figure 3.20.
- What type of unconformity is it? _________________________
- Are any other sedimentary layers missing from one of the outcrops? If so, explain why.
Fossils present in sedimentary rocks can also be used for correlation. This usually involves a fossil assemblage, which is just the group of fossils that are found in a rock layer. By comparing fossil assemblages from one rock outcrop to another, geologists can determine how the outcrops relate to each other in age. This can tell geologists that layers of rock may have been deposited at the same time or can be used to identify unconformities.
Certain fossils can be indicative of a narrow span of geologic time, called index fossils. These types of fossils are easily recognizable, abundant, existed for a short period of time, and have a wide geographic distribution. When you see an index fossil in a rock, it immediately gives you an idea of how old the rock is. For example, Belemnites (Figure 3.21), a type of cephalopod (squid), only existed during the Mesozoic era. So if you were to see a belemnite fossil in a rock, you know that rock is from the Mesozoic (between 252 and 66 Ma). We’ll have lots more on fossils in later chapters.
Fossils are very useful for geologic dating and correlation. The type of sediment deposited at a specific time in a region can vary. Sand particles (sandstone) can be deposited in one area while clay particles (shale) can be deposited in another. This means that two different sedimentary rocks will have the same age. In this case, correlating the rock units is more difficult because you can’t tell they are the same age. This is where fossils can help because they can tell you the sedimentary rocks were deposited at the same time. If both rock layers were deposited at the same time, then they should contain the same fossils.
- Correlate the sedimentary layers in Figure 3.22 based on the fossils they contain.
- Label where any unconformities could be interpreted.
- You can also use the assemblage of fossils in rocks to correlate sedimentary layers and determine age. Correlate the rock layers in Figure 3.23 based on the groups of fossils that are found.
- Label where any unconformities could be interpreted.
- Suppose the fossils have age ranges as shown in Figure 3.24. Label the geologic periods for each layer of sedimentary rock in both columns of Figure 3.23.
- Which of the fossils is the least useful for dating? __________________________
- Which of the fossils is the most useful for dating? __________________________
*Note the fossil age ranges in this exercise are fictional and used for educational purposes only.
3.4 Radiometric Dating
Geologists can determine numerical ages for the formation of rocks, minerals, and some fossils using radioactive isotope systems. Does radioactivity sound scary? It can be, but what we’re talking about is happening on such a small scale that you don’t have to worry. The next few paragraphs are a bit technical, but we will try to break it down for you.
An isotope is an atom of an element that has a different number of neutrons than protons in its nucleus. For example, a typical carbon atom on the periodic table of elements should have 6 protons and 6 neutrons in its nucleus, let’s call it carbon-12. Some atoms of carbon can have an extra neutron in the nucleus, called carbon-13. And some can have 2 extra neutrons in the nucleus, called carbon-14. If an atom has too many neutrons in its nucleus it becomes unstable because the nucleus is becoming too big; this is called a radioactive isotope. One analogy you can use to think about is air in a balloon. The balloon can hold different amounts of air, but if you add too much air it will pop. Atoms can hold different amounts of neutrons, but if you add too many they will pop; in this case “pop” means it becomes radioactive. Radioactive isotopes decay to a more stable form by way of alpha, beta, and/or gamma decay. We don’t need the particulars of these decay processes, but in short, radioactive atoms gain or lose protons, neutrons, and/or electrons to become stable. Carbon-12 and carbon-13 are stable atoms of carbon, but carbon-14 has one too many neutrons and is radioactive. Carbon-14 radioactively decays to nitrogen-14, a stable product.
|Parent Isotope||Stable Daughter Product||Half-life|
|Uranium-238||Lead-206||4.5 billion years|
|Uranium-235||Lead-207||704 million years|
|Thorium-232||Lead-208||14.0 billion years|
|Rubidium-87||Strontium-86||48.8 billion years|
|Potassium-40||Argon-40||1.25 billion years|
|Samarium-147||Neodymium-143||106 billion years|
Radioactive atoms don’t immediately decay to a stable form, instead, they spontaneously decay at a rate that we describe with the term half-life. Half-life is the time it takes for half of the unstable atoms (parents) to decay to stable forms (daughters). The length of time for a half-life is different for each isotope system. For example, the half-life for carbon-14 is 5,730 years, but the half-life for potassium-40 is 1.25 byrs. Table 3.4 shows some common isotope systems used for dating.
To calculate the age of geologic material, geologists analyze the chemistry of the material and figure out the ratio between parent and daughter atoms. The ratio tells geologists how many half-lives, or what fraction of a half-live, have passed. So, if you know the ratio between parent and daughter atoms, and you know the rate of decay for that isotope system, you can calculate how old geologic material is.
Geologists have to use a few assumptions when using radiometric dating. One assumption is that when the rock or mineral first formed, there were no daughter atoms present. This means all of the daughter atoms found in the rock have to come from radioactive decay. The second assumption is that the rock or mineral was a closed system. A closed system means there was no addition or removal of elements at any point, because that could change the parent-daughter ratio and affect the age you calculate. With that being said, geologists know that these assumptions may be incorrect for some of the material they work with, and they have methods that can help correct for that. But that is beyond our scope. That’s it for the technical stuff.
Radiometric dating tells you different things depending on which geologic material you are looking at. For minerals in igneous rocks, the radiometric age tells you when the mineral crystallized from magma. In metamorphic rocks, the radiometric age can give you the age metamorphism took place. For minerals in sedimentary rocks, the radiometric age only tells you when that mineral crystallized or metamorphosed in its original rock; it doesn’t tell you when the sedimentary rock formed. The only way to determine the age of sedimentary rocks is to combine the principles of relative dating with radiometric dating of igneous or metamorphic events that cross-cut the sedimentary rocks, such as a dike.
To determine the age of a mineral, geologists need to know a few things, including how long a half-life is for the isotope system being used and the ratio of parent to daughter isotopes is in the mineral, which can be measured using scientific instruments like a mass spectrometer.
- t = time (age of the material)
- ln = natural log (function on your calculator)
- P = parent ratio (this is measured)
- D = daughter ratio (this is measured)
- λ = lambda is the decay constant (different for each isotope system, a function of the half-life)
Let’s get some practice by calculating the ages below:
- Complete the following calculations to determine the ages of minerals in Table 3.5 (Hint: most cell phones have additional calculator functions when you turn it horizontal. There are also free scientific calculator apps in the Apple and Google stores.)
Table 3.5 – Worksheet for Exercise 3.7 Isotope System Decay Constant Parent:Daughter Age Carbon-14 1.21×10-4 1 : 1.3 Potassium-40 5.34×10-10 1 : 0.25 Uranium-235 9.72×10-10 1 : 2.34
- Look back to Figure 3.16. Let’s say a geologist collected a sample of G (gneiss) and MD (dike) to radiometrically date, with the ultimate goal of trying to constrain ages in this region. She used the potassium-40 isotope system to date metamorphism of hornblende grains and measured a parent-daughter ratio of 1:0.89. For the igneous dike, she used the uranium-235 isotope system on zircon grains and measured a parent-daughter ratio of 1:0.28.
- How old is the gneiss? ____________________
- How old is the igneous dike? ____________________
- What is age range of the fault PF? ____________________
- What is the age range of sedimentary layers D and F? ____________________
3.5 Other Uses for Geologic Dating
Dating in geology has many uses other than determining when a rock or mineral formed. One common use for dating is determining the source rock for sediment, called provenance. Interpreting sediment provenance can tell a geologist a lot about the history of an area, including where the sediment came from, where erosion was taking place, and how sediment was transported. Dating of certain sediment grains can also tell geologists when rocks were uplifting during an orogeny.
Even though radiometric dating on mineral grains in sedimentary rocks doesn’t give you the age the sedimentary rock formed, it still tells you very useful information about the geologic history of the area. One such use is determining the sediment provenance, which is the source rock of the sediment. Remember that the age you determine from sediment grains is the age they formed in their original source rock.
Let’s say you collect some biotite grains from the bottom of the lake in Figure 3.25 and you want to figure out where they came from. There are two potential sources for those grains, a set of metamorphic rocks to the west and a set of granites to the south. Little is known about the geology in this area though, so you need to figure out the ages of the potential source rocks and determine where the lake sediment came from.
- You collect 5 biotite grains for Source 1, 5 biotite grains from Source 2, and measure them using the potassium-40 system. What are the ages of these source-rock biotite grains? Fill this out in Table 3.7
Table 3.6 – Worksheet for Exercise 3.8a Samples from Source 1 Parent:Daughter Age 1a 1 : 0.808 1b 2 : 1.706 1c 1 : 0.825 1d 3 : 2.349 1e 2 : 1.596 Samples from Source 2 Parent:Daughter Age 2a 2 : 0.626 2b 4 : 1.236 2c 1 : 0.323 2d 3 : 0.903 2e 2 : 0.596
- What is the average age of the rocks at Source 1? _______________
- What is the average age of the rocks at Source 2? _______________
- The lake sediment grains you collected have the following ages. Determine which source area they most likely came from by filling out Table 3.7.
Table 3.7 – Worksheet for Exercise 3.8d Lake Sediment Age Source Area 1 523 Ma 2 1,157 Ma 3 517 Ma 4 521 Ma 5 486 Ma 6 1,116 Ma 7 513 Ma 8 501 Ma 9 523 Ma 10 494 Ma
- What percentage of the lake sediment is coming from Source 1? _______________
- What percentage of the lake sediment is coming from Source 2? _______________
- Critical Thinking: What could be the reason(s) why you see this sediment distribution in the lake?
Cross-cutting relationships is a universal principle. That means it not only applies to our planet, but applies to other planets. Let’s take a look at some pictures taken from the surface of Mars; these show features such as craters, rivers, and fractures (a type of fault). These types of images are used to decipher the tectonic, volcanic and impact history of our solar system. For example, lunar craters are surrounded by dust. So, planetary geologists propose that all impacts affected solid rock. Many martian craters are surrounded by fluidized (mud-like) ejecta which some propose is evidence that impacts occurred into icy terrain that melted on impact.
- Look at Figure 3.26.
- Which is older, Crater B or Crater S? ____________________
- Explain why.
- Which is older, Fracture F or Crater B? ____________________
- Which is older, Fracture F or Crater S? ____________________
- Put the craters in relative order from oldest to youngest. ______________________________
- Look at Figure 3.28.
- Which is older, Crater C1 or River R1? ____________________
- Which is older, Crater C2 or River R2? ____________________
Daniel Hauptvogel, Virginia Sisson, Carlos Andrade
a device that converts ground movement (velocity) into voltage, which is recorded as a seismic response and is analyzed for structure of the earth.