Geologic Time in the National Parks

Absolute Age

OBJECTIVES

• Understand Radioactive Parent and Stable Daughter ratios as they relate to decay rates
• Be able to calculate radiometric age dating ratios and the age of a rock unit
• Combine relative and absolute age dating methods to determine age ranges
• Identify geologic time principles in several National Parks
• PARKS: Yellowstone NP, Wyoming, Pikes National Forest, Colorado, and Petrified Forest NP, Arizona

MATERIALS

• Ruler, Calculator and a Pencil

INTRODUCTION

Rocks, glaciers, trees and fossils have long and often complex stories. Geologists have numerous methods for determining the age of many of Earths unique features. Time telling methods are often divided into Relative age dating methods which allow us to determine the age of a feature relative to another feature, for example, a fault must be younger than the rocks that is breaks, and older layers of sedimentary rocks are found below younger layers. Another method for dating rocks is called Absolute age dating. This method is quantitative and allows scientists to add numerical ages to rocks. Most of these methods rely on the radioactive decay process of certain isotopes (the same element but with a different number of neutrons in the nucleus) found in rocks. Since radioactive isotopes decay (become stable atoms) in a predictable way we can use this process to add numerical ages to rock units. In this lab, we will review the relative age dating methods and explore the absolute age dating method. Several National Parks such as Yellowstone NP Wyoming, the Pike and San Isabel National Forests, Colorado and the Petrified Forest NP, Arizona will serve as examples of this process.

FIRST: REVIEW OF RELATIVE AGE DATING PRINCIPLES

Within the list of relative age dating principles below, discuss the principle and make a drawing to represent this method. Label the oldest and youngest layers.

Principle of Superposition

Principle of Lateral Continuity

Principle of Original Horizontality

Principle of Cross-cutting

Principle of Intrusion

Principle of Inclusions

Principle of Faunal Succession

Index Fossils

LET’S PRACTICE:

The following diagram has 8 components. Place them in order from oldest (bottom) to youngest (top). Then name the relative age dating principle that allows you to determine this sequence. Both B and D are Igneous rocks. The blue and brown swirly unit at the bottom is Metamorphic (list it as metamorphic rock - M), A and F are faults, C and E are sedimentary layers. The black swirly line is an unconformity list it as “UN” (name the specific type of unconformity). Figure 1.

Figure 1. A practice geologic sequence. Image CC BY SA 1.0 by Woudloper https://commons.wikimedia.org/w/index.php?curid=6526213

YOUNGEST _________

_________

_________

_________

_________

_________

_________

OLDEST _________

GEOLOGIC TIME SCALE

Geologists have used many methods to reconstruct geologic time in order to map the major events in Earth’s history as well as their duration. Scientists studying rocks were able to piece together a progression of rocks through time to construct the Geologic Time Scale. This time scale was constructed by lining up in order rocks that had particular features such as rock type (lithology), environmental indicators (paleostratigraphy, geomorphology), chemical signatures (isotope geochemistry), or fossil (paleontology). Scientists looked at clues within the rocks and determined the ages of these rocks in a comparative sense - Relative age dating. The Geologic Time scale developed over many centuries as scientists attempted to understand the complexities and the vastness of time seen in the rocks of the Earth (figure 2). Many scientists in the 1700’s attempted to create a geologic time scale that could be applied to rock sequences around the world. Rocks were initially divided into four categories: Primary, Secondary, Tertiary and Quaternary. Once the identification of unique fossils found in specific rock layers (strata) was understood geologists such as William Smith, George Cuvier, Charles Lyell and others were able to divide the earths layers more precisely.

Figure 2. The four major Eon divisions of geologic time. Note they are note equal in their duration but are divided based on significant events in the rock record. Image:

https://opengeology.org/historicalgeology/geologic-time/ Image by Jonathan R. Hendricks. From: https://www.digitalatlasofancientlife.org/learn/geological-time/geological-time-scale/. Creative Commons Attribution-ShareAlike 4.0 International License.

The current time scale is divided from large (Eons) into increasingly smaller units (Eras) and subunits (Periods). You will notice that the duration of time in each unit is not the same. Each of the divisions are based on unique features found in the rock units and divided by specific events such as mass extinctions. Recall that the time scale was developed long before numerical dates were determined. The primary division is an Eon: Hadean, Archean, Proterozoic and Phanerozoic. Eons are divided into Eras, then periods, epochs and ages. Current understanding of the age of the Earth places Earth’s age at 4.54 Billion years old (figure 3). During the 1800’s mostly British and European geologists began naming time units that reflected a dominant location and rock type. For example, the “Devonian” was named for the English county of Devon and the “Jurassic” was named by a French geologist for the marine limestone exposures in the Jura mountains. The first global geologic time scale was published in the mid 1800’s and standardized the Eras of Paleozoic (old life) and Mesozoic (middle life). It wasn’t until the early 1900’s when the discovery of radioactivity led to the use of radiometric age dating (Absolute age dating) that actual numerical ages could be applied to the existing Geologic time scale. The geological time scale is currently maintained by the International Commission on Stratigraphy (ICS), which is part of the International Union of Geological Sciences.

Figure 3. The Geologic Time Scale. Note: not to scale as the Precambrian, which represents over 4 billion years is compressed. Image public domain nps.gov. https://www.nps.gov/subjects/geology/time-scale.html

ACTIVITY USING THE GEOLOGIC TIME SCALE:

Notice that the scale is not proportioned, meaning that it is not to scale with equal space representing equal amounts of time. Figure 1 represents the proportions of the 3 divisions within the Precambrian.

TASKS:

1.You are tasked with determining the proportion of the three remaining eras relative to the age of the Earth. For example, the Precambrian occurs from 4,600 million years until 541 million years ago for a duration of 4,059 million years (4,600-541). This represents 88% of the entirety of the Earths time (4,059/4,600). Calculate the duration and proportion of time for the three ERAS: Paleozoic, Mesozoic and Cenozoic.

Paleozoic Duration

Paleozoic Percentage

Mesozoic Duration

Mesozoic Percentage

Cenozoic Duration

Cenozoic Percentage

2. Then determine the space needed to draw them on the time scale below. Measure the length of the bar below. Mark the 88% representation of the Precambrian, and the three Eras.

4.6 bya Today

ABSOLUTE AGE DATING

Relative age dating is an essential tool for telling geologic time. While it cannot provide a numerical age, it remains an essential tool for providing the framework for adding numerical dates to rocks. Early geologists could only surmise that the earth was very old and ever changing. In 1896 French scientist Henri Becquerel discovered radioactivity. By the early 19^th century shortly after the discovery of radioactivity, scientist such as Ernest Rutherford, Paul Villard, Wilhelm Rontgen and Pierre and Marie Curie developed this new tool using radioactive isotopes to determine an absolute or numerical age of a sample. Radioactive decay is based on the natural elements in the periodic table (figure 4). Many of the elements in the periodic table occur in a variety of forms known as isotopes. Isotopes are elements that have the same number of protons but have different mass amounts. This means that they have more neutrons than the common version of the element. For example, the element Carbon is element number 6 which means that it has 6 protons in its nucleus. All Carbon atoms contain 6 protons. Most Carbon atoms have 6 neutrons with the 6 protons in their nucleus, and thus have a mass of 12. Some Carbon atoms have 7 or 8 neutrons with the 6 protons in their nucleus giving them a mass of 13 and 14 respectively. These specific varieties of Carbon are Isotopes. The Carbon atom with 8 neutrons is unstable, making it radioactive. Radioactive elements decay and break down until they are in a stable form. There are several ways for a radioactive atom to decay: Alpha decay, Beta decay, and Electron capture among others (figure 5). You may have learned about these in lecture.

Figure 4. The Periodic table, with elemental number. This tells the number of protons in the nucleus. Image: Public Domain from Wikimedia CCo 1.0 universal

Figure 5. The decay chain of radioactive parent isotope Uranium-238 to stable daughter product of Lead (Pb) 206. Notice then many stages between the isotopes. Image:http://opengeology.org/textbook/7-geologic-time/#721_Radioactive_Decay/ is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

A Half-life

All radioactive elements decay to stable products in much the same way. The radioactive decay of any individual atom is completely unpredictable and random at any specific moment, but over time and with large numbers of radioactive isotopes a very predictive pattern of decay called a half-life emerges. This is why we are able to use many radioactive elements to tell time. The decay process works like this: The half-life is the time it takes for half of the existing radioactive isotopes to decay into stable products. The length of a half-life remains constant for each specific isotope. It is the amount of radioactive material that decreases as the amount of stable product increases. For example, in figure 6, the amount of radioactive Potassium (K) 40 that is initially present is 100% (represented by the red to purple line), after one half-life half of the radioactive potassium has decayed (turned into the stable product). This results in 50% of the radioactive isotope existing with 50% of the stable products. During the second half-life, only half of the existing radioactive isotope can decay. Half of 50% equals 25% (figure 7). During each half-life you can only decay half of the remaining radioactive isotope until there is too little to measure.

Figure 6. Radioactive decay curve representing the half-life decay of radioactive parent isotope Potassium (K) 40 into stable daughter products. Image: S Earle, https://opentextbc.ca/geology/chapter/8-4-isotopic-dating-methods/

Figure 7. Blue diamonds represent radioactive isotopes in a magma (orange) and solid rock (grey). Once the rock solidifies the ratio is literally set in stone. As time passes the decay of the radioactive parent isotopes begins and isotopes transmute into stable (red square) atoms. Image by Jonathan R. Hendricks. Creative Commons Attribution-ShareAlike 4.0 International License. https://opengeology.org/historicalgeology/geologic-time/

Radioactive Decay Curve

The ratio of radioactive parent isotope to stable daughter product can be used to determine the number of half-lives that have passed. As the amount of radioactive isotope decreases by half the stable product will increase by that amount. For example, initially when there is 100% of the radioactive isotope there is 0% of the stable daughter product. After one half-life half of the radioactive isotope has been transformed into 50% stable daughter, therefore there is 50%/50% ratio (this is a 1:1 ratio). After a second half-life has passed there is only 25% of the original radioactive isotope and 75% stable product (this is a 1:3 ratio)(figure 8).

Figure 8. The percentages of the stable daughter product (red squares) produced by radioactive parent (blue diamonds) decay increases as the amount of radioactive isotope decreases. Image by Jonathan R. Hendricks. Creative Commons Attribution-ShareAlike 4.0 International License https://opengeology.org/historicalgeology/geologic-time/#Uniformitarianism

Since all radioactive isotopes decay in much the same manner, the only difference relates to the duration of each isotopes half-life. Once you have used the radioactive to stable product ratio to determine how many half-lives have passed simply multiply the number of half-lives by the duration of the half-life. See Table 1 for some common isotope half-life durations.

Table 1. Common Isotope systems and their half-lives.

NATIONAL PARKS WITH UNIQUE FEATURES THAT CAN BE AGE DATED WITH ABSOLUTE AGE DATING METHODS

Many of the unique features seen in Yellowstone NP, Wyoming are created by the Hot Spot Supervolcano beneath the surface, such as the world largest concentration of geysers and hydrothermal features (figure 9). Igneous, specifically volcanic rocks are a great choice for determining absolute age dates from rocks. Geologists take samples and trace the movement of the Hot Spot that is currently beneath Wyoming (figure 10). As the North American plate moves slowly over the volcano source, igneous rocks are produced at the surface (figure 11). Igneous rocks often contain radioactive isotopes such as Potassium-Argon and Uranium-Lead that enable age dating. This type of age dating is also a significant connection to plate tectonics and the movement of continental crusts.

Figure 9. There are more than 10,000 Geothermal Springs in Yellowstone NP, Wyoming. Image: author

Figure 10. A depiction of 16 million years (numbers within each igneous location in million years) of volcanic activity in the Yellowstone-Snake river volcanic system. The current hot spot is located beneath Yellowstone NP in Wyoming and is responsible for the hydrothermal geyser system. The largest concentration of geothermal hot springs in the world. Image: https://www.nps.gov/yell/learn/nature/volcano.htm

Figure 11. The Yellowstone River carved down more than 1,000 feet to create the Grand Canyon of the Yellowstone exposing the Yellow Rhyolitic igneous (volcanic) rock used for age dating and for which the park is named. Image author.

Igneous rocks from Pike National Forest (part of the USDA Forest Service) in the front range of Colorado west of Colorado Springs includes Pikes Peak (14,115 feet) contains the famous pink-orange Pikes Peak Granite. Igneous rocks are one of the best rocks for absolute age dating as they contain many of the necessary isotopes. The granite rocks have been age dated to 1.03 billion years (figure 12).

Figure 12. The famous pink-orange granites of Pikes National Forest, Colorado (bottom portion of image) are age dated to over 1 billion years. Image: (https://www.fs.usda.gov/main/psicc/maps-pubs. https://en.wikipedia.org/wiki/Pikes_Peak_granite#/media/File:Precambrian-Cambrian_nonconformity_(Sawatch_Sandstone_over_Pikes_Peak_Granite;_Ute_Trail,_Manitou_Springs,_Colorado,_USA)_3.jpg By James St. John - https://www.flickr.com/photos/47445767@N05/49270380632/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=85214138

The Petrified Forest NP Arizona contains sedimentary rocks and the famous fossilized wood from approximately 200 million years ago before the break-up of the supercontinent Pangaea. This unique combination of geologic conditions led to a large collection of mineralized wood as a forest was felled and rapidly buried by mud and sand. This became the Chinle formation. This rapid burial cut off the organic trees from sufficient air and sunlight preventing decomposition. The trunks were gradually permeated by water carrying dissolved silica and varying amounts of iron, manganese and copper that replaced atom for atom the organic matter in the original tree, turning the tree to stone (figure 13).

Figure 13. A petrified log from the Petrified Forest National Park, Arizona. Image: By Kumar Appaiah - Flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=23353296

https://en.wikipedia.org/wiki/Petrified_Forest_National_Park#/media/File:Fossilized_wood_at_Petrified_Forest.jpg

Figure 14 shows a portion of the geologic time scale shows a number of National Park locations with unique formations and their geologic age ranges. I hope you visit or have visited many of them.

Figure 14. A portion of the Geologic Time scale with examples of fossil age dated features at several National Parks. Image:

https://www.nps.gov/bibe/learn/nature/images/dinotimeline.jpg

Let’s Practice

Fill-in the grid below by determining the decreasing radioactive parent percentage (column A) to the increasing stable daughter percentage (column B) to determine the daughter/parent ratio (column C) for each of the 5 half-lives. Note: Column A plus Column B will always equal 100.

1. Reduce column A radioactive percentage by half with each half-life, while increasing the Stable daughter percentage. Note Radioactive and Stable must equal 100 when combined. You always keep the same amount of material it just converts from radioactive to stable.
2. Divide the Daughter amount by the Parent amount to determine the Daughter/Parent ratio.

1. Using the graph plot the Daughter/Parent ratio (Column C) on the Y-axis with the number of half-lives (X-axis). Carefully in pencil draw in a decay curve.

1. Determine the Daughter/Parent ratio using the data found in the Table above.
2. Using the newly drawn curve determine the number of half-lives that have passed by determining where the Daughter/parent ratio intersects the curve. Read the number of half-lives from the X-axis.
3. If the half-life of the material used has a duration of 12,000 years. Determine how old each sample is.
4. Discuss why radioactive decay is difficult to use if more than 7 half-lives have passed.