Geologic Time

OBJECTIVES

• Understand and apply the basic principles used to determine relative ages of geological sequences
• Understand the role of fossils in determining the age of rock units
• Identify and describe how unconformable boundaries are formed
• Be able to calculate radiometric dating and the age of a rock unit
• Combine relative and absolute age dating methods to determine age ranges

MATERIALS

• Ruler
• Calculator
• Pencil

INTRODUCTION

Time is the dimension that sets geology apart from most other sciences. Geological time is vast, and the conditions on Earth have changed enough that some rock types that formed in the past could not even form today (Banded iron formations and Komatiites). Even though most geological processes are very, very slow, the vast amount of time (4.6 billion years for Earth) that has passed has allowed for the formation of extraordinary geological features, such as the Grand Canyon and the Atlantic Ocean. Scientists have numerous ways of measuring geological time. We can tell the relative ages of rocks (whether one rock is older than another) based on their spatial relationships; we can use fossils to date sedimentary rocks because we have a detailed record of the evolution of life on Earth; and we can use a range of isotopic techniques to determine the absolute age (in millions of years) of igneous and metamorphic rocks.  In this lab, we will explore each of these types of age dating methods.

PRINCIPLES OF RELATIVE DATING

Relative dating is the process of placing geological events in sequential order.  For example, one rock unit is older or younger than another.  This type of age dating does not tell you how old a unit is numerically, only its order relative to other units. Many of the principles that we understand today were developed in the late 17th century by Danish physician Nicholas Steno (figure 1).  Steno observed rocks in the area of Italy and developed many of the principles of stratigraphy.  Many of these principles seem like common sense but they implied that the Earth was much older than was believed at the time and that it was a dynamic system that underwent changes both small and large.  

Figure 1.  Nicolas Steno circa 1670 CE (Danish born Niels Steensen).  He was among the first scientists to establish the theoretical basis for stratigraphy and therefore stating that the Earth was much older than originally believed and in a constant state of change.  He later became a Catholic Bishop. Image opengeology.org CC BY S.A. 4.0. Original painting unsigned but attributed to court painter Justus Sustermans.

Principle of Superposition

The Principle of superposition states that in an undeformed sequence of sedimentary rocks the oldest rocks will be at the bottom of the sequence while the youngest layer will be on top (figure 2).  Imagine a pile of newspapers or mail being piled one on top of the other. Sedimentary rocks are also deposited one on top of the other. Rock layers are frequently referred to as Strata.

Figure 2. Undeformed layer of sedimentary rock called strata.  The oldest layer is at the bottom while the youngest layers are on top. Image opengeology.org/textbook. Public Domain. Wilson 44691

 Principle of Lateral Continuity

The principle of lateral continuity states that layers within a depositional sequence will be continuous in all directions until they thin out at the edges or are stopped by a physical topographic barrier (figure 3).  Therefore starta cut by a canyon are continuous across the canyon. The missing canyon section was caused by erosion and uplift after the original layers were deposited (figure 4).

Figure 3. The layers on either side of the gap would have originally been deposited at the same time.  Note the dashed lines across the gap. Time and erosion have caused the gap. Therefore the layers were originally continuous across laterally. Image: Woudloper, Public domain, via Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Principle_of_horizontal_continuity.svg

Figure 4. The sedimentary sequences in the Grand Canyon, Arizona showing vast lateral continuity.  The Colorado river cut through top, newer/younger sequences to expose deep older layers at the bottom, along with considerable tectonic uplifting.  Image CC BY S.A. 3.0 International License, John Kees Commons.Wikimedia.org. John Kees, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

 Principle of Original Horizontality

The Principle of original horizontality states that undeformed sedimentary rock is deposited horizontally.  The deposition of sediment is controlled by gravity and on the whole sediment will deposit in relatively flat layers.  Therefore, if sedimentary rock layers are tilted or folded it must first have been horizontal and folded or tilted later.  This makes the layers older than the folding or tilting of them (figures 5 and 6).

Figure 5. Drawing shows originally horizontal layers that have been uplifted and tilted.  The uplift and tilt process must be younger than the rock layers themselves. Image oer.galileo.usg. Geology commons.

 Figure 6. Originally horizontal layers left. Colorado Plateau southeastern Utah in Glen Canyon National Recreation Area.  Tilted layers of sandstone and siltstone, Anglesey Wales. right.  Image (left) By Matt Affolter (QFL247), CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=14715935 Right: Wiki Commons By Jonathan Wilkins, CC BY-SA 2.0, https://commons.wikimedia.org. https://upload.wikimedia.org/wikipedia/commons/b/b8/Rock_Strata_-_geograph.org.uk_-_420447.jpg

Principle of Cross-cutting and Intrusion

The Principle of cross-cutting states that when two geologic features intersect, the one that cuts across the other is younger (figure 7).  This principle relates to faults that cut existing rocks being younger than the rocks they cut.  This principle also includes igneous rock intruding into or cutting across another rock layer. The layers must have been present for the fault or igneous rock to intrude or cut across.

Figure 7. Dark igneous intrusion cuts across paler rock.  The dark igneous rock is younger than the paler rock that it intrudes across. Bottom image shows a large dark diabase dike cross-cutting paler limestone near Winkelman, Arizona.  Image (top) opengeology.org/textbook. CC BY S.A. 4.0 International license. T Eliasson of the geological survey of Sweden. Image bottomDiabasic dike in limestone bed near Winkelman, Arizona. Jstuby at English Wikipedia, Public domain, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Dike_diabase_AZ.jpg

LET’S PRACTICE APPLYING A FEW PRINCIPLES

What principle is shown in these two images? Explain. 

Figure 8. Identify the principle shown in the image.

Image: Franck Bouttemy http://www.geodiversite.net/auteur197 - http://www.geodiversite.net/media1015, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=30139674 https://en.wikipedia.org/wiki/Fault_(geology)#/media/File:Fault_in_Seppap_Gorge_Morocco.jpg

Figure 9. Identify the principle in the image of these Silurian age limestone layers in Saaremaa Estonia. Image: Sandatlas.org siim seep with permission

Age sequence the 5 parts in the image – Include proofs.

Figure 10. Cartoon image representing Igneous dike D, Sedimentary layers A, B and C and fault E.  Image CC BY-SA 3.0 Kurt Rosenkrantz. http://cafreetextbooks.ck12.org/science/CK12_Earth_Science_rev.pdf (page 420) If the above link no longer works, visit http://www.ck12.org and search for CK-12 Earth Science., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11017657

YOUNGEST         _____

                _____

                _____

                _____

OLDEST        _____

Principle of Inclusions

The Principle of Inclusions represents the relative age relationship between small weathered bits of rock material that are included within the boundaries of other rocks. A rock fragment that is included within another rock unit must be older than the rock in which it is included. In other words, the small fragment must have existed before it could be surrounded by the younger rock (figure 11).

Figure 11. The principle of inclusion states that the small inclusions (bits of other rocks) are older than the larger rock surrounding them. Note coin for scale. Right image of an older pink granite (hammer for scale) included in a younger black basalt.  Image: (left) CC BY S.A. 4.0 international license S. Earle opentextbc.ca. Image (right): Rygel, M.C. CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11838946 https://simple.wikipedia.org/wiki/Xenolith#/media/File:Included_fragment_mcr1.jpg

LET’S PRACTICE

Using the four images A-D below try to identify which of the relative age principles is being illustrated.

Figure 12. Four Principles A-D illustrated. Image  cafreetextbooks.ck12.org/science/CK12_Earth_Science_rev.pdf CC BY-SA 3.0,commons.wikimedia Kurt Rosenkrantz

A.

B.

C.

D.

Principle of Faunal Succession and Index Fossils.

William “Strata” Smith, worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s. While doing his work, he had many opportunities to look at the Paleozoic and Mesozoic aged sedimentary rocks of the region, and he did so in a way that few had done before. Smith noticed the textural similarities and differences between rocks in different locations, and more importantly, he discovered that fossils could be used to correlate rocks of the same age. He noticed that organisms (fossils) appear and disappear from the rock record, indicating appearance, change, and extinction. Therefore, rocks of the same age where likely to contain the same type of fossil (index fossil).  Smith is credited with formulating the Principle of faunal succession (the concept that specific types of organisms lived during different time intervals and there was a successive change of fossil type over time), and he used it to great effect in his monumental project to create a geological map of England and Wales, published in 1815. For more on William Smith, including a large-scale digital copy of the famous map (figure 13), see http://en.wikipedia.org/wiki/William_Smith_%28geologist%29. Inset into Smith’s great geological map is a small diagram showing a schematic geological cross-section extending from the Thames estuary of eastern England all the way to the west coast of Wales. Smith shows the sequence of rocks, from the Paleozoic rocks of Wales and western England, through the Mesozoic rocks of central England, to the Cenozoic rocks of the area around London (Figure 14).

Figure 13. William Smith’s initial geologic stratigraphic map of England.  He was the first stratigrapher to create and use this type of mapping. Image wiki Commons released to the Public Domain. British Library.

Figure 14. William Smith’s “Sketch of the succession of strata and their relative altitudes,” an inset on

his geological map of England and Wales. Image S. Earle after: earthobservatory.nasa.gov/Features/WilliamSmith

Although Smith did not put any dates on these maps — because he didn’t know them — he was aware of the principle of superposition (the idea, developed much earlier by the Danish theologian and scientist Nicholas Steno) that young sedimentary rocks form on top of older ones, he knew that this diagram represented a stratigraphic column. And because almost every period of the Phanerozoic is represented along that section through Wales and England, it is a primitive geological time scale.

Some fossils, more than others, are particularly useful in telling time, these are called Index Fossils (Figure 15). These are organisms that we are likely to find because they were abundant when they were alive and were likely to become fossilized.  They often likely have a wide geographic range so that they can correlate rocks over a large distance.  However, they should have a short geologic existence so that we can precisely narrow their life span. The use of animal and plant fossils in combination with other relative age principles can help scientists build a precise time sequence.

Figure 15. Fossil succession showing correlation among strata (rock layers) and Index fossils. These fossils can be used to correlate rocks that may be far apart in distance. Image opengeology.org/textbook. CC BY S.A.

If we can identify a fossil to the species or at least genus level, and we know the period that the organism lived, we can assign a range of time to the rock. That range might be several million years, especially if that organism lived for a long period of time (figure 16).  If a specific organism is found, it can be postulated that the rocks are of that specific time frame.  If a rock unit contains several fossils, we can determine the range of time that all the fossils lived simultaneously (figure 17).  This may narrow down the time range considerably.  So instead of needing only species that lived for a short period of time, we can use the overlap of several organisms to determine the timeframe of the rock.

Figure 16. Foraminifera are common and highly variable marine organisms.  Many can be used as index fossils to help determine a specific age rock. Select index foraminifera from the Cretaceous and Paleogene. Figure is from the Geologic Timescale Foundation. https://timescalefoundation.org/. Modified by Shelley Jaye

https://opengeology.org/historicalgeology/geologic-time/#Uniformitarianism https://opengeology.org/historicalgeology/geologic-time/#Uniformitarianism

Figure 17. Species A, B, C, and D are four distinct species of fossils.  Each lived during the age range indicated by the different colored bars.  If all 4 fossils are found in a single rock unit that unit can only have been deposited between 7.0 and 8.3 million years ago as this is the only overlap between all four fossils. Image CC BY S.A. S Earle. opentextbc.ca. https://opentextbc.ca/physicalgeology2ed/chapter/8-3-dating-rocks-using-fossils/ is licensed under: CC BY 4.0

UNCONFORMITIES

The many previous stratigraphic principles relate to physical layers and units of rocks.  One other tool that can be useful in building relative time sequences is what is missing from a sequence of rocks.  Unconformities are boundaries between rock layers. They are surfaces that represent significant weathering and erosion which results in missing or erased time. They also often represent plate tectonic forces such as mountain building and uplift.  As a result, significant amounts of geologic time are often necessary to produce the three unconformable boundaries: Disconformity, Nonconformity, and Angular Unconformity.  Unconformities represent interruptions in the process of sedimentary rocks.  They are boundaries not specific rock units and frequently represent significant amounts of geologic time where individual rock layers are not deposited (figure 18). If the rock type is different above and below, the boundary is called a nonconformity.  For example, this results when an igneous rock is uplifted and exposed at the surface then covered with sedimentary rock. If the rocks above and below the boundary are both sedimentary but they have different orientations, (they are not parallel to each other) the boundary is an angular unconformity (figure 19). This occurs when sedimentary rocks are folded or tilted then eroded and new sedimentary rocks deposit on top. If the rocks above and below are parallel to each other the surface is a disconformity.  This boundary occurs when sedimentary layers are deposited and then removed by erosion and then new sedimentary rocks form on top.  

Table 1. Unconformities and their description with image. Image CC BY S.A. S. Earle opentextbc.ca

Figure 18. Sequence of geologic events producing the three unconformities. Follow the arrows from left to right as the processes of uplift and erosion result in unconformable boundaries. Image CC BY S.A. S. Earle opentextbc.ca

Figure 19. The detailed evolution of an Angular Unconformity in four stages.  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: Utah Geological Survey, Public Domain https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/unconformity

LET’S PRACTICE

Using the three drawings (A, B, C) to identify which type of unconformity is represented.

Figure 20. Three images of unconformities for identification. Image: By Woudloper - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3860901

Using the color photo below, try to draw in the Unconformable boundary.  Then Explain its possible formation.

Figure 21. Identify the unconformity. Image Anne Burgess, CC BY-SA 2.0, Wiki Commons. https://upload.wikimedia.org/wikipedia/commons/3/30/Siccar_Point.jpg

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 (figure 22).  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 fossils (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.  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 earth's layers more precisely.  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 lead to the use of radiometric age dating (Absolute age dating) that actual numerical ages could be applied to the existing Geologic time scale.

The current time scale is divided into increasingly smaller units and subunits (figure 22). 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.  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.  

The geological time scale is currently maintained by the International Commission on Stratigraphy (ICS), which is part of the International Union of Geological Sciences. The time scale is continuously being updated as we learn more about the timing and nature of past geological events. You can view the ICS time scale online. It would be a good idea to print a copy (in color) to put on your wall while you are studying geology.

Figure 22.  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.htm

LET’S PRACTICE  

1. Age sequence the layers in each of the images below from oldest at the bottom to youngest on top.  
2. Explain the Stratigraphic principle, such as cross cutting for each layer in the sequence.  
3. Identify the unconformities by specific type. Explain.

The top image (figure 23) has 8 sedimentary layers, 2 unconformities (Gu and Su), and RD is a cross-cutting dike.

Figure 23. Practice sequencing this geologic structure. Image: Daniel Hauptvogel, CC BY-NC-SA.

https://uhlibraries.pressbooks.pub/historicalgeologylab/chapter/chapter3-geologic-time/

Figure 24. G is metamorphic, MD is an igneous dike cross-cutting, all others are sedimentary layers. PF is a fault There are 3 unconformities Lu, Bu, Hu

Image: Daniel Hauptvogel, CC BY-NC-SA.

https://uhlibraries.pressbooks.pub/historicalgeologylab/chapter/chapter3-geologic-time/

Figure 25. G and I are Igneous, D is a fault, there is one unconformity and one inclusion. Image CC BY SERC.Carleton.edu https://serc.carleton.edu/details/images/3677.html

YOUNGEST        ______

                ______

                ______

                ______

                ______

                ______

                ______

                ______

                ______

                ______

                ______

OLDEST        ______

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 the early 19th century shortly after the discovery of radioactivity, scientists 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 26).  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 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 27).  You may have learned about these in lecture.

Figure 26. The Periodic table, with elemental numbers.  This tells the number of protons in the nucleus.  Image: CC BY- SA 4.0 Sandbh commons.wikimedia.org Public Domain 

Figure 27. The decay chain of radioactive parent isotope Uranium-238 to stable daughter product of Lead (Pb) 206. 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 28, 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%.  During each half-life you can only decay half of the remaining radioactive isotope until there is too little to measure.

Figure 28. 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/

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 (figure 29).  After one half -life half of the radioactive isotope has been transformed into 50% stable daughter, therefore there is 50%/50% ratio.  After a second half-life has passed there is only 25% of the original radioactive isotope and 75% stable product.  

Figure 29. 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. This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License https://opengeology.org/historicalgeology/geologic-time/#Uniformitarianism

Since all radioactive isotopes decay in 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 2 for some common isotope half-life durations.

 Table 2. Common Isotope systems and their half-lives.

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.

3. 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 a decay curve.

4. Determine the Daughter/Parent ratio using the data found in the Table above.

5. 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.

6. If the half-life of the material used has a duration of 12,000 years. Determine how old each sample is.

7. Discuss why radioactive decay is difficult to use if more than 7 half-lives have passed.