Unit 2-Historical Geology
Outcomes:
 Recognize that uniformitarianism is a fundamental principle of geology and contrast this principle with
catastrophism
 Define uniformitarianism
 Explain the appropriate applications of absolute and relative dating
 Distinguish between absolute and relative time
 Demonstrate an understanding of the principles and laws used to establish relative time.
Include:
(i) Superposition
(ii) cross-cutting relations
(iii) Horizontality
(iv)Inclusions
(v) Fossil succession (index fossils)
(vi)Unconformities
 Construct and interpret cross-sectional diagrams of Earth using geological concepts. Include:
(i) Horizontality
(ii) Superposition
(iii) Correlation
(iv)Cross-cutting relationships
(v) Unconformities
(vi)Inclusions
(vii) Folding and faulting
(viii) Metamorphism
 Explain how scientific knowledge evolves as new evidence comes to light and as laws and theories are
tested and subsequently restricted, revised, or replaced
• Analyse natural and technological systems to interpret and explain their structure and dynamics
• Compile and organize data, using appropriate formats and data treatments to facilitate interpretation
of the data
• Communicate questions, ideas, and intentions, and receive, interpret, understand, support, and
respond to the ideas of others
• Select and use appropriate numeric, symbolic, graphical, and linguistic modes of representation to
communicate ideas, plans, and results
• Work cooperatively with team members to develop and carry out a plan, and troubleshoot problems
as they arise
• Describe theories and evaluate the limits of our understanding of Earth’s internal structure explain the
appropriate applications of absolute and relative dating
 Demonstrate an understanding of the processes and features used to establish absolute time.
Include:
(i) varves
(ii) growth rings
(iii) radioactive dating
 Explain how the half-lives of radioactive elements are used in estimating ages of materials (332-4)
 Define half-life
 Define isotope
 Identify parent and daughter Elements
 Determine the age of a sample using radiometric data
 Evaluate the sources of error and limitations in estimating radiometric age
 Explain the importance of communicating the results of a scientific or technological endeavour, using
appropriate language and conventions
• Distinguish between scientific questions and technological problems (115-1)
• Analyse and describe examples where scientific understanding was enhanced or revised as a result of
the invention of a technology
• Analyse and describe examples where technologies were developed based on scientific understanding
• Analyse the knowledge and skills acquired in their study of science to identify areas of further study
related to science and technology
• Identify limitations of a given classification system and identify alternative ways to classify to
accommodate anomalies
• Identify and explain sources of error and uncertainty in measurement and express results in a form
that acknowledges the degree of uncertainty (214-10)
 Describe how fossils are used to distinguish geologic time
 Define fossil
 Describe three conditions necessary for fossilization
 Describe the formation of various types of fossils.
Include:
(i) Petrifaction by replacement
(ii) Carbonization
(iii) Mould and cast
(iv)Preserved intact (frozen, amber)
(v) Imprints (soft tissue)
(vi)Trace fossils (e.g., dinosaur eggs, coprolite)
 Explain how scientific knowledge evolves as new evidence comes to light and as laws and theories are
tested and subsequently restricted, revised, or replaced
 Identify and describe the work of palaeontologists
 Use instruments effectively and accurately for collecting data
 Identify limitations of a given classification systems and identify alternative ways of classifying to
accommodate anomalies
 Identify and explain sources of error and uncertainty in measurement and express results in a form that
 acknowledges the degree of uncertainty (214-10)
 Describe geological evidence that suggests life forms, climate, continental positions, and Earth’s crust
have changed over time
 Illustrate the geologic time scale and compare to human time scales (332-4)
 Identify that the geologic time scale is divided into eons, eras, periods, and epochs
 Recognize that Precambrian time represents the greatest part of Earth history
 Recognize that the Phanerozoic eon represents the emergence of complex life forms
 Distinguish between Precambrian time and the Paleozoic, Mesozoic, and Cenozoic eras
 List the dominant life forms present at each era. Include:
(i) single-celled and other simple life forms (Precambrian)
(ii) Invertebrates (early Paleozoic)
(iii) Fishes (middle Paleozoic)
(iv)First land plants (between early and middle Paleozoic)
(v) Amphibians (late Paleozoic)
(vi)Reptiles (Mesozoic)
(vii) Birds (Mesozoic)
(viii) Flowering plants (Mesozoic)
(ix) Mammals (Cenozoic)
List the time frame that correlates with the dominant life form on Earth. Include:
(i) Cenozoic -Age of Mammals
(ii) Mesozoic -Age of Reptiles
(iii) Paleozoic (late) -Age of Amphibians
(iv) Paleozoic (middle) -Age of Fishes
(v) Paleozoic (early) -Age of Invertebrates
Recognize that life forms, climate, continental positions, and Earth‘s crust have changed over time
Identify two mass extinction events in Earth’s history.
Include:
(i) Permian-Triassic boundary
(ii) Cretaceous-Tertiary boundary
Catastrophism Vs Uniformitarianism:
During the 17th and 18th centuries the doctrine of Catastrophism influenced people’s thinking about
Earth. Proposed by James Ussher, Catastrophists thought that Earth’s physical features (mountains,
canyons, and almost all landforms) formed by sudden spectacular events (catastrophes) produced by
unknowable causes that no longer operate and cannot be explained by nature
These ideas attempted to fit the rate of change of Earth processes into a relatively young aged Earth.
These scientists thought Earth was millions and not Billions of years old.
There is a tendency sometimes to reject the concept of catastrophism as an antiquated and
preposterous notion. Be careful of this. There are many good reasons to suggest that catastrophism may
have some ability to account for certain events. Mass extinctions such as the one that removed the
dinosaurs, for example, can be accounted for with catastrophic models.
One idea that helped scientist better understand the processes’ acting on Earth is the idea of
Uniformitarianism.
Plays an important role in understanding current and past geologic events.
This idea was first recognized by a Scottish Geologist named James Hutton
After years of studying land forms and rocks, Hutton came to the conclusion that,
“The present is the key to the past.” And “that, the physical, chemical, biological laws that
operate today to shape Earth also operated in the past.”
This statement included two concepts:
1) The geologic processes at work today were also active in the past.
2) The present physical features of Earth were formed by these same processes, at work over long
periods of time.
Determining Geologic Time
Many geologic events that Earth Scientists study occurred millions of years ago. The ages of these
events can be determined in two different ways.
1) Relative Dating
2) Absolute Dating
Both of these dating methods are needed to accurately record geologic time chronologically and to
organize the geologic rock record.
Relative Dating
Places events in a sequence of formation, but does not identify their actual date of occurrence. Often
done by comparing events.
This method of dating can’t tell us how long ago something happened, only that it followed one event
and preceded another
Ex: Mrs. McDonald is a little older than her students. (just a little)
Absolute Dating
Identifies the actual date of an event, & pinpoints the exact time in history when something took place.
For example, the extinction of the dinosaurs about 66 million years ago and the age of Earth is
approximately 4.6 Billion years.
Ex: Mrs. McDonald is 18 yrs older than her students. 
Relative dating techniques include;
1) Principle (Law) of Superposition
2) Principle of Original Horizontality
3) Principle of Cross-Cutting Relationships
4) Principle of Inclusions
(5) Fossil succession (index fossils)
(6) Unconformities
Law of Superposition
States that in any undisturbed sequence of sedimentary rocks, a sedimentary layer is older than the
layers above it and younger than the layers below it. The youngest is always at the top.
Principal of Original Horizontality
- states that most layers of sediment are deposited in a horizontal position. If rock layers are folded or
inclined, then the layers must have been moved into that position by crustal disturbances.
Law of Crosscutting Relationships
- states that an igneous rock or geologic feature is younger than the rocks it has intruded, or cuts across.
Two examples of cross-cutting in this diagram:
Fault cuts rock units A, B, C, D & dike
.
Igneous Dike cuts rock units A, B, & C.
Law of Included Fragments
states that pieces of one rock found in another rock must be older
than the rock in which they are found.
Rock fragments from rock unit “D” is included in layer “E” above it.
Unconformities:
When we observe layers of rock that have been deposited essentially without interruption, we call them
conformable. …..No place on earth is completely conformable.
An unconformity: represents a long period of time during which deposition ceased, erosion removed
previously formed rocks and then deposition resumes. They represent significant geologic events in
earth’s history.
Angular Unconformity:


Most easily recognizable
Consist of tilted or folded sedimentary rocks that are overlain by younger more flat-lying
strata.
 Indicates that during the pause in deposition, a period of deformation and erosion
occurred.
Disconformities:

More common but usually far less conspicuous because the strata on either side are essentially
parallel.
 Rocks above and below might be similar.
 Little evidence of erosion.
 Can represent a significant time gap.
Nonconformity:



The break here separates older metamorphic or intrusive igneous rock from younger
sedimentary strata.
Intrusive igneous and metamorphic rocks originate far below the surface.
Thus, nonconformities must develop in periods of uplift and erosion of the overlying rocks.
Sedimentation occurs above.
Fossils:
Fossils can help us to determine how organisms evolve over time. As one observes fossils in layered rock
it is often evident how organisms changed through time. Catastrophic events such as the extinction of
the dinosaurs can also be found in the fossil record.
Fossils occur in a consistent vertical order in sedimentary rocks all over the world. Fossil succession
occurs as a result of the natural appearance and disappearance of species through time.
CORE LAB #1 “Interpreting Historical Geologic Events”.
Absolute dating
Methods include:
1) Tree Rings - The age of a tree is found by counting the total number of rings.
2) Varves - any sediment layer that shows a yearly cycle. Varves are often seen in glacial lakes dating
back to the ice age
Note: A varve is a pair of sedimentary layers that are deposited in one year in a glacial lake in an area
that experiences strong seasonal contrast. Geologists count the pairs of sedimentary layers (varves) to
determine the number of years of deposition. For example, 308 sedimentary layers means that there
are 154 glacial varves, which represents 154 years of sediment deposition. Each varve contains a dark,
fine-grained sedimentary layer and a light, coarse-grained sedimentary layer. The dark, fine-grained
layer forms in the fall and winter when water is most likely frozen and very little sediment deposition is
occurring. The dark colour is due to the abundance of humus material that settles out of the water body
during these seasons. The light, coarse-grained layer forms in the spring and summer when meltwater is
abundant and is flowing and eroding large quantities of sediment. The light colour is attributed to the
abundance of sediment and the relatively small amount of humus material.
3) Radiometric Dating - calculating absolute ages of rocks and minerals that contain radioactive
isotopes.
Radioactive Dating
A radioactive sample is referred to as the Parent Material and the decayed product is called the
Daughter Material. When both are added together it equals 100%.
Because rocks on Earth range in age, several different dating methods can be used to find the age of
different rocks. Some of these dating methods and corresponding half-lives include;
1) Uranium-238 decays to Lead2) Uranium-235 decays to Lead3) Potassium-40 decays to Argon4) Carbon5) Rubidium-
-
0 years
-
Radioactive elements (Isotopes)• These are elements which are unstable in nature and give off
radiation as they undergo radioactive decay to become stable. This continues until the element formed
is stable, or not radioactive.
Radioactive elements decay at constant rates and are thought to start decaying as soon as the rock has
formed. Most radioactive elements exist in igneous rocks. The ratio of the amount of unstable, parent
material to the amount of stable, daughter material can be used to determine the absolute age of the
rock. The rate at which a radioactive element decays is called its half-life.
These questions could make reference to the radioactive parent isotope in;
Fraction Form (ex. 1/16th)
Percent Form (ex. 25%)
Amount in Grams (360 grams)
Example: Calculate the age of a rock given isotope half-life and amount of parent material (radioactive
isotope).
Question:
Calculate the age of a rock using the K - 40 à Ar – 40 dating method (which has a half – life of 1.3 billion
years), if you know that 12.5% of the parent material now remains in the rock sample.
Information Given in Problem:
Half-life of radioactive sample à 1.3 Billion Years
Parent material remaining à 12.5%
The key to solving radioactive problems is that the number of half-lives (represented by “N”) must be
found. To find the number of half-lives (N) that passed when 12.5% of the radioactive sample remains
we can use a chart and follow the following steps:
Note:
The original amount before any radioactive material decayed was
100%. This is represented in the chart as zero half-lives.
Find how many half-lives the radioactive sample has to go through so
that 12.5% remains.
After 3 half-lives 12.5% of radioactive sample remains. Thus,
“N” = 3
To calculate the Age of the radioactive sample, use the following formula;
Age = “N” x # of years per half-life
Age = 3 x 1.3 billion years
Age = 3.9 billion years
A common error student’s make when calculating this type of problem is:
Students when calculating the number of half-lives, as previously shown, count the “0" which implies
100% of the sample, as one of the half-lives. This would give an incorrect number of half-lives (N = 4),
which results in an incorrect answer.
Sources of error and limitations of radioactive dating
Include:
Metamorphism resetting the radioactive clock
Addition and/or loss of parent or daughter isotopes, (e.g. hydrothermal fluids) (e.g. leaching)
Applicability with sedimentary rock (due to their formational nature of being composed of sediment that
was weathered and eroded from various sources),
Appropriateness of certain parent-daughter pairs in their application (e.g. carbon-14 can only be used to
date living or once-living things).
Core STSE “Labrador Zircons and their Link to Radiometric Dating and Absolute Time”.
Fossils: Key to interpretation of past events.
FOSSILSS ARE: The remains, traces, impressions, or any other evidence of plants and animals preserved
in rock.
Fossils can help us to determine how organisms evolve over time. As one observes fossils in layered rock
it is often evident how organisms changed through time.
Catastrophic events such as the extinction of the dinosaurs can also be found in the fossil record.
When plants and animals die they get buried in sediment and the soft parts usually decay with the hard
parts being fossilized when the sediment turns to solid rock.
Conditions necessary for fossilization include:
1) Hard body parts.
Fossils of organisms that contained hard parts are abundant in the fossil record, but only rare
traces of soft tissue organisms are seen as fossils.
2) Rapid burial.
When an organism dies its soft parts are eaten by scavengers or is decomposed by bacteria.
However, if the organism is quickly buried by sediment where it is protected from the
environment, evidence of the organism’s remains can be preserved in the rock.
3) Low oxygen levels.
Prevents decomposition.
Fossils provide the basis by which the subdivisions of the Geologic Time-scale are divided.
Because fossilization is dependent on special conditions, the record of life in the geologic past is biased.
The fossil record shows an abundance of organisms that contained hard parts and lived in environments
of high sedimentation. However, only glimpses of the numerous other life forms that did not meet the
special conditions that favour fossilization exist in the fossil record.
Fossils have been recognized for centuries, but it was not until the early 1800’s that an English scientist,
William Smith, noticed that the same fossils were identified in the same rock types. This evidence was
the background work for one of the fundamental principles of historical geology known as the principle
of fossil succession.
This principle states, “Fossil organisms succeed one another in a definite and determinable order, and
therefore any time period can be recognized by its fossil content.”
What information can be gathered from fossils?
Fossils indicate the age of sedimentary rocks.Within each of the ages there are many subdivisions
based on certain species of fossils. For example the divisions of the geologic time scale is subdivided
according to the presence and absence of fossils. This same succession of organisms preserved as fossils
is seen on every major landmass.
Fossils indicate the environments in which rocks formed. Knowing the nature of life forms that existed
at a particular time may indicate the environment in which the sedimentary rock formed. Studying the
nature and characteristics of sedimentary rocks and the fossils they contain can indicate past
environments. For example, if clamshells are found in limestone, a geologist could assume that a
shallow sea covered the region, because that is where clams are found today. This assumption coincides
with the idea of uniformitarianism Fossil characteristics reveal what type of environment the organism
lived in the past
Fossils are used to match up (correlate) rocks from different places that are the same age. Once fossils
were recognized as time indicators, they became a useful means of correlating rocks of similar age in
different regions.
Scientist use fossils called index fossils, which are widespread geographically and are limited to a short
span of geologic time. The presence of these fossils is important when matching rocks of the same age.
If index fossils are not present, then groups of fossils in the same rocks are used to correlate rocks of the
same age.
Fossils are used to interpret the geologic past. By studying characteristics of certain fossils and the type
of fossil present in sedimentary rocks, different aspects of the geologic past can be interpreted by
geologist. Things such as, temperatures, climate, type of environment, etc… can be determined.
Fossils can also indicate evolutionary pathways. With an understanding of the principle of fossil
succession, when fossils are arranged according to their age by applying the law of superposition, fossils
in the rocks show a progressive change demonstrating the evolution of life through time.
For example, an Age of Invertebrates, such as the trilobites, are recognized early in the fossil record.
Then, in succession, paleontologists recognize an Age of Fishes, an Age of Amphibians, an Age of
Reptiles, and an Age of Mammals. Thus, it is thought that Invertebrates evolved to form Fish, which
evolved into Amphibians, which evolved into Reptiles, and finally Mammals.
Types of fossils.
Include:
1. petrifaction by replacement
2. carbonization
3. mould and cast
4. preserved intact (frozen, amber)
5. imprints (soft tissue)
6. trace fossils (e.g., dinosaur eggs, coprolite)
1)
Petrification: Occurs when the small internal cavities and pores of the original structure are
filled with precipitated mineral matter. This occurs when cell walls and solid material are removed and
replaced by mineral material carried by underground water. Sometimes internal details and structures
are retained.
2)
Carbonization: Occurs when fine sediment encloses delicate matter such as leaves in a oxygen
poor environment. As time passes, pressure squeezes out the liquid and gaseous components of the
organism leaving behind a thin residue of carbon
3)
Mold and Cast: Often preserve a replica of a plant or animal in sedimentary rocks. An organism
is buried in sediment and then dissolved by underground water leaving a hollow depression or an
impression, called a mold. The mold shows only the original shape and surface markings of the
organism; it does not reveal the internal structure. When minerals or sediment fills the hollow
depression or impression it forms a cast.
4)
Preservation-Ice, Mummification, and Amber: Original remains can be preserved in ice or in
amber (tree sap). Both ice and amber protects the organism from decay (oxygen free environment) and
from pressures that would crush the organisms. The entire animal has been preserved, even the soft
parts that usually decay and disappear.
Examples:
(1) Woolly Mammoths preserved in ice in Alaska and Siberia.
(2) Insects preserved in tree sap (amber).
5) Imprints (soft tissue):
6)
Traces: Trace fossils can include many different forms and should include: eggs, coprolites,
gastroliths, footprints and burrows/tunnels.
Show traces left in the rock by an animal, such as;
1)Tracks - animal footprints made in soft sediment that latter formed solid sedimentary rock.
2)Burrows - animal trails made in soft sediment that latter formed solid sedimentary rock.
3)Coprolites - Fossil dung (feces) and stomach contents.
Core Lab #2: Estimating Dinosaur Size and speed from Trackways.