Ch. 9 - Geologic Time

9.1 Creating a Time Scale: Relative Dating Principles

Distinguish between numerical and relative dating and apply relative dating principles to determine a time sequence of geologic events.

The Importance of a Time Scale

Presentation content:

• The Importance of a Time Scale
  • Interpreting Earth’s history is an important goal of the science of geology
  • Rocks record geologic and evolutionary changes throughout Earth’s history
  • Without a time perspective, these events have very little meaning
• Numerical and Relative Dates
  • Numerical dates specify the number of years that have passed since an event occurred
    • Example: The limestone is 250 million years old
    • Prior to the discovery of radioactivity, geologists had no reliable method for numerical dating
  • Relative dates place rocks in a sequence of formation
    • Example: The Hermit Shale is older than the Coconino Sandstone
    • Uses a few basic principles, still accurate today

Notes from the lecture:

• Zircon Crystals: This beach on Mauritius contains zircon crystals believed to be from a lost continent.
• It is important for understanding history and what the environment looked like in the past as well as to understand evolution and extinction events.
• We can put numerical values (years) to rocks

Principle of Superposition

Presentation content:

• In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below
• This principle also applies to surface features like lava flows and beds of ash
• Developed by Steno in 1669

Notes from the lecture:

• As sediment falls from an hour glass, those sediments in the bottom are the oldest, those at the top are the youngest (as you move up).
  • This only holds true in the case that this rocks that have not been undeformed

Principle of Original Horizontality

Presentation content:

• Layers of sediment are generally deposited in a horizontal position
• Rock layers that are flat have not been disturbed
• Rock layers that are deformed, must have been deformed after deposition

Notes from the lecture:

• Sedimentary rocks originally lie in horizontal layers
• Then later on these rocks were compressed

Principle of Lateral Continuity

Presentation content:

• Beds originate as continuous layers that extend in all directions until they eventually thin out or grade into a different sediment type
• When a river carves a canyon, we can assume that similar strata on either side were once connected across the span of the canyon

Notes from the lecture:

• Continuous layers
• The basin is a bath tube, this is were all the sediment is accumulating and deposited as layers
• Down cutting of a river -> erosion

Principle of Cross-Cutting Relationships

Presentation content:

• Younger features cut across older features
• Features that cut across rocks (faults, intrusions) must have formed after the rocks they cut through

Notes from the lecture:

• Example: Volcanic dikes cross-cut older rocks
• Layers should be nice continuous layers, but if we get this fault cross over, the fault is younger, and the other rock is older.
  • In the oldest rocks you get the greatest amounts of deformation
• Example: Volcanic dikes cross-cut older rocks

Principle of Inclusions

Presentation content:

• Inclusions are fragments of one rock unit that are enclosed within another rock unit
• The rock containing the inclusion is younger
  • When magma intrudes a rock mass, blocks of that rock may become dislodged and incorporated into the magma
  • These inclusions are called xenoliths

Notes from the lecture:

• Igneous intrusion undergoing erosion
• Intrusions closest to the second sedimentary unit are much colder.

Unconformities

Presentation content:

• Layers of rock that have been deposited without interruption are called conformable layers
• An unconformity is a break in the rock record produced by nondeposition and erosion of rock units
  • Uplift and erosion is followed by subsidence and renewed deposition
  • Three basic types: angular unconformity, nonconformity, disconformity

Notes from the lecture:

• Wither a time were sediments were deposited, or they got eroded away
• Surface (little curvy line) is your unconformity.

Types of Unconformities

Presentation content:

• Angular unconformity
  • Tilted rocks are overlain by flat-lying rocks
• Disconformity
  • Sedimentary strata on either side of the unconformity are parallel
• Nonconformity
  • Sedimentary strata overlay metamorphic or igneous rocks

Notes from the lecture:

• Angular unconformity
  • Wrosion happened after layers lied down
• Disconformity
  • Layer on top
  • Subsequent layers are parallel to one another
• Nonconformity
  • Igneous rock/metamorphic rock being weathered and lied down in sedimentary rocks

• Try interpreting these events.
• First based on original horizontality
• Then we consider inclusions of silt, still horizontal
• Then we have tilting (a tectonic event going on)
• Angular unconformity
• Deposition of sedimentary layers lied down horizontally

9.2 Fossil: Evidence of Past Life

Define fossil and discuss the conditions that favor the preservation of organisms as fossils. List and describe various types of fossils.

Fossils

Presentation content:

• Fossils are traces or remains of prehistoric life preserved in rock
• Paleontology is the study of fossils
• Helps researchers understand past environmental conditions
  • Fossils play a key role in correlating rocks of similar ages from different places on Earth

Notes from the lecture:

• Dominant organisms that belong to some specific periods

Types of Fossils

Presentation content:

• Permineralization
  • Mineral-rich groundwater flows through porous tissue (e.g., bone or wood) and precipitates minerals
  • Petrified literally means “turned to stone”
• Molds and Casts
  • A mold is created when a shell is buried in sediment and then dissolved by underground water
  • A cast is created when the hollow spaces of a mold are filled with mineral matter
• Carbonization and Impressions
  • Carbonization occurs via burial and compression of organic matter, leaving a thin film of carbon behind
    • Effective at preserving leaves and delicate animals
  • Impressions remain in the rock when the carbon film is lost
• Amber
  • Amber is the hardened resin of ancient trees
    • Effective at preserving insects
• Trace Fossils
  • Indirect evidence of prehistoric life
    • Includes tracks, burrows, coprolites, and gastroliths

Notes from the lecture:

• Permineralization:

  • Bone and wood, wood acts like a sponge and starts to precipitate
  • Indicates that once upon a time there was a kind of 'swamp' area

• Molds and Casts

  • Pression in the sediment
  • Take playdooh and a shell and press it, that pression tells us what type of organism it is

• Trace fossils

  • Not actual fossils of the organism but evidence of the life of this organism
  • Dungs

  

Conditions favoring preservation

Presentation content:

• Most organisms are not preserved
  • Rock Record has a Preservation Bias
• Preservation requires two special conditions:
  • Rapid burial and
  • Possession of hard parts

Notes from the lecture:

• Most organism don't make it to the museums, they are very rare to get preserved
• Mammals carried away by a flood (an example of mammals that got preserved)

9.3 Correlation of Rock Layers

Explain how rocks of similar age that are in different places can be matched up.

Correlation

Presentation content:

• Correlation involves matching of rocks of similar ages regionally
• Correlation provides a more comprehensive view of the rock record
  • Often accomplished by noting the position of the bed in a sequence of strata
  • Involves matching of rocks of similar ages from different regions
  • To correlate over larger areas, fossils are used for correlation

Notes from the lecture:

• There are large brackets (cover several million years)

Fossils and Correlation

Presentation content:

• Principle of Faunal Succession
  • Used by William Smith, British canal builder
  • The principle of fossil succession states that fossils are arranged according to their age
    • Example: Age of Trilobites, Age of Fishes, Age of Reptiles, Age of Mammals
• Index Fossils and Fossil Assemblages
  • Index fossils are widespread geographically and limited to a short period of geologic time
  • Fossil assemblages can be used to identify a rock bed that does not contain an index fossil

Notes from the lecture:

• Once an organism is non-extinct, we will no see it in any older rocks (periods)
• Once it goes extinct, it won't appear in younger rocks anymore
• Index fossils
  • Organisms that are widespread over the world but only live in certain periods
  • You can use that to age rocks in Texas and middle east and many other locations
• Look at multiple organisms, look when they lived and look at that overlap

Environmental Indicators

Presentation content:

• Fossils can be used to infer information about past environments
  • Shells of organisms can be used to infer positions of ancient shorelines and seawater temperatures
  • Corals can be used to indicate former temperature of the water

Notes from the lecture:

• Time window of different organisms
• Look at where these different organisms co-existed to get an age

9.4 Numerical Dating with Nuclear Decay

Discuss three types of radioactive decay and explain how radioactive isotopes are used to determine numerical dates.

Reviewing Basic Atomic Structure

Presentation content:

• The mass number is the number of protons and neutrons in a nucleus
• Isotopes have
  • Same number of protons
  • Different numbers of neutrons
  • Different atomic mass

Notes from the lecture:

Radioactivity

Presentation content:

• Radioactivity is the spontaneous decay in the structure of an atom’s nucleus
  • Unstable radioactive isotope is called the parent
  • Isotopes resulting from the decay of a parent are termed the daughter products
  • The ratio between parent and daughter isotopes in a rock is used to determine its numerical age

Notes from the lecture:

• If you change the atomic number (number of protons) - you change the element.
• Mass change is negligible to an electron

• This is the case series of Uranium
• Showing the different decays (alpha, omega) and the element that it changes to
• Decays over time and the entire process takes about 4.5 billion years
  • A really large time frame
  • Not suitable but we can use hthings like carbon 14 instead

Half-Life

Presentation content:

• A half-life is the amount of time required for half of the radioactive isotope to decay
  • Radioactive parent isotopes decay to stable daughter isotopes
  • When the ratio of parent to daughter is 1:1, one half-life has passed

Notes from the lecture:

• For most cases, after about 6 half lives, you will not be able to age things

A Complex Process

Presentation content:

• A Complex Process
  • Determining the quantities of parent and daughter isotopes must be precise
  • Some radioactive materials do not decay directly into stable daughter isotopes
    • Uranium-238 has 14 steps to ultimately decay to the stable daughter lead-206
• Sources of Error
  • The system must be closed
    • No external addition or loss of parent or daughter isotopes
    • Fresh, unweathered rocks are ideal to use for radiometric dating

Notes from the lecture:

• Age dating is the best process we have but it is definitely not perfect, it is very complex
• There are many sources of error
  • The system must be closed
    • If you open the system, isotopes can escape, so you are adding an external source, mixing materials that are different ages together
    • This can obscure your age dates.

Radiocarbon dating

Presentation content:

• Radiocarbon dating uses the radioactive isotope carbon-14
  • The half-life of carbon-14 is 5730 years
  • Can be used to date events as old as 70,000 years
  • Carbon-14 is produced in the upper atmosphere from cosmic-ray bombardment
    • Carbon-14 is incorporated into carbon dioxide and absorbed by plants through photosynthesis
    • Carbon-14 is only useful in dating organic matter
    • All organisms contain a small amount of carbon-14

Notes from the lecture:

• If we got plant fossils we can actually use Carbon-14 to age date them
• We can actually age date the paint that early humans used on rocks because it has organic components on it.

9.5 Determining Numerical Dates for Sedimentary Strata

Explain how reliable numerical dates are determined for layers of sedimentary rock.

Rarely dated by radiometric means

Presentation content:

• Sedimentary rocks can rarely be dated directly by radiometric means
  • Geologists must rely on igneous rocks in the strata
    • Radiometric dating determines the absolute age of the igneous rocks
    • Relative dating techniques assign date ranges to sedimentary rocks
    • This is referred to as “bracketing” various episodes in Earth’s history
    • Necessary to combine absolute dating methods with relative dating principles

Notes from the lecture:

• We have 8 brackets to constraint the ages of this one.

9.6 The Geologic Time Scale

Distinguish among the four basic time units that make up the geologic time scale and explain why the time scale is considered to be a dynamic tool.

The Geologic Time Scale

Presentation content:

• The Geologic Time Scale encompasses all of Earth’s history
  • Subdivides geologic history into units with meaningful time frames
  • Originally created using relative dates
  • Numerical dates applied to it in the twentieth century
• Structure of the Time Scale
  • An eon represents the greatest expanse of time
    • The Phanerozoic eon (“visible life”) is the most recent eon, which began about 542 My.
  • Eons are divided into eras
    • The Phanerozoic eon is divided into three eras
      • Paleozoic era (“ancient life”)
      • Mesozoic era (“middle life”)
      • Cenozoic era (“recent life”)

End of Chapter 9 - Concept Checks

9.1 Creating a Time Scale: Relative Dating Principles

1. Distinguish between numerical and relative dates.
2. Sketch and label four simple diagrams that illustrate each of the following: superposition, original horizontality, lateral continuity, and cross-cutting relationships.
3. What is the significance of an unconformity?
4. Sketch and explain the difference between an angular unconformity and a nonconformity.

9.2 Fossils: Evidence of Past Life

1. Describe several ways that an animal or a plant can be preserved as a fossil.
2. List two examples of trace fossils.
3. What conditions favor the preservation of an organism as a fossil?

9.3 Correlation of Rock Layers

1. What is the goal of correlation?
2. State the principle of fossil succession in your own words.
3. Along with being important in correlation, how else are fossils useful to geologists?

9.4 Numerical Dating with Nuclear Decay

1. List three types of radioactive decay. For each type, describe how the atomic number and atomic mass change.
2. Sketch a simple diagram to explain the idea of half-life.
3. For what time span does radiocarbon dating apply?

9.5 Determining Numerical Dates for Sedimentary Strata

1. Briefly explain why it is often difficult to assign a reliable numerical date to a sample of sedimentary rock.
2. How might a numerical date for a layer of sedimentary rock be determined?

9.6 The Geologic Time Scale

1. List the four basic units that make up the geologic time scale.
2. What term applies to all of geologic time prior to the Phanerozoic eon? Why is this span not divided into epochs as is the Phanerozoic eon?
3. Explain why scientists occasionally change the geologic time scale.