Physical Geology Chapter 8 Geologic Time

Relative time vs. numerical age



  Several principles for determining relative time:


Original horizontality

      most sedimentary rocks were deposited in flat-lying, or nearly flat-lying, layers (horizontal)



  in a flat-lying sequence, the youngest rocks are on top


Lateral continuity (of sediment layers)

  for most depositional systems, layers of sediment are laterally continuous over a large area

  examples:  sediments in Lake Erie, continental shelf


Cross-cutting relationships

  the geologic unit that cuts across others must be younger (the other units already existed)


A geologic section somewhere out west

Figure 8.1

First question: What was the sequence of events?







First phase of deposition – a shallow seaway    Figure 8.3

Principles involved in this section:

  Original horizontality


  Disconformities between formations


Paleogeographic reconstruction of the Cretaceous mid-continent seaway – This is a good analogy for the geologic section shown in Figures 8.1-8.11


Figure 8.4  A granite intrusion

   Principle involved:  Cross-cutting relations


Next event:  Regional tectonics – tilting






Figure 8.5 

Regression and regional erosion



Figure 8.6  A second marine incursion



  Principles involved:  Erosional truncation

    Forms an angular unconformity



Figure 8.8

  Intrusion of a basalt dike



Figure 8.9 

Erosion again;  basaltic dike is exposed at the surface


Figure 8.10 

Flooded by seawater a third time


Figure 8.11  Exposed as land surface,

 possibly millions of years later



Bringing it up to the present:  A canyon cuts through it

regional uplift and continued river erosion





Angular unconformities

  Figure 8.14




James Hutton 1788 – interpreted this section along the coast of Scotland as showing cycles of erosion & deposition


The prevailing world view in the 1600’s and 1700’s was that the Earth was no older than about 6000 years


Hutton’s idea was that geologic features could be explained by modern processes that operated over very long periods of time


The modern concept of uniformitarianism

The same processes have operated through

  Earth history…

but the rates and magnitudes may be greater or smaller



For example:

  erosion of the land surface

    transport of sediment by rivers

      deposition in the ocean


But these processes were not understood in the 1700s:


  mountain building

    seafloor spreading



Intrusions & relative ages  Figure 8.12

  local source of clasts where the top of the intrusion was eroded



  Figure 8.13


Using a sequence of fossils to correlate sediment layers

            overlapping time ranges for different species  Figure 8.20



Geologic timescale


  Table 8.2 and Figure 8.26


{ *** know these major divisions of geologic time highlighted in blue  *** }



Eons – major divisions, in billions of years (or large fractions)

Phanerozoic  “known life”


Precambrian  “before the Cambrian”

  the Cambrian is the first Period in the Phanerozoic


Proterozoic    “earlier life” 


The Eras of the Phanerozoic:  Cenozoic  “recent life”

Mesozoic  “middle life”

Paleozoic  “ancient life”



The Phanerozoic

  Informal geologic ages:

            Cambrian & Ordovician   Age of Marine Invertebrates

            Permian & Carboniferous  Age of Amphibians

            Mesozoic  Age of Dinosaurs

            Cenozoic  Age of Mammals






Analogy:  Time across the U.S.  Figure 8.27








Major events in the history of life

  Time in billion years:

            4.0  first cells (bacteria)

            3.6  oldest preserved fossils (bacteria)

            2.5  free oxygen in the atmosphere from photosynthesizing cyanobacteria

            1.5  first eucaryotic cells (advanced single-celled organisms)

            0.7  radiation (great increase in diversity) of multicellular organisms

            0.5 to 0.4  rapid evolution of plants and animals


Earliest bacteria


Archæbacteria and cyanobacteria (blue-green algae)


Banded iron formations – free oxygen produced by mats of cyanobacteria oxidize the iron dissolved in seawater


The earliest (known) Chordate

  From the Burgess Shale in Canada  505 MY (Cambrian Period)


Telling time 

  Figure 8.24

Numerical ages for geologic features

Linear decay rate for the candle:  2.5 g / hour

  All of the wax is consumed in 4 hours


But what if the candle burned a certain percentage of the remaining wax per hour?

  100% initially, then 50% after 1 hour









, 25% after 2 hours, 12.5% after 3 hours

    approaches zero very slowly


{ *** know the difference between linear decay and exponential decay *** }


Exponential decay   Radiometric decay


  Parent isotope becomes daughter isotope


{ *** know what a half-life is *** }



  One half-life is the amount of time for 50% (half) of the existing atoms of the parent isotope to decay radioactively to produce the daughter isotope


Example in Figure 8.23 of uranium-238 decaying to produce lead-206

  in this case, over several steps with intermediate isotopes


Modes of radioactive decay 

{ *** too detailed, don’t worry about this information about radioactive decay (highlighted in green) for the exam *** }

Figure 8.22


  Loss of an alpha particle  2 protons 2 neutrons

   Loss of a beta particle – electron emitted from a neutron, which becomes a proton


Creating a radioisotope

Uranium already has several isotopes (naturally) that decay radioactively



Radioisotopes of other elements may be created – for example, carbon-14 is produced in the upper atmosphere when a high-energy electron strikes a proton in the nucleus of a nitrogen atom, the proton plus the electron become a neutron, which changes the element from N to C


Uranium decay series


  Figure 8.23

    { labeled wrong in the book: alpha & beta decay are reversed }



  shows all of the intermediate steps in the decay chain from uranium-238 to lead-206


Different radioisotopes for different ages

Table 8.3  { note: these are not all of the radioisotopes }


Different parent-daughter pairs can be used to measure rates of geologic processes from hours and days to billions of years


Add numerical ages to the geologic section

Figure 8.25 

Radiometric ages of the igneous rocks

  (igneous rocks are most easily dated by radiometric methods)



 { *** understand the reasoning for determining both relative and numerical time *** }


First, what is the sequence of events?


Second, add some radiometric dates














What are the age constraints for the sediment layers?



Add some fossils

 (in the sediment layers)


The members of the horse evolutionary lineage and ages:

Equus           modern

Pliohippus     Pliocene

Merychippus  Miocene

Mesohippus   Oligocene

Eohippus       Eocene


Also found in the lower sediment layers:

            Ptilodus – an early mammal; extinct by the end of the Eocene

            Coelophysis – an early dinosaur from the Triassic


This additional information allows the geologist to determine the geologic epochs


The ages of the sediment lay








How much time is “missing” between Y & Z?

End of Triassic = 213 MY     Beginning of Paleocene = 65 MY