Several principles for determining relative time:
Original horizontality
most sedimentary rocks were deposited in flat-lying, or nearly flat-lying, layers (horizontal)
Superposition
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)
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
Superposition
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
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
Erosion again; basaltic dike is
exposed at the surface
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
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 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
local source of clasts where the top of the intrusion
was eroded
Using a sequence of fossils to correlate sediment layers
overlapping time ranges for different
species Figure 8.20
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
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)
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 *** }
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
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
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
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
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
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
ers:
Oligocene
Eocene
Paleocene
Triassic
How much time is “missing” between Y & Z?
End of Triassic = 213 MY
Beginning of Paleocene = 65 MY