Chapter 12 - Deep Time
“The Present is the Key to the
Past”
Geologic age - the younger, sedimentary layers on the left overlay the older,
granite on the right.
The concept of geologic time.
In the past, many people framed the concept of geologic time in terms of the
human life span or historical time. During the Rennaissance, some people observed
there were preserved shapes in rocks that were obviously very ancient and yet looked
like contemporary species.

Nicholas Steno observed that fossilized shark’s teeth were the same as
modern shark’s teeth. Therefore, he reasoned, the fossils found in stone were relics of
ancient life. His observation implied that geologic time was very, very old.
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Hutton’s principle of uniformitarianism stated that the same physical
processes we observe today also existed in the past and that these processes were
responsible for the geologic features and outcrops we see today. He reasoned that
since geologic processes operate very, very slowly, then the Earth must be far older
than generally assumed.
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Relative age is the age of one feature relative to another.

Absolute or numerical age is the age of that feature in years.
An illustration of the principle of uniformitarianism. The processes that formed mud
cracks today (left) are the same processes that formed fossil mud cracks (right).
This figure illustrates the principle of superposition. The oldest sand is at the
bottom, the youngest at the top.
The principle of original horizontality. Sediments laid down in flat layers (left), if
undisturbed, can remain in that order for millions of years (right).
The principle of original continuity. Continuous sediments laid down when
sea levels rose and flooded the land are older than the canyon that formed when
sea levels fell and rivers eroded the canyon.
Principle of cross-cutting relations: the sedimentary beds existed first and were
later cut by igneous intrusions which later eroded away leaving an igneous
dike.The dike must be younger than the sedimentary layers because it is doing
the cutting.
Principle of inclusions
The sill includes xenoliths
derived from the sandstone and
limestone layers. Therefore, it
must be younger.
The igneous flow layer is older
than the layer above it because
pieces of the igneous layer are in
the sandstone layer above it.
Principle of baked contacts: the plutonic intrusion creates a metamorphic
layer around it and also cross-cuts the bedding layers. Therefore, the pluton
must be younger than the surrounding rocks.
Summary of the physical principles defining relative age

principle of uniformitarianism (processes operating today also
operated in the past).

principle of superposition (in undisturbed sedimentary layers, the
oldest layer is on the bottom, the youngest on the top)

principle of original horizontality (layers of sedimentary deposition
are horizontal - folded and tilted beds indicate geologic forces that occurred after
deposition occurred).

principle of original continuity (sediments tend to accumulate in
sheets)

principle of cross-cutting relations (if one feature cross-cuts another,
the one doing the cutting is the younger feature)

principle of inclusions (if an igneous intrusion contains xenoliths, the
intrusion must be younger than the fragments contained within it)

principle of baked contacts (if a plutonic intrusion bakes the rock
surrounding it, the intrusion must be younger than the baked rock)
Determining geologic history.
Beds 1-7 were deposited first. Next came intrusion of the sill, then folding,
intrusion of the granite pluton, faulting, intrusion of the dike, and lastly erosion.
The principles of fossil succession
William Smith, considered by
many to be the father of modern
geology, worked as a British
surveyor in the late 1700’s and early
1800’s He observed that different
fossil species were aggregated in
layers and that fossils tended not to
be found in the strata above or
below. This is the principle of fossil
succession.
An unconformity is a period of erosion or nondeposition. Dry land existing now
will be an unconformity of the future. The interval between the deposition of the
youngest rock below an uncomformity and the oldest rock above is called a hiatus.
An angular unconformity is
where the rocks below the
unconformity were folded or
tilted before the area was
submerged and new
deposition took place.
A nonconformity is
where sedimentary
rocks overlie
igneous or
metamorphic rocks.
Pebbles from the
igneous or
metamorphic rocks
are typically found
in the lowermost
bed of the
sedimentary
sequence.
A disconformity is where the sea level rises and falls and creates parallel
sedimentary beds, but between the regressions and transgressions, the exposed
surface was eroded away before a new deposition period began.
Because of unconformities, no single
location on Earth contains a complete
record of Earth’s history. A stratigraphic
column summarizes information about a
particular location. The boundary between
two formations is called a contact. A
formation can consist of many beds and a
single rock type or multiple rock types.
Several related formations may be called a
group. Lithologic correlations is where
geologists relate formations between
nearby regions using rock type. For
example, the strata on the south side of
the Grand Canyon are related to the strata
on the north side because they contains
the same rock types in the same order.
The succession of rocks in the Grand Canyon are divided into formations, based on
rock type and fossil assemblages.
The Grand Canyon sequence of
strata. Some rock types are more
resistant to erosion than others.
Lithologic correlation means matching rock types by location. In formation C, the
Rufus limestone has been eroded away causing an unconformity. The Franklin
Ash is called a key bed because it is very distinctive and can be correlated with
certainty.
Here, fossil X,
appears in 2
locations. An entire
sequence of strata
present in the Las
Vegas location has
been eroded or
never accumulated
in the Grand
Canyon strata.The
boundary between
Redwall and Temple
Buttle in the G.C.
Column represents
an unconformity.
An angular uncomformity
showing horizontal strata
above and tilted beds below.
Map shows what it would
look like from above.
The geologic column was constructed by determining the relative ages of
stratigraphic columns from around the world.
Life evolution in the context
of the geologic column.
Radiometric Dating
How do we measure geologic time?
Many common minerals contain radioactive elements
 Ex: feldspars, mica, hornblende, zircon, garnet
 Useful elements for radiometric dating include 238U (decays to 206Pb); 235U
(>207Pb); 40K (>40Ar).
 Radiometric clock
 Half-lives of element are constant
 Rate of decay (ratio of parent:daughter isotopes) allows us to calculate
mineral age
The decay of radioactive isotopes starts when crystals cool enough for
parent/daughter isotopes are locked into the lattice.This is called the blocking
temperature. In the case of igneous rock, radiometric dating tells us when it cooled
enough to form a solid. For metamorphic rocks, r.d. tells us when the rock cooled
from the high temperature of metamorphism to a lower temperature. We can’t date
sedimentary rocks directly because the minerals making up the sedimentary rock
crystallized as part of a previous igneous or metamorphic rock.
 Radiometric dating protocol: (1) collect fresh unweathered rocks, (2) separate
out the minerals, (3) extract the parent and daughter isotopes, (4) analyze the parentdaughter ratio.
 Carbon-14 dating is only used for organic material < 70,000 yrs old.
What are isotopes?
Isotopes are atoms with same # of protons but different # of neutrons.
 Isotopes have same atomic number but different atomic weights
 example: Uranium-238 vs. Uranium-235
 Radioactive isotopes undergo decay, converting them to a different element
 Parent isotope – undergoes the decay process
 Daughter isotope – the product of the decay
 Physicists can measure how long it takes for half of a group of an element’s
isotopes to decay
An example of isotope decay:
At time zero (1st diagram), there are 16 parent isotopes. At time 2, half (8) of
those have decayed to form 8 daughter isotopes. At time 3, half of the 8 parent
isotopes (4) have decayed to form 4 more daughter isotopes (8+4=12). At time
4, half of the remaining 4 (2) have decayed to form 14 daughter isotopes
(8+4+2=14), etc.
Other methods of numerical dating:
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counting tree rings (dendrochronology) or sediment layers
organic productivity of lakes and seas (varies according to season)
sediment supply carried by rivers
growth rate of chemically precipitated rocks (i.e. travertine)
growth rate of shell-secreting organisms
proportions of snow to dust in cold climates
Magnetostratigraphy compares the sequence of magnetic reversals in strata
with the sequence of reversals in a global reference column to determine the
age of the strata.
A fission track is
when the
radioactive decay
of a mineral
damages the
crystal lattice
leaving a track.
The number of
tracks relates to the
crystal age.
Numerically dating sedimentary rocks
Geologists date sedimentary rocks by looking at the relationship between sedimentary
rocks and datable (using radiometric methods) igneous or metamorphic rocks in a
formation. As an example, see below where layers of sediment deposited over an
igneous pluton are younger than the pluton. The 80 mya dike cuts across the sandstone
so the sandstone is older than the basalt but younger than the granite, and so on...
A moon rock.
End of Chapter 12