Geologic Time
Historical Geology Lab # 2
Introduction
• Geology is different from most other
sciences in its relationship with time. It is
necessary to appreciate the immensity of
geologic time in order to understand the
physical and biological history of Earth.
Geologists use two different frames of
reference when speaking of geologic
time.
Relative Age
• Placing geologic events in chronological
order
• Determined from their position……
– when discussing relative age, one rock is
simply older or younger than another.
Absolute Age
• Various methods of producing an actual
age of a rock unit expressed in years.
• Most useful and widely used method is
radiometric dating.
Relative Ordering Geologic Events
• Relative ages are determined by their position
in relation to other rocks and structures
• 1600’s – Nicholas Steno first described the
principles of determining relative age
• 1700’s – James Hutton; unconformities
• 1800’s – William Smith and Georges Cuvier;
fossils
Nicholas Steno
• Principle of
Superposition
• Principle of Original
Horizontality
• Principle of Lateral
Continuity
• These principles enabled
workers to develop the
first geologic maps
James Hutton
• “Father of Geology”
• Formulated the
Principle of CrossCutting Relationships
• The first to recognize
unconformities, or
gaps in the geologic
record
William Smith & Georges Cuvier
• Recognized the
value of using fossils
in determining
relative age of widely
separated regions
Principle of Original Horizontality
• States that sedimentary layers are
generally deposited in a horizontal or
near horizontal position
• If the rock layers are dipping in the cross
section, then some episode of
deformation must have occurred after
deposition
“Something”
happened here
Principle of Superposition
• States that in a sequence of undisturbed
sedimentary rock, the oldest beds are on
the bottom and the layers above become
successively younger
Superposition
Principle of Lateral Continuity
• States that sedimentary deposits extend
laterally in all directions until they thin or pinch
pit against the edges of the depositional basin.
•
(Using this principle we can assume the rock
units on one side of a canyon were at one time
laterally continuous across the canyon, and
later eroded. This allows for determining if
widely separated units are equivalent.)
•Principle of Lateral Continuity
Principle of Cross-Cutting
Relationships
• States that an igneous intrusion or fault
must be younger than whatever unit or
structure it cuts across.
•Cross-Cutting Younger Intrusion
Principle of Inclusions
• Used to determine relative age
• States that if there are fragments of one rock
type contained within another rock, the
fragments must be older
• Therefore, if there are igneous rock fragments
within the surrounding sedimentary rocks, the
igneous rock must be older. (This type of
contact is called a nonconformity)
•Principle of Inclusions - Nonconformity
•Igneous Intrusion
Principle of Unconformities
• An unconformity is an erosional or
nondepositional surface which represents
a time period for which we are missing
geologic information
• On geologic cross sections these are
indicated with wavy lines
Types of Unconformities
• Three types – all separate younger
rock layers from older rock layers:
- Disconformity
- Angular unconformity
- Nonconformity
Disconformity
• A type of unconformity where older layers below
the erosional surface are parallel to the younger
layers above the erosional surface
Angular Unconformity
• A type of
unconformity in
which the older
layers below the
erosional surface are
not parallel to the
younger layers above
Nonconformity
• The type of unconformity which forms
when an older igneous or metamorphic
rock is exposed and eroded and the
younger sedimentary rocks are deposited
above
Nonconformity
Correlating with Fossils
Absolute Age
• Isotopes are atoms of the same element (same
atomic number) with different atomic masses
(different number of neutrons).
• Unstable isotopes emit nucleic particles to try to
become more stable
• The release of nuclear particles is known as
radioactive decay
Natural Isotopes of Carbon
•Radioactive isotopes can be used to
determine the absolute ages of rocks because
the rate of radioactive decay is constant for
each particular isotope
Half-Life
• The amount of time required for one-half of the
original parent radioactive isotope to decay to is
stable daughter isotope
• The amount of parent isotope will never reach
zero. (This means if we graph the ratio of
parent to daughter changes with age, we will
obtain a decay curve, which approaches, but
never intersects the horizontal axis.)
Radioactivity and Time
Dendochronlogy
• Science of studying annual tree rings to
obtain absolute ages
• Annual tree rings have different sizes and
densities, depending on rainfall and
temperature, allowing climatologists to
determine times of warming, drought, etc.
Varves
• Type of sedimentary deposit
• Alternating layers that form by seasonal
variations in sedimentation
• May form in lakes that freeze each winter
depositing fine silt; summer deposits are
coarser; each pair of layers represents
one year
Varves
• May also form in evaporite deposits
• Permian Castile Formation of rock
gypsum in eastern New Mexico and west
Texas have well-developed varves of
dark calcite and white gypsum
• Pair of light and dark layers indicate one
year
Varves
Exposure of
varved clays in
Ontario, Canada.
Assignment
• Lab # 2 Geologic Time
• Information page 7 – 16
• Lab Activity page 17 - 22