Geologic Time
Chapter 8
Determining geological ages
Relative age dates – placing rocks and
geologic events in their proper sequence
Numerical dates – define the actual age of
a particular geologic event (termed
absolute age dating)
First principle of relative dating
Law of superposition
• Developed by Nicolaus Steno in 1669
• In an undeformed sequence of
sedimentary or volcanic rocks, oldest
rocks at base; youngest at top
Superposition illustrated by strata
in the Grand Canyon
2nd, 3rd principles of relative dating
Principle of original horizontality
• Layers of sediment are originally
deposited horizontally (flat strata have
not been disturbed by folding, faulting)
Principle of cross-cutting relationships
• Younger features cut across older ones
Cross Cutting Relationships in strata
Grand Canyon younger strata cutting across older ones
Cambrian Tapeats sandstone over Precambrian Unkar Group
Unconformities (loss of rock record)
• An unconformity is a break in the rock record
produced by erosion and/or nondeposition
• Types of unconformities
– Angular unconformity – tilted rocks overlain by flatlying rocks
– Disconformity – strata on either side of the
unconformity are parallel (but time is lost)
– Nonconformity – sedimentary rocks deposited above
metamorphic or igneous rocks (basement)
Formation of an angular unconformity
An angular unconformity at Siccar Point, England
Development of a Nonconformity
The basement-cover contact near
Boulder (Pennsylvanian sandstone
over Precambrian granite) is a
nonconformity (visible on Flagstaff
Road near bouldering area)
Nonconformity in the Grand Canyon - Strata deposited over Schist
Correlation of rock layers
Matching strata of similar ages in
different regions is called correlation
Correlation of strata in
southwestern United States
Sections are
incomplete
Correlation of rock layers with fossils
Correlation relies upon fossils
• Principle of fossil succession – fossil
organisms succeed one another in a
recognizable order - thus any time period
is defined by the type of fossils in it
Determining the ages of
rocks using fossils
Geologic time scale
The geologic time scale – a “calendar” of
Earth history
• Subdivides geologic history into units
• Originally created using relative dates
Structure of the geologic time scale
• Eon – the greatest expanse of time
Geologic Timescale
Divisions based on fossils
Geologic time scale
Structure of the geologic time scale
• Names of the eons
– Phanerozoic (“visible life”) – the most recent
eon, began about 540 million years ago
– Proterozoic
– Archean
– Hadean – the oldest eon
This, believe it or not, is the rock with the oldest known minerals
ever found. From NW Australia, the rock (a conglomerate) is
about 3.0 Billion years old. The rock contains detrital grains of
zircon (a mineral formed in granite in the crust) that is 4.4 Billion
years old. Age of the Earth is 4.54 Billion (sample and age date
courtesy of Steve Mojzsis)
QuickTime™ and a Sore nson Vid eo d eco mpres sor a re ne eded to see this picture .
Geologic time scale
Structure of the geologic time scale
• Era – subdivision of an eon
• Eras of the Phanerozoic eon
– Cenozoic (“recent life”)
– Mesozoic (“middle life”)
– Paleozoic (“ancient life”)
• Eras are subdivided into periods
• Periods are subdivided into epochs
Geologic time scale
Precambrian time
• Nearly 4 billion years prior to the
Cambrian period
• Not divided into small time units because
the events of Precambrian history are not
know in detail
• Immense space of time (Earth is ~ 4.5 Ga)
Radioactivity (Used to age date rocks)
• Spontaneous changes (decay) in structure
of atomic nuclei
Types of radioactive decay
• Alpha emission
– Emission of 2 protons and 2 neutrons (an
alpha particle)
• Beta emission
– An electron (beta particle) is ejected from the
nucleus
• Electron capture
– An electron is captured by the nucleus
– The electron combines with a proton to form
a neutron
Neutron capture (A)
and Beta emission (B)
Using radioactivity in dating
Parent – an unstable radioactive isotope
Daughter product – isotopes resulting
from decay of parent
Half-life – time required for one-half of
the parent isotope in a sample to decay
A radioactive decay curve
Dating with carbon-14 (radiocarbon dating)
• Half-life only 5730 years
• Used to date very young rocks
• Carbon-14 is produced in the upper
atmosphere
• Useful tool for geologists who study very
recent Earth history (for me this is the
history of earthquakes).
Using radioactivity in dating
Importance of radiometric dating
• Allows us to calibrate geologic timescale
• Determines geologic history
• Confirms idea that geologic time is immense
How do we actually “date” a rock?
1. Collect sample (geologist as pack animal)
2. Process for minerals by crushing, sieve, separate
magnetically and/or with heavy liquids
3. Measure parent/daughter ratio of mineral
separates with a mass spectrometer
Dating sediments without fossils
In-class exercise: Determining the age of a young sedimentary rock with
carbon-14
1)
2)
3)
4)
Take out a piece of paper and put your name on it to obtain credit
Assume a half-life of 5730 years.
Determine the age of the sample after three half-lives
Determine the amount of parent material in the rock after three half lives
5)
6)
7)
8)
Determine the age of a Precambrian igneous rock using Uranium 238.
Assume a half life of 2.25 Billion years
Assume the rock is 4.5 Billion years old
Determine the relative amount of parent and daughter isotopes (or ratio
relative to one another.
End of Chapter 8