Visualizing Earth Science
By Z. Merali and B. F. Skinner
Chapter 10 – How Old is Old?
The Rock Record and Deep Time
Chapter Overview
• Relative Age
• The Geologic Column
• Numerical Age
• The Age of Earth
The Age Debate:
A change of perspective
• Catastrophism
– Large scale geologic features formed in catastrophic
events
– The age of Earth measurable in human historical
terms
• Uniformitarianism
– Past and present geological processes similar
– Discarded catastrophic formation
– Earth processes can be observed to understand past
– Proposed the Earth was ancient much beyond the
span of human history
Relative Age
•
•
Determination of sequence of
geological events
Use stratigraphy – Science of rock
layers and strata forming processes
– Law of original horizontality
• Beds or strata laid in water are laid
horizontal
Relative Age
•
The principle of stratigraphic superposition
-
Undisturbed strata have younger layers above older ones
Relative Age
• Principles of stratigraphy
– Principle of cross cutting relationships
• Feature cutting through or disrupting a stratum – younger than the
stratum
Unconformities: Gaps in the record
• Numerical age
– Time elapsed
since a feature
formed
• Unconformity
– A chronological
gap in
sequence of
rock layers
Correlating Layers
• Fossils
– Signs of ancient living
organisms
• Usually consist of preserved
hard parts
– Paleontology
• Study of fossils and ancient life
– Fossils useful to correlate rock
layers by age
– Fossil succession – determine
sequences in fossil records
Correlating Layers
Sequencing geologic time
• The geologic column
– Visual, suggestive representation
of geologic time
– Based on succession of rock strata
– Major division -- Eon
– Subdivisions – Eras, Periods and
Epochs
– Four eons
• Current eon: Phanerozoic –
visible life
– Eras: Paleozoic, Mesozoic,
Cenozoic
• Past: Hadean, Archean,
Protrozoic
Sequencing
geologic time
Periods and Epochs
Recent fossil record
•
Eras divided into periods
– Based on fossil groups
– Some period boundaries marked
by extinctions
•
The Cambrian period
– First of the Paleozoic era
– Animals with hard shells
– Period of “explosion” in biodiversity
•
•
Precambrian – the whole of
geologic time before the Cambrian
period
Periods last for multi-million years
– Subdivided into epochs
Numerical Age
• Attempts made at estimating the age of Earth
– Rate of sedimentation and thickness of strata
• Too much variability in estimates
– Ocean salt deposit rate (Halley)
• Flaw in assumption – salt content not varying
– Darwin’s estimate based on erosion processes
• Rejected on the basis of thermodynamic cooling of
Earth
– Discovery of radioactivity reveals internal heating of
Earth
– Modern estimates (> 4.5 billion years)
• Based on radiometric dating
Numerical Age
Radioactivity and Numerical Ages
• Rocks contain internal clock
– Radioactive matter decays at determinable rate
– Development of sensitive instruments measuring
• Rates of radioactive decay
• Concentrations of elemental substances
– Enabled measurement of numerical ages of rocks
Radioactivity and Numerical Ages
Radiometric dating
Half life
• Radioactive atoms decay
into daughter products
– Half life: time taken for half
the atoms in sample to
decay
• Known half life and known
initial concentrations of
– Parent atoms
– Daughter atoms
• Provide accurate
estimates of age when
final concentration are
measured
Radiometric dating
• Radiometric dating difficulties
– Need to find
• Rock samples with initially discoverable
parent/daughter ratios
• No contaminations to externally change parent
daughter ratio after rock formation
– Different dating techniques found for
• Rocks, fossils and biological material
– Based on estimated age and composition
Radiometric
dating
Radiometric dating and the geologic column
• Finding sedimentary
strata with igneous
inclusions
– Provides an
application of the
radiometric dating
technique to the
geologic column
• Fossil correlated
layers can be dated
– using the
principles of
stratigraphy
– Numerical dates
from radiometric
dating
Magnetic Polarity Dating
• Earth’s magnetic field
periodically reverses
– During rock formation
• Orientation of magnetized mineral
grains locked in
• Allows determination of magnetic
field polarity
– Field polarity measurements
along with radiometric dating
• Provide a timescale for more
recent geologic time
• Used to date history and
succession of hominid fossils
Modern estimates of the age of Earth
• Earth has volcanic,
geological and biogenic
activity
– Pristine rock from early Earth
difficult to find
– Oldest sedimentary rock mineral
dated to 4.4 billion years
• However sedimentation requires
prior rock formation and erosion
• Earth older than oldest rock
– Moon rock samples and
meteorites provide clues to age
of planetary formation
– Oldest meteorite 4.56 billion
years old
Chapter Summary
• Relative Age
- The Age Debate
- Stratigraphy: Principles and Application
- The Fossil Record
• The Geologic Column
- Sequencing Geologic Time: Eons, Eras, Epochs and Periods
• Numerical Age
- Numerical Age: The Age of Earth Problem
- Radioactivity and Radiometric dating
- Magnetic Polarity Dating
• The Age of Earth
- Modern estimates of the Age of Earth