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Earth Science
Chapter 6
Earth’s History
Geologic Events
• Geologic history – is the study and
interpretation of the Earth’s past.
• Relative age versus absolute age
– The relative age of a rock or event is
determined by comparing the ages of rocks or
events in a sequence.
– The absolute age is the actual age of the
rocks or events.
Geologic Events
• Principle of uniformitarianism – geologic
processes that are occurring today also
occurred in the past.
• Principle of original horizontality – sediments are
deposited in horizontal layers parallel to the
surface on which they were deposited.
• Principle of superposition – in a series of
undisturbed layers the oldest rock is on the
bottom and each layer above becomes
progressively younger.
Geologic Events
Geologic Events
• Igneous intrusions and extrusions
– An intrusion is when magma forces its way
into cracks in crustal rocks and solidifies
beneath the Earth’s surface.
• Since the rocks through which the magma
moved existed prior to the intrusion, they
must be older than the intrusion.
Geologic Events
– An extrusion is when lava solidifies above the
Earth’s surface.
• Rock layers below the extrusion are older
and rock layers above it are younger.
• To determine if igneous rock is an intrusion
or extrusion, look for contact
metamorphism.
• If contact metamorphism exists, the
igneous rock is younger than the
surrounding rock, if not, it is older.
Geologic Events
Geologic Events
• Faults, joints and folds
– Younger than the rocks they are found in
Determining Geologic Ages
• Unconformities – buried erosional surfaces
that indicate gaps or breaks in the
geologic time record.
– Indicate that some of the layers in a rock
record are missing.
– Useful in determining the relative ages of
rocks because they can explain why a rock
can occur between two layers in one location,
but be missing from another.
Determining Geologic Ages
Correlation Techniques
• Correlation – the process of matching rocks
and events at one location with rocks and
events in another location.
– Rocks at different locations can often be
tentatively matched based on similarities in
appearance, color, and composition.
– Volcanic eruptions can deposit a thin layer of
ash over a wide area which aids in correlation.
Correlation Techniques
Correlation Techniques
– Index fossils – fossils of organisms that lived
for a short period of time and were dispersed
over a wide geographic area.
Correlation Techniques
• Anomalies – deviations from what is
expected.
– Example: two very similar rock formations
may actually be of different ages.
– Careful observation and cautious
interpretations can minimize errors due to the
presence of anomalies.
Radioactive Decay
• Radioactive decay – occurs when the
nuclei of unstable atoms break down,
giving off particles and energy.
– Changes the original atoms to atoms of
another element.
– The rate of radioactive decay is measured in
terms of half-life.
– The half-life is the time it takes for one half of
the atoms to decay to another element.
Radioactive Decay
– Different radioactive substances have
different half-lives (see ESRTs).
– The age of a rock can be inferred from the
relative amounts of undecayed radioactive
substance and the decayed product.
– Radioactive decay has helped determine
thousands of dates for events in Earth history.
– Geologists have inferred the age of the earth
to be over 4.5 billion years.
Radioactive Decay