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

Earth ScienceEarth Science

Chapter 6 Chapter 6

Earth’s HistoryEarth’s History

Geologic EventsGeologic 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.


• 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.



• 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.


– 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.



• Faults, joints and folds– Younger than the rocks they are found in

Determining Geologic AgesDetermining 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 AgesDetermining Geologic Ages

Correlation TechniquesCorrelation 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.



– Index fossils – fossils of organisms that lived for a short period of time and were dispersed over a wide geographic area.


• 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 DecayRadioactive 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.


– 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.


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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