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Earth’s History
Geologic Events
“The present is the key to the past”.
This statement summarizes one of the basic
principles of geologic history – that the
geologic processes going on today, such as
weathering, erosion, volcanism, and
earthquakes, also went on in the past.
This concept is known as the
principle of uniformitarianism.
Sequence of Geologic Events
Relative Age versus Absolute Age
The relative age of a rock or event is
the age as compared to other rocks
or events.
The absolute age of a rock or event
is the actual age.
Chronology of Layers
There are two major principles used
when interpreting geologic history:
• Principle of original horizontality;
• Principle of superposition.
Principle of Original Horizontality
• The principle of original horizontality
states that sediments are deposited in
horizontal layers that are parallel to the
surface on which they were deposited.
• This implies that tilted or folded layers
indicate that the crust has been
deformed.
Original
Horizontality
Principle of Superposition
• The principle of superposition states
that, in a series of undisturbed layers,
the oldest layer is on the bottom and
each overlying layer is progressively
younger with the youngest layer on the
top.
Superposition
Evidence of Events
• Several things provide geologists with
evidence of events:
• Igneous intrusions and extrusions;
• Faults, joints and folds;
• Internal characteristics.
Igneous Intrusions and Extrusions
When magma forces its way into cracks or
crevices in crustal rock and solidifies, it
forms a mass of igneous rock called an
intrusion.
When lava solidifies at the surface it forms a
mass of igneous rock called an extrusion.
Since the rock that the magma moved
through, or over, existed prior to the
intrusion, (or extrusion), it must be older.
Igneous Intrusions
Faults, Joints, and Folds
• Features such as faults, joints,
and folds must be younger than
the rocks in which they are
found.
– A joint is a crack in a rock
formation, similar to a fault but
without any displacement.
Internal Characteristics
• Cracks, veins, and mineral
cement are younger than the
rocks in which they appear.
• A vein is a mineral deposit
that has filled a crack, or
permeable zone, in existing
rock.
Correlation Techniques
• Correlation is the process of matching
rocks and geologic events in one
location to the rocks and events in
another location.
• Methods used for correlation include:
– Continuity of rocks
– Fossil evidence in rocks
– Volcanic time markers in rocks
Correlation
Bedrock and Outcrops
• Bedrock is the solid, unbroken rock of
the crust.
• An outcrop is bedrock that is exposed
at the Earth’s surface.
– Outcrops provide opportunity for
geologists to directly study the layers
of the bedrock, tracing them from
one location to another – called
“walking an outcrop”.
Bedrock / Outcrop
Fossil Evidence
• Index fossils are fossils or
organisms that lived over an
extensive area, preferably over the
entire Earth, for relatively short
periods of time.
• Index fossils are useful in
correlating the sedimentary rocks
in which they are found.
Index Fossils
Fossil Correlation
Volcanic Time Markers
• Severe volcanic eruptions can deposit a
thin layer of volcanic ash over the
surface of the entire Earth.
• These layers within a rock sequence
may remain distinguishable and
provide a time marker. (Similar to
index fossils)
Anomalies
• Anomalies are differences from
what is expected.
Determining Geologic Ages
• Without the rock record there would be
no geologic history.
• The older the rock the more difficult to
determine absolute age.
• The geologic history of an area is
determined primarily by fossil evidence,
the age of the rocks, and the erosional
record in the rocks.
Geologic Time Scale
• Geologic time is subdivided divided into
units based on fossil evidence.
• There are 4 major divisions:
– Precambrian – represents the first 85% of
Earth’s history (mostly devoid of fossils).
– Paleozoic Era – represents ~ 8.5% of Earth’s
history (invertebrates, fishes, amphibians,
vertebrates and land plants first appear).
– Mesozoic Era – rep. ~ 3.5% of Earth’s history
(dinosaurs, earliest birds, and mammals).
– Cenozoic Era – rep. ~ 1.4% of Earth’s history
(humanoids show up late ~0.04% of history).
Erosional Record
• Buried erosional surfaces, called
unconformities, indicate gaps or breaks in
the geologic time record.
• 4 steps combine in sequence to form
unconformities:
– Uplift, erosion, submergence and deposition
• 3 most common types of unconformities are:
angular unconformities, parallel
unconformities, and nonconformities.
Radioactive Decay
• Radioactive decay occurs when the nuclei of
unstable atoms break down, changing the
original atoms into atoms of another
element.
• The rate of radioactive decay is measured in
terms of half-life.
– Half-life is the amount of time it takes for half
the atoms of a substance to decay into another
element.
– Different substances have different half-life’s
• Examples are Uranium 238 and Carbon 14.
History and Evolution of the
Earth’s Atmosphere
• Earth is about 4.5 billion years old.
• The primary source of gases for the
earliest atmosphere are thought to be
from outgassing by volcanoes.
• Water vapor in the outer atmosphere
would have been broken down into
hydrogen and oxygen.
Fossil Record
• Fossils preserved in the rocks
provide evidence that many
kinds of animals and plants
have lived on Earth in the past
under a variety of different
conditions.