•Relative time -- Subdivisions of Earth's geologic
history are related to actual rock units arranged in a
specific order based on relative age relationships
(most commonly the stratigraphic position). These
subdivisions of the geologic time scale are given
names (based on places where particular rocks were
first described). These subdivisions are recognized
globally, usually on the basis of fossil content.
•Absolute time -- Numerical ages in "millions of
years ago". These dates are most commonly obtained
from radiometric dating methods using carefully
chosen rock samples.
Geological Views of Time
• States that the
today have been
Earth formed. Only
the rate, intensity,
and scale have
Principle of Original Horizontality
Sedimentary rock layers (and
lava flows) are formed in a
horizontal orientation. Any
folding or tilting must have
happened sometime after the
layers were originally formed.
The Law of Superposition is one of
the most basic principles of geology .
This law states that younger rock
layers will be deposited on top of
older layers, during normal
conditions of deposition. This law is
the basic principle of stratigraphy,
the study of sedimentary rock layers.
Stratigraphy is still the single best
method that geologists have for
determining the relative ages of rock
Law of Superposition
The Principle of
that an igneous
intrusion (or a fault)
is always younger
than the rock it cuts
In the diagram at the left, the
igneous intrusion (E) must be
younger than rock layers A,
B, C, and D because it
contains inclusions from
The Principle of Inclusions states that the rock unit
that contains inclusions is younger than the rock
that the inclusions came from.
Gneiss in Granite
• An erosional
become buried by
the deposition of
• This buried
results in a gap in
the rock record.
2 Types of Unconformities
• Disconformity: an
(erosion) between two
• Angular Unconformity: An
unconformity between 1
horizontal rock layer and 1
diagonal (angled) rock layer.
outcrops of one
• Occurs near and
1. Deposition of sediment to form Sandstone (E).
2. Deposition of sediment to form Shale (B).
3. Deposition of sediment to form Limestone (D).
4. Tilting and Erosion
5. Deposition of sediment to form Conglomerate (A).
6. Faulting (F)
7. Intrusion of magma, cooling to form Basalt (C).
1. Deposition of Sediment to form Shale (C)
2. Deposition of Sediment to form Sandstone (A)
3. Deposition of Sediment to form Limestone (L)
4. Deposition of Sediment to form Sandstone (B)
5. Intrusion of Magma, Cooling to form Granite (G)
6. Faulting (F)