Geologic Time
“Consider the Earth’s History as the old
measure of the English yard, the
distance from the king’s nose to the tip
of his outstretched hand One stroke of
a nail file on his middle finger erases
human history.”
John McPhee, 1980, Basin & Range
Absolute or Relative?
• Anthony is the youngest.
• Melony is 4 years old.
• Michael is older then
Sasha.
• Sasha is 16 years old.
• Ashley is older than
Melony, but younger than
Sasha.
Relative Dating
• Used to determine the order of
events and the relative age of
rocks by looking at the position of
rocks in a sequence
• Numbers are not used
• Ex: “I am older than you.”
Law of Superposition
• In an undisturbed layer of rock,
the oldest rocks are on the bottom
and the younger rocks are toward
the top
Law of Original
Horizontality
• Layers of sediment and lava are
deposited in horizonatal sheets.
• If layers are not horizontal, they
have been moved
Folding
Law of Inclusions
• Inclusions are rocks, crystals, or
fossils in another kind of rock.
• Any inclusion is older than the
rock that contains it.
Law of Cross-Cutting
• Any feature that cuts across a
rock or sediment must be younger
than the rock through which it
cuts.
• Faults, veins, and igneous
intrusions
Unconformity
• Gap in the geological record
–Due to erosion
–Layers not deposited
–Sediment on igneous or
metamorphic rock
Clues from Fossils
• Fossils in layers of rock
show definite order.
• Index fossils
ID ages of different rocks