Transcript Relative Age of Rocks

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