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