Geologic History

One exciting thing from geologic history is the fossil records of coral.

Key Words in Geologic History

Word	Definition
Original horizontality	rock layers were originally horizontal when they were formed by deposition of sediments.
Superposition	the lower layer is older, and the layer above is younger, when the layers were not disturbed or overturned.
Radioactive half-life period	the time for half of the atoms in a sample of radioactive isotope to decay
Index fossils	fossils that are easy to recognize, existed for a brief period of time, and are found over a large geographic area.

Key Things to Know in Geologic History

Earth's early atmosphere formed from outgassing of volcanoes.
The half-life of a radioactive element can't be changed.
Index fossils are good time markers (widely spread, lived a short time).
Undisturbed layers - bottom layer is oldest (this is the Law of Superposition).
Intrusion and faults are younger than the rock they are in.
Unconformity means erosion (time gap in the layers).
Uranium 238 is used to date old rocks.
Carbon 14 is used to date recent living objects.

Example 1

Which graph best represents human existence on Earth, compared with Earth’s entire history?
exercise Q 5 picture

ESRT p. 8. shows humankind starts at 1.8 mya while the Earth is 4600 mya. People presence is less than 0.04%. (4)

Exercises

1. How old is a fossil that has radioactively decayed through 4 half-lives of carbon-14?
(1) 5,700 years (3) 22,800 years
(2) 17,100 years (4) 28,500 years

Hint: Apply the concept of radioactive half-life period.