Q1: Lesson 2: Relative and Absolute Dating + Fossils

Relative Dating

Places events or rocks in chronological sequence without knowing the actual age

Absolute Dating

Determines actual age of rocks or events from the decay rate of certain radioactive isotopes

Original Horizontality; Superposition; Cross-Cutting Relationship; Inclusion

Enumerate the four (4) principles of Relative Dating

Original Horizontality

• Sedimentary rocks are deposited as horizontal or nearly horizontal layers

• Deviation indicates deformation after deposition

Superposition

In the sequence of sedimentary rocks, the bottom layer is the oldest

Cross-Cutting Relationship

Geologic features like faults or igneous intrusions are younger than the rocks they cut across

Inclusion

Fragments within another rock layer are older than their embedded rock layer

True Form; Mold; Cast; Trace (Ichnofossils)

Enumerate the four (4) types of fossils

True Form

• Entire animals or plants

• Preserved in ice, tar, or other materials

• E.g. bones, flies in amber, etc.

Mold

• Hollow impression of a living thing in a rock

• Only reflects the shape

• E.g. shell mold on rocks

Cast

• Created when mold fossils get filled with mineral

• Replica of organism

• E.g. whole rock shaped as a shell with no hole for the organism

Trace

• Impressions on rocks that showed their various activities

• E.g. footprints, eggs, droppings, nests, etc.

Half-life

Amount of time necessary for one-half of the nuclei in a sample (parent) to decay into a stable isotope