Discovering Earth’s History 12.1

Uniformitarianism

The concept that processes that have shaped Earth past are essentially the same as those operating today.

Relative Dating

Process by which rocks are placed in their proper sequence or order; Only the chronological order of events is determined, not the absolute age in years.

Key concept

What are the key principle of relative dating?

Relative Dating tells us the sequence in which events occurred, not how long they occurred.

Law of Superposition

Process by which any sequence of sediments or rocks that has not been overturned, the youngest sediments or rocks are at the top of the sequence and the oldest are at the bottom.

Principle of Original Horizontality

Layers of sediment, when originally deposited, are fairly horizontal.

Principle of Cross Cutting Relationship

The principle of cross-cutting relationships

states that an igneous intrusion is always younger than the rock it cuts across.

Key Concept

What do unconformities represent?An unconformity represents a long period

during which deposition stopped, erosion removed previously formed rocks, and then reposition resumed.

Unconformity

A surface that represents a break in rock record, caused by erosion or lack of deposition.

Correlation Establishing the equivalence of rocks of

similar age in different areas.

Key Concept

How do rocks allow geologists to interpret Earth’s history?

Rocks record geological events and changing life forms of the past.

Key Concept

How does uniformatarianism help explain Earth’s features?We can understand the past by understanding

forces and processes that we observe in the present

Key Concept

How do geologists use relative dating in their work?Relative dating tells us the sequence in which

events occurred, not how long ago they occured