Earth Science: Relative Dating Principles & Geologic Time Scale

What is Relative Dating?

A method used to determine the sequence of geologic events without providing exact numerical ages.

What does the Law of Superposition state?

In undisturbed sedimentary rock layers, the oldest layers are at the bottom and the youngest are at the top.

What is the Law of Original Horizontality?

Sediments are originally deposited horizontally; if layers are tilted or folded, they were disturbed after deposition.

What is the Principle of Lateral Continuity?

Sedimentary layers extend sideways until they thin out or meet a barrier, indicating that matching layers were once continuous.

What does the Principle of Cross-cutting Relationships indicate?

A rock or fault that cuts across another rock is younger than the rock it cuts.

What is the Principle of Fossil Succession?

Fossils occur in a predictable vertical order in rock layers and can be used to correlate rock layers in different locations.

What does the Principle of Inclusion state?

If rock fragments (inclusions) are found inside another rock, the inclusion is older than the surrounding rock.

What are unconformities in geology?

Gaps in the rock record due to erosion or lack of deposition.

What is a Disconformity?

Layers above and below are parallel, but a gap or erosion surface exists.

What is a Nonconformity?

Sedimentary rocks lie on eroded igneous or metamorphic rocks.

What is an Angular Unconformity?

Tilted or folded layers are overlain by horizontal layers.

What is the Geologic Time Scale (GTS)?

A system that divides Earth's 4.55-billion-year history into structured time units based on relative dating principles.

What are the major divisions of the Geologic Time Scale?

Eon, Era, Period, and Epoch.

What era marked the first appearance of dinosaurs?

Mesozoic Era, specifically the Triassic Period.

What period marked the extinction of dinosaurs?

Cretaceous Period, at the end of the Mesozoic Era.

What significant event occurred during the Paleozoic Era?

The Cambrian Explosion, which marked rapid evolution of major animal groups.

What was the Great Dying?

The Permian Extinction, which resulted in the loss of 96% of marine life.

When did the first mammals appear?

During the Triassic Period, approximately 220 million years ago.

What is the significance of the Snowball Earth event?

It occurred from 850 to 635 million years ago and marked a period of severe glaciation.

What major evolutionary milestone occurred in the Cenozoic Era?

The eventual evolution of hominids, including Australopithecus.

What is the sequence of major events in Earth's history from oldest to youngest?

Snowball Earth, First Fish, First Amphibian, First Reptile, Formation of Pangaea, Permian Extinction, Triassic-Jurassic Extinction, First Mammal, First Bird, Cretaceous-Tertiary Extinction.

What is the basis for the Geologic Time Scale?

It is based on major geological and biological events such as extinctions and evolutionary milestones.