Reading Geologic Layers

AZ State Standards

• Concept 3: Origin and Evolution of the Earth System

• Analyze the factors used to explain the history and evolution of the Earth.

• PO 4. Interpret a geologic time scale.

• PO 5. Distinguish between relative and absolute geologic dating techniques.

Content Objectives

• SWBAT use the laws of superposition, original horizontality, and lateral continuity to interpret the relative ages of geologic layers in a cross-section.

Fault Types

Law of Superposition

Unless moved by faulting, the higher layers are the younger layers because they were deposited on top of the previously existing strata

The Layers on Top Are the Youngest

Oldest

Youngest

Principle of Original Horizontality

• All geologic layers are originally deposited horizontally due to gravitational action spreading them out.

• If there is a slope, then the material will move downhill until there is no longer any gradient.

• They can only become angled, tilted, or twisted through later geologic processes.

Erosion Brings Sediment Down From Above

Sediment from the Ridges Have Accumulated onto the Valley Floor

Principle of Lateral Continuity

• If there is a break in the layers due to erosion, they can still be connected laterally and be assumed to be the same formation.

Erosion

• Erosion always works hardest on the highest points.

• Once things are flattened out, new layers can be deposited horizontally on that.

Violations of Original Horizontality • This principle does not hold for volcanic dikes

(since they are liquid when formed).

Violations of Original Horizontality

• It also does not hold for large grained sediments that can grip to each other due to friction such as sand dunes and alluvium.

Law of Inclusions • If a chunk of rock is found in a sedimentary

layer, then it must be older than the sediments it was deposited in.

Reading the Rock Layers • Angular unconformity: Unconformity between

sedimentary strata where the older underlying layers are tilted and the erosional surface is irregular.

• Disconformity: Unconformity between parallel layers of sedimentary rock which is recognizable due to an irregular erosional surface.

• Nonconformity: An irregular erosional surface between strata and younger igneous or metamorphic rock.

Unconformity

• The surface between two different sedimentary strata. It represents an interval of time during which erosion occurred rather than deposition.

Conformities

List Each Formation In Order from Oldest to Youngest

Answers

• A

• D

• C

• E

• B

• G

• F

• H

List Each Formation In Order from Oldest to Youngest

The Principles of Geologic Layers Can Also Be Applied to Archaeology

Content Objectives

• SWBAT use the laws of superposition, original horizontality, and lateral continuity to interpret the relative ages of geologic layers in a cross-section.