Determining Relative AgeDetermining Relative Age

The beginning of modern The beginning of modern geologygeology

James Hutton (1726 – 1797)James Hutton (1726 – 1797)

• Father of Modern Father of Modern GeologyGeology

• Formulated Formulated UniformitarianismUniformitarianism

• Also formulated Also formulated Plutonic TheoryPlutonic Theory

• Thought that the Thought that the Earth was very Earth was very old.old.

UniformitarianismUniformitarianism

• A principle that assumes geologic processes that A principle that assumes geologic processes that occurred in the past can be explained by current occurred in the past can be explained by current geologic processes.geologic processes.– Examples: Examples:

• Past volcanoes act just like modern Past volcanoes act just like modern volcanoes.volcanoes.

• Erosion occurs today just like it did long Erosion occurs today just like it did long ago.ago.

• Mountains are forming and eroding just Mountains are forming and eroding just like they did millions of years ago.like they did millions of years ago.

Hutton and the Earth’s AgeHutton and the Earth’s Age

• In Hutton’s day most In Hutton’s day most people thought that people thought that the Earth was about the Earth was about 6,000 years old6,000 years old

• Hutton observed Hutton observed geologic changes on geologic changes on his farm were very his farm were very slow and determined slow and determined that the Earth must that the Earth must be infinitely old.be infinitely old.

MOST GEOLOGISTS TODAY AGREE THAT THE EARTH IS ABOUT 4.6 BILLION YEARS OLD

Relative AgeRelative Age

• Layers of rock, called Layers of rock, called stratastrata, show the , show the sequence of events sequence of events that took place in the that took place in the past.past.

• Relative age indicates Relative age indicates that one layer is that one layer is older or younger older or younger than another layer.than another layer.

RELATIVE AGE DOES NOT GIVE A LAYER’S AGE IN YEARS

Law of SuperpositionLaw of Superposition

• A sedimentary rock layer, or bed, is older than A sedimentary rock layer, or bed, is older than the layers above it and younger than the layers the layers above it and younger than the layers below it if the layers are not disturbed.below it if the layers are not disturbed.

Principle of Original HorizontalityPrinciple of Original Horizontality

• Sedimentary rocks left undisturbed Sedimentary rocks left undisturbed will remain in horizontal layers.will remain in horizontal layers.

• Sedimentary rock layers that are not Sedimentary rock layers that are not horizontal must have been acted on horizontal must have been acted on by forces within the Earth.by forces within the Earth.

• Sometimes these forces can force Sometimes these forces can force older rock on top of younger rock.older rock on top of younger rock.

Law of Superposition and the Principle of Law of Superposition and the Principle of HorizontalityHorizontality

Clues to Original HorizontalityClues to Original Horizontality

• Graded BeddingGraded Bedding– Larger particles should be on the bottom of the Larger particles should be on the bottom of the

layers.layers.• Cross BedsCross Beds

– Caused by soft sediments, like sand, slumping to Caused by soft sediments, like sand, slumping to form curved layers with respect to the bedding form curved layers with respect to the bedding plane. The bedding plane is the boundary between plane. The bedding plane is the boundary between two layers, or beds.two layers, or beds.

• Ripple MarksRipple Marks– Ripple marks, formed by water, point up in Ripple marks, formed by water, point up in

undisturbed layers.undisturbed layers.

Clues to Original HorizontalityClues to Original Horizontality

UnconformitiesUnconformities

• A break in the geologic record created A break in the geologic record created when rock layers are eroded or when when rock layers are eroded or when sediment is not deposited for a long sediment is not deposited for a long period of time.period of time.

• There are three types of unconformitiesThere are three types of unconformities– NonconformityNonconformity– Angular unconformityAngular unconformity– DisconformityDisconformity

NonconformityNonconformity

• When stratified rock (sedimentary) When stratified rock (sedimentary) rests on unstratified rock (igneous).rests on unstratified rock (igneous).

Laws of Crosscutting RelationshipsLaws of Crosscutting Relationships

• A fault is a break or crack in the Earth’s crust A fault is a break or crack in the Earth’s crust along which rocks shift their position.along which rocks shift their position.

• An intrusion is a mass of igneous rock which is An intrusion is a mass of igneous rock which is injected into rock and then cools and solidifies.injected into rock and then cools and solidifies.

• A fault or igneous intrusion is always younger A fault or igneous intrusion is always younger than the rocks it cuts through.than the rocks it cuts through.

Laws of Crosscutting RelationshipsLaws of Crosscutting Relationships

Laws of Crosscutting RelationshipsLaws of Crosscutting Relationships

Angular UnconformityAngular Unconformity

• When horizontal layers of rock rest When horizontal layers of rock rest on top of tilted layers of rock.on top of tilted layers of rock.

DisconformityDisconformity

• Occurs when younger rock is Occurs when younger rock is layered on top of older rock that has layered on top of older rock that has already been eroded.already been eroded.

http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization

Principle of Inclusions• Included rock fragments must be older than

the rock in which they are found.

Principle of Faunal SuccessionPrinciple of Faunal Succession

• Fossils in rocks occur in a fixed, worldwide sequence.

• These fossils can be used to correlate and relatively date rocks in which they are found. Index Fossil – widespread and is

associated with a specific span of geologic time