Determining Relative Age The beginning of modern geology

James Hutton (1726 – 1797) Father of Modern GeologyFather of Modern Geology Formulated UniformitarianismFormulated Uniformitarianism Also formulated Plutonic TheoryAlso formulated Plutonic Theory Thought that the Earth was very old.Thought that the Earth was very old.

Uniformitarianism A principle that assumes geologic processes that occurred in the past can be explained by current geologic processes.A principle that assumes geologic processes that occurred in the past can be explained by current geologic processes. –Examples: Past volcanoes act just like modern volcanoes.Past volcanoes act just like modern volcanoes. Erosion occurs today just like it did long ago.Erosion occurs today just like it did long ago. Mountains are forming and eroding just like they did millions of years ago.Mountains are forming and eroding just like they did millions of years ago.

Hutton and the Earth’s Age In Hutton’s day most people thought that the Earth was about 6,000 years oldIn Hutton’s day most people thought that the Earth was about 6,000 years old Hutton observed geologic changes on his farm were very slow and determined that the Earth must be infinitely old.Hutton observed geologic changes on his farm were very slow and determined that the Earth must be infinitely old. MOST GEOLOGISTS TODAY AGREE THAT THE EARTH IS ABOUT 4.6 BILLION YEARS OLD

Relative Age Layers of rock, called strata, show the sequence of events that took place in the past.Layers of rock, called strata, show the sequence of events that took place in the past. Relative age indicates that one layer is older or younger than another layer.Relative age indicates that one layer is older or younger than another layer. RELATIVE AGE DOES NOT GIVE A LAYER’S AGE IN YEARS

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

Principle of Original Horizontality Sedimentary rocks left undisturbed will remain in horizontal layers.Sedimentary rocks left undisturbed will remain in horizontal layers. Sedimentary rock layers that are not horizontal must have been acted on by forces within the Earth.Sedimentary rock layers that are not horizontal must have been acted on by forces within the Earth. Sometimes these forces can force older rock on top of younger rock.Sometimes these forces can force older rock on top of younger rock.

Law of Superposition and the Principle of Horizontality

Clues to Original Horizontality Graded BeddingGraded Bedding –Larger particles should be on the bottom of the layers. Cross BedsCross Beds –Caused by soft sediments, like sand, slumping to form curved layers with respect to the bedding plane. The bedding plane is the boundary between two layers, or beds. Ripple MarksRipple Marks –Ripple marks, formed by water, point up in undisturbed layers.

Clues to Original Horizontality

Unconformities A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.A break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time. There are three types of unconformitiesThere are three types of unconformities –Nonconformity –Angular unconformity –Disconformity

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

Laws of Crosscutting Relationships A fault is a break or crack in the Earth’s crust along which rocks shift their position.A fault is a break or crack in the Earth’s crust along which rocks shift their position. An intrusion is a mass of igneous rock which is injected into rock and then cools and solidifies.An intrusion is a mass of igneous rock which is injected into rock and then cools and solidifies. A fault or igneous intrusion is always younger than the rocks it cuts through.A fault or igneous intrusion is always younger than the rocks it cuts through.

Laws of Crosscutting Relationships

Angular Unconformity When horizontal layers of rock rest on top of tilted layers of rock.When horizontal layers of rock rest on top of tilted layers of rock.

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

Principle of Inclusions Included rock fragments must be older than the rock in which they are found.

Principle 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