How do we know what happened first? Relative Dating How do we know what happened first?

Historical Developments James Hutton (1726-1797) “Father of Modern Geology” native of Edinburgh, Scotland educated as a medical doctor in Leiden (1749) passionate about scientific inquiry “Theory of the Earth” -- processes are slow; take a long time Charles Lyell (1795-1875) Scotsman who attended Oxford University father was an avid naturalist rebelled against prevailing thought of “catastrophism”/”Neptunism”. “Principles of Geology” -- popularized Hutton’s views idea of “uniformitarianism” -- same processes operating today occurred in the past ….the present is the key to the past….

The Key to the Past Relative time Absolute time order of events or objects from first (oldest) to last (youngest) she is older than he is; she was born first and he was born last Absolute time age of events or objects expressed numerically she is twenty-one and he is nineteen study of timing of geologic events and processes is geochronology

The Key to the Past Relative Time- “this rock is older than that” Principles Used to Determine Relative Age Unconformities Correlation The Standard Geologic Time Scale Index Fossils Absolute Time- “this rock is 28 million years old” Principles of radioactive decay

relative time and relative order apply simple concepts to determine… • original horizontality • superposition • lateral continuity • cross-cutting relationships • inclusions • unconformities

relative age dating concepts original horizontality all beds originally deposited in water formed in horizontal layers sediments will settle to bottom and blanket the sea floor

Superposition: within a sequence of undisturbed sedimentary or volcanic rocks, layers become younger, upward

Lateral Continuity: original sedimentary layers extend laterally until it thins out at edges rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.

Relative Age Dating Concepts cross-cutting relationships a disrupted pattern is older than the cause of the disruption e.g. an intrusion is younger than the rocks it intrudes

Relative Age Dating Concepts inclusions fragments of other rocks contained in a body of rock must be older than the host rock e.g. xenoliths in granite are older than granite and 2) pieces of rock in conglomerate are older than conglomerate

Relative Age Dating Concepts unconformities A gap in the geologic record -- “gap” may be an amount of time or amount of missing section conformity • relatively continuous deposition • deposition of a sequence of parallel layers • contacts between formations do not represent significant gaps in time

conformity from: http://www.elohi.com/photo/grandcanyon

Relative Age Dating Concepts different types of unconformities 1. angular unconformity • contact separates overlying younger layers from tilted older layers • sequence of layers is not parallel • contacts between formations may represent significant amounts of time angular unconformity

angular unconformity from: http://www.uakron.edu/envstudies/parks/rmgcan2.html

angular unconformity

Relative age dating concepts different types of unconformities 2. disconformity • contact separates beds (formations) that are parallel • sequence of layers is parallel • contacts between formations may represent significant amounts of time • missing time is difficult to recognize

Relative age dating concepts different types of unconformities 3. nonconformity • strata deposited on older crystalline (metamorphic/igneous) rock • erosion surface on igneous/metamorphic rock covered by sedimentary rocks • large gap in geologic record nonconformity

Unconformity Types Using Grand Canyon as Example

Relative Age: Correlation How is this done? faunal succession (correlation by fossils) fossil species succeed one another through the layers in a predictable order index fossil short-lived organism; points to narrow range of geologic time fossil assemblage group of fossils associated together