Chapter 4 Geologic Time Concepts and Principles Grand Canyon When looking down into the Grand Canyon we are really looking all the way back to the early history of Earth Grand Canyon More than 1 billion years of history are preserved like pages of a book in the rock layers of the Grand Canyon Reading this rock book we learn that the area underwent episodes of mountain building advancing and retreating shallow seas We know these things by applying the principles of relative dating to the rocks and recognizing that present day processes have operated throughout Earth history What is time We are obsessed with time using clocks calendars appointment books Mostly we don t have enough of it Our common time units are seconds hours days weeks months years Ancient history involves hundreds of years thousands of years But geologic time involves millions of years even billions of years Concept of Geologic Time Geologists use two different frames of reference when discussing geologic time Relative dating involves placing geologic events in a sequential order as determined from their position in the geologic record It does not tell us how long ago a particular event occurred only that one event preceded another For hundreds of years geologists have been using relative dating to establish a relative geologic time scale Relative Geologic Time Scale The relative geologic time scale has a sequence of eons eras periods epochs but no numbers indicating how long ago each of these times occurred Concept of Geologic Time The second frame of reference for geologic time is absolute dating Absolute dating results in specific dates for rock units or events expressed in years before the present It tells us how long ago a particular event occurred giving us numerical information about time Radiometric dating is the most common method of obtaining absolute ages Such dates are calculated from the natural rates of decay of various natural radioactive elements present in trace amounts in some rocks Geologic Time Scale The discovery of radioactivity near the end of the 1800s allowed absolute ages to be accurately applied to the relative geologic time scale The geologic time scale is a dual scale a relative scale and an absolute scale Changes in the Concept of Geologic Time The concept and measurement of geologic time has changed through human history Early Christian theologians conceived of time as linear rather than circular James Ussher 1581 1665 in Ireland calculated the age of Earth based on recorded history and geneologies in Genesis He announced that Earth was created on October 22 4004 B C A century later it was considered heresy to say Earth was more than about 6000 years old Changes in the Concept of Geologic Time During the 1700s and 1800s Earth s age was estimated scientifically Georges Louis de Buffon 1707 1788 calculated how long Earth took to cool gradually from a molten beginning using melted iron balls of various diameters Extrapolating their cooling rate to an Earth sized ball he estimated Earth was 75 000 years old Changes in the Concept of Geologic Time Others used different techniques Using rates of deposition of various sediments and thickness of sedimentary rock in the crust gave estimates of 1 million to more than 2 billion years Using the amount of salt carried by rivers to the ocean and the salinity of seawater John Joly in 1899 obtained a minimum age of 90 million years Relative Dating Principles Six fundamental geologic principles are used in relative dating Principle of superposition Nicolas Steno 1638 1686 In an undisturbed succession of sedimentary rock layers the oldest layer is at the bottom and the youngest layer is at the top This method is used for determining the relative age of rock layers strata and the fossils they contain Relative Dating Principles Principle of original horizontality Nicolas Steno Sediment is deposited in essentially horizontal layers Therefore a sequence of sedimentary rock layers that is steeply inclined from horizontal must have been tilted after deposition and lithification Principle of Horizontality Illustration of the principles of superposition and original horizontality Horizontality These sediments were originally deposited horizontally in a marine environment This outcrop is Chattanooga Shale Tennessee Principle of Superposition Illustration of the principles of superposition and original horizontality Superposition The youngest rocks are at the top of the outcrop and the oldest rocks are at the bottom Relative Dating Principles Principle of lateral continuity Nicolas Steno Sediment extends laterally in all direction until it thins and pinches out or terminates against the edges of the depositional basin Principle of cross cutting relationships James Hutton 1726 1797 An igneous intrusion or a fault must be younger than the rocks it intrudes or displaces Relative Dating Principles Principle of inclusions discussed later in the term Principle of fossil succession discussed later in the term Cross cutting Relationships North shore of Lake Superior Ontario Canada A dark colored dike has intruded into older light colored granite The dike is younger than the granite Cross cutting Relationships Templin Highway Castaic California A small fault displaces tilted beds The fault is younger than the beds Neptunism Neptunism All rocks including granite and basalt were precipitated in an orderly sequence from a primeval worldwide ocean proposed in 1787 by Abraham Werner 1749 1817 Werner was an excellent mineralogist but is best remembered for his incorrect interpretation of Earth history Neptunism Werner s geologic column was widely accepted Alluvial rocks unconsolidated sediments youngest Secondary rocks rocks such as sandstones limestones coal basalt Transition rocks chemical and detrital rocks some fossiliferous rocks Primitive rocks oldest including igneous and metamorphic Catastrophism Catastrophism proposed by Georges Cuvier 1769 1832 dominated European geologic thinking The physical and biological history of Earth resulted from a series of sudden widespread catastrophes which accounted for significant and rapid changes in Earth and exterminated existing life in the affected area Six major catastrophes occurred corresponding to the six days of biblical creation The last one was the biblical flood Neptunism and Catastrophism Were Eventually abandoned These hypotheses were abandoned because they were not supported by field evidence Basalt was shown to be