KEY POINTS: Thinking About Time• Geologic time is measured in time intervals spanning thousands, millions, even billions of years (= “Deep Time”).• Earth is estimated to be 4.6 billion years old.• The change from a biblical age for Earth to the concept of geological time is an example of a paradigm shift.
KEY POINTS: The History of Relative Time• The concept of relative time deals with placing events in the order in which they occurred.• Several rules used to determine the sequence of geologic events are: original horizontality, superposition, cross-cutting relationships and inclusions.• The fossil record is also used to determine the order of events.
ArchbishopJames UssherIn 1650, an IrishAnglican archbishop,James Ussher, whoattempted to correlatedages of biblical figureswith real time. Heconcluded that the Godcreated the Earth,beginning on 22 October4004 B.C., or 6,000 yearsago.His chronology wasincluded in early KingJames versions of theBible and was widelycirculated.
Nicholas Steno(1638-1686)Steno, a Danishphysician and priest,initiated thedetermination of Earth’sage from a scientificstandpoint in the late1600s, not long afterBishop Ussher’spronouncement of 4004BC.After dissecting a shark’shead, he believed theshark teeth found inseaside cliff sedimentswere real teethdeposited ages ago.
Steno’s 1st Principle: Original Horizontality• Steno hypothesized that objects like sharks teeth were preserved when they sank to the bottom of a body of water and were covered up by sediment.• He reasoned that if rock layers were deposited in water, they must have originally been horizontal.• This observation –that sedimentary rocks are deposited in nearly horizontal layers—became known as the principle of original horizontality.• Sedimentary rocks that are no longer horizontal must have gone through an episode of deformation after forming (such as uplifting via mountain building).
Steno’s 1st (B, C, D) and 2nd Principle (A): Superposition(In a series of sedimentary rock layers that are not deformed, the rocks at the bottom of the stack are oldest, rocks at the top are youngest.)
Principle 2 Appliesto VolcanicProcessesVolcanic eruptionscan produce layers(strata) from lavaflows and whenash and otherejecta fall back toEarth.
Hutton’s Principles• Hutton developed our modern concept of geologic time.• He suggested that same slow-acting geologic processes operating today must have operated in the past.• So it must take a very long time for those processes (e.g. weathering, erosion) to produce and significant change in the shape of the Earth’s surface (longer than 6000 years).• This process is know as the uniformitarian model.• Secondly, Hutton proposed that all land should be worn flat unless some process acts to renew the landscape by forming new mountains, which then renews the slow cycle of destruction.
Siccar Point • Siccar Point, in Scotland, is where Hutton discovered nearly flat rocks overlaying nearly vertical layers. He hypothesized that the lower layers of rock formed first (superposition) as flat layers (original horizontality) and were later pushed up and tilted by mountain uplift processes. • These layers were then worn down by erosion and buried beneath new horizontal deposits of sediment. • Hutton had found an illustration of his idea of cyclical processes.
Hutton called the rock formation at Siccar Point an “Unconformity” in this 1796 Book, The Theory of the Earth.
Relative Time• The study of “relative time” involves placing events in the order in which evidence indicates they occurred.• In the context of geology, it means describing the order or sequence of geologic events.
Investigating theOrigin of theGrand CanyonA one-armed Civil Warveteran, John WesleyPowell led twoexpeditions down theColorado River throughthe Grand Canyon (1869and 1871).He was the first person tomake a detailed record oflandscape formation.Powell interpreted therocks in the canyon wallsto be the result of long-lived geological processesthat had been operating“through ages too long forman to compute.”
Grand Canyon. Rock layers exposed representintervals of time stretching back millions of years. The canyon is 19 miles across in the well-layered sedimentary rocks but is much narrower in the metamorphic and igneous rocks of the inner gorge.
Grand Canyon:Oldest toYoungest LayersSchist, a metamorphicrock, is the lowestand oldest (once wassedimentary).Granite (an igneousrock) cuts across theschist and is thereforeyounger, but stillsecond oldest.Third oldest is theSuper-group, a seriesof sedimentary rocks.Uplift tilted them afterformation.
Grand Canyon:Oldest toYoungest LayersFourth up is a progressionof sandstone, to shale, tolimestone, typical of thesequence of rocks formedin a shallow sea alongcontinental margins,followed by uplift.Next are a series ofsedimentary layersindicative of shallowmarine and deltaconditions, formed as sealevel rose and fell.The Coconino sedimentarylayer was formed bywindblown sand anddrying desert conditions.
Grand Canyon:Oldest toYoungest LayersThe youngest layer (toplayer), the KaibabFormation, is a series ofsedimentary (sea bottom)deposits.After this came a period ofuplift, raising the landsome 6,500 feet,permitting the ColoradoRiver to cut the canyon.The last geologic event wasthe formation of a series ofcinder cone volcanoesright at the top (north rim)after the canyon wasformed.