Time & Geological Record

1. Time & Geological Record Associate Professor John Worden DEC University of Southern Qld

2. Time & Geological Record • Geology’s greatest contribution- immensity ofTIME. • Earth is 4.56 Billion years old. • Earth processes slow& occur mostly over millions of years. • Have to expand our conception of time to study Earth processes. • Time approached in two ways by geoscientists: • RELATIVE TIME & ABSOLUTE TIME- • Relative time identifies the oldest/ first eventfollowed by progressive events in a sequence, until the last/ youngest event. • Absolute time measured in years B.P (before present) since an event .

3. Time & Geological Record • Relative Geological Time: • James Hutton in 1788, first to appreciate concept when he recognised anAngular Unconformity at Siccar Point, Scotland. • He realised that underlying rocks had to be sourced from yet older pre-existing rocks by weathering, erosion, transport, & deposition. They had then been buried, lithified, tilted, uplifted, exposed, eroded, & later had the overlying rocks deposited on top of them. • Hutton was first to grasp the significance of the ‘Rock Cycle’, it’s slow steady progression & huge amounts of time for it’s completion. • He advanced the “Principle of Uniformitarianism” • “Rates of Geological Processes do not change with time.” • Today , we know that : • Rates of processes do change with time within limits.

4. Time & Geological Record • Relative Geological Time: • Hutton recognised that the rock (geological) record held many majordiscontinuities, when time was not recorded by rock sequences (these are termed ‘Unconformities’ & there arethreedistinct types). • Later sedimentary rocks overlying igneous rocks = ‘Nonconformity’. • Much younger sediments overlying older sediments without apparent break = ‘Disconformity’;(confirmed by different fossil assemblages). • Horizontal younger sediments overlying inclined strata =‘Angular Unconformity’. • Hutton’s work led Charles Lyell to define fiveprinciples for determining relative time. • Law of Original Horizontalitystates that: • “Water-laid sediments are deposited horizontally.”

5. Time & Geological Record • Principle of Superpositionstates that- “in any sequence of sedimentary strata, the oldest strata are at the base and the youngest at the top.” • Principle of Cross-cutting Relationships- “Igneous intrusions & faults are younger than the rocks they cut.” • Principle of Faunal Succession- “groups of fossils(animals + plants) occur in the geological record in a definite and determinable order & that a geological period can be recognised by it’s characteristic fossils.” • English Surveyor Smith used this to predict location & properties of sub-surface rocks during canal construction, before Darwin’s Theory of natural selection. • Principle of Inclusion- • “Any fragment of rock incorporated or included inanother is older than its host rock.” • These then used to construct the Geological Column.

6. Time & Geological Record • Geological Column: • Places fossil-controlled sequences in relative chronological order from oldest to youngest. • As rock formations named for localities where they are best exposed type localities), they became standard names for portions of column. • These names ranked as ‘EONS’, ‘ERAS’, ‘PERIODS’ etc, in decliningorder. Four Eons- Hadean, Archean, Proterozoic & Phanerozoic. • Proterozoic Eon is a large time interval with only trace fossils (< 1400 Ma), • Paleozoic Era- “ancient life”. • Mesozoic Era- “middle life”. • Cainozoic Era- “recent life’. • Jurassic Period - part of the Mesozoic Era. • Fossils permit correlation of areas worldwide.

7. Time & Geological Record • Relative geological time permitted development of a worldwide time scale which placed all rock formations in their correct chronological sequence. • However, it is still desirable to know the ‘Absolute Age’ of the Earth, that of any Eon, Era, Period, and of any individual geological event. • Absolute Time: • Early attempts to measure length of geologic time were indirect. • All failed due to incorrect assumptions: • Thickness of sedimentary strata- estimates between 3 Ma  1.5 B yrs. • Sea salt concept - 90 Ma. • Cooling Earth model - 100 Ma • Only measured by an independent process that: • is constant, unidirectional, & independent of T & P.

8. Time & Geological Record • Thickness of Sedimentary Strata Concept: • Wide-ranging estimates obtained depending on “average sedimentation rate”,( 0.3 m/ 1000 years); • Problem- Gaps in Sedimentary Record yield a minimum estimate. • Sea-Salt Concept: • Basic premise- Oceans initially fresh water; • Progressively polluted by common salt from weathering of Continents; • Estimate total river runoff & contained salt content Age of Earth. • Problems- Ignored evaporite deposits that lock away vast quantities of Salt, and • Failed to consider ‘Cyclic Salt’ (removal of salt from Oceans to Continents by prevailing winds).

9. Time & Geological Record • Cooling Earth Model: • Assumption- Planet formed in molten state & cooled rapidly ever since. • Radioactivity unknown when concept used to estimate the Age of Earth, so:an important source of heat overlooked, • Radioactive decay of elements releases heat that has contributed to lowering the actual cooling rate for the Earth. • Hence concept flawed and the estimate was too low. • Time is measured by any regularly recurring event, provided the event is measurable. • At least three natural events satisfy these terms:- Tree rings, - Varves, and - Ice sheets of Antarctica, Greenland, etc.

10. Time & Geological Record • Tree rings (Dendrochronology): • Early in growing season, trees produce cells with thin walls that appear light in color, along the outermost circumference of bole. • Later in growing season, cells are small with thick walls and appear dark in color. • Thus two growth rates appear as varying width bands or rings, and • Each couplet represents a year. • Oldest trees date back < 5000 years B.P.  Little use geologically! • Varves: • Geological equivalent of tree rings. • Seasonally -based mountain lake deposits, formedfrom glacial stream- transported detritus. • During winter, stream runoff reduced & only transports fine ground “rock flour”.

11. Time & Geological Record • Summer Thaw with Melt-water runoff. Transports larger quantities of coarser sediment into lake. • Results insediment couplet representing each year. • By coring lake bottom sediments, varves can be counted, & used to date events. Important in recentClimate studies, but limited use in Geology. • Ice Sheets: • Seasonally -controlled snowfalls compact to different thickness layers. • Annual couplets measured in ice cores from IceSheets. • Below particular depth, ice deforms, recrystallises & flows destroying the record. • Time resolution to >200,000 years B.P . • Vital toClimate studies; very limited geological application.

12. Time & Geological Record • Absolute Time: • Radioactivity is a constant, unidirectional & independent process. • Vast majority of Isotopes are stable, but a few are unstable and spontaneously decay to lighter isotopes = ‘Radioactive Decay’. • All radioactive decay follows an exponential curve. • Decay rates unaffected by changes in chemical & physical environment. • Totally independent of geological processes. • Decay rates expressed as decay constants () , or • As half lives. (T= 0.6931/  ) • The time needed for the number of parent atoms to be reduced by one half. • Basic equation: t (age)= loge ( 1+ Nd/Np)/  • Where Np= Amount of Radioactive parent now present: Nd= Daughter product.

13. Time & Geological Record • Absolute Time: • Equation assumes two conditions; • that the Decay Constant () is constant, and • the system being determined has remained closed, ie no addition or loss ofparent or daughter atoms in response to internal/external factors. • Strength- reflects nuclear processes independent of T& P & geological processes. • Which isotopic systems are useful for isotopic dating? • Carbon (C14) Dating: • C14 forms in upper atmosphere by cosmic ray bombardment of N14 (slow thermal neutron capture) ieN714 + n C614 + p. Subsequently, C614  N714 + . • Production & decay of C14 in equilibrium, & rapid mixing as CO2 ,ensures constant in all carbon reservoirs.

14. Time & Geological Record • Absolute time: • C14 has a half life of 5730 +/- 30 years. • Too short for significant geological use. • Used for time range 100 -80,000 years only. • Potassium/Argon (K/Ar) dating: • K1940 +   Ar1840 by K-electron shell capture ( ie p +   n). • This decay scheme is important for terrestrial heat generation in the Earth.. • Since Argon (Ar) is a noble gas, it does not bond with other elements! • When K-bearing minerals crystallise, they include K40. • At high T, Ar40 diffuses out of mineral & is lost. • Retention of Ar40 is an important assumption. • Therefore, prefer K/Ar ages to be measured on-randomly -orientated materials like Basalt, Hornblende.

15. Time & Geological Record • Absolute Time: • Rubidium-Strontium (Rb/Sr) dating: • Rb3787 Sr3887 + ie n  p +  • Decay constant () = 1.41 x 10-11 yr –1 (recently reviewed & modified). • Rb like K, is widely distributed in rocks and minerals & offers many dating possibilities. • Use suites of rocks from same formation (to obtain variable Rb/Sr ratio spread). • Best-suited to igneous rocks, both intrusive & extrusive. Will give ‘cooling ages’ for metamorphic rocks, or time when contained minerals cooled through their ‘closure temperatures’. • Least-suitable for detrital sedimentary rocks. • Has advantage that does not involve gas decay product.

16. Time & Geological Record • Absolute time: • Uranium-Thorium-Lead (U-Th-Pb): • Both Uranium & Thorium decay to Lead with very long half lives. • Three decay schemes- three potentially discrete dating methods + fourth internal check by comparing U235 & U238 decay as Pb207/Pb206 ratio. • If the Pb207/Pb206 age differs from the two U-Pb ages, then the mineral or rock has experienced a later geological event. • Use: • U238  Pb206 + 8 + 6 , t ½ = 4.51 x109 yrs; • U235  Pb207 + 7 + 4 , t ½ = 0.71 x109 yrs; • Th232  Pb208 + 6 + 4 , t ½= 13.9 x109 yrs.

17. Time & Geological Record • Paleomagnetism: • Earth’s liquid outer core generates a magnetic field; • As a result, planet behaves like a giant ‘bar magnet’; • Iron-containing minerals in forming sediments/igneous rocks align to prevailing magnetic field(like a compass needle); • But periodically, Earth’s magnetic poles switch (at irregular intervals); • Any rock sequence forming over time preserves a record of polarities: • Dated rock sequences used to reveal history of magnetic reversals: • Can be used to determine age of other sequences by matching magnetic reversals pattern; • Measure rock’s residual magnetism; • Used worldwide to date Mesozoic-Cainozoic rocks.

18. Time & Geological Record • Absolute time & Geological time scale: • Radiometric dating best applied to igneous rocks, but sedimentary rocks with their contained fossils define thegeological time scale. • With careful & critical examination of intrusive & extrusive igneous rocks and their related sedimentary rock hosts, it is possible to assign absolute ages to various Eras, Periods, etc of the geological time scale. • Careful & often tedious work of 19th Century geologists proven correct by absolute time radiometric dating, as well as length of time that the rock cycle has been active. • Oldest Earth age is 4.1-4.2 B yrs for zircon from: • Narryer metamorphic gneisses, Western Australia. • Oldest rocks are Greenland gneisses (3.8-3.9 B yrs). • Hadean Eon not represented on Earth, but in Lunar rocks and Meteorites. As Earth & Moon formed at same time, Age = 4.56 B yrs.

19. Time & Geological Record • Age of the Earth: • If Meteorites have remained closed systems since condensation of the solar system, then their contained Pb isotope ratios will yield the age ofplanet formation. • Iron meteorites contain extremely small quantities of U & Th and their Pb ratios are essentially primeval Lead. • Stony meteorites have higher U & Th producing variable mixtures of primeval & radiogenic Pb. (Stony meteorites contain silicate minerals). • Yield age of formation of meteorites = 4.56 B yrs. • Lunar Highlands samples plot on same isochron. • Deep ocean sediments also plot on same isochron. • They are best ‘average’ samples of Earth’s Pb comp. • Therefore the Earth developed it’s Pb isotopic compat 4.56 B yrs, and is the same age as the GEOCHRON.