geologic time

time is critical for geologic processes Rockies and Alps are ~3000 m tall -- mountains grow at ~1 meter per 5000 yrs (0.2 mm/yr) -- 3000 m x 5000 yr/m = 15,000,000 (yrs necessary) Atlantic Ocean is ~5000 km across -- today, seafloor spreading in Atlantic is ~4 cm/yr -- 6000 km = 6000 km x 1000 m/km x 100 cm/m = 600,000,000 cm -- 600,000,000 cm / 4 cm/yr = 150,000,000 years for comparison: fingernail grows at 1 cm/yr

age of the Earth prior to 19th century, accepted age from religious beliefs -- 6,000 years for Western culture (Christian) …Bishop Usher from geneology in the Bible -- old beyond comprehension (Hindu/Buddhist/Chinese) -- age not certain (Islam) during 19th century, length of time required for geologic processes to occur was recognized -- fundamental contribution of geology to scientific knowledge

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”. “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 relative time vs. absolute time relative 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

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

relative age dating concepts superposition youngest within a sequence of undisturbed sedimentary or volcanic rocks, oldest rocks are at the bottom and youngest at the top ….young upward… lateral continuity oldest original sedimentary layers extend laterally until they thin at edges continue continue

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 contact between sedimentary formations that represents a gap in the geologic record -- “gap” represented is variable (i.e. amount of time or the amount of missing section) different types of unconformities conformity • relatively continuous deposition • deposition of a sequence of parallel layers • contacts between formations do not represent significant amounts of time

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

relative age dating concepts different types of unconformities 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 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 (may need other information--paleosol?)

relative age dating concepts different types of unconformities 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

what events occur? angular unconformity

what events occur? nonconformity

now that we know all this…what happened?

deposition

intrusion

tilting and erosion

subsidence and renewed deposition

missing formation (time)?

dike intrusion

erosion and exposure

subsidence and deposition

uplift/sea level fall and river deposition

relative ages of the formations

relative age: correlation correlation -- determining time equivalency of rocks within a region, between continents, etc. how is this done? physical continuity physically following a continuous exposure of a rock unit --most direct; easily done in some locations, not in others e.g. within the Grand Canyon lithologic similarity assuming similar sequences of rocks formed at same time -- inaccurate if common rocks are involved e.g. the Grand Canyon and Zion National Parks

physical continuity -- Coconino Sandstone in Grand Canyon

lithologic similarity -- Coconino and Navajo Sandstones

lithologic similarity -- Coconino and Navajo Sandstones Navajo is much younger!

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

use of index fossils/fossil assemblages permits global correlation similar units found in India, Africa, S. America, Australia, Antarctica.

established initially as a relative scale using sedimentary rocks and fossils absolute ages were determined later with radiometric dating

absolute time natural clock is necessary -- radiometric dating (nuclear clock: decay of radioactive isotopes) -- dendrochrolonology -- astronomical methods

age of the Earth early methods: long debated • 1625: Archbishop Usher determined Earth was created in 4004 B.C. by counting generations in the Bible • Hindus regarded Earth as old: 2000 A.D. is 1.97 million years according to Hindu calendar • 1866: Lord Kelvin calculated age by assuming that Earth was molten and cooled to a solid; age between 20-40 million years old. - did not know about radioactive decay (makes heat) - assumed all heat dissipated by conduction early isotopic methods (radioactivity known in 1896) • 1905: first crude estimates yielded 2 billion year age • meteorites gave dates of 4.5 to 4.6 billion years old • modern uranium/lead methods yield values of 4.55 billion years

radioactive isotopes have nuclei that spontaneously decay -- emit or capture subatomic particles parent: decaying radioactive isotope daughter: decay daughter parent daughter loss or gain loss or gain of neutron converts parent to daughter of same element loss or gain of proton changes parent into entirely new daughter

electron capture (e- + p+ = n0) 3 primary ways of decay alpha decay (Z ≥ 58) particle has 2 neutrons and 2 protons U238 Th234 92 protons 90 protons beta decay (n0 = p+ + e-) breakdown of neutron into an electron and a proton and loss of the electron to leave a proton (result is gain of one proton) K40 Ca40 19 protons 20 protons electron capture (e- + p+ = n0) capture of an electron by a proton and change of proton to neutron (result is loss of proton) K40 Ar40 19 protons 18 protons

radiometric dating uses continuous decay to measure time since rock formed only possible since late 1890’s -- radioactivity discovered in 1896 as minerals crystallize in magma; they trap atoms of radioactive isotopes in their crystal structures radioactive isotopes will decay immediately and continuously as time passes, rock contains less parent and more daughter

half-life amount of time it takes for half the atoms of the parent isotope to decay different radioactive isotopes have different and distinct half-lives if rock has 12 parents and 12 daughters--ratio of 1:1 …original rock had 24 parents and one half-life has elapsed… …after another half life, rock will have 6 parents and 18 daughters… …ratio of 1:3---note that total number (24) remains the same regardless of isotope, the ratio of parent to daughter atoms is predictable at each half-life

predictable ratios at each half-life exponential decay (half always remains)

exponential decay: never goes to zero linear

(each has its own half-life) example: Uranium 238 decay to Lead 206 (stable) several steps (each has its own half-life)

most common dating systems • uranium-thorium-lead dating (previous example) U-238, U-235, Th-232 each of these decays through a series of steps to Pb U-238 to Pb-206 half-life = 4.5 by U-235 to Pb-207 half-life = 713 my Th-232 to Pb-208 half-life = 14.1 my • potassium-argon dating …argon is a gas--may escape (ages too young--daughter missing) K-40 to Ar-40 half-life = 1.3 by • rubidium-strontium dating Rb-87 to Sr-87 half-life = 47 by

basic geochronological assumptions decay constants constant through geological time -- good reasons to believe this is correct from nuclear physics -- measurements of decay sequences in ancient supernovae yield the same values as modern lab measurements system closed to adding or subtracting of parent/daughter -- isotopic system and type of mineral (rock) are important -- careful procedure is essential to correct analysis igneous rocks are most reliable for dating …metamorphism may cause loss of daughter products… …sedimentary rocks will give ages of source rocks…

Instruments and Techniques Mass Spectrometry: measure different abundances of specific nuclides based solely on atomic mass. Basic technique requires ionization of the atomic species of interest and acceleration through a strong magnetic field to cause separation between closely similar masses (e.g. 87Sr and 86Sr). Count individual particles using electronic detectors. TIMS: thermal ionization mass spectrometry SIMS: secondary ionization mass spectrometry - bombard target with heavy ions or use a laser Sample Preparation: TIMS requires doing chemical separation using chromatographic columns.

Clean Lab - Chemical Preparation http://www.es.ucsc.edu/images/clean_lab_c.jpg

Thermal Ionization Mass Spectrometer From: http://www.es.ucsc.edu/images/vgms_c.jpg

Schematic of Sector MS

Zircon Laser Ablation Pit

Rate Law for Radioactive Decay Pt = Po exp - (to –t) 1st order rate law

Rb/Sr Age Dating Equation

Rb/Sr Isochron Systematics

Independent Checks on Radiometric Ages Correlation of erosion with age on Hawaiian Island Chain: Dates increase in age to the NW as does erosion. Annual growth bands in Devonian corals: 400/yr yields date that is similar to radiometric date. Consistent with slowing of Earth rotation with time. Independent determination of Pacific plate motion yields age progression that is consistent with K/Ar dates of the island chains formed by “hotspots”. Agreement between magnetic “age” from deep marine sediments and radiometric ages of tuffs in East African Rift

Other dating methods: dendrochronology annual growth of trees produces concentric rings …dating back to 9000 years is possible… rings need to be calibrated against C-14 dates to yield “true” numerical age other information may also be obtained from rings, including rainfall and temperature can develop composite chronologies for specific regions of interest for climate studies photo © H.D. Grissino-Mayer

relative and absolute dates combined same example as in relative age

geological time scale eons, eras, periods, epochs Oldest rocks: Greenland gneisses Oldest rock fragments: W. Australia detrital zircons

earliest life cyanobacteria: primitive single-celled organisms found in Australia and dated at 3.7 billion years old modern equivalents in Shark’s Bay, Australia

proportional time scale

combine relative and absolute time for geologic time scale