Dating methods and the age of the Earth Geology 103

Two types of dating Relative dating asks “Is a given event older or younger than another event?” Numerical (absolute) dating asks “How many years ago did an event take place?”

What is an “event”? A discrete occurrence that can be inferred from the rock or fossil record Examples: the deposition of a sedimentary layer, or the death of an organism

Relative dating Steno’s principles (1669) are all very common sense but do order strata from oldest to youngest Stratum = layer Strata = layers Stratigraphy = study of the order of events in sedimentary rocks

Principle of original horizontality Any distortion of sedimentary strata is due to deformation after the strata were deposited

Principle of lateral continuity Strata are deposited in a basin and thus will be continuous from end to end Any breaks are due to erosion or deformation after deposition

Principle of superposition Younger strata overlie older strata True of all gravity-driven deposits

Principle of cross-cutting relationships The body that cuts through is younger than the body that got cut through True for dikes, faults, joints and any non-concordant stratum

Principle of inclusions Sometimes called “xenoliths” The inclusion is older than the rock in which it is included

Correlation Similar strata separated by gaps, were once part of a continuous layer Led to the idea of “formations”: a mappable unit of rocks sharing a common origin event

Faunal succession William Smith and George Cuvier (around 1800): There is an order to the fossils found in strata Smith and Cuvier did not think of evolution but the principle led Charles Darwin to think of how the order might arise

Evolution of geologic timescale

Modern geologic timescale

Points about the timescale Divided up by major changes in the fossils found in the strata (Phanerozoic): mass extinctions mark the boundaries Major divisions: eon, era, period, epoch Rocks don’t care about life: that is, one period ≠ one formation Thus, lithostratigraphy is not biostratigraphy

Unconformities An unconformity is a time gap in the rock record Classified into different types, but all represent significant missing time

“The Great Unconformity”

Numerical dating Two types: radiometric and non-radiometric Radiometric dating always involves the decay of a radioactive isotope Non-radiometric dating always involves a change in the number or condition of a material

Basic atomic structure Atoms are the basic unit of matter with chemical properties Made of sub-atomic particles: electrons, protons and neutrons All elements (atoms of one type) have isotopes that differ in mass Some isotopes are unstable and will decay (radio-isotopes)

Radiometric dating Typically need to know: how much of the radioactive isotope (radio-isotope) was present when the “geo-clock” started (i.e., the event occurred), how much of the radio-isotope is present currently and/or the half-life of the radio-isotope

Radioactive decay series Not all decays are as complicated as this! Start with the principal radio-isotope and end with the stable isotope

The “geo-clock” The shape of the curve is called “exponential decay”, because half of the radioisotope (“parent isotope”) is con-verted to the daughter isotope each successive half-life.

Many different systems

Non-radiometric dating Examples: tree ring dating (dendrochronology), lichenometry, fission track dating, electron spin resonance (ESR) dating In the photo above, the path of an alpha particle ejected from a radioisotope nucleus is called a “fission track”. By enhancing these tiny tracks, counting them allows the researcher to figure out how old numerically the sample is.

Two key questions about numerical dating 1. Is the dating method appropriate for the estimated age and composition of the dated material? 2. What is the event being dated? In other words, when does the geo-clock start?

Test case You discover early homonid (i.e., pre-Homo sapiens) skeletons in a sandstone deposit How do you obtain the numerical age? What method(s) would you use?

What are appropriate materials? Radiometric dating relies on closed systems – typically this means a material that does not gain or lose components, like radioisotopes. Examples are: igneous rocks, some metamorphic rocks (note they are all crystalline) Notable bad material: Sedimentary rocks

Use boundary (bounding) numerical ages If one can find a datable material stratigraphically near a fossil, then the age of the datable material can be used to constrain the age of the fossil. In the example to the right the ash layer was dated (U-Pb) at 563 ± 3 My, so fossils are at least that old.

So how old is the Earth? And how do we know?

Count the days of recorded history James Ussher, Archbishop of Armagh, uses biblical history to determine the first day of Genesis. It is a complex calculation. Calculates Sunday, October 23, 4004 BCE as the start date.

Estimate the Earth’s cooling William Thomsen, Lord Kelvin (University of Glasgow, Scotland), 1897. Reasoned that the Earth was initially molten and had cooled off over time, enough to make a crust. Measured the rates of cooling of iron spheres. Age: 20 to 40 million years.

Measure the ocean’s salinity John Joly (Trinity University, Ireland), 1899. Reasoned that the oceans had started off without any sodium content, and thus, by measuring the amount of sodium brought to the ocean annually by all the rivers in the world and measuring the ocean’s salinity, one could calculate the age of the ocean, which is close to the age of the Earth. Age: 80 to 100 million years

Biologists (and geologists) had a problem with these ages As Charles Darwin, originator of the theory of evolution by means of natural selection pointed out, even 100 million years was not long enough to give the mechanism of evolution time to generate the diversity of Earth’s life.

Radioactivity provides the key As mentioned earlier, the phenomenon of radioactivity generates heat – something that Kelvin could not have predicted. Henri Becquerel (Museum of Natural History, Paris) found radioisotopes emitting invisible rays (1896). Ernest Rutherford (McGill University, Montreal) suggested uranium isotopes might be useful in dating earth materials (1905).

Refining the technique Inspired by a talk by Rutherford, Bertram Boltwood (Yale, 1907) reasons that by measuring the amount of lead, the stable end product of uranium decay, one could determine the age of a rock sample. Ages ranged from 250 to 1300 million years.

...to the age of the Earth Arthur Holmes (Imperial College, London, 1913) realizes that the decay of thorium also contributes to the final lead amount and refines Boltwood’s technique. By 1927, he publishes the age of the Earth as about 1.6 to 3.0 billion years. In 1931, he co-authors a report the establishes radiometric dating as the only reliable method to determine the Earth’s age.

The current age Earth materials were not solid for a while after the Earth formed – big problem, as radioisotopes are not “fixed” in a crystal until after crystallization. Oldest rock: Acasta gneiss from the Slave craton, North America: 4.031 ± 0.003 Gyr Oldest mineral: Zircon from Jack Hills, Western Australia: 4.401 ± 0.008 Gyr Other solar system bodies cooled much more quickly after formation: Canyon Diablo meteorite (shown in photo): 4.53 to 4.58 Gyr Also useful: Lunar rocks, meteorites from Mars Age of solar system can be independently determined by helioseismology, which uses the distribution of helium in the Sun’s core to figure out how long the Sun has been active.