In combination, the principles of stratigraphy were useful for determining a global relative time scale, but questions of numerical time were still unresolved.
Discovery of radioactive decay at the dawn of the 20th Century gave the key:
A way around the problem of Lord Kelvin's short physical estimate of Earth's age, because a new natural heat source for keeping Earth's interior molten was now known
Also, radioactive decay itself forms a "clock" usable for determining age of rocks.
Radiometric Dating: the single most important method of determining numerical rock ages.
Radioactive materials decay at predictable rate, known as the half-life
Atoms decay from one form (parent) to another (daughter product), releasing energy and particles
After one half-life has passed, half the original parents in the material will have decayed into the daughter product; after two half-lives, only one-quarter of the parent material remains, with three quarters daughter product; after three half-lives, 1/8 to 7/8; after four half-lives, 1/16 to 15/16; and so on
Can thus date rocks:
Compare the ratio of parent product to daughter product
Radiometric dates will only be effective for igneous rocks, since those are the ones that form by cooling and locking atoms into place
In sedimentary rocks, can date the individual grains of sediment: tells you age of source rock, but not deposition
In metamorphic rock, recrystallization redistributes atoms and obscures signal
Eliminates need for closed system and need for absence of original daughter product
Need three measurements from a rock: the amount of a radiogenically-produced isotope of an element; the amount of a non-radiogenically produced isotope of that element; the amount of the parent of the radiogenically-produced isotope.
Plot ratios of [daughter/stable] vs. [parent/stable]
At T=0, [daughter/stable] will be a single number, but [parent/stable] will vary from mineral to mineral in the rock, depending on how either the radioactive parent element or the non-radioactive stable isotope of the same element and the daughter product fits into different crystal structures in those minerals.
Thus, at T=0 (cooling of the melt), the points will be on a straight horizontal line
As T increases, the radiogenically-produced daughter value increases relative to both the stable and the parent product, so the points move together away from the horizontal.
Because of the nature of the ratios involved, as points move away from horizontal they should do so in a straight line with the same Y-intercept as the original line. This straight line is the isochron, and its slope is a function of the number of half-lives that have passed. The steeper the slope, the older the rock sample.
If points do NOT fit on a straight line, indicates either gain or loss of one of the parts of the system; allows for a measure of uncertainty.
The following is an animated demonstration of the isochron method, from Talkorigins.org:
(Incidentally, while the two main geological radiometric systems (40Ar/39Ar and 238U/206Pb) are now highly precise, the numbers they yielded were slightly different based on traditional calibrations. There was already reason to suspect that Ar-dating was slightly miscalibrated on geochemical arguments. Recalibrating the Ar-clock based on astrochronology (see below) suggests that this technique
as typically used yield numbers about 1% too young. As a consequence, dates from before the 2010s often don't quite match up with our current understanding...)
Other methods of numerical dating:
Fission track: etching of crystal by decay products of uranium 238 in individual zircon crystals.
As 238U decays, particles thrown off smash crystal lattice.
Number of tracks already present are counted in acid-etched crystals. This indicates the number of decay events that have happened so far.
Crystals are then put in a neutron field, which causes it to decay completely
Number of new tracks are counted, which gives the number of parent that was present when the sample was collected.
Radiocarbon (Carbon 14) dating:
NOT the same as radiometric dating!!
In life, organisms take up both 12C (stable) and (radioactive).
When dead, no new carbon added, and 14C breaks down with half-life of 5730 years.
With short half-life, only useful for objects less than ~70,000 years old.
Some other methods of relative dating:
Transgression-regression patterns: On scale of regional depositional basins can be very useful, as long as no additional uplift on only one part of basin.
Eustatic (global) sea level changes: As above, but can correlate globally. In both cases: only accurate in environments (shorelines, mainly) where sea-level changes are recorded. We will come back to this when we look at sequence stratigraphy later on.
Marker beds: One-time events (volcanic eruptions, asteroid impacts, etc.) may send particular types of material over wide region (even globally). These record EXTREMELY short periods of time: essentially instantaneous!
Stable Isotope Stratigraphy: Some stable isotopes of various elements (carbon, oxygen, strontium) vary over time relative to each other due to a number of factors (productivity, glaciers, temperature, erosion, etc.). When examined against a time scale, these form irregular curves. Individual samples or series of samples can be compared to the known-curve of these isotopes to see where they fall.
Astrochronology: a relatively new field. Looks at the varying thickness of strata within sedimentary packages to calibrate their changing thicknesses to some astronomically-controlled cycles (such as tidal, daily, monthly, annual, or longer term cyclicity.) Not good for calibrating against a time scale as such, nor for correlation, but very useful for determining the duration in numerical time of packages of sedimentary rock.
The magnetic (but NOT the geographic) poles have "flip-flopped" throughout geologic time, so that sometimes a magnet's north pole points towards geographic North, and sometimes toward geographic South.
Magnetic polarity can be recovered by some iron-bearing rocks (sedimentary and igneous).
Because based on the Earth's magnetic field, the changes occur everywhere on the planet at the same time.