Potassium-Argon dating is a form of radiometric dating which tells us the absolute age of rocks. Practically all elements are radioactive to some extent, some more than others. Moreover, different isotopes of elements are more radioactive than others. Isotopes are forms of a particular atom that varies in the number of neutrons in the nucleus. For instance, K-40 is a naturally occurring isotope of Potassium is far more radioactive than its siblings K-39 and K-41 because K-40 is much less stable. K-40 has a half-life of approximately 1. 35 billion years and is found in essentially everything on Earth.
K-40 can undergo beta decay and can transform to two different atoms: Calcium-40 and Argon-40. Approximately 10. 72% of beta decay is K-40 transforming to Ar-40 via electron capture. Electron capture occurs when a proton-saturated atom absorbs an electron from the inner shell and thus transforming a positively charged particle (proton) into a neutral particle (neutron). Since lava is liquid, any argon-40 gas previously trapped in lava will be able to escape as gas is less dense than liquid. When there is no argon gas in the lava, it is known as the zero point (zero argon trapped in lava).
As soon as the lava solidifies, there are no outlets for argon-40 gas to escape and subsequently begins to collect in the solidified lava (rock). We can measure the amount of argon-40 gas in rock and thus using the known halflife of K-40 and decay constant of K-40, we can work out the time the rock solidified and estimate the time each volcanic eruption occurred. Using the mathematical equation that demonstrates exponential decay: N(t) = NOe-kt Where N(t) is the present amount of potassium, NO is the original amount of potassium, -k is the constant at which potassium decays, and t is the time it took the potassium to decay.
We can calculate how long the rock has been there for. Since we know the half life of potassium is 1. 35 billion years, we can work out the constant at which potassium is decaying (k). The working shows that potassium has a decay constant of 5. 134 x 10-10 yrs-1 We can test this value using given potassium ages and percentages: By testing GA No. 1467: 1. 329 x 10-10 is relatively similar to 1. 437 x 10-10, which is the value taken from New Zealand Journal of Geology and Geophysics, therefore, we have reasonable evidence to believe our value for potassium decay constant is relatively accurate.
Paleomagnetism: Relative Dating Paleomagnetism is the study of the Earth’s history of magnetic fields. It can be used to determine the relative age of magnetic rock such as basalt (basalt is iron rich, iron is magnetic, thus basalt is magnetic). The Earth has a naturally occurring magnetic field believed to be generated from the core. The Earth’s inner core is mostly comprised of solid iron while the outer core is made up of 2,000 kilometre thick layer of liquid iron, nickel, and other metals. The differences in density, temperature, and characteristics of the molten metal in the outer core causes convection currents.
This is where dense matter sinks whereas warmer, less dense matter rises. Additionally, the Coriolis effect, a phenomenon caused by the Earth’s rotation that makes moving substances/objects’ path bend relative to the Earth’s rotation, also causes a churning vortex of molten metal. The effect of moving liquid iron induces currents which can also produce magnetic fields. When the other charged metals passes through these fields, this can also induce current and produce more individual magnetic fields, this domino effect is called advection.
Since advection would endlessly generate a massive magnetic field, it is stabilised and balanced via diffusion. Diffusion is where the field “leaks”from the core and thus destroyed; this overall process is known as the geodynamo. Moreover, the churning vortexes caused by the Coriolis effect also creates separate magnetic fields that are aligned in a similar direction as the magnetic fields generated by the geodynamo; this magnetic field constructively interferes with the magnetic field created by the geodynamo and results in a brobdingnagian magnetic field that envelops our Earth and generating our “North” and “South” poles.
However, these poles are not permanent and reverse quite often (in a geological sense). Pole reversal is an attribute of the geodynamo. The Earth’s inner core is solid, therefore it cannot generate a magnetic field via advection. However, any field generated by the outer core can circulate and disperse into the inner core. The advection process in the outer core periodically tries to reverse, however, the field that was diffused in the inner core prevents the reversed field generated by the outer core’s reversal process and thus a reversed magnetic field cannot establish throughout the entire core.
The inner core refuses new fields diffusing in and it’s estimated that one in ten reversals are successful. When basaltic lava settles, the rocks will have an angular magnetic inclination which points in the direction of the closest pole, this angle can help us determine the direction of the pole at the time and by comparing the direction to the Paleomagnetic Time Scale, we can determine the age of the rock.
The Paleomagnetic Time Scale is simply a recording of the different position of poles historically. It was initially created by measuring the magnetic inclination of different areas with seafloor spreading. The lava from seafloor spreading settles and a collection of the magnetic inclination from different locations helped us determine the age and pole locations at the time (see figure 1).