• Joly missed that salt is removed from the oceans by various processes.
• Kelvin could not have know that new heat is generated inside the Earth by radioactive decay (nuclear fission), because the process was unknown.

The discovery of radioactivity: Ironically, radioactive decay, which frustrated Kelvin's purpose, ended up providing the true key to the absolute dating of rocks.

• Antoine Becquerel (1852-1908): Discovered natural radioactivity (1896). In the following years, a large number of radioactive isotopes and their daughter products became known.

• Pierre (1859-1906) and Marie (1867-1934) Curie: Discovered that radium continuously releases newly generated heat. With this discovery, it became clear that the decay of radioactive substances provided a continuous source of new heat that Thomson hadn't accounted for. The Earth might, indeed, be much older than anticipated. But how old?

Radiometric dating: In 1905, Bertram Boltwood noted a parent-daughter relationship between 235U, a radioactive isotope, and Pb suggesting that one decayed into the other. It develops that each radioactive isotope has a characteristic decay rate, transforming from a parent radioactive substance to a daughter decay product product at a constant stochastic rate. This means that each atom of the radioactive substance has an equal chance of decaying in a given interval. The overall sample, however, displays a constant half life - the amount of time it takes for half of a given sample to decay, regardless of the sample size. Thus, if we know a substance's half-life and can measure the proportions of parent and daughter substances, we can calculate the time at which the crystal containing the substances solidified from a melt. Ernest Rutherford calculated decay rate from U to Pb. This enabled the first radiometric dating. Arthur Holmes: First used radioactive decay as a means of dating rocks (1911). Result is that oldest known Earth rocks are aprox 4.2 billion years old (abbreviated "Ga") Oldest in Solar System 4.56 Ga.

Radioactive isotope	Daughter substance	Half Life	Applicable range
Uranium 238	Lead 206	4.5 Ga	10 m.y. to 4.6 Ga
Potassium 40	Argon 40	1.3 Ga	100,000 to 4.6 Ga
Carbon 14*	Nitrogen 14	5,730	to 70,000

* used in plant material only, not rocks. Note that the effective range of these dating systems is limited by the degree of error in measurement.

Which rocks are useful? When you radiometrically date a mineral grain you are determining when it crystallized. Thus The best rocks to use are igneous rocks in which all crystals are roughly the same age. The age of new minerals crystallizing in metamorphic rocks can also be determined by radiometric dating. Because metamorphism can occur over long intervals, they are somewhat less ideal. Also, we must use minerals that incorporate the radioactive isotope and daughter product in known proportions to begin with. Thus, sedimentary and metamorphic rocks can't be radiometrically dated. Note: relatively young plant material can be dated with 14C. Although only igneous rocks can be radiometrically dated, ages of other rock types can be constrained by the ages of igneous rocks with which they are interbedded.

The Earth generates a magnetic field that encompasses the entire planet. In the last fifty years, a new dating method has emerged that exploits two aspects of rocks' interactions with the Earth's magnetic field. It is, in essence a form of relative dating. Paleomagnetism: Some magnetic minerals, such as magnetite occur naturally in igneous rocks. When their grains form, they align themselves with the Earth's magnetic field. The Earth's magnetic field changes quickly (i.e. on the scale of a human life.) Nevertheless, because of the orientation of their magnetic minerals, their intrinsic magnetic field records the orientation of the Earth's field as it existed when they formed. Such ancient magnetic fields are called remnant or paleomagnetism. ("Paleomag" in geological slang.) Magnetic reversals: The Earth's magnetic field has a north and south pole. For unknown reasons, at intervals of (very) roughly 500,000 years, the north and south poles trade places. The result is that the paleomagnetic polarity of igneous rocks is either: Normal: Magnetic north coincides roughly with geographic north. Reversed: Magnetic north coincides roughly with geographic south. But note: magnetic reversals don't occur with clock-like regularity. If we drill a core form layers of rocks with paleomagnetism, and color-code ones with normal and reverse polarity, we get a pattern like a bar code. Any interval of time we designate will display a unique pattern of paleomagnetic reversals.

What kinds of rocks retain paleomagnetism:

• Sediments consist of very small grains that settle slowly from water
• Sediments include magnetic minerals
• Sediments were deposited in very quiet body of water, like a lake.

The fact that sediments can record paleomagnetism is very useful. Remember, we have no means of directly measuring the radiometric age of sediments that aren't preserved in association with igneous rocks. We can, however, hang a numerical age on them if their paleomagnetic "fingerprint" can be matched with that of a sequence of igneous rocks that can be radiometrically dated.

The utility of paleomagnetism:

• Radiometric dates are always subject to margins of error, whereas a rock's paleomagnetic polarity is absolute. Knowing the paleomagnetic polarity of a sample can, therefore, give an independent means of constraining its age.

• Most rocks that preserve paleomagnetism (igneous) can also be radiometrically dated. Because some sedimentary rocks can also retain paleomagnetism, then by knowing their polarity, we can assign them more reliable absolute dates by correlating them with igneous rocks of the same paleomagnetic chron.

• Since paleomagnetic chrons are not of the same duration, paleomagnetic time charts resemble sections of tree rings in which the differing thicknesses of adjacent rings provide a "fingerprint" of a time period. Similarly, the differing duration of adjacent chrons gives each period of Earth history a distinct paleomagnetic fingerprint.