Age dating metamorphic rocks

Radiometric dating uses the decay of isotopes of elements present in minerals as a measure of the age of the rock: to do this, the rate of decay must be known, the proportion of different isotopes present when the mineral formed has to be assumed, and the proportions of different isotopes present today must be measured.

This dating method is principally used for determining the age of formation of igneous rocks, including volcanic units that occur within sedimentary strata.

Deep inside the Inner Gorge of Grand Canyon, northern Arizona, are the crystalline basement rocks that probably date back even to the Creation Week itself.

Clearly visible in the canyon walls are the light-colored granites, such as the Zoroaster Granite, which are stark against the darker, folded strata of the Vishnu Schist and the other metamorphic rock units of the Granite Gorge Metamorphic Suite (see lowest purple and green shading in diagram).

This is different to relative dating, which only puts geological events in time Most absolute dates for rocks are obtained with radiometric methods.

These use radioactive minerals in rocks as geological clocks.

These were originally basalt lava flows several meters to tens of meters thick.

Up to this time estimates of the age of the Earth had been based on assumptions about rates of evolution, rates of deposition, the thermal behaviour of the Earth and the Sun or interpretation of religious scriptures.

Shortly after Becquerel's find, Marie Curie, a French chemist, isolated another highly radioactive element, .

The realisation that radioactive materials emit rays indicated a constant change of those materials from one element to another.

When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.

Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.