Geological dating radioactive elements

They have clearly fallen to Earth from outside, often gouging out huge craters such as that called New Québec (61°17´ N, 73° 41' W).Rubidium-strontium, potassium-argon, uranium-lead and samarium-neodymium dating all show that the meteorites formed about 4.6 billion years ago.For centuries people have argued about the age of the Earth; only recently has it been possible to come close to achieving reliable estimates.In the 19th century some geologists realized that the vast thicknesses of sedimentary rocks meant that the Earth must be at least hundreds of millions of years old.The other key dating techniques involve uranium-235 transforming to lead-207 at a rate of one-half every 713 million years, uranium-238 becoming lead-206 at one-half every 4.5 billion years, potassium changing to argon (and calcium) at one-half every 1.3 billion years and samarium-147 becoming neodymium-143 at one-half every 106 billion years.These radioactive processes present a set of natural clocks which reveal when the rock was formed, or when it was last heated severely.The Earth certainly must be older than the oldest terrestrial rocks found.

Those rocks available for analysis (ie, the oldest ones) have been heated and squeezed many times in their GEOLOGICAL HISTORY, because for billions of years continents have been drifting over the Earth's surface, colliding and producing mountains and new ocean floors.It has even been possible to work out a time scale of the reversals of the Earth's magnetic field.This "radiometric" approach has superseded all other techniques for determining absolute ages. Their nuclei tend to emit particles spontaneously - ie, they are radioactive.Using the uranium-lead technique they dated zircon crystals from the gneiss (located southeast of Great Bear Lake in the NWT) and showed that it was formed almost 4 billion years ago.Therefore it is clear that the Earth is over 4 billion years old.

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