The earth has been determined by radiometric dating to be

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The accumulating evidence pointed to an extraordinary new idea: that the history of Earth goes back much, much further than any human memory.

In 1788, Scottish geologist James Hutton published his “Theory of Earth,” which introduced the world to the idea of “deep time.” The implications of the treatise were revolutionary: Not only was the Earth not young, but it was not static, Hutton said.

This spawned several earnest — if not entirely successful — attempts to determine the age of the Earth based on ongoing natural processes.

One calculated how long it would take rivers to deliver enough dissolved minerals to the ocean to give it its current saltiness (answer: 90 million to 100 million years).

Certain isotopes are unstable and undergo a process of radioactive decay, slowly and steadily transforming, molecule by molecule, into a different isotope.

This rate of decay is constant for a given isotope, and the time it takes for one-half of a particular isotope to decay is its radioactive half-life.

Using fossils as guides, they began to piece together a crude history of Earth, but it was an imperfect history.

Rather than assume the planet was the product of bygone catastrophes, such as a massive global flood, scientists could explain the ancient rock record with phenomena that exist today.These atoms will decay as the rock ages, and by measuring the ratio of radioactive isotopes within the rock, scientists can figure out how long it has been around.In 1913, geologist Arthur Holmes published “the Age of the Earth,” the first major effort to date the planet using radiometric dating.But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.

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