Dating rocks Zircon clocks A beginning Commentary

Bertram Boltwood geology

Dating rocks

• Christian scholars thought the earth
• had been created in 4004 BCE.
• Scottish and British geologists
• gradually pushed this date back in time.
• Bertram Boltwood, who suggested
• that uranium eventually decays into lead,
• noted that as a rock ages
• its ratio of uranium to lead would decrease,
• and so he become the first person
• to date rocks. He tested different rocks
• and found some were 2200 million years old,
• which isn’t any longer the oldest rock.
• The oldest known Earth rock is a zircon crystal
• that formed 4.4 billion years ago.

Zircon clocks

• When a zircon crystal forms
• uranium atoms may replace zirconium atoms
• but no lead atoms fit in the crystalline structure.
• At this point the uranium is sealed into the crystal.
• Uranium-235, decaying to lead-207,
• has a half-life of 700 million years
• and uranium-238, decaying to lead-206,
• has a half-life of 4.5 billion years,
• so each sample provides two clocks,
• a built-in cross-check to correct for loss of lead.

A beginning

• For those whose dislike of infinity
• is unquestioned, yes, the earth had a beginning;
• therefore, and why not, let’s say
• the universe had a beginning, too,
• without being too precise
• about what we mean by universe.
• If we cannot see, if we cannot date
• anything farther away, or further back in time,
• look how easily our “at least”
• starts to look like an absolute beginning.