Age Of The Earth

Age of the Earth

• Early ideas: • 1644 - Bishop Lightfoot - Earth created on Sept. 17, 3928BC at 9 am. • 1658 - James Usher, Anglican Bishop of Ireland – earth created on October 26th, 4004 BC. • 1750 - Buffon (French) Early Earth would require 75,000 years to cool. • 1800 - James Hutton - Recognized that the Earth was created by the same processes that we see in operation today - "The present is the key to the past" - recognized the enormity of geologic time. • 1830 - Sir Charles Lyell - wrote "Principles of Geology" and coined term "uniformatarianism" to describe the earlier idea of Hutton concerning the enormous amount of time recorded in the geologic record.

• 1830-1900 - numerous attempts to determine the age of the Earth: • (1) Use the thickness of sediments and the rate of sediment accumulation to calculate the time required to produce the observed sedimentary record - Estimates from 3 million to 1.5 billion years.

• (2) Use the amount of NaCl in the ocean and the salt content of rivers flowing into the ocean to calculate how long it would take to accumulate all the salt in the sea. John Joly used this method to calculate an age of 90 million years. • (3) Lord Kelvin - English physicist - Earth would require <100 million years to cool; the Suns energy could only last 20 to 40 million years. • 1895 - Radioactivity discovered. • 1906 - first radioactive dates suggested that the Earth must be billions of years old.

• Modern Age Dates: • Oldest rocks on Earth = ~3.7 billion years: West Greenland granite; Minnesota gneiss. • Oldest minerals on Earth = ~4.1 to 4.2 billion years: zircons in Australian sedimentary rock. • Oldest rocks on Moon = ~4.3 billion years. • Most meteorites = 4.55 billion years. • Generally accepted age of the Earth = 4.55 billion years.

• Radioactive Dating Methods: • Decay paths (a = 4He; b = 1 electron; half life = time for 1/2 of total to decay) • 238U -> 206Pb + 8a half life = 4.5 billion • 235U -> 207Pb + 7a half life = 0.71 billion • 232Th -> 208Pb + 6a half life = 14.1 billion • 87Rb -> 87Sr + b half life = 47 billion • 40K + b -> 40Ar half life = 1.3 billion • 14C -> 14N + b half life = 5,730 years

• Examples: • Radiocarbon Dating • 14C is an unstable isotope that forms in the upper atmosphere from cosmic ray bombardment on nitrogen molecules. • Not use to directly date rocks but organic materials produced by once living organisms. • Living plants absorbed 14C and 12C during photosynthesis and animals that ate plants will also have 14C in their bodies.

• Living plants and animals have ratio of 14C/12C equal to the atmosphere. • When plants and animals died they stopped taking in 14C. • 14C will then decay back to N. • 5,730 yrs. after death the 14C/12C is 1/2 that of the atmosphere; • 11,460 yrs. after death 1/4 that of the atmosphere; • 34,380 yrs. after death 1/64 that of the atmosphere; etc.

• K-Ar Dating • If we assume a mineral contained lots of K and no Ar when it formed (generally a good assumption since Ar is an inert element), the K/Ar ratio gives the age. • But if Ar that was formed escaped this will give younger age. • Can give the age of reheating of rocks.

• U-Pb Dating • If we assume a mineral contained lots of U and no Pb when it was formed, the Pb/U ratio gives the age. • Since there are two isotopes of U and two of Pb, we can test the date to see if it is accurate. • A plot of 206Pb /238U vs. 207Pb /235U will fall on a known curve (called concordia) if the system has not been disturbed. • The position on the curve gives the date.

• Rb-Sr Dating • When a rock crystallizes, • - some minerals have lots of Rb and little Sr, • - others have lots of Sr and little Rb, but all have the same 87Sr/86Sr. • Since 87Rb decays to 87Sr over time, minerals with a high Rb/Sr will increase in 87Sr/86Sr over time much faster than minerals with a low Rb/Sr. • A plot of 87Sr/86Sr vs. Rb/Sr for minerals from the same sample will fall on a straight line (called an isochron) if the system has been undisturbed since the sample was formed. • The slope of the line gives the date.

• What Then the Age of the Earth? • Theoretically we should be able to determine the age of the Earth by finding and dating the oldest rock that occurs. • So far, the oldest rock found and dated has an age of 3.96 billion years. • But, is this the age of the Earth? • Probably not, because rocks exposed at the Earth's surface are continually being eroded, weathered, subducted, re-melted and thus, it is unlikely that the oldest rock will ever be found. • But, we do have clues about the age of the Earth from other sources:

• Meteorites • These are pieces of planetary material that fall from outer space to the surface of the Earth. • Most of these meteorites appear to have come from within our solar system and either represents material that never condensed to form a planet or was once in a planet that has since disintegrated. • The ages of the most primitive meteorites all cluster around 4.6 billion years. • Moon Rocks • The only other planetary body in our solar system that we have samples of are moon rocks (samples of Mars rocks have never been returned to Earth). • The ages obtained on Moon rocks are all within the range between 4.0 and 4.6 billion years. • Thus the solar system and the Earth must be at least 4.6 billion years old.

• Why is the Earth looks younger than the moon and meteorites? • Earth is geologically active – plate tectonics. • Has a hot, molten interior. • Rocks are re-melted and their internal clocks are reset. • Also, rocks on Earth's surface are acted on by erosion and weathering. Rocks on Earth surface are not as old as the Earth, they are "recycled" rock materials • Rocks broken down into sediments (gravel, sand, silt, clay). • Sediment will turn into sedimentary rocks over time. • Older rocks were buried deeply under younger rocks.