ESS 106 Lecture 05 June 2013

This means the ratio of 14c to 12c decreases by half

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Unformatted text preview: rengthens the C14 dating method Radiogenic Isotopes: Principles of Carbon-14 Dating •  3 isotopes of Carbon; one is radioactive Isotopes: SAME number of PROTONS (and thus, the same element) DIFFERENT number of NEUTRONS (and thus, a different atomic mass) Carbon-14 is COSMOGENIC! •  Born when a “cosmic ray” interacts with an atom already in the earth’s atmopshere –  In this case, the cosmic ray is a neutron •  14C is formed when a neutron bombards a 14N atom •  Relatively constant flux of neutrons from outer space n 1 4N Free neutron (cosmic ray) collides with a Nitrogen atom in the atmosphere p 1 4C Expels a proton from the nucleus, atom is converted into Carbon BUT… 14C wasn’t meant to be It is UNSTABLE And RADIOACTIVE Principles of Carbon-14 Dating 1 4C has a half life of 5,730 years. THIS MEANS: The ratio of 14C to 12C decreases by half every 5,730 yrs. Uses of Carbon-14 Dating: Carbon dating can be used on anything which was once alive: 1.  Animal (or human) remains, including skin, fur and bone. 2.  Plant remains, including wood, natural fibers (cotton, silk, wool, cloth, rope), seeds and pollen grains. Limitations of Carbon-14 Dating: • Carbon dating cannot be used on things which have never lived. • The amount of carbon-14 in samples is very small (1 in 850 billion atoms of carbon) and after 9 or 10 half-lives the amount of carbon-14 is too tiny for accurate measurement. • Therefore, limit of carbon dating is about 60,000 years old. • The method of carbon dating uses an assumption that the atmospheric carbon-14 in the past is known (by calibration) and that there has been no contamination. You cannot date dinosaur bones with 14C! OTHER Radioactive isotope systems • We date old lavas (eruptions) in order to figure out how old they are. • Carbon dating limit is ~60,000 year (~10 half lives) • Doesn’t give us too much to work with! • There are other ways. 40Ar / 39Ar : half life = 1.25 billion years U/Pb: half life = 4.6 billion years Oldest known hominid footprints (no knuckle impressions) Tanzania, Africa 3.6 Ma volcanic ash Dating using the K/Ar method (precursor to the Ar/Ar method) Oldest known piece of earth Tiny grain (the mineral zircon) in a sandstone 4.4 BILLION YEARS OLD Western Australia Dating using the U/Pb method Another geologic clock: Cater Counting •  Establishes a relative age of a planetary surface •  Lots of craters –  Older •  Less craters –  younger •  Normalized to known ages of moon rocks and the crater density in those regions Moon impact theory •  Mars sized body collided with earth –  After planetary differentiation –  Mostly clipped the outer portions of earth –  The moon’s Fe-Ni core is very small •  4.5 billion years ago •  Evidence for moon’s earth origin –  O isotope similarity –  Earth’s spin and moon’s orbit are identical –  Moon was once molten Surface of the Moon Two distinct terrain types • Light, cratered highlands made of anorthosite a plutonic igneous rock that are around 4 billion years old Light highlands • Darker, blotchy mare basins of low viscosity basalt flows that are very extensive, with flow fronts 20-30 m high. Most flows 3 – 3.5 billion years old. Basically a lunar flood basalt • Named maria (latin for sea) by early astronomers who mistook them for actual seas No obvious volcanoes; some rare volcanic domes Mare basins Venus •  Close in size and density to Earth, but closer to Sun; water couldn‘t form oceans •  Covered in clouds of CO2 •  Surface temp 500o C and pressure 90 bars •  Exceeds both Earth and Mars in number of preserved volcanic landforms, over 22,000 on just 25% of the surface area •  Most abundant are small shields (2-8 km dia.) with small central crater, in large fields or colles Jupiter s Moons (the Galilean Satelites) Io Europa Ganymede Callisto Io: Pillan Patera 140 km - high plume the most volcanically active body In the Solar System Prometheus 75 km - high plume •  Radioactive decay is not sufficient to heat interior Io •  Tidal forces- from an eccentric orbit around Jupiter- create gravitational kneading that keeps its combined metal and silicate interior hot and partially molten Visible light image Infrared image Photos by NASA’s Galileo spacecraft, 2001 The Lowdown: Planetary Volcanism •  Volcanoes are not specifically an earthly phenomenon •  Any planet experiencing heating/heat loss •  Io and Earth have the only “active” volcanoes in the solar system –  There’s plenty of “cryovolcanism” which is basically geyser activity •  Evidence for past volcanism on the Moon, Mars, Venus –  Biggest volcano in the solar system: Olympus Mons, Mars NASA, Voyager 1 GOOD LUCK!! teach your friends about volcanoes!...
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This document was uploaded on 02/24/2014 for the course ESS 106 at University of Washington.

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