Transcript Earth
Deep Time How old is Earth? Key Concepts: Different Age Determinations Ages based on human history Pre-20th century physical estimates. Radioactive Isotope Dating Best current estimate: 4.6 ± 0.1 Ga Geo-speak for “billion years” Historical Age Determinations Based on the concept of creation Paradigm: Human History = Physical History of Earth Traditional Jewish calendar starts from 3760 BC www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html Historical Age Determinations Eusebius of Caesarea (AD 260-340) 3184 BC, based on biblical chronology James Ussher (1581-1656) Evening of Sunday, October 23, 4004 BC (Julian) www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html Pre-20th century physical estimates George-Louis Leclerc, Comte de Buffon (1707-1788) Cooling of the Earth: 75,000 years Lord (William) Kelvin (1824-1907) Refined Buffon's calculation in 1897 to obtain an age of 25 Ma Why is this number wrong? Radioactivity! www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html Radioactivity Radioactive isotopes are unstable: over time they decay to produce stable isotopes Radioactive decay occurs in the nucleus of an atom Different types of radiation are emitted by this process (, and rays) Nuclear Decay Parent Daughter Exponential decay of the parent P = P0e-t is called the decay constant Half life (t1/2) is more commonly used Half life The time required (in a closed system) for the concentration of a parent isotope to be reduced by half. t1/2 = ln(2)/ = 0.693/ Each radioactive isotope has a unique half life Example: U-Pb system t1/2 = 4.5 Ga t1/2 = 0.70 Ga http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit5/deeptime.html Radiometric age dating: the Geological Hour Glass Parent D t1/2 Uranium-238 Lead-206 4.5 Ga Uranium-235 Lead-207 704 Ma Potassium-40 Argon-40 1.25 Ga Carbon-14 Nitrogen-14 5568 years Radiocarbon Potassium-Argon Uranium-Lead method method method (useful (useful (useful for geologically for for very intermediate-age oldyoung igneous rocks) (< igneous 50,000 rocks) years), previously living material) How big is Earth? In 200 BC, Eratosthenes Eratosthenes 276-194 BC estimated Earth’s circumference by geometry. He used the length of a building shadow in Alexandria at noon on the summer solstice. He knew that, simultaneously, sunlight was hitting the bottom of a water well in Aswan. How big is Earth? Alexandria His estimate (250,000 stadia) is only slightly larger than today’s known global circumference (40,075 km) Aswan Journey to the Centre of the Earth The earth is divided into four main layers: the inner core, outer core, mantle, and crust. Radius: 6,378 km Why is Earth differentiated? When Earth formed, immense amount heat released by meteorite bombardment melted the planet. How hot does it get? Early Earth during the Hadean period showing the so-called magma ocean. http://rst.gsfc.nasa.gov/Sect19/Sect19_2a.html Global heat flow Highest heat flow at mid-ocean ridges Thermal energy production from Earth: 3.8x1013 W Energy use of U.S.A. 3.0x1011 W http://geophysics.ou.edu/geomechanics/notes/heatflow/global_heat_flow.htm Geothermal gradient On continents, the near-surface geothermal gradient is ~ 16oC / km The geothermal gradient decreases with depth Extrapolating temperature into Earth’s interior Sources of Earth’s internal heat Primordial (from planet formation) Decay of radioactive elements Primordial Heat Gravitational contraction Core formation (heats mantle by 1000o C) Heavy meteorite bombardment during the Hadean Period (4.5 - 3.8 Ga) http://www.palaeos.com/Hadean/Hadean.htm Formation of the Moon Leading theory: moon formed by collisional ejection of mantle material at 4.45 Ga, due to collision of Mars-sized body