Deep Time How old is Earth? Key Concepts: 4 Different Age Determinations Ages based on human history Pre-20th century physical estimates. Radioactive Isotope Dating 4 Best current estimate: 4.6 ± 0.1 Ga Geo-speak for “billion years”

Historical Age Determinations www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html 4 Based on the concept of creation 4 Paradigm: Human History = Physical History of Earth 4 Traditional Jewish calendar starts from 3760 BC

Historical Age Determinations www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html 4 Eusebius of Caesarea (AD ) 3184 BC, based on biblical chronology 4 James Ussher ( ) Evening of Sunday, October 23, 4004 BC (Julian)

Pre-20th century physical estimates www-astronomy.mps.ohio-state.edu/ ~pogge/Ast161/Unit5/deeptime.html 4 George-Louis Leclerc, Comte de Buffon ( ) Cooling of the Earth: 75,000 years 4 Lord (William) Kelvin ( ) Refined Buffon's calculation in 1897 to obtain an age of 25 Ma 4 Why is this number wrong? Radioactivity!

Radioactivity 4 Radioactive isotopes are unstable: over time they decay to produce stable isotopes 4 Radioactive decay occurs in the nucleus of an atom 4 Different types of radiation are emitted by this process ( ,  and  rays)

Nuclear Decay Parent Daughter

Exponential decay of the parent P = P 0 e - t   is called the decay constant 4 Half life (t 1/2 ) is more commonly used

Half life 4 The time required (in a closed system) for the concentration of a parent isotope to be reduced by half. t 1/2 = ln(2)/ = 0.693/ 4 Each radioactive isotope has a unique half life

Example: U-Pb system t 1/2 = 4.5 Ga t 1/2 = 0.70 Ga

Radiometric age dating: the Geological Hour Glass ParentDt 1/2 Uranium-238 Lead Ga Uranium-235 Lead Ma Potassium-40 Argon Ga Carbon-14Nitrogen years Uranium-Lead method (useful for very old igneous rocks) Potassium-Argon method (useful for intermediate-age igneous rocks) Radiocarbon method (useful for geologically young (< 50,000 years), previously living material)

How big is Earth? Eratosthenes BC 4 In 200 BC, Eratosthenes estimated Earth’s circumference by geometry. 4 He used the length of a building shadow in Alexandria at noon on the summer solstice. 4 He knew that, simultaneously, sunlight was hitting the bottom of a water well in Aswan.

How big is Earth? 4 His estimate (250,000 stadia) is only slightly larger than today’s known global circumference (40,075 km) Alexandria Aswan

Journey to the Centre of the Earth 4 The earth is divided into four main layers: the inner core, outer core, mantle, and crust. Radius: 6,378 km

When Earth formed, immense amount heat released by meteorite bombardment melted the planet. Why is Earth differentiated?

How hot does it get? Early Earth during the Hadean period showing the so-called magma ocean.

Global heat flow 4 Highest heat flow at mid-ocean ridges 4 Thermal energy production from Earth: 3.8x10 13 W 4 Energy use of U.S.A. 3.0x10 11 W

Geothermal gradient ! On continents, the near-surface geothermal gradient is ~ 16 o C / 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 1000 o C) ! Heavy meteorite bombardment during the Hadean Period ( Ga)

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