Jacob et al., 2014

Geersen et al., 2015

Wei et al., 2013

Duputel et al., 2012

GEOL 460 LECTURE 8 TODAYS MATERIAL: Geochronology Geochronology PREVIEW MATERIAL: Heat Flow Heat Flow Note Supplemental Reading on website:

The averaged shape of the Earth, calculated by assuming that the Earth is a symmetrical about its rotation axis (solid line), compared with a spheroid of flattening 1/ (dashed line).

The relations between the geoid, the spheroid, topography and anomalous mass. From: G.R. Foulger and C. Peirce

variations in the strength of the Earth’s gravity and magnetic fields due to variations in crustal density and magnetism. Note the simpler form of the variations in gravity compared with those in the magnetic field.

Gravitation field of a sphere. (a) Gravity measured on a horizontal surface above a dense spherical source (b). (c) Variation in the vertical component of gravity along the principal profile (X-X’) due to the combined effects of the Earth’s field and that of the sphere. (d) The radially directed gravity field of the source and its vertical component. Note that the strength of the Earth’s gravity field is many times greater than that of the source, but has been reduced here for clarity.

Local isostatic compensation according to: (a)the Airy–Heiskanen model (b)the Pratt–Hayford model; Regional isostatic compensation according to: (c) according to the elastic plate model of Vening Meinesz

GEOCHRONOLOGY Time is relative… The earliest clocks were developed by the Egyptians and were later introduced to Greece and later Rome. About 2000 BC the Egyptians invented a water clock (clepsydra). The progress of time would be measured by collecting the amount of water that would escape a small hole. Sundials came later. The earliest clocks were developed by the Egyptians and were later introduced to Greece and later Rome. About 2000 BC the Egyptians invented a water clock (clepsydra). The progress of time would be measured by collecting the amount of water that would escape a small hole. Sundials came later. Mechanical clocks were invented around 1000 AD but reliable pendulum clocks did not come onto the scene until the 17 th century. Sundials were used to calibrate mechanical clocks until the 19 th century. Mechanical clocks were invented around 1000 AD but reliable pendulum clocks did not come onto the scene until the 17 th century. Sundials were used to calibrate mechanical clocks until the 19 th century. An early 19th-century illustration of Ctesibius's (285–222 BC) clepsydra from the 3rd century BCE. The hour indicator ascends as water flows in. Also, a series of gears rotate a cylinder to correspond to the temporal hours. /media/File:Clepsydra-Diagram-Fancy.jpeg

Two kinds of geologic time: Relative Relative Numerical (Absolute) Numerical (Absolute) What are they based upon? Relative: Relative: Principles of: Principles of: Superposition Superposition Original Horizontality Original Horizontality Lateral Continuity Lateral Continuity Cross-Cutting Relations Cross-Cutting Relations Inclusions Inclusions Unconformities: Unconformities: Angular Unconformity Angular Unconformity Disconformity Disconformity Nonconformity Nonconformity Paraconformity Paraconformity What does conformable mean? What does conformable mean? Continuous deposition of rocks Continuous deposition of rocks Numerical: Numerical: Radioactive Decay Radioactive Decay

By Paul Nadar - Portrait of Antoine-Henri Becquerel ( ), Physicist, Public Domain,

Do electrons have mass? If so, how much compared to protons and neutrons? ~1/2000 times

What is radioactive decay? When nuclei are unstable because the forces that bind protons and neutrons together are not strong enough, the nuclei break apart, or decay (creating isotopes). Parent isotopes give rise to daughter isotopes at a constant rate, called a half life. For every half life, half of the parent isotopes have decayed.

N P P P P N N N E

All plots should have their axes labeled and that is the first thing one should observe whenever they look at any plot. The vertical axis on this plot represents the number of atoms (in percent). The horizontal axis represents the number of half lives, or elapsed time. In other words, this plot shows the number of parent and daughter isotopes that exist after a given number of half lives.

Scienceblog Image credit: Press & Silver

This figure shows how 14 C is formed and how 14 C decays. The upper panel shows a neutron from outer space being captured by a Nitrogen 14 atom. The lower panel shows how 14 C decays to 14 N when an Neutron converts into a Proton and an electron is emitted.

Assumptions: Initial Concentration (Po) Half Life (Decay Rate (λ) Clock Setting Time (time when system closes) Closed System (no contamination with new parent isotopes)

Lowrie

Normalized radioactive decay curves of some important isotopes for dating the Earth and solar system. The arrows indicate the respective half-lives in 109 yr (data source: Dalrymple, 1991)

Schematic design of a mass spectrometer, and hypothetical mass spectrum for analysis of strontium isotopes (after York and Farquhar, 1972).

14 C Age Modeling Green is the terrestrial curve Blue is the marine curve. 1.Radiocarbon years to Calendar Years (different) 2.Marine Reservoir Correction (younger) 3.Gap Correction (younger) 4.Stratigraphic Correction = A > B > C (depends)

Rb/Sr isochron for the Uivak gneisses from eastern Labrador. The slope of the isochron gives an age of Ga (after Hurst et al., 1975).