How Scientists Measure Time.

"Common sense” tactile measures of layering.


Stratigraphy. Layer process, sequence of sedimentary rocks.  Tiers of geological strata gives a rough idea of age.  Not precise or absolute. 
Dendochronology.  Annual tree rings. 
Varves, a Swedish term referring to the annual layers of sediment and silt in riverbes and lake floors.

Self-referencing Dating.

This uses fossils of known age or grains of pollen common to period to time to date a deposit. 
Or archaeomagnetic based on the wanderings of magnetic north around the rotational north pole.

Radioactive Decay Clocks.


Carbon 14 is stable in the atmosphere and maintained in living plants which take in air or animals that consume those plants.  When they die, however, the atomic structure changes into nitrogen 15, gives off radioactivity.  Half of it is gone in 5,730 years, half of what’s left decays in another 5,730 years.  This measures how long ago the organism died. If you have a quarter of the carbon 14, it’s 11,460 years old…up to 1/64 to make it 34,380 or 1/28 to be 40,110.  Calibrated by dendrochronology. 

Potassium 40 in molten lava begins to turn into argon 40 as it solidifies, half life is 1.25 billion years and measures lava flows and volcanic ash above and below fossil-bearing rock.  Also,

Rubidium 87 into strontium with half life of 48.8 billion years, and

Uranium 238 into lead 206, with a half life of 4.51 billion years..

Thermoluminescence transformation of minerals that form clay.  As fired in a kiln, it causes an unstable subatomic arrangement to become more stable, producing light.  The finished ceramic slowely returns to its unstable form, creating a clock mechanism. Measure the light when ceramic is refired.

Electron spin resonance (ESR) measures the extent to which a background radiation dislodges electrons from normal positions in atoms, particularly in quartz and flint as far back as 500 million years.

Obsidian hydration.  Depth at which moisture can be detected in an obsidian arrowhead, for example, measures its age.

Amino acid racemization. Very precise, based on behavior of amino acids.  Subject to light that is polarized, made to vibrate in one direction.  As amino acids crystallize, they affect incoming beam of light by rotating it to the left, but when an organizism dies, the crystals begin to realign and bend light to the right. ..racemization.  This takes about 60 million years.

Human Evolution.