Absolute dating rocks
VARVES ARE LAYERS OF LIGHT AND DARK SEDIMENT THAT SHOW YEARLY CYCLES. Each spring or summer when the glacier was melting the glacier deposits a ton of sediment it was carrying into strems of water that are melting off of it.When the meltwater reaches a lake the heaiest sediments (sand and silt) sink to the bottom quickly and eventually formed thick layers of light colored rock.For really old dates we need to use radioactive isotopes.RADIOACTIVE ISOTOPES ARE UNSTABLE VERSIONS OF NORMAL ELEMENTS.Simply stated, each bed in a sequence of sedimentary rocks (or layered volcanic rocks) is younger than the bed below it and older than the bed above it.
Following this law, sedimentary rocks can be “dated” by their characteristic fossil content.
Carbon-14 is a method used for young (less than 50,000 year old) sedimentary rocks.
This method relies on the uptake of a naturally occurring radioactive isotope of carbon, carbon-14 by all living things.
THE ORIGINAL RADIOACTIVE ISOTOPE IS CALLED THE PARENT ELEMENT.
THE MORE STABLE FORM IT TURNS INTO IS CALLED THE DAUGHTER ELEMENT.