earth+geologic+history

=**Earth History**= = = =**It is estimated that the earth formed along with the solar system 4.6 billion years ago.**= = = =**Geologic time scale**= =**summary of the major events in earths history**= =** EON- largest segment of geologic time**= =** ERA **= =** PEROID **= =** EPOCH - smallest segeminent of geologic time**= = = =**Reading the rock record**= =** __Relative time vs. Absolute time__ **=

**Example of absolute- the exact times at which you did theses things**
=**FINDING THE AGE OF ROCKS WITH RELATIVE TIME**= =**law of superposition in undisturbed sedimentary rocks the oldest rock layers at the bottom and the youngest are at the top.**= = = =**law of cross cutting relationishemts- An ignoeus intrusion is younger than the rock it has intruded into __Law of including fragments__ - if fragments of one type of rock are found in another rock layer the rock fragments must be older than the rock layer in which they are found. __Faulted and folded layers__ __-__ layers of rock that have been faulted or folded must have been present before the actions of faulting or folding took place. unconformity- a place in the rock record where layers of rock are missing because of uplift and erosion. The result can be a large age difference between the rocks above and those below the erosional surface (It appears like a squiggly line in a cross difference)**=



=Rock correlation=

A key bed is a thin, widespread layer usually of volcanic ash that can be used to correlate an exact point of time
= Fossils =

Trace fossils- edvince of earlier life other then remains which includes any impressions left in the rock ( trails, footprints, tracks, burrows)
=Index fossils-= = =
 * ===Easily identifiable===
 * ===Short- lived===

__measuring absolute time__
Tree rings- each ring represents a single year (spring/fall) The width of the ring depends upon the temp and rainfall

varves- glacial lake deposits. a thick light colored layer in the summer and thin dark layer in the winter

Radioactivity dating- used to date far back in time. certain rocks contain radioactive isotopes

Radioactive isotopes are atoms of elements that give off radiation from their nuclei

Radioactive decay is the process by which isotopes changes into a new stable element

HALF LIFE

the rate at which a radio active element decays it is the time it takes for halftime atoms of the radioactive element to decay to a stable end product

At the end of each half time half of the time radioactive material remains parent isotopes- the radioactive isotopes that begins

Daughter isotope- The stable isotope that has been changed

Radiocarbon dating- uses the radioactive isotopes carbon-14 found in all living things. because carbon-14 is continually absorbed by food and water it stays constant in living things when the living thing dies the percentage of carbon-14 decreases at the rate ofits halftime. can be used to date back about 100,000 years

uranium lead method- is useful to date older rocks older than 10 million years can be used only on iginoeus rocks that contain the right kind of uranium

rubidium strontium method- can also be used to date older rocks because of its long half-life it is also very commonly foundin igeous rocks

POTASSIUM argon method- is very usefull since potassium 40 can be found in metamorphic sedimentary and igenous rocks. It can date older rocks but may also date rocks as young as 50,000 years