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



EPOCH- smallest segeminent of geologic time

Reading the rock record

Relative time vs. Absolute time

relative- places events in a sequence but does not identify their actual date of occurrence

Absolute- identifies the actual dates of geologic events

Example of relative a list in chronological order of what you have done today up until this time

Example of absolute- the exact times at which you did theses things


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

matching of rock layes that can be seen at the earths surface over a large area.

An outcrop is exposed rock layers at the earths surface

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


A fossil is any evidence of earlier life preserved in the rock

original remains- the actual unchanged remains of the plant or animal are preserved

Replaced remains- The soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material (petrified wood)

Molds and casts- fossils shells or bones are dissolved complety out of the rock leaving a hollow depression in the rock. new minerial material fills the mold it forms a cast of the original fossil

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

Wide spread occurrence


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


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