Geologic+History

GEOLOGIC HISTORY:DD ** E A R T H H I S T O R Y : D D

**

A SUMMARY OF THE MAJOR EVENTS IN EARTH'S HISTORY.
EON- largest segment of geologic time ERA --PERIOD --EPOCH- smallest segment of geologic time.

== FINDING THE AGE OF ROCKS WITH RELATIVE TIME -Law of superposition in undisturbed sedimentary rocks the oldest rock layers are at the bottom and the youngest are at the top. LAW OF CROSS- cutting relationships- an igneous intrusion is younger than the rock it has intruded into. (look at the whiskers!) 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- section.
 * Law of included 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.

ROCK CORRELATION - matching of rock layers that can be seen at the earth's surface, over a large area. - an **//outcrop//** is exposed rock layers at the earth's surface. - A key bed is a thin, widespread layer, usually of volcanic ash, that can be used to correlate an exact point of time.

=FOR PRACTICE:=

http://regentsprep.org/Regents/earthsci/rockcorrelation.htm



FOSSILS a **f**//**ossil**// is any evidence of earlier life preserved in the rock.
 * Original Remains** (rare) 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 * Replaced Remains -** the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material.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**- fossil shells or bones are dissolved completely out of the rock leaving a hollow depression in the rock. new mineral material fills the mold it forms a cast of the original fossil.
 * Trace Fossils** - evidence of life other than remains, which includes any impressions left in the rock (trails, footprints, tracks, burrows.)

THIS ABOVE HERE IS A REPLACED REMAIN.

Trace fossils- evidence of life other than remains, which included any impressions left in the rock.

EASILY IDENTIFIABLE. ---SHORT-LIVED -WIDESPREAD OCCURRENCE.
 * INDEX FOSSIL-**

Measuring absolute time - each ring represents a single year (spring/fall) the width of the ring depends upon the temperature and rainfall.
 * Tree Rings**

VARVES- glacial lake deposits. a thick light colored later in the sumer and a thin dark layer in the winter.

RADIOACTIVE 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 a radioactive isotope changes into a new stable element.

HALF-LIFE - the rate at which a radioactive element decays. it is the time it takes for half of the atoms of the radioactive element to decay to a stable end product. - at the end of each half-life, half of the radioactive material remains. parent isotope= the radioactive isotope that begins. daughter isotope= the stable that has been changed.

radiocarbon dating- uses the radioactive isotope 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 of its half-life. can be used to date back about 100,000 years

Rubidium strontium method- can also be used to date older rocks because of its long half-life. it is also very commonly found in igneous rocks. potassium argon method- is very useful since potassium 40 can be found in metamorphic, sedimentary, and igneous rocks. it can date older rocks but may also date rocks as young as 50,000 years.