Earth+Geologic+History


 * __Earth History__

-It is estimated that the Earth formed along with the solar system 4.6 billion years ago (4,600 MYA) - Geologic time Scale- Eon- largest segment of geologic time Era Period Epoch- smallest segment of geologic time

__Reading the Rock Record__ ** Places events in a sequence Identifies the actual but doesn't identify their dates of geologic events actual date of occurrence
 * __Relative Time vs. Absolute Time__**

-**law of superposition is undisturbed sedimentary rocks the oldest 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
 * Finding the age of Rocks with relative Time

-matching of rock layers that can be seen at the Earth's surface, over a large area - outcrop- exposed rock layers at the Earth's Surface //** -key bed-thin, widespread layer, usually of volcanic ash, that can be used to correlate an exact point of time
 * //Rock Correlation

//Fossils// - //**// 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are preserved - ** replaced remains- ** the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material.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) **// -**trace fossils-**evidence of life other then remains, which includes any impressions left in the rock. (trails, footprints, tracks, burrows) -**index fossil-**easily identifiable, short-lived, widespread occurrence**
 * // -any evidence of earlier life preserved in a rock
 * -**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

//Measuring Absolute Time// - varves- glacial lake deposits. a thick light colored layer in the summer and a thin dark layer in the winter
 * -**Tree Rings- each ring represents a single year (spring/fall) the width of the ring depends upon the temperature and rainfall

-used to date far back in time. certain rocks contain radioactive isotopes - radioactive isotopes- atoms of elements that give off radiation from their nuclei - radioactive decay- the process by which a radioactive isotope changes into a new stable element
 * //Radioactive Dating//**

-// the rate at which a radioactive element decays. it is the time it takes for a half of the atoms of the radioactive element to decay to a stable end product (see page 1 of the //ERST). at the end of each half-life, half of the radioactive material remains // **
 * //Half-Liife

-**parent isotope-=the radioactive isotope that begins**

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 things die the percent of Carbon-14 decreases at the rate of its half-life. Can be used to date back about 100,000 years**

Uranium Lead Method-**is useful to date rocks older than 10 million years. Can be use only on igneous rocks that contain the right kind or uranium.** Rubidium-Strontium Method-**can also be used to date older back 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 metamorphix