Earth+Geologic+History

 EARTH HISTORY

It is time estimated that the earth formed along with the solar system 4.6 billion years ago (4,699 MYA)

Geologic Time Scale- A summary of the major events in earth's history Eon - largest segment of geologic time

Era - A major division of time that is a subdivision of an eon and is itself subdivided into periods

Period - asubdivision of an era and is itself subdivided into epochs, corresponding to a system in chronostratigraphy. Epoch - Smallest segment of geologic time

Relative Time VS. Absolute Time-
 * Reading the Rock Record :**

Relative Time : Places events in a sequence but does not identify their actual dat or occurrence Absolute Time : Identifies the actual dates of geologic events EXAMPLE Relative Time : A list in chronological order of what you have done today up until this time Absolute Time : The exact times at which you did these things

Law of superposition- in undisturbed sedimentary rocks the oldest rock layers are at the bottom and the youngest are at the top.
 * Finding The Age Of Rocks With Relative Time

Law of cross-cutting relationships- an igneous intrusion is younger than the rock it has intruded into. (Look at the whiskers)! ** 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

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)

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, widesoread layer, usually of colcanic ask, that can be used to correlate an exact point of time.
 * Rock Correlation

Fossils : A **fossil** 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 * Replaced Remains-** The soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material.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 then remains which includes any impressions left in the rock. (Trails, footprints, tracks, burrows.)
 * Index Fossil** - easily identifiable, short-lived, widespread occurrence

Measuring Absolute time
 * Tree Rings** - each ring represents a single year (spring/fall) the width of the ring depends upon the temperature and rainfall.
 * Varves** - Glacial lake deposits. A thick light colored layer in the summer 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 (see page 1 of erst) At the end of each half-life, half of the radioactive material remains. Parent Isotope: the radioactive isotope that begins Daughter isotope: the stable isotope that had 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 says 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. Is is also bery commonly found in igneous rocks.
 * Potassium-Argon Method** is very usful 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