Geologic+History

It is estimated that the Earth formed along with the Solar System 4.6 Billion years ago (4,600 Mya)

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

Era

Period

Epoch- smallest segment of geologic time __Relative Time vs. Absolute Time__ ** Relative- Places events in a sequence but does not identify their actual date of occurrence.
 * Reading the Rock Record

Absolute- Identifies the actual dates of Geologic Events

EXAMPLE

Relative- A list in chronological order of what you done today up until this time

Absolute- The exact times at which have 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. - Law of Cross-Cutting relationships- an igneous intrusion is younger than the rock it has intruded into. - Law of included fragments- if fragments of one type of rock are found in another rock layer the rock fragment 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)
 * __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. -An outcrop is exposed rock layers at the earth's surface - *A Key Bed is thin, widespread layer, usually of volcanic ash, 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 Remans (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 * Replaced Remains**- **the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material.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 others 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

Used to date far back in time. Certain rocks contain radioactive isotopes.
 * __Radioactive Dating__**
 * 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

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 the ESRT) 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 has been changed
 * Half- Life


 * 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.


 * Uranium Lead Method** is useful to date rocks older than 10 million years. Can be used only on igneous 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 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 ** **