Earth+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 segment of geologic time era period epoch: smallest segment of geologic time ___ A list in chronological The exact times at which order of what you have you did these things done today up until this time
 * EXAMPLE**

Finding the age of rocks with relative time - Law of Superposition: in undisturbed sedimentary rocks the oldest rock layers are at the bottom of the youngest are at the top -Law of Cross: cutting relationships-an igneous intrusion is younger than the rock it has introduced into (look at the whiskers!)

the rock layer in which they are found or folding took place difference between the rocks above and those below the erosional surface (it appears like a squiggly line 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
 * Faulted and Folded Layers**: layers of rock that have been faulted or folded must have been present before the actions of faulting
 * 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

Rock Correlation: Matching of rock layers that can be seen at the earths surface, over a large area An outcrop is exposed rock layers at the earth's surface A keybed is a thin, widespread layer, usually of volcanic ash, that can be used to correlate an exact point of time

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 * Replaced remains**- the soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material.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// - easy 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 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
 * Radioactive Dating**

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 (page 1, 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

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 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

The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below
 * Air Masses**: An air mass is a huge section of the lower troposphere that has the same kind of weather (temperature and moisture properties) throughout

Air masses have **two** characteristics:
 * **temperature** depends on whether it comes from the tropics or polar regions
 * **humidity** of the air mass depends on whether it comes from land or sea
 * air masses are named from their source region

c - continental : dry m - marine T - tropical P - polar A - arctic

- fronts are the boundaries between air masses are called fronts

COLD FRONT
 * 1) cold air is advancing and replacing warm air
 * 2) cold fronts are steeper and move faster than warm fronts
 * 3) the air rises upward rapidly forming cumulonimbus
 * 4) heavy precipitation and thunderstorms which start and end quickly are associated with cold fronts
 * 5) precipitation covers 75 - 100 miles and occurs both before and after a cold front passes at the surface
 * 6) cold fronts travel at speeds between 25-30 MPH
 * 7) as soon as the front passes the temperature will decrease and the wind speed may rise



**WARM FRONT**

 * 1) warm air is advancing and replacing cold air
 * 2) warm fronts move slower; warm air moves up a gentle frontal surface
 * 3) warm air may travel 1000 KM before rising 2 or 3 KM
 * 4) first cirrus and cirrostratus clouds form and then there are altostratus clouds. finally, steady rain falls from nimbostratus clouds
 * 5) precipitation can occur for 225-275 miles ahead of where the front touches the ground
 * 6) warm fronts travel at speeds between 20-25 MPH
 * 7) warmer temperatures follow the passing warm front




 * OCCLUDED FRONT**
 * 1) A cold front is advancing and combing with a warm front
 * 2) Occluded fronts move the slowest of all (20 MPH)
 * 3) These fronts are associated with cirrus and stratus clouds in front of nimbostratus and cumulonimbus clouds
 * 4) Precipitation occurs for about 400 miles, most of which is in front of the frontal boundary
 * 5) Cool air is in front, as warm air is forced up by the cold air which follows the passing occluded front