Record in the Rock

Record in the RockWhat Processes Shape our Earth?GeologyGeology: the scientific study of the origin, history, structure, and composition of the Earth Importance:Understanding of the forces that shape our Earth, so we can better forecast potential disastersEarths internal structureCharacterized by a gradual increase in temperature, pressure, and density with depth

Crust is the outer most layer of Earth, consists of two types:Continental: makes up continents, older than oceanic crust, thickOceanic: makes up the ocean floor youngest rocks, thin, mostly made of basalt (igneous rock) Age of The Earth & Geologic TimePrinciple of Uniformitarianism:The order in which layers are deposited is how they relate to time.The same processes that are at work today were at work in the past

*The present is the key to the past! Age of The Earth & Geologic TimeRelative Dating: Finding the age of something, compared to something else. We use a number of principles and laws to do this:Law of Original Horizontality soil is deposited horizontally, then forms rock layersLaw of Superposition - the layer below is older than the layer above. Scientist use law of superposition by using relative age (something compared to the age of something else)Problem: uncomformity buried surface that represents a break in the rock record.

Age of The Earth & Geologic TimeRelative Dating3. Lateral Continuity- layers of sediment extend in all directions when they form.4. Law of Cross-cutting Relationships - A rock is older than any rock across which it cuts. Folds & faults are younger than the layers they cut across.

Age of The Earth & Geologic TimeInclusions: the inclusions (rock pieces) are older than the surrounding rockFaunal Succession: fossils can be used to identify relative layers of rockIndex fossil: Lived in a certain time span in many placesLived in great numbersDistinct features to identify Age of The Earth & Geologic TimeCorrelation: matching rocks by index fossil in different places 1st person to use correlation was William SmithSmith & others developed the Geologic Column a diagram of the sequence of rock layers in an specific area, in order of ageEarly Estimates of Earths Age MethodsSedimentation estimated the total thickness of the earths sedimentary rocks (rate of sedimentation) Range from 3 million to 1.5 billion yearsProblemsRate can vary at different times & placesNo accurate way to measure thicknessStarted with different measurementsEarly Estimates of Earths Age2. Salt Method estimated the salt content of the oceans, then compared it with the rate at which salt is being added to the oceans. Range from 9 million to 2.5 billion yearsProblems1. assumed at first the oceans contained fresh water2. did not account for all way salt is added to or removed from the oceans3. Each scientist estimated amounts of salt Early Estimates of Earths Age3. Kelvin Method assumed Earth was hot molten rock, he measured rate of Earths cooling to present. Also, took into account heat coming from within the Earth and from the Sun. Range from 20 million years to 100 million yearsProblemsNo one knew about radioactivityAbsolute Age DatingAbsolute Dating: Enables scientists to determine the exact numerical age of rocks and other objectsRadioactive Decay : Radioactive isotopes break down into other elements by emiting radiationAn element is defined by the number of protons it containsAs the number of protons changes with each emission, the original radioactive isotope (parent) gradually converted to a different element (daughter)Ex: Uranium 238, parent, changes into Lead -206, daughterRadiometric Dating: Dating an object using radioactive isotopesAs the number of parent atoms decrease during radioactive decay the number of daughter atoms increaseRate of decay never changes, but is based on elements half lifeHalf life: time it takes for half of the isotope to break down into another elementUranium 238 use to determine age of non-living thingsCarbon used to determine age of once living things

Dendrochronology: Dendrochronology: use tree rings to determine absolute ageUses tree rings called annual rings (early & late season) The width of the rings depends on certain conditions in the environmentTrees from the same geographic region tend to have the same patterns of ring width for a give span of timeBy matching the rings in these trees scientists have established tree ring chronologies up to 10,000 yearsIce Cores: Ice Cores: they contain a record of past environmental conditions in annual layers of snow depositionSummer ice tends to have more bubbles and larger crystals than winter iceIce-core chronologies study glacial cycles and climate

Varves: Varves: bands of alternating light and dark colored sediments of sand, clay and siltOccur with seasonal deposition of sediments around lakes usuallySand-sized particles in summer (more) and thinner, fine-grained particles in winter (few)Scientists can date cycles of glacial sedimentation over periods as long as 120,000 years

Radioactive Half-LifeHalf Life: The amount of time required for half of a substance to decayAfter one half life there is 1/2 of original sample left.

After two half-lives, there will be 1/2 of the 1/2 = 1/4 the original sample.Example 1You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years. How many grams are left after one half-life? Answer:50 g How many grams are left after two half-lives?Answer:25 g

Half-Life ExampleThe half-life of iodine-131 is 8 days. If you start with 36 grams of I-131, how much will be left after 24 days?24 days/8days = 3 half lives

36 g 18 g 9 g 4.5 g1 half life2 half lives3 half lives