3How geologists explain Earth’s rock record Neptunism: all rocks were precipitated from an ocean (1780’s)Werner (p. 6)In line with biblical floodAppeared to agree with rock layeringProblems:Where did the water go?What about volcanoes?Too simple!
4How geologists explain Earth’s rock record Catastrophism (1800’s)-Ch.1All of Earth’s history resulted from sudden, widespread catastrophes6 major catastrophesLast catastrophe was the floodProblem: need more than 6 events
5How geologists explain Earth’s rock record Uniformitarianism (1800’s)-in Ch. 1James HuttonCharles LyellPresent is the key to the pastProcesses occurring today have occurred throughout geologic history AT THE SAME RATESCan’t explain all rocks found on earthRocks formed in conditions that no longer existConditions still exist, but at great depthConditions exist, but not observable over human lifetime
6Relative dating Establishing relative sequence of events Based on uniformitarianismDoesn’t put a specific age on eventThis is what your homework is!
7Applying Uniformitarianism to rocks Horizontal sedimentary rocksWhich rock units are older?
8Principle of superposition With horizontal sed rocks, the oldest are on the bottom
9Near vertical Sedimentary Rocks Were these sed rocks deposited like this?
10Priciple of lateral continuity A water-laid layer of sediment, at the time it was formed, must continue laterally in all directions until it thins out as the result of non-deposition or until it abuts against the edge of the original basin of deposition.
11Principle of original horizontality Sediments are deposited nearly horizontal. If they aren’t today, they must have been moved or deformed
22Rock ID Quiz Wed. 30 Jan Look at your rock and answer the following Describe your rock. Include the texture, color and composition of your rock. (4pts)Is your rock igneous, sedimentary or metamorphic? (3pts)What is the name of your rock? (3pts)Bonus (2pts):What is a mineral (you must have all characteristics to get the points)?
23Stratified sedimentary rocks Stratigraphy: study of layered rocksRight: pyroclastics in Oregon so not only sed rx are layered
24Bedding planes in sed rocks Layers separated by surfaces called bedding planes
25Relative dating and stratigraphy Which is younger?
26Disturbed (deformed) sed rocks Which is younger? Which way is up?
27Relative dating and stratigraphy Which way is up?
28Principles of relative dating Assumes constant deposition of rocksWhat if not constant?What if rocks removed from record?Unconformity: break in geologic time
29Unconformities Missing time in the rock record Caused by…what? 3 kinds DisconformityAngular unconformityNonconformityRelative dating and stratigraphy
30Erosion and unconformities How do unconformities occur?ErosionOccurs when rocks are above sea level…why is this??
31DisconformityErosional surface is in between parallel beds
32DisconformityDescription: A disconformity - unconformity, or gap in geologic time, caused by the process of erosion.
33Angular unconformityTilted beds overlain by younger, horizontal beds
34Angular unconformityTitled beds overlain by younger horizontal beds
35NonconformityBedded rocks rest on an eroded surface of crystalline rocks (what are those types of rocks?)
36NonconformityDescription: An nonconformity. Crystalline granite in contact with Mesozoic sandstone, near Colorado Springs.T
49Applying stratigraphy and relative dating Rocks from Saskatchewan Canada. What is sealevel doing? Introduce the concept of sea level fluctuating back and forth
50Early attempts to date the Earth St. Augustine of Hippo ( A.D )Established idea of A.D. and B.C.
51Archbishop Ussher in 1600’s Counted geneologies in the bible back to Adam and EveEarth formed: October 22, 4004 B.C.
52Buffon in 1700’s Assumed Earth started molten and cooled to today He measure the cooling rate of melted balls of ironEarth ~ 75,000 years oldProblems:Assumes no new source of heat
53Other techniques to estimate age: Deposition rates of sedimentsAges range from less than 1 million to more than 2 billionProblems:Assumes constant rates of depositionDoesn’t take erosion into account
54Other techniques to estimate age: Salt content in oceansAge of earth ~ 90 million yearsProblems:Assumes oceans start as fresh waterAssumes all salt comes from riversAssumes no salt is removed from oceans
55Lord Kelvin’s age of Earth (1866) Returned to cooling Earth calculationsEarth was hot when first formed, cooling ever sinceEarth looses heat through timeAge: 20 million to 40 million years oldProblem:Too young to account for uniformitarian interpretationDiscovery of radioactivity defeated this idea
56Where does this leave us? Earth is old with slow processesEarth is younger than 400 million with fast processesDiscovery of radioactivity changes thingsRadioactivityDecay of elements through timeMajor heat source for Earth
57Modern uniformitarianism Less literal uniformitarianismProcesses are the same, rates changeUniformitarianism and catastrophism combinedOccasional catastrophic events do happenEarth is 4.6 billion years old
58Help with relative dating exercise Geologic time/relative datingPicture with solution