Mountain Building Folding vs. Faulting Geologic time & dating.

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Presentation transcript:

Mountain Building Folding vs. Faulting

Geologic time & dating

Relative Dating Process of placing events in the sequence in which they occurred does NOT identify actual dates Geologic History Sequence

Uniformitarianism concept that the same processes that occur today also occurred in the past changes were slow over time, not quick catastrophic events “the present is the key to the past” George Hutton

Original Horizontality concept that most sedimentary rocks are deposited as a horizontal layer

Superposition concept that in an undisturbed sequence of sedimentary strata, the oldest rock layer will be at the bottom, and the youngest on top oldest youngest

Cross-cutting concept that an igneous intrusion or fault is always younger than the rock it cuts across

Embedded Fragments concept that rocks embedded in another rock must be older than rock in which it is found Pieces of igneous rock, left when the igneous was partially eroded, are in the green sedimentary rock. Logically, then, the igneous pebbles must be older than the sedimentary rock they are included in

Uplift Over time, rocks can be tilted or deformed by regional or local metamorphism If rocks are tilted, then the metamorphism that caused it is younger than the rocks tilting

New, younger rocks form on top of the tilted strata tilting Unconformity- gap in geologic record caused by erosion Unconformity

A B C D Layer A is the oldest. Layer D is the youngest. Practice

Layer A is the oldest. Layer E is the youngest. Tilting & erosion occurred after D, but before E. A B C D E Practice unconformity

F E D C B A Layer A is the oldest, then Layers B, C, D and E. Folding & erosion occurred after E but before F. Practice unconformity

Layer A is the oldest. Layer F is the youngest. Tilting & erosion occurred after D, but before E. F E D C B A Practice unconformity Anticline / Dome Mtn.

1. limestone 2. sandstone 3. shale 4. limestone 5. sandstone 6. reverse fault 7. Igneous Intrusion Practice

Unconformity Fault 6. Geologic Column

E G L C H M D J A N K B FYoungest Oldest

Oldest Youngest MEGAKZCFDNBLTRJ

FPBRMHAEXDKJS

VCOMXKEBSZJGLFTADPHNR

Fossils help geologists study Earth’s past & determine…  Approximately when life began  What plants & animals were 1 st to live on land  When organisms appeared/disappeared  How organisms live Trilobite

Fossils Can be in the form of: REMAINS BONES TEETH TRACES

How Fossils form  The body must be protected from scavengers & bacteria  Hard parts have a better chance of becoming fossils  Fossils are found in sedimentary rocks

INDEX FOSSILS  Certain fossils can help find the relative age of rocks.  Since each organism comes from a specific time period, we can use a fossil to determine the age of the rock it is found in.  These special fossils are called index fossils.  They are abundant, widely spread and must have lived only during a short part of earth's history.

INDEX FOSSILS Ex: Graptolites an ancient plant - alive mya. Ex: Trilobite an ancient animal - alive mya.

Some Index Fossils

ABSOLUTE Dating

Absolute Dating Determining the actual dates for events Gives ages of rock in number of years

Counting the Years Tree rings (up to 10,000 yrs) Varves (yearly glacial layers) How does this help us understand the past?

Radioactive dating The most accurate method for determining age. Radioactive elements give of particles and energy as they decay and new elements form. Scientists know how long it takes for them to wear down, so they are like natural clocks.

Radioactive decay Half-life: the time it takes for half the radioactive atoms in a sample to decay to a stable product 100 kg 50 kg 25 kg 12.5 kg 0123

Absolute dating Most absolute dating is done by measuring the amount of… –Carbon 14 left over in fossils of plants & animals less than 50,000 years old. –Uranium 238 left over in rocks older than 50,000 years old.

Isotopes Used in Radiometric Dating Parent IsotopeDaughter Isotope Half-life (years) Effective Range (yrs) Possible Materials for Dating Carbon-14Nitrogen ,000Once-living matter Uranium-238Lead billion>10 millionUranium-bearing minerals (zircon) Rubidium-87Strontium-8747 billion>10 millionMicas, feldspars, metamorphic rocks Potassium-40Argon billion>50,000Micas, amphiboles, feldspars, volcanic rocks The instrument that measures radioactivity is a Geiger counter.

Bibliography l.jpg rohan.sdsu.edu/~rhmiller/geologictime/GeologicTime.htm 01.jpg mainz.de/~pfeiffer/home_for_old_atoms.jpg Modified by J Fehr & L Bell