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Geologic Time

Geologic Time. The Foundation: Fossils and Relative Time Understanding how both are used to get a better understanding of our Earth’s past (and its future). *The Beginning: 3 Basic Rock Types*.

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Geologic Time

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  1. Geologic Time The Foundation: Fossils and Relative Time Understanding how both are used to get a better understanding of our Earth’s past (and its future)

  2. *The Beginning: 3 Basic Rock Types* • Sedimentary Rock- Rocks made by erosion of rock;( limestone, sandstone, shale and they are water soluble) • Igneous Rock – rocks made by magma; granite, marble, and basalt and are water resistant) • Metamorphic rock- sedimentary rocks that due to being under pressure and heat turn into more igneous nature

  3. Determining the age of rocks: 2 ways • Absolute age -Actual Age (determined by Carbon dating) • Relative Age -Age compared to other objects (gives you an estimate of its age; index fossils or depth provide this)

  4. *Absolute Age* • It tell the actual age or how long ago an even occurred • They use Radioactive dating • Radioactive dating-measures the age of a material by comparing the amount of a radioactive form of an element in a rock or fossil with the amount of its decay product • Precise

  5. *Relative age* • It tells if something happened earlier or later than others without giving a definite date

  6. Did these Rocks fall before or after the rock layers formed?

  7. *Law of superposition* • Formed by undisturbed sedimentary rock layers • Older rock layers lie beneath younger rock layers: (Grand Canyon at first glance)

  8. Exceptions to the Rule: 4 • Overturned Bed • Angular Bed • Intrusions • Unconformity

  9. Law of Superposition EXCEPTIONS: Overturned bed • Sedimentary Rock layers totally misplaced • Commonly caused by Compression forces and found in mountains

  10. Law of Super Position EXCEPTIONS: Angular bed • Horizontal rocks are younger than the tilted rocks • Tilted rock caused plate-tectonic forces

  11. Law of Superposition EXCEPTIONS : IntrusionCaused by Magma • Igneous rocks are younger than any sedimentary rocks they cut

  12. *Law Of Superposition EXCEPTIONS*: Unconformity • Missing layer or gap in the rock record: caused by magma intrusion or erosion • Makes it hard to understand how Earth has changed during a specific period of time Missing Layer destroyed by magma

  13. *2 Types of Erosion of Rocks* • Mechanical Weathering : • water freezes and unfreezes causing Rocks to split open from ice the expanding in the cracks; • also wind blowing sand against rocks

  14. *2 Types of Erosion of Rocks: Chemical Weathering * • Chemical Weathering- Acid rain dissolves away the rock

  15. Why do We Care about Rocks? • Rocks provide us with a “Picture” of the Earth’s Past History • Fossils

  16. *Fossils* • Important to understand the history of life on Earth • Older rock—simpler organisms and Younger rock-more complex organisms • Shows how species have changed: EVOLUTION • Shows how species are related to another or changed to environmental change

  17. *How Fossils Form* • 1 Quick Burial of organism in soft sediment: Mudslide, volcanic ash, quicksand, mudpits, tarpits • 2 Organism remains undisturbed for long period of time • 3. Water slowly enters area soft tissue is dissloved and water slowly eats away bone/shell • 4. If water flow is “fast” complete organism will be eaten away creating “hole” in the rock. Leaves basic shape of organism • 5. If water eats away very slowly minerals in water can replace bone creating a Cast. Minerals recrystalize as soon as bone is dissolved creating a PERFECT COPY! • 6. Fossil is then found millions of years later: FACT-Only 17 T-rex Skeletons have ever been found

  18. Ways Fossils form

  19. Molds • Hollow area in sediment in the shape of an organism or part of organism • Hard part of organism buried in sediment • Water carries dissolved minerals and sediment may seep into the empty space of the mold

  20. Cast/Recrystalization • Solid copy of the shape of an organism: bones replaced by minerals in water. EXTREMELY ACCURATE; VERY RARE

  21. Petrified • Fossils where minerals replace all or part of an organism • Turns into stone • Water rich with mineral seep into spaces

  22. Carbonization • Oils from organism turn into carbon imprint on rock • Common in plants and Fish

  23. Trace fossils • Fossilized footprint or trail

  24. *INDEX FOSSILS* • Index fossil- commonly found fossil worldwide that can give you an approximate age and environment of the area then. (Trilobites)

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