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Cosmic Calendar. Reading Rock Records. Telling Time. Relative Time – places events in a sequence but does not identify their actual date of occurrence. Absolute Time – identifies the actual date of an event. Laws .

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Telling Time

  • Relative Time – places events in a sequence but does not identify their actual date of occurrence.

  • Absolute Time – identifies the actual date of an event


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Laws

  • Law of superposition - states that in a sequence of undisturbed sedimentary rock, the oldest rocks are on the bottom, while the youngest is on the top.

  • Law of cross-cutting – states that an igneous rock is younger than the rock that it has intruded, or cut across

  • Law of included fragments – states that pieces of one rock found in another rock must be older than the rock in which they are found.

  • Unconformity – is a place in the rock record where layers of rock are missing.


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Geologic Timetable

  • EON – the longest segment of time

    • Eras – the segment of time EONS are divided into

      • Periods – era are divided into these

        • Epochs – period are divided into these



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How are they formed?

  • Fossils – any evidence of earlier life preserved in a rock.

    • Original remains

      • The actual, unchanged remains – of the plant or animal are preserved.

    • Replaced remains

      • The soft parts of the animal are no longer there, but the hard parts are still visible

    • Molds and Casts

      • Mold is where you have left a hollow depression in the rock

      • Cast is where you have a new mineral fill a mold

    • Trace Fossils

      • Evidence of life other than the remains of a plant or animal






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Index Fossils

  • Fossils that are used to determine ages of rock that they come from

  • Also known as guide fossils





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Decaying

  • Radioactive Decay

    • When an element loses mass as it ages

  • Half life

    • The rate at which that process happens

  • Radiocarbon dating

    • An isotope that measure the amount of carbon decay