CK12 Earth Science

(Marvins-Underground-K-12) #1

Lesson Summary



  • Nicholas Steno first formulated the principles that allow scientists to determine the
    relative ages of rocks in the 17th century. Steno stated that sedimentary rocks are
    formed in continuous, horizontal layers, with younger layers on top of older layers.
    A century later, James Hutton discovered the law of cross-cutting relationships: a
    fault or igneous intrusion is younger than the rocks that it cuts through. Hutton also
    was the first to realize the vast amounts of time that would be needed to create an
    unconformity, a place where sedimentary rocks lie above an eroded surface.

  • Other methods come into play when comparing rock layers that are separated by a
    large distance. Many sedimentary rock formations are large and can be recognized
    across a region. Distinctive rock layers, called key beds, are also useful for correlating
    rock units. Fossils, especially index fossils, are the most useful way to compare different
    rock layers. Changes of fossils over time led to the development of the geologic time
    scale.


Review Questions



  1. In the 15thcentury, a farmer finds a rock that looks exactly like a clamshell. What did
    the farmer probably conclude about how the fossil got there?

  2. Which of Steno’s Laws is illustrated by each of the following images inFigure11.18?

  3. What is the sequence of rock units inFigure11.19, from oldest to youngest?

  4. What kind of geological formation is shown in the outcrop inFigure11.20, and what
    sequence of events does it represent?

  5. The three outcrops inFigure11.21are very far apart. Based on what you see, which
    fossil is an index fossil, and why?


Further Reading / Supplemental Links


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