If you wanted to consult the geologic period for when the first vertebrates evolved, where would you look?
a. in the section for the Ordovician period of the Paleozoic era.
b. in the section for the Quaternary period of the Cenozoic era.
c. in the section for the Triassic period of the Mesozoic era.
d. in the section for Precambrian time.
After a rock layer is eroded, which additional event must occur to create an unconformity, or a gap in the geologic record?
a. an earthquake or motion along a fault.
b. new deposits of sediment must build up on the eroded area.
c. folding of sedimentary rock layers.
d. a volcanic eruption.
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To consult the geologic period for when the first vertebrates evolved, you would look in the section for the Ordovician period of the Paleozoic era (option a). The Ordovician period, which lasted from about 485 to 444 million years ago, is known for the diversification of marine life, including the appearance of the first vertebrates.
After a rock layer is eroded, the additional event that must occur to create an unconformity, or a gap in the geologic record, is that new deposits of sediment must build up on the eroded area (option b). This process is known as deposition. When new layers of sediment accumulate over the eroded surface, it creates a gap in the geologic record because the time and geological history represented by the eroded layer is not recorded in the new layers of sediment.