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Use Index Fossils
Transcript of Use Index Fossils
What are index fossils?
Why are index fossils useful to geologists?
Index fossils are the remains or traces of organisms that lived for only a short time, but in many places around the world.
When geologist find an index fossil in a rock layer, they know it is about the same age as all other rock layers that contain the same index fossil.
What is the difference between remains and traces?
->Remains are physical parts, like shells, bones, leaves, and so forth.
->Traces are impressions the organism made on its environment, like footprints.
Kiabab Limestone (Formation) and Z1 (limestone, sandstone)
Z7 (shale, sandstone, limestone) and B5 (shale, sandstone)
Younger. B3 comes between layers B5 and B1, which contain identifable index fossils. B5 contains
fossils from the late Jurassic. B1 contains fossils from the early Triassic. Z1 and the Kaibab in the
Grand Canyon contain the same index fossils from the late Pennsylvanian. So, B3 has to be older than
early Triassic, making it younger than the Supai Group.
Younger. Z1 contains Permian index fossis. B1 contains fossils from
the early Triassic, making it younger than Z1. B2 is on top of B1, so it
is younger than B1. So it is also younger than Z1.
The area was probably swampy or on a floodplain, since there is coal
and sandstone in the layer.
Read: A Fossil Primer
Hutton came up with the idea that the present is the
key to the past. This idea is call uniformitarianism. His ideas showed
people they could observe processes today to come up with ideas
about how to interpret past environments from evidence captured in
studied animals without backbones. He demonstrated how to use
Lamarck was the first person to define the word fossil. He
the evidence from rock types, fossils, and the principle of
uniformitarianism to reconstruct prehistoric environments.
Smith figured out how to use index fossils to come up
with a relative age for rocks. He helped develop the science of
stratigraphy, the study of the order and correlation of Earth's rocks
and the study of historical geology.
Certain fossils provide evidence for how old rocks are. These fossils
are called index fossils. If a geologist can identify index fossils in a
rock layer, he or she can be fairly sure how old the rock is. It's
somewhat like an index in a book, which helps your locate certain
chapters or subjects.
or in could have eroded away. Fossil b might not have lived in this
The rock layer that contained fossil b might not have bee deposited,
area, or no specimens were preserved here. You might look to see
where fossil b is found and see ifyou could trace the layer as far as it
Maybe the rocks turned upside down, or these weren't very good index fossils. You could follow
the layer to see if it turned upside down somewhere or look for another place in the area where
the fossils were in the "right" order. Or you might need to do some more explorations to show that these were not good index fossils because they wer found over a longer time period than was first thought.
rock layers. The rock layers had to be there first. Rocks B, C, D, and E were deposited on top
The basalt dike is younger than rock layer B, C, D, and E because it passes though all of these
of rock A, so this means they are younger then rock A. So the ages of rocks B, C, D, and E
are somewhere between 200 million years old and 225,000 years old, with rock B the oldest
and rock E the youngest.
Since the magma had to flow though the crack to get to the surface, it was there before lava
erupted on the surface, forming the volcano. If it cooled first, then the dike is older than the
Z2 (shale, limestone) and B1 (shale, conglomerate)