absolute age
relative age
niformitarianism
Study Sheet for Geologic History
superposition
index fossil
extrusion
unconformity
intrusion
evolution
orogeny
radioactive decay
half-life
Things to Study:
1. Humans have been on Earth for only for a very small fraction of time.
2. We observe processes happening today and assume they happened the same way in
the past – so we can infer what processes formed present day features.
(uniformitarianism)
3. In sedimentary rock layers, the oldest are on the bottom (if they haven’t been
overturned, it they aren’t igneous intrusions that came later)
4. Sedimentary layers form horizontal layers – if they are no longer horizontal they must
have moved after they formed.
5. Any rock layers with contact metamorphism must have been there BEFORE the
igneous rock pushed through (they are older).
6. Any layers that are folded and/or faulted must have been there first (they are older
than the fold or fault)
7. Sediments are older than the rock they are found in, mineral veins are younger than
the rock they are found in
8. Correlation means matching layers in one area with layers in another area
9. Walking the outcrop means visually following an exposed rock layer from one
location to another.
10. Walking the outcrop only works in places where rocks are exposed over a long
distance.
11. Rocks can be correlated if they have similar color, mineral composition, and grain
size.
12. An index fossil is one from a species of organism that existed only a short period of
time on Earth, but lived over a large geographic area.
13. Volcanic ash can be used for correlation because it covers a large area over a short
period of time and becomes a part of the rock record.
14. Steps in the formation of an unconformity: Rocks layers form, rock layers are pushed
up and eroded, rock layers later sink and are covered with new rock layers (but now
something is missing – this is a buried erosional surface).
15. Be able to use ref. tab. P 8-9.
16. Estimated age of the Earth is: 4600 million years (4.6 billion years)
17. The largest division of geologic history is the Precambrian (by a lot)
18. There are not many fossils from the Precambrian because few organisms lived, those
that did live had soft bodies that don’t make good fossils.
19. Geologic time is broken into different divisions based on changes in life forms –
based on fossil evidence
20. On p. 8 RT – rock record in NYS: if there is a black line in the column we have
them, if not, we don’t.
21. NYS has a nearly complete rock record from the end of the Precambrian up to the end
of the Devonian.
22. If NYS does not have rocks from a certain period it means they were either eroded, or
there were not rocks being formed during that time period (we were not covered by
water).
23. To find index fossils on p. 8-9 RT: find the names across the top of the chart – see
which letter, find that letter in the column labeled “Time distribution of fossils”.
24. To match the information on p. 3 to p. 8-9 RT: On p. 3 find the symbol on the map –
check the key to see which period of time – turn to p. 8-9 to find the same period –
look for information about that time period
25. Be able to identify the landscape regions from p. 2 and match them up with p. 3 in the
reference tables.
26. Mountains have: High elevation, distorted structure, metamorphic rock.
27. Plateaus have: Medium to high elevation, horizontal structure, sedimentary rock.
28. Plains have: Low elevation, horizontal structure, sedimentary rock.
29. Number of species that have become extinct is far greater than the number of species
that exist today.
30. Evolution: Gradual change in organisms over a period of time. If a slight difference
in one organism makes it better able to survive it will live longer and pass on the
genetic information for that characteristic to offspring.
31. Be able to use the chart on p. 1 RT to find the decay products of radioactive
substances.
32. NOTHING changes the half life of an element.
33. To find half life from a graph: On the vertical (Y) axis find the point that represents
one half of the original amount (it may be listed as 50%), go over to the line, go down
to find the number of years.
34. 1 half-life: 1:1 ratio; 50% radioactive, 50% product; ½ radioactive, ½ product
2 half-lives: 1:3 ratio; 25% radioactive, 75% product; ¼ radioactive, ¾ product
3 half-lives: 1:7 ratio; 12.5% radioactive, 87.5% product; 1/7 radioactive, 7/8 product
35. Uranium-238 has just reached its first half life.
36. Carbon-14 is used to date recent remains of living things.