Review Sheet- Earth’s History


Fossils – the remains or the evidence of a living thing
A. Types of Fossils
1. mold- an empty space in the shape of the organism
2. cast- occurs when minerals fill a mold and harden into the shape of the original organism
3. imprints- occurs when leaves and light weight organisms leave an impression in soft sediment like mud, that later hardens into rock
4. amber- occurs when insects are trapped and become embedded in resin
5. ice- occurs when an organism is preserved in ice (woolly mammoth)
6. tar- occurs when animals are trapped in tar pits (LaBrea Tar Pits -Cali)
7. Petrified- occurs when minerals dissolved in ground water gradually replace the original tissues of plans/animals
8. carbonaceous film- occurs when carbon in tissues of organisms leave a residue/thin film of carbon on sediment which then hardens into rock
B. Why do we study fossils?
1. Fossils indicate the variety of life forms that once existed on Earth
2. Fossils show how organisms have evolved over time
3. Fossils indicate how Earth’s surface has changed over time (uplifting)
4. Fossils give clues to Earth’s past climate.
5. Fossils tell us about the appearance and activities of extinct organisms

II. Relative Dating

1. Original Horizontality- Due to gravity, sediments are deposited horizontally, parallel to Earth’s surface.
2. Principle of Superposition- In undisturbed sedimentary rock layers, the oldest rocks are found at the bottom, and the youngest are found on top.
3. If the rock layers were disturbed by faulting, then the faulting occurred after the last layer which was broken by the fault.
4. If the rock layers have been disturbed by folding, then the folding occurred after the last which has been folded.
5. If an intrusion of magma occurs within a group of rock layers, the intrusion is younger than the rock layers it goes through.
a. intrusions with “hairs” show contact metamorphismà rocks is younger than rocks above it.
b. intrusions “without hair” à are older than rocks above it.
6. A rock that has been glued and cemented together is younger than the rock pieces that make it up.

7. An unconformity is a surface of erosion and is usually represented by a wavy line.     Unconformities show a gap in Earth’s history.
a. angular unconformity- erosion of tilted rock strata
b. disconformities- erosion of parallel rock strata
III. Rock Correlation- matches the age of rock layers from different locations

         - must have a wide distribution geographically
2. Walking the outcrop/observing -physically seeing a connection
3. Similar Sequence- similar patterns in areas like age, order, and composition
4. Key Beds- presence of a distinctive layer; ex: volcanic ash

IV. Radioactive Dating- determining the absolute age of a rock or fossil

1. Radioactive/parent element- an atom that has an unstable nucleus that changes spontaneously or “decays”. (to the left of the à)
2. Decay/daughter- the stable element that is the result of the spontaneous change in a radio active element. (to the right of the à)
C. Half-Life- the time it takes for one-half of a radioactive element to change into a decay element (not affected by any outside condition)
*Half life and disintegrations are found on the ESRT pg. 1
* Amount of radioactive material decreases as half lives increase in parent element

V. Geologic Time Scale- divides Earth’s history into sections of time.
A. The units or sections of time include
1. Eon (longest)
2. Era
3. Period
4. Epoch (shortest)
B. The boundaries between geologic time intervals represent major extinctions.
C. You must be able to interpret ESRT pg 8 & 9 and calculate
a. time lengths (start date– end date)
b. % of time (specific time period / earths age (4600 my) x 100
c. fossils and rock type found in different parts of nys & landscape regions

Earth's History

1. Paleontology- the study of life that existed during pre-historic times.

2. mold- a hollow depression in rock formed when a fossil dissolves out of rock; shows the original shape of the surface of the fossil.

3. cast- an object created when minerals seep into and fill a mold, forming a copy of the original fossil.

4. Trace fossil- any indirect evidence of life preserved as an impression in rock; trails, tracks, footprints, burrows, and bite marks.

5. Index fossils- the fossilized remains of organisms that lived and died within a particular time segment of Earth’s history that can be used to correlate rock layers.

6. strata- the layers of sedimentary rock that form after particles settle out of a fluid and are compressed over time.

7. unconformity-the layer or layers of missing rock from a strata sequence.

8. Correlation- the matching of rock layers from one area with those of another.

9. Uniformitarism- same natural laws and processes that operate in the universe now, have always operated in the universe in the past.

10. Law of superposition-Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top.

11. Original horizontality- Due to gravity, layers of sediments are originally deposited horizontally.

12. Key bed- a single, widespread rock layer that is unique and easily recognizable used to correlate rock layers.

13. Relative dating- the process of placing events in the sequence in which they occurred; does not identify actual dates.

14. radioactive decay- the process by which radioactive isotopes emit or capture tiny particles; includes alpha decay, beta decay, and electron capture
15. Parent isotope- any sediment that is deposited on a yearly cycle .

16. daughter isotope- an element that is the product of radioactive decay.
17. half life- The time it takes for half the atoms in a sample or a radioactive element to decay to a stable end product.

18. geologic time scale- a summary of major events in earths history that are preserved in the rock.