Geologic History









Relative Time:

  • Ages of events are placed in order of occurrence.
  • No exact date is identified.
    • Ex. WWI and WWII
    • "I am the second child in my family."

Absolute Time- identifies the exact date of an event.

  • Ex. 65 Million Years Ago
  • 1990

Finding age with relative time:

1. The Law of Superposition- in a sequence of undisturbed sedimentary rocks, the oldest rocks will be at the bottom.

2. The law of cross-cutting relationships- an igneous rock is younger than the rocks that it has intruded into. This also applies to faults.

3. The law of included fragments- the individual fragments that make up a rock are older than the entire sample.

4. The Law of Folds or tilts in rocks are younger than the rocks themselves.

5. Original Horizontality- Rocks are usually deposited flat and level.

6. Cole’s Law- Thinly sliced cabbage.

Other Guidelines for figuring out a sequence:

  • Sedimentary rocks are usually formed under water.
  • Weathering and erosion usually happen above water (on dry land).

Speed Links

Correlation

Radioactive Dating

Evolution

Worksheets
&
Downloads

Vocabulary

Relative Dating Craters

Sequence Puzzles:

Sequence 1

Sequence 2

Sequence 3

Sequence 4

Sequence 5

Sequence 6

Sequence 7

DBQ on Pole Reversals

DBQ on Dinosaurs

Lab on Index Fossils

Lab on Radioactive Decay

DBQ on The Natural History of LI

Worksheet on Radio Dating

Sequence 1: Uplift & Erosion

  1. Limestone deposited
  2. Sandstone deposited
  3. Shale Deposited
  4. Uplift
  5. Erosion

Sequence 2: Faulting

  1. Limestone deposited
  2. Sandstone deposited
  3. Shale deposited
  4. Faulting

Sequence 3: Folding

  1. Limestone deposited
  2. Sandstone deposited
  3. Shale deposited
  4. Folding of all the layers together

Sequence 4: Igneous Intrusion

  1. Limestone deposited
  2. Sandstone deposited
  3. Shale deposited
  4. Igneous Intrusion
    Note: contact metamorphism is the same event as the intrusion.

Formation of an Unconformity- an unconformity is a "missing layer of rock" or a gap in the record. Simply put, it is a "buried erosional surface."

  • Layers are formed according to superposition.

  • Something happens to uplift the area- (folding,faulting, etc).

  • Erosion wears away the uppermost layers

  • Area submerges and deposition begins again.

Correlation- Matching a layer in one location with a layer formed at the same time in another location.

Correlation Techniques:

  • "Walking the outcrop"- following an outcrop and mapping all the structures that it touches.
  • Similar rock characteristics- layers can be matched on the basis of similar colors, compositions, a unique feature, or by the same series of rocks "sandwiching" it.
  • Index fossils- a fossil that lived in a wide area but for only a short time.
  • Volcanic time markers - a layer of volcanic dust covering layers.

When a violent eruption of a volcano occurs it may send dust high into the atmosphere where it can spread over the entire planet. It settles out of the air and forms a layer over wide regions at the same time.

  • Asteroid impacts can have the same effect.
    • Example: the layer that marks the extinction of dinosaurs has characteristics of an asteroid impact.

Ways of Measuring Absolute Time

  • Tree rings- every year a tree grows a new ring. When the tree dies, the rings can be counted and events such as forest fires can be dated. (Evidence of the same fire may be seen as a layer of ash in the rocks to correlate a rock layer with the tree rings). Tree rings can be used to date events up to 3000 years ago.

  • Varves- Layers in sedimentary rock that were formed in a lake. Some lakes have seasonal deposits (ex.. sand in the spring from high rain volume and silt deposited in the winter when the top is covered in ice and water flow stops.)

Radioactive Dating

Radioactive elements decay (fall apart) at steady rates.

Key points of Radioactive Decay:

  • One "half-life" of time has passed when exactly half of the element remains.
  • Half lives differ from element to element.
  • When they fall apart they form a different material.
  • A ratio between the original material (parent material) and the decay product (daughter material) can be used to determine how many half-lives the material has undergone.
  • The radioisotope used should have a half - life that is around the age of the object being dated.
    • Ex: C14 is used for objects thousands of years old
    • C14 can be used to date back to around 50,000 years.

Some notes on C14 :

  • C14 has a half-life of 5,700 years old.
  • It is only used to date organic remains.
  • One setback of using C14 is that levels in the atmosphere depend on solar strength and have not been consistent in history resulting in inaccuracies in dating. Levels are calibrated using other radioisotopes which do not have such variations.

Evolution- the gradual changing pattern of life forms.

Natural Selection-First proposed by Charles Darwin-life forms best adapted to survive will continue to reproduce. Life forms will gradually change over time.

  • Life begins as single cells about 3.5 Billion Years ago.
  • Humans arrived about 1.6 Million Years ago.
  • Most of the life that has evolved (>99.9%) has become extinct. This is not necessarily a bad thing. Many creatures are extinct because they have become something else.
    • Ex.: Cro-Magnon Man is extinct because they evolved into modern Homo-sapiens.

Mass Extinctions

There have been several mass extinctions that have caused widespread endings to species.
Ex.:

  • 65 MYA dinosaurs became extinct.
  • 245 MYA 70% of all life on the planet became extinct.
  • Today Approximately 10 species per day go extinct because of human involvement.
  • Mass extinctions may have been caused by environmental stresses such as disease, climate changes, or the introduction of a new predator.

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