Geologic Time

GeoClassroom Physical Geology Historical Geology Structure Lab

Physical Geology

Geologic Time

Geologic time uses quite a different scale than human time.

- age of the Earth is estimated to be around 4.6 billion years old.

- first one celled organisms - beginning of life - is about 3.2 billion

- Jurassic Park and the dinosaurs happened anywhere from 65 - 245 million years ago.

- one must adjust the way they think when talking about geologic time. Many of the processes in geology take thousands and millions of years to work, such as the formation of a mountain range.

- For example, the Appalachian Mtns formed around 200 million years ago and were the size of the Himalayan Mtns at the time. Since then the actions of water and wind have worn down the Apps to what they are today. The coastal plane is the remnants of the great mountain range.

To grasp the scale of geologic time, use the following as an example.

1) compress the entire 4.6 billion year history of Earth into a single year.

2) the oldest rocks found would date from mid-March

3) living things in the oceans appear in May

4) land plants and organisms appear in late November

5) dinosaurs were dominant in mid-December, disappeared on the 26th

6) "manlike" animals appear on the evening of the 31st

7) ice age ended 1 min 15 sec before midnight on the 31st

8) Columbus discovers America 3 sec before midnight

The geologic time scale:

Eons separate great spans of time.

Phanerozoic = life visible in the rocks as fossils

Precambrian = barren rocks

Only in the past few decades have actual dates been assigned to these time periods through radiometric dating.

Rocks before were dating using relative dating, that is, they were classified into which rock was older than another rock, but not giving absolute dates through radiometric dating.

Relative dating through:

1) Law of Superposition

- In undeformed sequence of sedimentary rx, each layer is older than the one below.

- Also applies to lava flows

- Layers of sediment are deposited horizontally.

2) Principle of cross-cutting relationships

- A cross-cutting rock is younger than the rock it cuts

- Magma chamber is younger than the country rock

- Dikes are younger than country rock

3) Inclusions

- Frags on one rock enclosed in another

- Piece of country rock trapped in magma chamber

- Inclusions are older

4) Unconformities (results of erosion)

- A period of time when deposition ceased, then erosion removed previously formed rx or seds, then deposition resumed.

- Interrupts a nice sequence of depositing sedimentary rx

- 3 types of unconformities:

A) Angular unconformity

B) Disconformity:

- tough because strata on either side are still parallel.

C) Nonconformity:

- separates older meta or ign rx from younger sed rx deposited on top of them.

Correlation:

- The process by which rx of similar age are matched up, without the help of radiometric age dating.

- Use through the help of fossils.

- Study of fossils is called Paleontology.

- Fossils are the remains or traces of organisms preserved in the rock record.

-Principle of fossil succession:

- Fossils succeed one another in a very definite and determinable order, from less developed in older rx to complex life forms in newer rx.

- Age dating later confirmed the older -> younger sequence.

- Certain fossils were found to be very good age indicators, and these were used as index fossils to correlate the ages of rocks across continents.

Geologic time scale was further subdivided into the eras:

- Paleozoic = ancient life

- Mesozoic = middle life

- Cenozoic = recent life

These were further subdivided into Periods and then Epochs

Note: Precambrian represents >85% of the Earth's history.

Radiometric Dating:

Some elements decay into other elements by radioactivity.

- they loose protons, neutrons, e- to form a different atom

- also give off heat

- Decay rates are well known for some elements

- they decay very slowly in terms of half-lives

- exponential decay

The amount of a radioactive element in a crystal which has just formed is called parent isotope.

- The amount of the decayed product, or new element, is called daughter isotope.

Another important one is carbon 14 dating (radiocarbon).

- Every living thing maintains a fixed amount of C 14 while alive, and it is constantly replenished by the living organism,

- After death, 14 C is not replenished, so it decays to nitrogen 14.

- Time since death can be measured by knowing the half-life of 14 C (5730 yrs), then looking at the relative proportions of 14 C to 14 N left in the organism.