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Principles & Fossils
Transcript of Principles & Fossils
How we learn from the past
The fundamental rules of Geology
Evidence of past life preserved in rock.
also called Guide Fossils
Help geologists determine the approximate age of the rocks surrounding the fossil. Also helps them correlate one rock layer to another across large distances.
Can be any type of fossil, but they must be:
1) easily recognized
3) widely distributed geographically
4) represent a species that existed for a relatively short period of time
Fossils & Scale
Long before radioactive dating, fossils were the best way to identify one time segment in multiple locations around the world.
By using index fossils, geologists were able to slowly develop a record of life throughout Earth’s history, which became the Geologic Time Scale.
No numbers yet...
based on names.
Dinosaurs, lizards, & birds
Fish & marine life
Then came technology...
In the early 1900s, Ernest Rutherford invented radiometric dating techniques.
The technology was expanded when the mass spectrometer began to be used in the 1950s, improving accuracy.
Adding Dates to the Scale
Geologists have several ways of expressing ages:
Thousand years ago = 1x10 years = KA (kilo annum)
Million years ago = 1x10 years = MA (mega annum) = mya (million years ago)
Billion years ago = 1x10 years = GA (giga annum) = bya (billion years ago)
Geologists use the ash from past volcanic eruptions to get accurate age measurements. Volcanic ash is important because they contain freshly crystallized zircon crystals (tiny time-capsules).
Radiometric dating of the ash allows geologists to know how long ago each eruption event happened.
By comparing the relative locations of index fossils & the dated ash beds, the Scale slowly gained numbers.
1) Relative Dating
2) Absolute Dating
The process of placing geologic events in their proper chronological order with NO regard to when the events took place in number of years ago.
Examples: Breakfast comes first, then lunch, then dinner. It doesn't matter what times you eat each meal.
The rock formations in Garden of the Gods are in the following order: Fountain Fm on bottom, then the Lykins Fm, with the Morrison Fm on top.
The process of using radioactive decay and radiometric dating techniques to assign an age in years before present to geologic events.
Examples: I married my husband on October 2, 2010.
Pikes Peak Granite was dated to 1.08 billion years old.
Importance of Ash
The clam must have lived some time between 495 and 510 mya
The trilobite must have existed between 520 and 545 mya.
The Future of the Scale
As technology continually improves radiometric dating results, the dates on the Scale will continue to adjust.
Every decade or so, a new Scale is published by the US Geological Survey or the ICS (International Commission on Stratigraphy).
Don't be surprised if you see slightly different dates on different geologic time scales.
In a sequence of sedimentary rocks layers, the bottom layers are older than the top layers because the bottom layers were deposited first.
The law of gravity will deposit sediments horizontal to the Earth's surface.
Original Lateral Continuity
Layers of sediment initially extend laterally in all directions and can be traced over large distances.
The oldest fossils in a series of sedimentary rock layers will be found in the lowest layer. Progressively younger fossils will occur in higher layers.
Rocks that have formed by consolidated sediment deposited in layers. Sediments are broken pieces of other rocks (like sand and gravel)
Evaporite Sedimentary Rocks
Rock that formed from the evaporation of isolated bodies of water.
Biochemical Sedimentary Rocks
Some minerals dissolve in acidic water. Organisms will absorb the minerals to make their body parts (shells, teeth, bones). When the organism dies, the concentrated minerals in their body parts will form limestone. Biochemical rocks can also form when dead vegetation is compressed underground, forming coal.
Rock that formed from broken fragments of previously existing rocks.
Rock fragment = clast
The geologic processes that change the Earth's crust today have worked in much the same way over geologic time.
"The Present is the key to the Past."
The geologic feature which cuts another is the younger of the two features.
In a rock that contains fragments of other rocks, the fragments are older than the rock containing them.
Things to look for:
More rounded clasts indicate a larger distance from the source rock. Angular clasts have not had the time to be weathered, and so must be very close to the source.
Also gives clues to the distance from the source rock. Larger sediments will be found close to the source (conglomerate will form near the mountains), while small sediments will be far from the source (siltstone or shale will form at the bottom of the ocean.
Sorting refers to the variation in clast size. A well sorted sedimentary rock will contain clasts that are all the same size. Poorly sorted rocks contain clasts of varying sizes. The amount of sorting indicates the energy, rate, and/or duration of deposition, as well as the transport process (river, wind, glacier, etc.) responsible for laying down the sediment.
Generally indicates a marine or a shallow sea environment (limestone)
Or, it can indicate a swamp or other environments with dense vegetation (lignite and coal)
Generally indicates a salt lake or a past ocean.
2 general types of fossils:
Trace fossils preserve the activity of an animal
(For example, footprints, animal burrows, or even poop [called coprolites])
Remains of an animal or plant
Cast & Mold fossils - imprints of the plant of animal that later fill in with rock. (Most dinosaur skeletons or big bones we see)
Replacement - the organic material of the plant or animal is replaces with rock. (For example, petrified wood)
Whole body fossils - the original organic material remains but is preserved (For example, a mammoth frozen in ice, or an insect trapped in amber)