GEOLOGICAL SCIENCES 100
CORAL REEFS MINI-COURSE
KYGER C LOHMANN

 
9.  Geological versus Evolutionary Time:  The  Framework for Reef Evolution

Basic Concepts:

Methods for Determining Relative Time:

1. Superposition: (what is on top is youngest)
2. Original Horizontality: everything starts off horizontal -- if it is tilted, tilting happened later)
3. Cross-Cutting relations:  If a layer (#1) is cut by another feature (#2), then #2 is younger than #1
4. Evolutionary Trends:  Use of the changes that occur in the types of animals
Method for Determining Absolute Time:
The decay series of radioactive isotopes of several elements can be used to determine absolute time.  In effect, if one knows the rate of radioactive decay, the relation between the number of parent (radioactive) atoms that remain and the number of the daughter atoms that are produce can be used to directly determine the real age (in years) of a material.   It is on this basis the the absolute time estimates for the age of the earth and of various geologic events is based.
Some Key points to Remember:
 
1. Know the key dates of death (extinctions) and the kinds of animals that made of reefs just prior to these extinctions.  Also, note the age of the earth is 4.6 billion year old based on the radioactive isotope measurements from meteorites.

2. First Skeletons preserved in the rock records were at the beginning of the Paleozoic (550 million years ago).  This is also the end of the PreCambrian.

3. The first reefs were made of algae and are on the order of 3.5 billion years old.  These are in the PreCambrian time period.   Note also that whenever other types of organisms have over specialized, and subsequently go to extinction, algae always are around to form reefs.  Therefore, the winning reef organism is ALGAE.

4. The first coral reefs were in the Paleozoic (around 400 million years ago).  In Michigan, these corals occur in reefs that are exposed in the area around Petosky.  You should know that Petosky stones are examples of "old corals" that became extinct about 250 million years ago at the end of the Paleozoic and beginning of the Mesozoic.

5. In the Mesozoic, the time of the Dinosaurs, corals of the type that comprise present day reefs were alive, but did not construct the major reef systems.  Rather, clams (large conical shaped or horn shaped shells) of the group called Rudists were the primary reef formers.   Unfortunately, at the end the Mesozoic, about 65 million years ago, a global catastrophe occurred (we think that this event is related to the impact of a large meteor or comet with the Earth).

6. The Scleractinia corals (recent stony or hard corals) only began to form the major reef systems after the demise of its competitors at the end of the Mesozoic.  In the Cenozoic (new times), the new corals finally take over as the primary reef builder.




 

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