A Look Back in Geologic Time Dating Rocks and Fossils A Look Back in Geologic Time

Rocks Tell a Story!!!

How do we know what happened? Relative Dating – tells which one is older, not actual age Absolute Dating – actual age

RELATIVE DATING Layers of rock called strata tell the story of what came first. A relative age (older vs. younger) can be determined with a few simple laws.

Law of Original Horizontality Sedimentary rocks left undisturbed will remain in horizontal layers.

Law of Inclusions Rock fragments (in another rock) must be older than the rock containing the fragments.

...granite is older. ...sandstone is older.

Law of Crosscutting Relationships A fault or an intrusion is always younger than the rock layers it cuts through.

UNCONFORMITIES Gaps in the geologic record when deposition stopped, rock was eroded away, and then deposition restarted. There are 3 types: 1. Angular Unconformity 2. Disconformity 3. Nonconformity

Angular Unconformity Boundary between a tilted layer and a horizontal layer

Disconformity Nearly horizontal Boundary between layers of rock that have not been deposited continuously A large time gap exists.

Nonconformity Boundary where stratified (layered) rock rests on unstratified rock (igneous or metamorphic)

Possible Solution 1. Deposition of shale 2 Possible Solution 1. Deposition of shale 2. Deposition of conglomerate 3. Deposition of sandstone 4. Deposition of lithic sandstone 5. Deposition of limestone 6. Deposition of quartz sandstone 7. Deposition of limey shale 8. Deposition of mudstone 9. Intrusion of granite. This may have occurred earlier. However, as granites are emplaced at depth, it is assumed all the sequence was deposited and lithofied prior to emplacement. 10. Uplift and erosion to form current land surface. 11. Deposition of alluvium

Possible Solution 1. Deposition of conglomerate 2 Possible Solution 1. Deposition of conglomerate 2. Deposition of sandstone 3. Deposition of limestone 4. Deposition of shale 5. Deposition of mudstone 6. Faulting 7. Intrusion of gabbro

Index Fossils Certain fossils are found only in the rock layers of a particular geologic age. Therefore, they are used to date rock layers These are known as INDEX FOSSILS. Paradoxides: Cambrian

To Qualify as an Index Fossil: Must be “world-wide.” Must be distinctive. Must have a “short” life span. Must be numerous! Echioceras: Jurassic Period

Absolute Dating Some fossils/rocks are dated using absolute methods such as radioactive decay Gives the fossil a “birthday”!

Radioactive Decay The loss of particles and energy from unstable (radioactive) elements. Radioactive elements (such as uranium) decay at their own unique, constant rate to form new elements…

Once that rate is known, scientists compare the amount of original radioactive material (“parent”) in a rock sample to the amount of newly created element present (“daughter”)… …and the age of the sample can be determined by plotting amount versus time on a logarithmic graph!

Half-Life Definition: the time it takes for half of a radioactive sample to decay to a different stable element. Example: it takes 10 g of U-238 4.5 b.y. to “become” 5 g of Pb-206 and 5 g of U-238.

Half-Life

Problem… Uranium, Potassium, & Rubidium work great with very old (more than 10 million year-old) ROCKS… …but what about younger rocks and objects not containing radioactive elements (like once-living things)?

Carbon Dating Carbon dating is used for younger samples that contain carbon such as organic material and some sedimentary rocks. The half-life of C-14 is 5,730 years. Works for wood, bone, shells, early humans up to 50,000 years old.