Chapter 29: Geologic Time THE RULES OF THE GAME

Time What is time?

Time is… Either relative or absolute Time is: a measured period during which an event, process, or condition exists or continues.

Dating Techniques One way in which geologists look at time: Relative Dating

Relative Dating is.. … the measure of a sequence of events without knowing the exact date at which the events occurred. What happened first? What happened next?

Absolute Dating Absolute age— age, in years, of a rock or other object; determined by properties of atoms.

Principles of Relative Dating Original Horizontality Superposition Cross-Cutting Relationships Inclusions Unconformities Fossil Succession

Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position. Thus, if the rocks are still flat, then they have not been disturbed and still have their original horizontality.

But, if the layers are folded or inclined at a steep angle they must have been moved into that position by crustal disturbances sometime after their deposition.

Principle of Superposition In an undeformed sequence of sedimentary rocks, each bed is older than the one above and younger than the one below.

In other words, the older rocks on the bottom were formed before the ones above. 1 = Oldest 5 = Youngest

Principle of Cross-Cutting Any feature that cuts across another rock or structure is younger than what it cuts across. This is because you can't cut something until that “something” exists!

Principle of Cross-Cutting What is younger? The rocks or the fault line? Q#2

Law of Included Fragments Rock layers which contain other rocks are younger than the inclusion. The inclusion (the other rocks) is older!

Law of Included Fragments: 1st - Igneous Intrudes Sediments Deposited Over Fragments Granite Worn Down

Principle of Unconformities Unconformities are surfaces that represent gaps in the geologic record. Usually represents long periods of erosion.

Principle of Unconformities missing rock layer = missing time There are 3 types of unconformities:

Angular Unconformity 1.Sedimentary rocks deposited horizontally 2.Sedimentary rocks tilted and eroded 3.New sedimentary rocks deposited.

Disconformity 1.Sedimentary rocks deposited horizontally 2.Long period of erosion occurs 3.New sedimentary rocks deposited.

Nonconformity 1.Igneous or metamorphic rock forms 2.Erosion occurs 3.Sedimentary rocks deposited.

Principle of Fossil Succession: Fossil organisms follow one another in a specific order, and therefore any time period can be recognized by its fossil content.

What happened first? How do you know?

Where is the youngest rock? How do you know?

Where is the unconformity? What type is it? How do you know?

What rock layer is the oldest? How do you know?

Does this diagram show an unconformity? If so, which type?

Stop Here!

WHAT ARE FOSSILS? Fossils are any remains, impressions, or traces of an organism preserved in the Earth’s crust. Fossils have been found on every continent on Earth.

Here's a flow chart of fossil formation:

mold fossils: a fossilized impression made in the sediment - a negative image of the organism cast fossils: formed when a mold is filled in trace fossils = ichnofossils: fossilized evidence of organism activity nests, gastroliths, burrows, footprints, etc. true form fossils: fossils of the actual animal or animal part Four Types of Fossils

Ways that organisms can turn into fossils: 1.unaltered preservation: Wooly Mammoth in Siberia Sloth in Desert Cave Mosquito in Amber Saber Toothed Tiger in Tar Pit 2.permineralization=petrification: Rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized

3.carbonization=coalification: Only the carbon remains in the specimen - other elements, like hydrogen, oxygen, and nitrogen are removed

TRACE FOSSILS or (ichnofossils) Record the movements and behaviors of an organism. Even the lack of trace fossils can yield information; the lack of tail- furrow fossils indicates an erect tail stance for dinosaurs that were previously believed to have dragged their tails.

Examples of Trace Fossils Trackways (sets of footprints) Toothmarks - Toothmarks generally appear in bones Gizzard Rocks - Some dinosaurs swallowed stones to help grind their food (modern birds do this also). Coprolites (fossilized feces) - Coprolites yield information an organism‘s diet and habitats. Burrows and Nests - Fossils burrows and nests can reveal a lot about behavior.

Index Fossil: Any fossil used to date strata. Index fossils have formed from organisms which had a wide geographic distribution and existed for a short period of time.

Absolute Ages of Rocks Absolute age— age, in years, of a rock or other object; determined by properties of atoms.

Radioactive Decay Isotope: An atom having the same number of protons in its nucleus as other varieties of the element but has a different number of neutrons.

Radioactive Decay Unstable isotopes break down into other isotopes and particles.

Beta Decay An isotope’s neutron breaks into a proton and an electron. The electron leaves the atom. Now the atom has an extra proton--it is a new element!

Alpha-Decay An isotope gives off two protons and two neutrons as an alpha particle The atom is now missing protons and neutrons—it is a new element!

Half-life The time it takes for half of the atoms in an isotope to decay. TimeAmount

Radiometric Dating Is the calculation of the absolute age of a rock using the ratio of parent isotope (starting element) to daughter product (ending element) and the half-life of the parent.

How to Do a ½ Life Problem An isotope of manganese (manganese-137) has a half-life of 30 years. If 2.0 mg of manganese- 137 disintegrates over a period of 90 years, how many mg of manganese-137 would remain? Isotope Name

Step 1: Make a Table Time ElapsedAmount of Substance

Step 2: Fill in the Time Elapsed Time ElapsedAmount of Substance 0 years 30 years 60 years 90 years Add 1 Half Life

Step 3: Plug in Initial Amount Time ElapsedAmount of Substance 0 years2 mg 30 years 60 years 90 years

Step 4: Calculate Substance Loss Time Elapsed Amount of Substance 0 years2 mg 30 years1 mg 60 years0.5 mg 90 years0.25 mg Divide by 2

Final Answer! Time Elapsed Amount of Substance 0 years2 mg 30 years1 mg 60 years0.5 mg 90 years0.25 mg There will be 0.25 mg of manganese- 137 left over after 90 years!

Radiometric Dating Potassium-Argon dating is used to date ancient rocks millions of years old. Carbon-14 dating is used to date bones, wood, and charcoal up to 75,000 years old.