Ch 29 Geologic Time & Dating



GEOLOGIC TIME

Divisions of Time: _EON_ (longest), _ERA_, _PERIOD_, _EPOCH_ (shortest)

_Cenozoic Era__ (Present – 65 mya): “Age of Mammals”

_Mesozoic Era__ (65 mya – 248 mya): “Age of Reptiles”

_Paleozoic Era__ (248 mya – 543 mya): “Age of Invertebrates”

_Pre-Cambrian Time__ (543 mya – 4.5 bya): “Time Before Life”

RELATIVE DATING

Process of placing events in the _Sequence_ in which they occurred

Does _Not_ identify actual dates

Uniformitarianism

The same processes that occur today also _occurred in the past_

__Changes happen slowly__ over time, not quick catastrophic events

Original Horizontality

Most __sedimentary___ rocks are deposited as __horizontal layers___

Superposition

In an undisturbed sequence of sedimentary strata, the _oldest_ rock layer will be at the _bottom_ , and the _youngest_ on _top_.

Cross-cutting

An igneous __intrusion__ or __fault__ is always __younger__ than the rock it cuts through.

Uplift

Over time, rocks can be _tilted_ or deformed by regional or local metamorphism.

If rocks are __tilted__, then the metamorphism that caused it is __younger__ than the rocks.

New, younger rocks form __on top_ of the tilted strata.

_Unconformity_ - is a gap in the geologic record caused by erosion.

Fossils - help geologists study Earth’s past & determine…

• Approximately _when_ life began

• Which plants & animals were _first_ to live __on land__

How Fossils form:

➢ Body must be _protected_ from scavengers & bacteria

➢ Hard parts (like _bones_ & _teeth_) have a better

chance of becoming fossils

➢ Fossils are found in _sedimentary_ rocks

Index Fossils

• Certain fossils can help find the _relative age_ of rocks.

• These special fossils are called __index__ __fossils__.

Ex: __Graptolite__ an ancient plant - lived 350-450 mya.

Ex: __Trilobite__ an ancient animal - lived 500-600 mya.

Types of Fossils

• Original Remains: Some substances (such as _Ice_, _Salt_, and _Amber_) can fossilize organisms while preserving their remains

• Replaced Remains: Over time, organic material can be replaced by __rock-forming minerals__ dissolved in water (Ex: Petrified wood)

• Molds & Casts: an organism is buried and then decomposes, leaving a cavity (or _Cast_) that may fill in with minerals or rock (a _Mold_)

• Trace Fossils: evidence of organisms without actual remains

Ex: __footprints__, _tracks_, _burrows__, _imprints_, bite marks

ABSOLUTE DATING

Determining the __actual age__ for events

Gives ages of rock in _number of years_

Radioactive dating

The most __accurate__ method for determining age.

Radioactive elements give of particles and energy as they decay and new elements form.

Scientists know how long it takes for certain radioactive elements to decay, so they are like __natural___ __clocks__.

Radioactive decay

__Half-Life__: the time it takes for half the radioactive atoms in a sample to decay to a stable product.

Most absolute dating is done by measuring the amount of…

_Carbon-14_ left over in fossils of plants & animals

__Uranium-238__ left over in rocks older than 50,000

The instrument that measures radioactivity is a _Geiger Counter__

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3 half-lives

1 half-life

2 half-lives

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