Ages Of RocksAges Of Rocks

Mr. TaylorMr. Taylor’’s Favorite 8s Favorite 8thth Grade Grade Class Class

• Relative time -- subdivisions of the Earth'sgeology in a specific order based upon relative relationships. These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils.

• Absolute time -- numerical ages in "millions of years" or some other increment of time. These are most commonly obtained via radiometric dating methods performed on appropriate rock types.

TimeTime

• Relative time tells of the sequence of the way eventsoccurred and not the exact time these events tookplace. The following list is placed in relative order.– I woke up this morning– I ate breakfast– I brushed my teeth.– I took a shower.– I got dressed.– I left for school

RELATIVE TIME

ABSOLUTE TIMEABSOLUTE TIME

• Absolute time tells the exact time an event occurred.

• I was born in 19__

• I started to walk 19__

• I started kindergarten 19__

• I started 6th grade 19__

RELATIVE DATING METHODSRELATIVE DATING METHODS

• SUPERPOSITION

• ORIGINAL HORIZONATALITY

• UNCONFORMITIES

• INTRUSIVE RELATIONSHIPS

• INCLUSIONS

• INDEX FOSSILS

Principle of SuperpositionPrinciple of Superposition

The principle ofsuperposition statesthat older beds are

covered by youngerbeds, so that in a

sedimentary sequencethe youngest unit is at

the top.

Original HorizontalityOriginal Horizontality

Sedimentary rocks were originally deposited in horizontal beds.

Sedimentary rocks thatare not horizontal havebeen affected by sometectonic process.

Intrusive RelationshipsIntrusive Relationships•Invading igneous (formed from melted rock) intrusions are always younger than the rock they intrude.

•Also any pebbles or fragments must be older than the host rock containing them

UnconformitiesUnconformities

A surface of non-deposition or erosionencompassing significant amounts ofgeologic time. Recognizing unconformitiesin the geologic record is important incorrelation and in determining relative age.There are three types of unconformities.

Angular UnconformityAngular Unconformity

• Angular unconformity: the beds below the unconformity dip at a different angle from the beds above it.

NonconformityNonconformity

A nonconformity occurs when igneous (once was molten or melted) rock is uplifted and erodedand new sedimentary rock layers are deposited.

DisconformityDisconformity

•A disconformityseparates totallydifferent rock types,such as sedimentaryrocks from metamorphicrocks in the photographon the right.

•In this type of unconformity one or more layers have been eroded away and are missing.

Index FossilsIndex Fossils•The principle of index fossils states thatorganisms evolve through time, and, therefore, particular forms can be usedas age markers wherever they are found. The kinds of animals and plants found asfossils change through time. When we find the same kinds of fossils in rocks fromdifferent places, we assume that the rocks are the same age.

•Remember that index fossils are of species that were abundant, widespread and were only around for “short” periods of time.

UNRAVELING RELATIVE TIME

• Beds P, K, M, S and A deposited

• Intrusion of R• A & R eroded• B, J and F deposited

Figure out the sequence of events.Figure out the sequence of events.

• Blue, Red and Green beds deposited

• Blue, Red and Green layers folded

• Blue, Red and Green eroded

• Brown, Flesh and Yellow deposited

Order the followingOrder the following

• R deposited• P deposited• B deposited• I intrudes• F forms• B, I and F eroded

P

RF

I

B

TrilobitesTrilobites