Geologic TimeGeologic Time

The Foundation: Fossils and Relative TimeThe Foundation: Fossils and Relative TimeUnderstanding how both are used to get a better Understanding how both are used to get a better understanding of our Earth’s past (and its future)understanding of our Earth’s past (and its future)

*The Beginning: 3 Basic *The Beginning: 3 Basic Rock Types*Rock Types*

Sedimentary Rock- Rocks made by erosion of Sedimentary Rock- Rocks made by erosion of rock;( limestone, sandstone, shale and they rock;( limestone, sandstone, shale and they are water soluble)are water soluble)

Igneous Rock – rocks made by magma; Igneous Rock – rocks made by magma; granite, marble, and basalt and are water granite, marble, and basalt and are water resistant)resistant)

Metamorphic rock- sedimentary rocks that due Metamorphic rock- sedimentary rocks that due to being under pressure and heat turn into to being under pressure and heat turn into more igneous naturemore igneous nature

Determining the age of Determining the age of rocks: 2 waysrocks: 2 ways

Absolute ageAbsolute age -Actual Age (determined -Actual Age (determined by Carbon dating)by Carbon dating)

Relative AgeRelative Age -Age compared to other -Age compared to other objects (gives you an estimate of its age; objects (gives you an estimate of its age; index fossils or depth provide this)index fossils or depth provide this)

*Absolute Age**Absolute Age*

It tell the actual age or how long ago an It tell the actual age or how long ago an even occurredeven occurred

They use Radioactive datingThey use Radioactive dating Radioactive dating-measures the age of a Radioactive dating-measures the age of a

material by comparing the amount of a material by comparing the amount of a radioactive form of an element in a rock or radioactive form of an element in a rock or fossil with the amount of its decay productfossil with the amount of its decay product

PrecisePrecise

*Relative age**Relative age*

It tells if something happened earlier or It tells if something happened earlier or later than others without giving a definite later than others without giving a definite datedate

Did these Rocks fall before Did these Rocks fall before or after the rock layers or after the rock layers formed?formed?

*Law of superposition**Law of superposition*

Formed by undisturbed sedimentary rock Formed by undisturbed sedimentary rock layerslayers

Older rock layers lie beneath younger Older rock layers lie beneath younger rock layers: (Grand Canyon at first rock layers: (Grand Canyon at first glance) glance)

Exceptions to the Rule: 4Exceptions to the Rule: 4

Overturned BedOverturned Bed Angular BedAngular Bed IntrusionsIntrusions UnconformityUnconformity

Law of Superposition Law of Superposition EXCEPTIONS: Overturned EXCEPTIONS: Overturned bed bed

Sedimentary Rock layers totally Sedimentary Rock layers totally misplacedmisplaced

Commonly caused by Compression Commonly caused by Compression forces and found in mountainsforces and found in mountains

Law of Super Position EXCEPTIONS: Law of Super Position EXCEPTIONS: Angular bedAngular bed

Horizontal rocks are younger than the Horizontal rocks are younger than the tilted rockstilted rocks

Tilted rock caused plate-tectonic forcesTilted rock caused plate-tectonic forces

Law of Superposition Law of Superposition EXCEPTIONS : IntrusionEXCEPTIONS : IntrusionCaused by MagmaCaused by Magma

Igneous rocks are younger than any Igneous rocks are younger than any sedimentary rocks they cutsedimentary rocks they cut

*Law Of Superposition *Law Of Superposition EXCEPTIONS*: EXCEPTIONS*: UnconformityUnconformity

Missing layer or gap in the rock record: Missing layer or gap in the rock record: caused by magma intrusion or erosioncaused by magma intrusion or erosion

Makes it hard to understand how Earth Makes it hard to understand how Earth has changed during a specific period of has changed during a specific period of timetime Missing Layer destroyed

by magma

*2 Types of Erosion of *2 Types of Erosion of Rocks*Rocks*

Mechanical Mechanical Weathering :Weathering :

water freezes and water freezes and unfreezes causing unfreezes causing Rocks to split open Rocks to split open from ice the from ice the expanding in the expanding in the cracks;cracks;

also wind blowing also wind blowing sand against rockssand against rocks

*2 Types of Erosion of *2 Types of Erosion of Rocks: Chemical Weathering Rocks: Chemical Weathering **

Chemical Chemical Weathering- Acid Weathering- Acid rain dissolves away rain dissolves away the rockthe rock

Why do We Care about Why do We Care about Rocks?Rocks?

Rocks provide us Rocks provide us with a “Picture” of the with a “Picture” of the Earth’s Past HistoryEarth’s Past History

FossilsFossils

*Fossils**Fossils*

Important to understand the history of life Important to understand the history of life on Earthon Earth

Older rock—simpler organisms and Older rock—simpler organisms and Younger rock-more complex organismsYounger rock-more complex organisms

Shows how species have changed: Shows how species have changed: EVOLUTIONEVOLUTION

Shows how species are related to another Shows how species are related to another or changed to environmental changeor changed to environmental change

*How Fossils Form**How Fossils Form* 1 Quick Burial of organism in soft sediment: 1 Quick Burial of organism in soft sediment:

Mudslide, volcanic ash, quicksand, mudpits, tarpitsMudslide, volcanic ash, quicksand, mudpits, tarpits 2 Organism remains undisturbed for long period of 2 Organism remains undisturbed for long period of

timetime 3. Water slowly enters area soft tissue is dissloved 3. Water slowly enters area soft tissue is dissloved

and water slowly eats away bone/shelland water slowly eats away bone/shell 4. If water flow is “fast” complete organism will be 4. If water flow is “fast” complete organism will be

eaten away creating “hole” in the rock. Leaves basic eaten away creating “hole” in the rock. Leaves basic shape of organismshape of organism

5. If water eats away very slowly minerals in water 5. If water eats away very slowly minerals in water can replace bone creating a Cast. Minerals can replace bone creating a Cast. Minerals recrystalize as soon as bone is dissolved creating a recrystalize as soon as bone is dissolved creating a PERFECT COPY!PERFECT COPY!

6. Fossil is then found millions of years later: FACT-6. Fossil is then found millions of years later: FACT-Only 17 T-rex Skeletons have ever been found Only 17 T-rex Skeletons have ever been found

Ways Fossils formWays Fossils formFossilFossil ProcessProcess NotesNotes

Molds and castsMolds and castsAcids eat away the skeleton or Acids eat away the skeleton or

shell and leave an shell and leave an impression in the rockimpression in the rock

If the mold fills with minerals it If the mold fills with minerals it becomes a castbecomes a cast

Tracks and trailsTracks and trails Imprints are left in mud which Imprints are left in mud which later hardenslater hardens Dino tracks are commonDino tracks are common

CarbonizationCarbonizationOils leave the plant and the Oils leave the plant and the

remaining matter becomes remaining matter becomes a layer of carbona layer of carbon

fossilized plantsfossilized plants

PetrificationPetrification

Minerals in plant cells Minerals in plant cells crystallize; minerals enter crystallize; minerals enter openings or cavities in openings or cavities in shells or bonesshells or bones

Common with plantsCommon with plants

ReplacementReplacementObject buried in mud has its Object buried in mud has its

molecules replaced by molecules replaced by mineralsminerals

rarerare

RecrystallizationRecrystallization mineral aragonite in shells turns mineral aragonite in shells turns then into calcitethen into calcite

Preserves the general shape of Preserves the general shape of the animalthe animal

Soft tissue preservationSoft tissue preservation Mummification, frozen in iceMummification, frozen in ice Fossils are relatively you and do Fossils are relatively you and do not last longnot last long

TrapsTraps Whole organisms locked in tar Whole organisms locked in tar pits, asphalt, amber etc…pits, asphalt, amber etc…   

MoldsMolds

Hollow area in sediment in the shape of Hollow area in sediment in the shape of an organism or part of organisman organism or part of organism

Hard part of organism buried in sedimentHard part of organism buried in sediment Water carries dissolved minerals and Water carries dissolved minerals and

sediment may seep into the empty space sediment may seep into the empty space of the moldof the mold

Cast/RecrystalizationCast/Recrystalization

Solid copy of the shape of an organism: Solid copy of the shape of an organism: bones replaced by minerals in water. bones replaced by minerals in water. EXTREMELY ACCURATE; VERY RAREEXTREMELY ACCURATE; VERY RARE

PetrifiedPetrified

Fossils where minerals replace all or part Fossils where minerals replace all or part of an organismof an organism

Turns into stoneTurns into stone Water rich with mineral seep into spacesWater rich with mineral seep into spaces

CarbonizationCarbonization

Oils from organism Oils from organism turn into carbon turn into carbon imprint on rockimprint on rock

Common in plants Common in plants and Fishand Fish

Trace fossilsTrace fossils Fossilized footprint or trailFossilized footprint or trail

*INDEX FOSSILS**INDEX FOSSILS*

Index fossil-Index fossil- commonly found commonly found fossil worldwide that fossil worldwide that can give you an can give you an approximate ageapproximate age and and environment of environment of the areathe area then. then. (Trilobites)(Trilobites)