Determining the Determining the Age of RocksAge of RocksRelative Age of RocksRelative Age of Rocks

Types of RockTypes of Rock

Sedimentary rockSedimentary rock forms from sediment forms from sediment

((sedimentsediment- solid particles of rock - solid particles of rock produced by weathering and erosion by produced by weathering and erosion by water and wind)water and wind)

* * Igneous rockIgneous rock forms from volcanic lava forms from volcanic lava flowsflows

* * Metamorphic rock-Metamorphic rock- forms from intense forms from intense heat and pressureheat and pressure

Relative Dating of RocksRelative Dating of Rocks

The The relative age of rocksrelative age of rocks determines determines the the age of rock layers as younger or age of rock layers as younger or olderolder, but does not give the exact age., but does not give the exact age.

The principle of The principle of uniformitarianism uniformitarianism states that forces that shaped the Earth states that forces that shaped the Earth in the past continues to shape the earth in the past continues to shape the earth today (volcanoes and weathering and today (volcanoes and weathering and erosion by wind and water).erosion by wind and water).

Sedimentary RocksSedimentary Rocks

Sedimentary rocks formed from sediment Sedimentary rocks formed from sediment deposited millions of years ago. deposited millions of years ago. Sedimentary rocks are deposited in Sedimentary rocks are deposited in horizontal layers. (horizontal layers. (principle of original principle of original horizontality)horizontality)

Sedimentary rock is deposited in horizontal Sedimentary rock is deposited in horizontal layers over geologic time with layers over geologic time with the oldest the oldest layer on bottom and the youngest layer at layer on bottom and the youngest layer at the topthe top ( (principle of superposition)principle of superposition)

Sedimentary Rock LayersSedimentary Rock Layers

Remember -Remember -sedimentary rock sedimentary rock layers are layers are horizontalhorizontal with the with the oldest at the oldest at the bottom and the bottom and the young at the topyoung at the top unless the layers are unless the layers are disturbed by a disturbed by a faultfault or or igneous intrusionigneous intrusion

Faults Faults and and Igneous Igneous IntrusionsIntrusions

Sometimes sedimentary rock layers are Sometimes sedimentary rock layers are disturbed by geological forces. A crack in the disturbed by geological forces. A crack in the rock layer is called a rock layer is called a fault. fault. When igneous When igneous rock (volcanic lava) rock (volcanic lava) intrudesintrudes or cuts through or cuts through layers of sedimentary rock it is called an layers of sedimentary rock it is called an igneous intrusion.igneous intrusion.

The principle of cross-cutting relationshipsThe principle of cross-cutting relationships states that a states that a fault fault or or igneous intrusionigneous intrusion is is always always younger younger than the rock it cuts across.than the rock it cuts across.

FaultFault (Crack) in (Crack) in Sedimentary rockSedimentary rock

Here is a picture of a Here is a picture of a faultfault or or crackcrack in the in the rockrock

Igneous IntrusionIgneous Intrusion in Rock in Rock LayersLayers

Remember that Remember that igneous intrusionsigneous intrusions (lava) are always (lava) are always younger younger than the rock than the rock layers they cut layers they cut through. Note the through. Note the igneous intrusions igneous intrusions (volcanic rock) cutting (volcanic rock) cutting through the rock layer through the rock layer

Relative Age of RockRelative Age of Rock The The RRelative Age elative Age of the rockof the rock can be can be

determined by the sequence of the rock determined by the sequence of the rock layers using the layers using the Principle of SuperpositionPrinciple of Superposition (oldest on the bottom and youngest at the (oldest on the bottom and youngest at the top).top).

If the sedimentary rock layer has been If the sedimentary rock layer has been disturbed by a disturbed by a faultfault or or igneous intrusion,igneous intrusion, the the faultfault or or intrusion intrusion is always is always youngeryounger than than the rock layer they cut through (cross-cutting)the rock layer they cut through (cross-cutting)

SWBAT: Describe what an index fossil SWBAT: Describe what an index fossil is: is: Finding the Relative Age of Rocks using Finding the Relative Age of Rocks using Index FossilsIndex Fossils

Certain fossils called Certain fossils called Index fossilsIndex fossils help help geologists determine the geologists determine the Relative AgeRelative Age of of rocks.rocks.

To be a useful To be a useful Index fossil:Index fossil: - the fossil must be - the fossil must be widely distributedwidely distributed

different geographic areasdifferent geographic areas - and represent an organism that - and represent an organism that

existed only brieflyexisted only briefly

Index fossilsIndex fossils

Trilobites Trilobites were a group of hard shelled were a group of hard shelled animals that evolved in shallow sea more animals that evolved in shallow sea more than 500 million years ago (trilobites than 500 million years ago (trilobites became extinct at the end of the became extinct at the end of the Paleozoic Era)Paleozoic Era)

Trilobites Trilobites are useful as are useful as index fossilsindex fossils because they were because they were widely distributedwidely distributed geographically and represent an geographically and represent an organism organism that existed briefly.that existed briefly.

Example of an Example of an index fossilindex fossil (trilobite) in layers of (trilobite) in layers of sedimentary rock. sedimentary rock. How old How old is the bottom layeris the bottom layer??

Continental Drift TheoryContinental Drift Theory

All continents were once joined together All continents were once joined together in a single landmass (supercontinent) in a single landmass (supercontinent) called called PangeaPangea

Fossil evidence supports the Fossil evidence supports the Continental Drift Theory.Continental Drift Theory. FossilsFossils from from a fernlike plant a fernlike plant GlossopterisGlossopteris have been have been found in Africa, South America, Australia, found in Africa, South America, Australia, and Antarctica. and Antarctica. The seeds could not have The seeds could not have traveled across the span of the oceanstraveled across the span of the oceans

Absolute Age of RocksAbsolute Age of Rocks

To determine the To determine the Relative AgeRelative Age of Rocks of Rocks geologists use the geologists use the Principles ofPrinciples of Superposition,Superposition, Cross-Cutting Relationships, Cross-Cutting Relationships, and and Index fossils.Index fossils.

To determine the To determine the Absolute AgeAbsolute Age (exact age) of (exact age) of rock, geologists use rock, geologists use Radiometric Dating.Radiometric Dating.

How did geologist determine the Earth was How did geologist determine the Earth was 4.6 4.6 billion years old?billion years old?