view of the earth's past

A View of the Earth’s A View of the Earth’s PastPastThe Geologic Time ScaleThe Geologic Time Scale

Rock Record holds a fascinating story of the Rock Record holds a fascinating story of the evolution of life on earthevolution of life on earth

The Geologic Time The Geologic Time ScaleScale

Some organisms flourish then become Some organisms flourish then become extinctextinct with the changes that occur on earth with the changes that occur on earth such as mountains forming and eroding, such as mountains forming and eroding, oceans rising and recedingoceans rising and receding

Scientists use the Scientists use the geologic time scalegeologic time scale – – gives the sequence and length of time these gives the sequence and length of time these changes occurred changes occurred

The Geologic Time The Geologic Time Scale: The Geologic Scale: The Geologic

ColumnColumnReviewReview

Law of SuperpositionLaw of Superposition

Index fossilsIndex fossils

Relative ages of rocksRelative ages of rocks

Geologic Column – Geologic Column – the ordered arrangement the ordered arrangement of rock layers based on the ages of rocksof rock layers based on the ages of rocks

Law of Superposition


ColumnColumn Rock layers distinguished from each other Rock layers distinguished from each other

mainly by the types of fossils and the kinds of mainly by the types of fossils and the kinds of rocks found in the rock layersrocks found in the rock layers

Upper layers of rock layers contain modern Upper layers of rock layers contain modern types of plants and animalstypes of plants and animals

Lower layers have older plants and animal Lower layers have older plants and animal fossilsfossils Many contain EXTINCT animals and plantsMany contain EXTINCT animals and plants


ColumnColumn Original theory of geologic column thought Original theory of geologic column thought

the rates of deposition were constantthe rates of deposition were constant Later it was discovered that the rates of Later it was discovered that the rates of

deposition change over timedeposition change over time Radioactive dating development also helped Radioactive dating development also helped

refine the geologic column theoryrefine the geologic column theory


ColumnColumn According to the law of superposition, the According to the law of superposition, the

position of a given rock layer is a clue to its position of a given rock layer is a clue to its relative agerelative age

Relative Age of Rocks

The Geologic Time The Geologic Time Scale: Divisions of Scale: Divisions of

Geologic TimeGeologic Time Geologic history is divided into sections, like a Geologic history is divided into sections, like a

calendar, that represent major changes in the calendar, that represent major changes in the earth surface or climate changesearth surface or climate changes Geologists use these events to divide the Geologists use these events to divide the

geologic time scale into smaller unitsgeologic time scale into smaller units

The Geologic Time The Geologic Time Scale: ErasScale: Eras

EraEra – a – a very large very large unit of geologic timeunit of geologic time

Four geologic eras Four geologic eras

1.1.PrecambrianPrecambrian

2.2.PaleozoicPaleozoic

3.3.MesozoicMesozoic

4.4.CenozoicCenozoic


1.1. Precambrian Time Precambrian Time

- OLDEST layer- OLDEST layer

- much larger than other eras – 4 billion - much larger than other eras – 4 billion yearsyears

- not many fossils so difficult to divide into - not many fossils so difficult to divide into sectionssections


2. Paleozoic Era 2. Paleozoic Era

- Greek word meaning “- Greek word meaning “ancient life”ancient life”

- lasted 325 million years (570 million - lasted 325 million years (570 million years ago years ago to 245 million years ago) to 245 million years ago)

- large variety of marine, plant, and - large variety of marine, plant, and animal animal fossilsfossils

The Geologic Time Scale: The Geologic Time Scale: ErasEras

3. Mesozoic Era

- Greek for “middle life”

- existed for 180 million years

- more-complex life forms found like reptiles and birds


4. Cenozoic Era4. Cenozoic Era

- Greek meaning - Greek meaning “recent life”“recent life”

- - fossils of fossils of mammals mammals found in the rocks found in the rocks

- earlier forms of life are lower than - earlier forms of life are lower than more-more-complex formscomplex forms

The Geologic Time The Geologic Time Scale: Periods and Scale: Periods and

EpochsEpochsPeriods – Periods – shorter time units of each Erashorter time units of each Era

- - defined by specific fossilsdefined by specific fossils

- named by location where fossils were - named by location where fossils were first first found in geologic columnfound in geologic column

Example: Devonian Period named after Example: Devonian Period named after Devonshire, EnglandDevonshire, England

The Geologic Time The Geologic Time Scale: Periods and Scale: Periods and

EpochsEpochsEpochsEpochs - shorter time units of each era defined - shorter time units of each era defined by diversity of fossilsby diversity of fossils

- Cenozoic Era has the most diverse and - Cenozoic Era has the most diverse and detailed fossils so easy to divide rock layers detailed fossils so easy to divide rock layers from this era into periods from this era into periods

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