Geologic Time Scale

Earth Science Spring 2014

The Geologic Column

• Geologic column- arrangement of rock layers based on the ages of the rocks – No single area on earth a record of all geologic time – Observations from around the world were used to

create the geologic column– Represents a timeline of the earth’s history– Rock layers are distinguished from one another

primarily by the fossils they contain and the type of rock they are made of

Divisions of Geologic Time

• Earth’s history is divided by:– Major surface or climate changes– Extinction of various species – **these types of events are used to divide the

geologic time scale into smaller units

Divisions of Geologic Time

• Eras – Era- largest unit of geologic time – 4 geologic eras • Precambrian• Paleozoic• Mesozoic• Cenozoic

Divisions of Geologic Time

• Eras – Precambrian Era (Precambrian Time) • Earliest era

– Oldest layer of the geologic column

• Longest era (4 billion years) • Very few fossils

– Makes it difficult to divide into smaller units– Bacteria, algae, primitive worms, sponges, & corals

» Evidence that life started in the ocean

Divisions of Geologic Time

• Eras– Paleozoic Era• 2nd era• Means “ancient life”• 292 million years long• Fossils- wide variety of both marine & land plants &

animals

Divisions of Geologic Time

• Eras– Mesozoic Era• 3rd era• Means “Middle Life” • 183 million years long • Fossils- more complex organisms like reptiles & birds

Divisions of Geologic Time

• Eras– Cenozoic Era• Present geologic era• Means “recent life” • Fossil- mammals are common

Divisions of Geologic Time

• Periods- subdivision of a geologic era – Characterized by specific fossils – Named for the location in which the fossils were

first found• Epochs – subdivision of a geologic time period– Contain an extremely detailed fossil record – 2 period of the cenozoic era are divided into these

Geologic History

• Theory of evolution– Theory that organisms change over time and that

new organisms are derived from ancestral types. – First proposed by Charles Darwin by way of

natural selection (survival of the fittest) • Evidence:

– Fossil record – Examination of living organisms

Geologic History

• Theory of Evolution cont…– Geologic changes & climatic changes affect the

survival of organisms• Example of geologic change is a dramatic decrease in

the amount of the earth’s surface covered by water• Example climatic change is a decrease in atmospheric

temperature

Geologic History

• Precambrian Time – Makes up 88% of the earth’s history – Began with the formation of the earth 4.6 billion

years ago & ended 540 million years ago – Rocks from this era are difficult to interpret – Shields of precambrian rock are found on every

continent– Fossils are rare • Most common are stromatolites

• Stromatolites

Geologic History

• Paleozoic Era – Began 540 million years ago & ended about 248

million years ago – It is believed that Pangea formed by the end of

this era– Abundant fossil record• Rich in marine & land animal & plant fossils

– Divided into 7 periods

Geologic History

• Paleozoic Era- Cambrian Period – Contains the first advanced marine life– Most of the continents were covered with warm,

shallow oceans. • Marine invertebrates thrived here

– Invertebrates- animals without backbones – Most common- trilobites

– Brachiopods- shelled animals • 2nd most common type of animal to live during this time

– Still no evidence of land-dwelling plants or animals

Geologic History

• Cambrian Period

Geologic History

• Paleozoic Era- Ordovician Period – Number brachiopod species increased & number

of trilobites decreased – Large number of coral started to appear– Graptolite- useful index fossil – First vertebrate appeared • Vertebrate- animal with a backbone

– Still no plant life on land

Geologic History

• Ordovician Period

Geologic History

• Paleozoic Era- Silurian Period– Marine life continued to thrive & evolve during

this time.• Echinoderms became more numerous • Eurypterids- scorpion-like sea creatures were abundant

– Near the end, land plants & animals started showing up• Ex: spiders, millipedes

Geologic History

• Silurian Period

Geologic History

• Paleozoic Era- Devonian Period – Many kinds of bony fish • Lungfish- air breathing fish• Rhipidistians- land going fish

– 1st true amphibian showed up • Ichthyostega- like a huge salamander

– Land plants began to develop • Giant horsetails, ferns, cone-bearing plants.

Geologic History

• Devonian Period

Geologic History

• Paleozoic Era- Carboniferous Period (Pennsylvanian & Mississippian) – Climate was generally warm & the humidity was high all

over the world – Forests & swamps covered much of the land – Amphibians & fish continued to thrive– Crinoids were common in oceans – Insects were common on land – Reptiles appeared & resembled large lizards – Many of the coal, oil, & natural gas deposits are found in

rocks from this period

Geologic History

• Carboniferous Period

Geologic History

• Paleozoic Era- Permian Period– End of the Paleozoic Era– Mass extinction of many Paleozoic life forms– Nearly all continents had joined to form Pangea – Areas of desert & savannah formed in the interior

of the continent• Shallow seas evaporated • Many species of marine invertebrates became extinct

Geologic History

• Mesozoic Era – Began 248 million years ago – Ended 65 million years ago– Pangea broke up into the continents – Tectonic plates drifted & collided forming

mountain ranges – Shallow seas & marshes covered most of the land– Climate was warm & humid. • Favored reptiles

Geologic History

• Mesozoic Era- Triassic Period – Dinosaurs first appeared – Dinosaurs varied greatly in size– Lush forests of cone-bearing trees & cycads– New forms of marine life appear – Earliest mammals made their appearance

Geologic History

• Mesozoic Era- Jurassic Period– Dinosaurs were the dominant form of life• 2 major groups of dinosaurs evolved.

– Saurischians- both carnivores & herbivores – Ornithischians- herbivores

– Flying reptiles were common

Geologic History

• Mesozoic Era- Cretaceous Period – Dinosaurs continued to dominate the earth• Tyrannosaurus rex• Ankylosaurs-armored bodies• Ceratopsians- horned • Ornithopods- duck-billed

– 1st flowering plants appeared (angiosperms)– End of this period was marked by a mass

extinction

Geologic History

• Cenozoic Era (Age of the Mammals) – Began 65 million years ago – Continents in the beginning looked much like they

do today – Increased tectonic activity – Dramatic climate changes – Mammals became the dominant life form

Geologic History

• Cenozoic Era (Age of the Mammals) – 2 periods• Tertiary

– Time before the last ice age

• Quaternary– Began with the last ice age & includes present time

• ** divided into 7 epochs – Tertiary- Paleocene, Eocene, Oligocene, Miocene, & Pliocene – Quaternary- Pleistocene & Holocene

Geologic History

• Cenozoic Era: Paleocene & Eocene Epochs (Tertiary Period) – Paleocene Epoch• Many new mammals evolved

– Small rodents – 1st primates appeared

– Eocene Epoch• First flying squirrels, bats, and whales appeared • Smaller reptiles flourished • World wide temperature dropped 4 degreed Celsius

Geologic History

• Cenozoic Era: Oligocene & Miocene Epochs (Tertiary Period)– Oligocene Epoch

• Climate continued to become cooler & drier • Early mammals became extinct • Larger species of today’s common animals flourished• Himalaya mountains formed

– Miocene Epoch• Climate still dry & cool • Golden Age of Mammals • Common fossils include: deer, rhinoceros, & pig families • Largest known mammals lived • Modern polar ice caps started to form

Geologic History

• Cenozoic Era: Pliocene Epoch (Tertiary Period) – Hunting animals became fully evolved – Fossils of first modern horses– Great climatic changes occurred & continental ice

sheets began to spread – Land bridge between Eurasia & North America

appeared – North & South America became connected by the

central American Land Bridge

Geologic History

• Cenozoic Era: Pleistocene & Holocene Epochs (Quaternary Period) – Pleistocene Epoch

• Several periods of glaciation occurred over most of North America & Eurasia• Many animals became extinct • Fossils of early ancestors of humans

– Holocene Epoch • Began about 11,000 years ago as the last ice age ended• Sea level rose drastically as glaciers & ice sheets melted• Great Lakes were formed • Modern humans developed • Agriculture developed • Use of bronze & iron tools began