Geologic Time Scale: Eons, Eras, Periods, and Epochs

What does the Geologic Time Scale (GTS) refer to?

Sequential periods of time defined by fossil succession found in rocks.

Why was the Geologic Time Scale developed using relative dating?

Radiometric dating was unavailable at the time the scale was created.

What is the significance of the Geologic Time Scale?

It serves as a large-scale calendar from Earth's origins to the present.

Who maintains and updates the Geologic Time Scale?

The International Commission on Stratigraphy.

What is the largest subdivision in the geologic time scale?

Eon.

What is an Era in the context of the Geologic Time Scale?

The second-largest subdivision marked by major changes in life and climate.

What is a Period in the Geologic Time Scale?

A basic unit of geological time and a subdivision of an era.

What is an Epoch?

The smallest division in geologic time, characterized by specific environmental shifts.

What percentage of Earth's history falls under the Precambrian Eon?

More than 88%.

What significant events occurred during the Hadean Eon?

Earth's temperatures were extremely hot, leading to the solidification of the continental and oceanic crust, and formation of the Moon.

What characterized the Archean Eon?

Formation of continents and the evolution of Earth's first life forms, primarily simple single-celled organisms.

What major development occurred during the Proterozoic Eon?

The appearance of oxygen in Earth's atmosphere and multicellular organisms and Snowball Earth.

What marks the beginning of the Phanerozoic Eon?

An abundance of visible fossilized remains in rock layers, beginning about 542 million years ago.

What is the Paleozoic Era (Old Life) known for?

The explosion of diverse marine life and the largest mass extinction of marine organisms.

What significant events occurred during the Mesozoic Era (Middle Life)?

The rise of dinosaurs, mammals, birds, and flowering plants.

What does the Cenozoic Era (New Life) represent?

The most recent era, marking the age of mammals and the first human evolution.

What are the epochs of the Cenozoic Era?

Holocene, Pleistocene, Pliocene, Miocene, Oligocene, Eocene, and Paleocene.

How are many geological periods named?

Based on geographic names where the rocks of that age were first described.

What is a characteristic of periods in the Geologic Time Scale?

They are often named for prominent rock beds.

What does the term 'Snowball Earth' refer to?

Periods during the Proterozoic Eon when the planet experienced extensive glaciation.

What type of organisms are represented by the earliest identifiable fossils?

Stromatolites, formed by microbial mats of cyanobacteria.

What is the significance of trilobites in the Paleozoic Era?

They are one of the first four-limbed vertebrates and a key indicator of marine life diversity.

What major extinction event is associated with the end of the Mesozoic Era?

The mass extinction that led to the demise of the dinosaurs.

What does the term 'Phanerozoic' mean?

Derived from Greek, meaning 'visible life'.

What is the time frame of the Paleozoic Era?

From about 544 million years ago to 245 million years ago.

What is the time frame of the Mesozoic Era?

From about 245 million years ago to 65 million years ago.

What is the time frame of the Cenozoic Era?

From about 65 million years ago to the present.

The first four -limbed

vertebrates, and the origin of

land plants.

Trilobites

Cambrian Explosion; First complex animals

Cambrian

First fish; marine life flourishes; major glaciation

Ordovician

First land plants and animals; coral reefs expand

Silurian

"Age of Fishes"; first amphibians; first forests

Devonian

Coal-forming swamps; sharks abundant; first reptiles

Carboniferous/Mississippian and Pennsylvanian

Supercontinent Pangaea intact; greatest mass extinction

Permian

First dinosaurs and mammals; Pangea begins to split

Triassic

Age of giant dinosaurs; first birds

Jurassic

Flowering plants appear; dinosaurs go extinct

Cretaceous

Early primates; mammals diversity; mountain building; Laramide Orogeny ends

Paleogene — Paleocene, Eocene, Oligocene

Spread of grassy ecosystems; modern mammals; first hominins; isthmus of Panama links; North and South America

Neogene — Miocene and Pliocene

Ice Age glaciations; modern humans; extinction of large mammals and birds; glacial outburst floods

Quaternary — Pleistocene and Holocene