The Earth Through Time: An Introduction to the Geologic Time Scale

Flashcards covering the concepts, subdivisions, eons, eras, periods, epochs, evolutionary landmarks, naming origins, and climate shifts of the Geologic Time Scale.

What is the Geologic Time Scale (GTS)?

A sequential timeline of Earth's history defined by the fossil succession found in rocks, including animals, plants, landforms, and mass extinctions.

Why was the original Geologic Time Scale constructed using relative dating methods?

Because radiometric dating was unavailable at the time the scale was developed.

What international body is responsible for actively maintaining and updating the Geologic Time Scale?

The International Commission on Stratigraphy.

Why can standard human calendars not be used to measure geological time?

Geological time is so immense that human calendars cannot measure it, and most geological processes require vast stretches of time to form extraordinary rocks and features.

The greatest expanse of geologic time and the longest geologic subdivision.

Eon

The second-largest subdivision of geologic time; a division of an eon marked by major changes in life and climate.

Era

A basic unit of geological time and a subdivision of an era during which a specific kind of rock system is produced and distinct fossil changes occur.

Period

The smallest division in geologic time and a finer subdivision of a period, characterized by specific environmental shifts.

Epoch

What percentage of Earth's history falls under the Precambrian supereon, and what is its time span?

More than $88\%$ of Earth's history, spanning from $4.6\text{ billion years ago}$ to $540\text{ million years ago}$.

Temperatures were extremely hot and much of Earth was molten, the continental and oceanic crust solidified, and the Earth and moon formed.

Hadean Eon (4.6b years ago)

The Earth's crust cooled enough to allow continents to form, and Earth's first life forms evolved as simple single-celled organisms lacking nuclei, called Prokaryota.

Archean Eon (4b years ago)

What are the earliest identifiable fossils from the Archean Eon, and how were they produced?

Stromatolites, which are microbial mats formed in shallow water by cyanobacteria.

Oxygen appeared in Earth's atmosphere, and multi-celled animals and eukaryotes (cells with a nucleus) emerged, including soft-bodied multicellular organisms.

Proterozoic Eon

Periods during which the planet suffered below-zero temperatures, extensive glaciation, and a resulting drop in sea levels.

Snowball Earth (Proterozoic Eon)

Derived from Greek words meaning 'visible life,' it is marked by an abundance of visible fossilized remains preserved in rock layers.

Phanerozoic Eon (542 million years ago)

Meaning 'old life,' it was characterized by the explosion of diverse marine life, trilobites, the origin of land plants, the first four-limbed vertebrates, and the largest mass extinction of marine organisms.

Paleozoic Era (544m years ago)

The Cambrian Explosion, marking the emergence of the first complex animals (marine invertebrates), first chordates, and first fishes.

Cambrian Period (541.0-485.4 Ma)

The appearance of the first fish, flourishing marine life with a sudden diversification of metazoan families, and major glaciation.

Ordovician Period (485.4-443.8 Ma)

The first land plants (first vascular land plants) and land animals, accompanied by the expansion of coral reefs.

Silurian Period (443.8-419.2 Ma)

Jawed fishes diversified significantly; this period also saw the emergence of the first amphibians and first forests. Known as the 'Age of Fishes'?

Devonian Period (419.2-358.9 Ma)

What are the two sub-periods of the Carboniferous Period ($358.9 - 298.9\text{ Ma}$), and what characterized them?

The Mississippian ($358.9 - 323.2\text{ Ma}$) and Pennsylvanian ($323.2 - 298.9\text{ Ma}$); characterized by coal-forming swamps, abundant sharks, scale trees, seed ferns, and the first reptiles.

The supercontinent Pangaea was intact, reptiles diversified, and the greatest mass extinction in Earth's history took place.

Permian Period (298.9-251.9 Ma)

Meaning 'middle life' (beginning around $245\text{ million years ago}$), it is known as the 'Age of Reptiles,' dominated on land by dinosaurs, with the first appearances of mammals, birds, and flowering plants.

Mesozoic Era (252-66 Ma)

The appearance of the first dinosaurs and first mammals, and the beginning of the breakup of Pangaea.

Triassic Period (252.0-201.3 Ma)

Dinosaurs diversified and dominated as giant terrestrial forms, and the first birds appeared.

Jurassic Period (201.3-145.0 Ma)

Flowering plants and early primates appeared, followed by a mass extinction at $66\text{ Ma}$ that wiped out the dinosaurs.

Cretaceous Period (145.0-66.0 Ma)

Meaning 'new life,' it is the 'Age of Mammals,' dominated by mammals, angiosperms (flowering plants), shifting modern continents, and human evolution.

Cenozoic Era (66 Ma to Present)

What three periods comprise the Cenozoic Era?

The Paleogene ($66.0 - 23.0\text{ Ma}$), Neogene ($23.0 - 2.6\text{ Ma}$), and Quaternary ($2.6\text{ Ma}$ to present).

What epochs make up the Paleogene Period?

The Paleocene ($66.0 - 56.0\text{ Ma}$), Eocene ($56.0 - 33.9\text{ Ma}$), and Oligocene ($33.9 - 23.0\text{ Ma}$)

What epochs comprise the Neogene Period?

The Miocene ($23.0 - 5.3\text{ Ma}$) and Pliocene ($5.3 - 2.6\text{ Ma}$)

What epochs make up the Quaternary Period?

The Pleistocene ($2.6 - 0.01\text{ Ma}$) and Holocene ($0.01\text{ Ma}$ to present)

What are the two primary naming origins used for geologic periods?

Geographic names (locations where rock strata and fossils were first described) and rock types (prominent characteristic rock beds).

What are the type areas and name origins for the Cambrian, Ordovician, and Silurian periods?

Cambrian: Cambrian Mountains (Wales, UK); Ordovician: Ordovician region (Wales and England, UK); Silurian: Silurian region named after the Silures tribe (Wales, UK).

What are the type locations and name origins for the Devonian, Carboniferous, and Permian periods?

Devonian: Devonshire (Devon, England, UK); Carboniferous: Carboniferous region (Great Britain), named after carbon/coal-bearing rocks; Permian: Perm region (Perm Krai, Russia).

What are the type areas and origins for the names Triassic, Jurassic, and Cretaceous?

Triassic: Tyrol region, Austria (named after the threefold 'Trias' strata); Jurassic: Jura Mountains (France/Switzerland); Cretaceous: Cretaceous region in Normandy, France (named from 'creta,' meaning chalk).

It was the warmest period of the Cenozoic Era, featuring very warm, tropical conditions across many regions of the planet.

Eocene Epoch

During which epoch did glaciers first appear on Antarctica?

The Oligocene Epoch ($\approx 34 - 23\text{ Ma}$).

During which epoch did glaciers expand and appear on Greenland?

The Miocene Epoch ($\approx 23 - 5.3\text{ Ma}$).

What was the extent of glaciation during the Pleistocene Epoch ($\approx 2.6\text{ Ma} - 11.7\text{ ka}$), and when did the last glaciation end?

Ice sheets covered much of North America and Europe, and the most recent glaciation ended approximately $11{,}700\text{ years ago}$.

Marked by early primates, mammal diversification, mountain building, and the end of the Laramide Orogeny.

Paleogene Period

Marked by the spread of grassy ecosystems, modern mammals, the first hominins, and the Isthmus of Panama connecting North and South America.

Neogene Period

Characterized by Ice Age glaciations, modern humans, extinction of large mammals and birds, and glacial outburst floods.

Quarternary Period