Geologic Time

Note : There are many different ideas on the division of geologic time. Please use the geologic time scale found in your book for testing purposes. Some pictures in this slideshow may show a slightly different version. Neither is right or wrong, just different interpretations.

Geologic time is divided into giant chunks of time called eons.

There have been two eons in Earth’s history: the Precambrian and the Phanerozoic.

The Precambrian eon is divided into sub-eons called the Hadean, the Archean, and the Proterozoic.

Eons are broken into large chunks of time called eras.

Eras are broken up into smaller chunks of time called periods.

Periods are broken up into even smaller chunks of time called epochs.

Today, we are in the:• Phanerozoic eon• Cenozoic era• Quaternary period• Holocene epoch

Precambrian Eon

The Precambrian is an immense chunk of time in Earth’s history beginning 4.6 billion years ago until the start of the Cambrian period (in the Phanerozoic eon), about four billion years later.

The Precambrian spans about 87% of Earth’s history.

The Precambrian Eon saw the formation of the earth, the formation of an atmosphere, and the first one-celled and multi-celled organisms.

Very little is known about this ancient time because early rock history has been disrupted by plate tectonics, erosion, and deposition.

Consequently, this span of time has not been successfully divided into briefer time units and is missing many parts.

Rocks of this time are metamorphosed and deformed, extensively eroded, and obscured by overlying strata.

Most Precambrian rocks are devoid of fossils, which makes correlation of rock difficult.

Untangling the long, complex Precambrian rock record is a large task that is far from done.

Each continent has large “core areas” of Precambrian rocks dominate on the surface (mostly deformed as metamorphic rock). These areas are called shields.

Shields

North American Precambrian Shield

The North American shield encompasses about 7.2 million square kilometers (2.8 million square miles) – the equivalent of ten states of Texas.

Rocks from the middle Precambrian (1.2 – 2.5 billion years ago) contain most of Earth’s iron ore, mainly as the mineral hematite (Fe2O3).

These iron-rich sedimentary rocks probably represent the time when oxygen became sufficiently abundant to react with iron dissolved in shallow lakes and seas.

Later, after much of the iron was oxidized and deposited on lake and sea bottoms, formation of these iron-rich deposits declined and oxygen levels in the ocean and atmosphere began to increase.

Because most of Earth’s free oxygen results from plant photosynthesis, the formation of extensive Precambrian iron ore deposits is linked to life in the sea.

Earth’s atmosphere formed about 2 billion years ago.

Formation of Earth’s Atmosphere

Absent in the Precambrian are fossil fuels (coal, natural gas, oil). The reason is because this time was virtually absent of land plants to form coal swamps and of certain animals to form petroleum.

Precambrian Fossils

Today, our knowledge of Precambrian life, although far from complete, is quite extensive.

Precambrian fossils are disappointing if you expect plants or large animals.

The most common Precambrian fossils are stromatolites which are mounds or columns of calcium carbonate.

Stromatolites are not the remains of actual organisms, but are material deposited by algae. They are indirect evidence of algae because they closely resemble similar deposits made by modern algae.

Stromatolites did not become common until the middle Precambrian (2 billion years ago).

Most Precambrian ancient fossils are preserved in chert, a hard, dense chemical sedimentary rock.

Chert must be very thinly sliced and studied under powerful microscopes to observe bacteria and algae fossils within it.

Precambrian filamentous cyanobacteria

Cyanobacteria (Nostocales) from the Bitter Springs Chert of Central Australia, 850 million years old.

Chert

Microfossils have been found at several locations worldwide. Two notable areas are in southern Africa, where the rocks date to more than 3.1 billion years, and in the Gunflint Chert of Lake Superior, which dates to 1.7 billion years.

In both places, bacteria and blue-green algae have been discovered.

The fossils are of the most primitive organisms, prokaryotes (cells that lack organized nuclei, and reproduce asexually).

Microfossils are perhaps the most important group of all fossils — they are extremely useful in age-dating, correlation and paleoenvironmental reconstruction.

Phanerozoic Eon

The Phanerozoic eon consists of the Paleozoic, Mesozoic, and Cenozoic eras.

Approximately 540 million years ago to the present.

Paleozoic Era

The Paleozoic era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian Periods.

It extends from 540-248 million years ago.

Paleozoic EraCambrian Period – “Age of Trilobites”(540-500 million years ago)

"Age of Trilobites“ - Many marine invertebrates (marine animals with mineralized shells) - First vertebrates - Earliest primitive fish - Mild climate - The supercontinent Rodinia began to break into smaller continents (no correspondence to modern-day land masses) - Mass extinction of trilobites at the end of Cambrian (50% of all animal families went extinct), probably due to glaciation.

Paleozoic EraOrdovician Period(505-438 million years ago)

Primitive plants appear on land - First corals - Primitive fishes, seaweed and fungi - High sea levels at first, global cooling and glaciation, and much volcanism - North America under shallow seas - Ends in huge extinction, due to glaciation.

Paleozoic EraSilurian Period(438-408 million years ago)

The first jawed fishes and uniramians (largest group of anthropods - like insects, centipedes and millipedes) appeared during the Silurian (over 400 million years ago) - First vascular plants (plants with water-conducting tissue as compared with non-vascular plants like mosses) appear on land - High sea levels worldwide - Brachiopods, crinoids, corals.

Paleozoic EraDevonian Period – “Age of the Fish”(408-360 million years ago)

Fish and land plants become abundant and diverse - First tetrapods appear toward the end of the period - First amphibians appear - First sharks, bony fish - Many coral reefs, brachiopods, crinoids - New insects, like springtails, appeared - Mass extinction (345 mya) wiped out 30% of all animal families) probably due to glaciation or meteorite impact.

Paleozoic EraCarboniferous Period(360-280 million years ago)

Wide-spread coal swamps, corals, bryozoans, brachiopods, seed ferns, and other plants - Amphibians become more common.

Mississippian sub-period (360 to 325 mya) First winged insects.

Pennsylvanian sub-period (325 to 280 mya) First reptiles - Many ferns - The first mayflies and cockroaches appear.

Paleozoic EraPermian Period – “Age of Amphibians”(280-248 million years ago)

"The Age of Amphibians" - Amphibians and reptiles dominant - Gymnosperms dominant plant life - The continents merge into a single supercontinent, Pangaea - Phytoplankton and plants oxygenate the Earth's atmosphere to close to modern levels - The first stoneflies, true bugs, beetles - The Permian ended with largest mass extinction - Trilobites go extinct, as do 50% of all animal families, 95% of all marine species, and many trees, perhaps caused by glaciation or volcanism.

Mesozoic Era

The Mesozoic era consists of the Triassic, Jurassic and Cretaceous periods.

It spans from 248-65 million years ago.

This era is known as the “Age of the Reptiles”.

Mesozoic EraTriassic Period(248-208 million years ago)

The first dinosaurs and mammals appear - Mollusks are the dominant invertebrate - Many reptiles, for example, turtles - True flies appear - Triassic period ends with a minor extinction 213 mya (35% of all animal families die out - This allowed the dinosaurs to expand into many niches.

Mesozoic EraJurassic Period(208 - 146 million years ago)

Continental drift, Pangaea begins to break apart into Laurasia and Gondwana toward the end of the Jurassic Period.

Apatosaurus (formerly known as Brontosaurus), large, plant-eating dinosaurs of the late Jurassic

Many dinosaurs - The first birds appear (Archaeopteryx) - The first flowering plants evolve - Many ferns, cycads, ginkgoes, rushes, conifers, ammonites - Minor extinctions at 190 and 160 mya.

Mesozoic EraCretaceous Period(146 - 65 million years ago)

Lower epoch - (146-98 mya)

The height of the dinosaurs - The first crocodilians, and feathered dinosaurs appear - The earliest-known butterflies appear (about 130 million years ago) as well as the earliest-known snakes, ants, and bees - Minor extinctions at 144 and 120 mya.

100 million years ago: further continental drift, begins formation of modern continents

Upper epoch - (98-65 mya) High tectonic and volcanic activity - Primitive marsupials develop - Continents have a modern-day look - Minor extinction 82 mya that ended with large extinction of dinosaurs, ammonites, about 50 percent of marine invertebrate species, etc., probably caused by asteroid impact or volcanism.

Cenozoic EraMost recent era

Began 65 million years ago, continues today

The “Age of Mammals”

The Cenozoic era is divided into two periods: Quaternary and Tertiary

Some of the highest mountains (Alps, Andes, Himalayan) were lifted during this time.

66 million years ago: asteroid (10-20 mile diameter) impact? - causing extinction of the dinosaurs and other land animals

Cenozoic EraTertiary Period(65-1.8 million years ago)

The tertiary period includes Paleocene, Eocene, Oligocene, Miocene and Pliocene Epochs.

34 million years ago: separation of Antarctica from South America, begins formation of Antarctic ice

Paleocene epoch (65-54 mya)First large mammals and primitive primates

Eocene epoch (54-38 mya)Mammals abound - Rodents appear - Primitive whales appear

Oligocene epoch (38-24 mya)Starts with a minor extinction (36 mya) - Many new mammals (pigs, deer, cats, rhinos, tapirs appear) - Grasses common.

Miocene epoch (24-5 mya)More mammals, including the horses, dogs and bears - Modern birds

Pliocene epoch (5-1.8 mya)First hominids - Modern forms of whales.

Cenozoic EraQuaternary Period – “Age of Man”(1.8 million years ago - Present)

The quaternary period includes the Pleistocene and Holocene Epochs.

This period is known as the “Age of Man”.

Pleistocene epoch - The Last Ice Age (1.8 - .011 mya)The first humans (Homo sapiens) evolve - Mammoths, mastodons, saber-toothed cats, giant ground sloths - A mass extinction of large mammals and many birds happened about 10,000 years ago, probably caused by the end of the last ice age.

Holocene epoch - (11,000 years ago to today)Human civilization