ROCKS OF ALL AGES
The geologic time scale is one of the most significant achievements of all of science. It's taken considerable work by numerous geologists over many decades to come up with a useful geologic time scale for Earth with accurate boundary dates. Geochronological work is ongoing that continues to refine the dates of important boundaries.
Earth history is divided into three large time intervals - eons. From oldest to youngest, they are the Archean Eon, the Proterozoic Eon, and the Phanerozoic Eon. Some geologists want to insert a fourth eon at the beginning of Earth's history (variously called "Hadean" or "Priscoan").
These terms do not have fixed definitions. Some define them based on the oldest known preserved rocks - well, someone may find older ones. Note the recent discovery of 4.28 billion year old rocks from Hudson Bay - these are significantly older than the previous oldest known Earth rock at 4.03 billion years. Also note the recent indirect discovery of 4.45-4.55 billion years old mantle rocks below Baffin Island, Canada.
Or, some have suggested that these terms be defined based on speculative events (e.g., 1 - the collision event that produced the Moon, which may not have happened according to several lines of evidence, including 4.4 billion year old Earth zircons indicating a cool early Earth; 2 - the late heavy bombardment event, which is based on lunar mare evidence that can be interpreted in other ways).
So, the terms "Hadean" and "Priscoan" are rejected here.
Each eon can be subdivided into smaller units called eras. Eras can be subdivided into smaller units called periods. Periods can be subdivided into epochs (a.k.a. series). Epochs/series can be subdivided into stages.
Photos of example rocks from each major division of the geologic time scale are given on the linked pages below.
The "Precambrian" is an informal time term that combines all geologic time units predating the Cambrian. The Precambrian includes the Archean Eon and the Proterozoic Eon.
The term "Archean" simply means "ancient". The Archean starts at the beginning of Earth, 4.55 billion years ago. It ends arbitrarily at precisely 2.50 billion years ago. Traditionally, the end of the Archean was intended to represent the beginning of the rise of free atmospheric oxygen gas. Relatively few Archean-aged rocks are exposed at the Earth's surface. Many exposed Archean rocks are metamorphic or igneous. Very few Archean rocks are sedimentary.
Different folks subdivide the Archean in different ways. In general, geologists seem to want four subdivisions (eras) in the Archean (from oldest to youngest): Eoarchean, Paleoarchean, Mesoarchean, Neoarchean. Every time scale you look at usually has different dates on these boundaries.
Archean-aged orthogneiss (a metamorphosed granite), Beartooth Highway, Montana, USA.
"Proterozoic" means "hidden-life", referring to the presence of rare, microscopic fossils in some rocks of this age. The term was first proposed by Samuel Emmons in 1888. The Proterozoic starts arbitrarily at 2.5 billion years and ends at 544 million years ago (see additional comments about this date in the Cambrian section).
The Proterozoic has an abundance of preserved sedimentary rocks, in constrast to the Archean. This was a time of considerable continental consolidation, and the first appearance of widespread continental shelves. Such environments are necessary in order to get sedimentary rocks preserved for long intervals of time.
The Proterozoic is divided into three parts (eras) (from oldest to youngest): the Paleoproterozoic, the Mesoproterozoic, and the Neoproterozoic. The PP-MP boundary is often given as 1.60 billion years. The MP-NP boundary is often given as 1.0 billion years or 900 million years.
Proterozoic-aged quartzite (a metamorphosed quartzose sandstone), Lorrain Quartzite, Manitoulin Island, Ontario, Canada.
After the Precambrian, we've got the Phanerozoic Eon. "Phanerozoic" means "apparent-life", referring to the abundance of macroscopic body fossils, microscopic body fossils, and trace fossils in sedimentary rocks of this age. The Phanerozoic is subdivided into three eras (from oldest to youngest): the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era.
The Paleozoic has six periods (from oldest to youngest): Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian. In America, the Carboniferous is considered to be two periods, the Mississippian and the Pennsylvanian. Outside America, the Mississippian and Pennsylvanian are considered together and called the Carboniferous. In order to settle this disagreement, the IUGS has ruled that everyone on Earth has to use all three terms (a good compromise is one that no one likes). So, the "Carboniferous Period" is a valid term, and so are "Pennsylvanian Subperiod" and "Mississippian Subperiod".
The Cambrian was first named in 1835 by Adam Sedgwick. It dates from 544 to 490 million years. Many consider the base of the Cambrian to be at 542 million years. Based on available information and evidence, I'm not convinced that the 544 m.y. date is not accurate (there are correlation and isotope issues that haven't yet been addressed to my satisfaction). So, 544 m.y. it is!
The Cambrian begins with the most significant event in the history of Earth - the Cambrian Explosion. Despite the name, this does not refer to an impact event or a volcanic eruption or a natural disaster. The Cambrian Explosion refers to the rapid evolutionary appearance of abundant macrofossils, microfossils, and trace fossils near the Precambrian-Cambrian boundary. After years of study, the PC-C boundary was defined to be at the first appearance of complex trace fossils, as exemplified by Trichophycus pedum (aka Treptichnus pedum; aka Phycodes pedum). By the way, almost everyone thinks that T. pedum is the definition of the PC-C boundary. It is not. Read the original definition. The PC-C boundary is defined at the first appearance of complex trace fossils, as exemplified by T. pedum.
Cambrian rocks are often noticeably fossiliferous. The most common fossils found in Cambrian rocks are trilobites.
Cambrian-aged fossiliferous sandstone (with abundant trilobite fragments), Eau Claire Formation, Little Falls Dam, Wisconsin, USA.
The Ordovician was first named in 1879 by Charles Lapworth. It dates from 490 to 444 million years ago. During the Ordovician, Earth experienced the highest sea levels that we have evidence for. This was also a time of a global greenhouse climate. Ordovician rocks around the world are often richly fossiliferous.
Ordovician-aged fossiliferous limestone (with abundant crinoid stem pieces, plus trilobite fragments, bryozoans, tentaculites, etc.), Kope Formation, Covington, Kentucky, USA.
The Silurian was first named in 1839 by Roderick Murchison. It dates from 444 to 416 million years ago.
Silurian-aged dolostone (with a few fossil ostracods - see the oval-shaped structures near the bottom), Tymochtee Dolomite, Latham Limestone Quarry, Ohio, USA.
Devonian-aged fossiliferous limestone (with abundant brachiopods), Columbus Limestone, Columbus, Ohio, USA.
Mississippian Period (Subperiod)
The Mississippian (= lower Carboniferous) was first named in 1891 by Henry Shaler Williams. It dates from 359 to 318 million years ago.
Mississippian-aged fossiliferous limestone (partly chertified, with abundant crinoid stem pieces and a camerate crinoid calyx at right-center), Fort Payne Formation, Lake Cumberland, Kentucky, USA.
Pennsylvanian Period (Subperiod)
The Pennsylvanian (= upper Carboniferous) was first named in 1891, in the same publication by H.S. Williams that names and defines the Mississippian. It dates from 318 to 299 million years. Global sea levels during the Pennsylvanian were wildly fluctuating, due to the repeated waxing and waning of glacial ice sheets in Gondwana (= the modern-day southern continents).
Pennsylvanian-aged fossiliferous ironstone (with carbonized fern frond and plant stems), Nelsonville, Ohio, USA.
The Permian was first named in 1841 by Roderick Murchison. It dates from 299 to 251 million years ago. The end of the Permian is the same as the end of the Paleozoic. The end-Permian/end-Paleozoic marks the greatest mass extinction event in the history of planet Earth. There's tons of geologic data that help geologists interpret the causative factors behind the end-Permian mass extinction, but long story short - it's still a bit of a mystery. There's excellent evidence for one (or more) impact events (we've got pieces of the original meteorite). There's excellent evidence of the largest volcanic flood basalt event in history occurring at this time (the Siberian Traps). There's impressive evidence that Earth's atmosphere lost much of its free oxygen gas. The list goes on and on.
Permian-aged limestone, Bird Spring Formation, Arrow Canyon Range, Nevada, USA.
The Paleozoic Era ended with a severe mass extinction event. What followed next was the Mesozoic Era, which is subdivided into three periods (from oldest to youngest): Triassic, Jurassic, Cretaceous. The Mesozoic was the "Age of Reptiles". Reptiles were at the top of the food pyramid in the water, on the land, and in the air. The most famous Mesozoic fossils are the dinosaurs. They first appear in the Triassic, and they disappear at the end of the Cretaceous.
The Mesozoic dates from 251 to 65 million years ago (a 186 million year duration).
The Triassic was first named in 1834 by Friedrich August von Alberti. It dates from 251 to 200 million years.
Triassic-aged mudshale (redbed), Connecticut Valley, Connecticut, USA.
The Jurassic was first named in 1829 by Alexandre Brongniart. It dates from 200 to 145 million years.
Jurassic-aged rock gypsum, Arapien Formation, San Pitch Mountains, Utah, USA.
The Cretaceous was first named in 1822 by Jean-Baptiste-Julien d'Omalius d'Halloy. It dates from 145 to 65 million years. During the Cretaceous, Earth had the second-highest sea levels of all time (they were higher during the Ordovician). This was a time of a global greenhouse climate.
The Cretaceous literally ends with a bang. The Cretaceous-Tertiary boundary (often referred to as "K-T"; = Mesozoic-Cenozoic boundary) is placed at one of the most significant mass extinction events in history. Famous victims of this extinction include the dinosaurs, the pterosaurs, the plesiosaurs, the mosasaurs, the ichthyosaurs, etc. Abundant evidence has been accumulated since the late 1970s indicating a massive impact event occurred in the Yucatan Peninsula of Mexico. The extinction was a result of the severe climate effects of dust & debris blocking sunlight and possible acid rain. For more info., see the K-T clay.
Cretaceous-aged granite, Sierra Nevada Batholith, Lake Tahoe (Nevada side), USA.
The Cenozoic Era includes the modern day. It started 65 million years ago, at the catastrophic impact event that killed off the dinosaurs. Historically, the Cenozoic is divided into a long Tertiary Period (65 to 1.8 million years) and a short Quaternary Period (1.8 million years to now). More recently, the Cenozoic is divided more or less evenly into the Paleogene Period (65 to 23 million years) and the Neogene Period (23 million years to now).
The Cenozoic's period terminology is less important than the epoch terminology. The Cenozoic is divided into seven epochs (from oldest to youngest): Paleocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene.
The Paleocene was first named in 1874 by Wilhelm Schimper. It dates from 65 to 55.8 million years. The end of the Paleocene is marked by a dramatic climate warming event called the Paleocene-Eocene Thermal Maximum.
Paleocene-aged oncolitic limestone, Flagstaff Formation, San Pitch Mountains, Utah, USA.
The Eocene was first named in 1833 by Charles Lyell. It dates from 55.8 to 33.9 million years. The Eocene starts with a significant climate warming event. Things gradually cool down from the Eocene into the Oligocene.
Eocene-aged fossiliferous marlstone (with complete fossil herring, Knightia eocaena), Green River Formation, Kemmerer, Wyoming, USA.
The Oligocene was first named in 1854 by Ernst Beyrich. It dates from 33.9 to 23 million years.
Oligocene-aged porphyritic phonolite, Cripple Creek Diatreme, Cripple Creek, Colorado, USA.
The Miocene was first named in 1833 by Charles Lyell. It dates from 23 to 5.33 million years.
Miocene-aged basalt, Columbia River Flood Basalt Group, Rt. 90, Washington State, USA.
The Pliocene was first named in 1833 by Charles Lyell. It dates from 5.33 to 1.81 million years.
Pliocene-aged sandstone (diagenetic concretion of alunite- & jarosite-cemented sandstone), Camp Rice Formation, Picacho Mountain, New Mexico, USA.
The Pleistocene was first named in 1839 by Charles Lyell. It dates from 1.81 million to 10 thousand years. The Pleistocene is otherwise well known as the last Ice Age. It included four separate glacial intervals. Each phase of glaciation was separated from the next by a warm, interglacial interval. The last of the Pleistocene glaciations ended about 10,000 years ago. Two massive continental ice sheets still remain from the Pleistocene glaciations - one in Greenland, and one in Antarctica.
Pleistocene-aged fossiliferous limestone, Grotto Beach Formation, San Salvador Island, Bahamas.
The Holocene was defined in 1885 by the International Geological Congress. It dates from 10,000 years ago to now. The 10 ky date is arbitrary, but is traditionally considered to represent the end of the last Ice Age.
Holocene-aged scoriaceous basalt, Black Rock Desert, Utah, USA.
For those interested in all things related to the geologic time scale, the following reference (& its precursor) is highly recommended:
Gradstein, F., J. Ogg & A. Smith. 2004. A Geologic Time Scale 2004. Cambridge. Cambridge University Press. 589 pp. 1 poster.
Harland, W.B., R.L. Armstrong, A.V. Cox, L.E. Craig., A.G. Smith & D.G. Smith. 1990. A Geologic Time Scale 1989. Cambridge. Cambridge University Press. 263 pp.