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GEOLOGIC EONS, ERAS and PERIODS The period is the basic unit of geological time in which a single type of rock system is formed. Two or more periods comprise a geological Era. Two or more Eras form an Eon, the largest division of geologic time. Some periods are divided into epochs. The major periods in the geologic history of the Earth are (mya=million years ago): Era Period Quaternary (1.8 million years ago - Present) The second period of the Cenozoic era. It contains two epochs: the Pleistocene and the Holocene. It is named after the Latin word "quatern" (four at a time). Cenozoic (65 million years ago - Present) "Age of Recent Life" The several geologic eras were originally named Primary, Secondary, Tertiary, and Quaternary. The first two names are no longer used. Tertiary and Quaternary have been retained but used as period designations. An era of geologic time from the beginning of the Tertiary period to the present. Its name is from Greek and means "new life." Epoch Holocene (8,000 years ago Present) An epoch of the Quaternary period. It is named after the Greek words "holos" (entire) and "ceno" (new). Pleistocene (1.8 million - 8,000 years ago) "The Great Ice Age" An epoch of the Quaternary period. It is named after the Greek words "pleistos" (most) and "ceno" (new). Pliocene (5.3 - 1.8 million years ago) Tertiary (65 - 1.8 million years ago) The first period of the Cenozoic era (after the Mesozoic era and before the Quaternary period). Final epoch of the Tertiary period. It is named after the Greek words "pleion" (more) and "ceno" (new). Miocene (23.8 - 5.3 million years ago) A epoch of the upper Tertiary period. It is named after the Greek words "meion" (less) and "ceno" (new). Oligocene (33.7 - 23.8 million years ago) An epoch of the early Tertiary period. It is named after the Greek words "oligos" (little, few) and "ceno" (new). Eocene (55.5 - 33.7 million years ago) An epoch of the lower Tertiary period. Its name is from the Greek words "eos" (dawn) and "ceno" (new). Paleocene (65 - 55.5 million years ago) Earliest epoch of the Tertiary period. It is named after the Greek words "palaois" (old) and "ceno" (new). Era Mesozoic (248 - 65 million years ago) "Age of Medieval Life" An era of geologic time between the Paleozoic and the Cenozoic. The word Mesozoic is from Greek and means "middle life." Period Epoch Cretaceous Late or Upper (145 - 65 million years ago) "The Age of Dinosaurs" The final period of the Mesozoic era. The name is derived from the Latin word for chalk ("creta") and was first applied to extensive deposits of this age that form white cliffs along the English Channel between Great Britain and France. Jurassic (213 - 145 million years ago) Early or Lower Late or Upper Middle The middle period of the Mesozoic era. It is named after the Jura Mountains between France and Switzerland, where rocks of this age were first studied. Triassic (248 - 213 million years ago) The earliest period of the Mesozoic era. The name Triassic refers to the threefold division of rocks of this age in Germany. The Break-up of the continent Pangea ... MORE Era Early or Lower Late or Upper Middle Early or Lower Period Epoch Permian Late or Upper (286 - 248 million years ago) The final period of the Paleozoic era. It is named after the province of Perm, Russia, where rocks of this age were first studied. Pennsylvanian* (325 - 286 million years ago) Paleozoic (544 - 248 million years ago) "Age of Ancient Life" An era of geologic time, from the end of the Precambrian to the beginning of the Mesozoic. The word Paleozoic is from Greek and means "old life." Early or Lower Late or Upper Middle "The Coal Age" Carboniferous (360 - 286 million years ago) A period of time in the Paleozoic era that includes the Pennsylvanian and Mississippian periods. A period of the Paleozoic era. It is named after the state of Pennsylvania where rocks of this age are widespread. Mississippian* Early or Lower Late or Upper (360 - 325 million years ago) A period of the Paleozoic era. It is named after the Mississippi River valley, which contains good exposures of rocks of this age. Devonian (410 - 360 million years ago) Early or Lower Late or Upper Middle Early or Lower A period of the Paleozoic era. It is named after Devonshire, England, where rocks of this age were first studied. Silurian (440 - 410 million years ago) A period of the Paleozoic. It is named after a Celtic tribe called the Silures. Ordovician (505 - 440 million years ago) The second earliest period of the Paleozoic era. It is named after a Celtic tribe called the Ordovices. Cambrian (544 - 505 million years ago) The earliest period of the Paleozoic era. It is named after Cambria, the Roman name for Wales, where rocks of this age were first studied. Late or Upper Middle Early or Lower Late or Upper Middle Early or Lower Late or Upper Middle Early or Lower Precambrian (Beginning of earth - 544 million years ago) All geologic time before the beginning of the Paleozoic era. This includes about 90% of all geologic time and spans the time from the beginning of the earth, about 4.5 billion years ago, to 544 million years ago. Its name means "before Cambrian." About the geologic time scale Origins of a geologic time scale The first people who needed to understand the geological relationships of different rock units were miners. Mining had been of commercial interest since at least the days of the Romans, but it wasn't until the 1500s and 1600s that these efforts produced an interest in local rock relationships. By noting the relationships of different rock units, Nicolaus Steno in 1669 described two basic geologic principles. The first stated that sedimentary rocks are laid down in a horizontal manner, and the second stated that younger rock units were deposited on top of older rock units. To envision this latter principle think of the layers of paint on a wall. The oldest layer was put on first and is at the bottom, while the newest layer is at the top. An additional concept was introduced by James Hutton in 1795, and later emphasized by Charles Lyell in the early 1800s. This was the idea that natural geologic processes were uniform in frequency and magnitude throughout time, an idea known as the "principle of uniformitarianism." About the geologic time scale divisions The geologic history of the Earth is broken up into hierarchical chunks of time. From largest to smallest, this hierarchy includes eons, eras, periods, epochs, and ages. All of these are displayed in the portion of the geologic time scale shown below. Eon Era Period Epoch Phanerozoic Cenozoic Paleogene Oligocene The Phanerozoic Eon represents the time during which the majority of macroscopic organisms — algae, fungi, plants and animals — lived. When first proposed as a division of geologic time, the beginning of the Phanerozoic (542.0 million years ago)* was thought to coincide with the beginning of life. In reality, this eon coincides with the appearance of animals that evolved external skeletons, like shells, and the somewhat later animals that formed internal skeletons, such as the bony elements of vertebrates. The time before the Phanerozoic is usually referred to as the Precambrian and is usually divided into the three eons shown. The Phanerozoic is subdivided into three major divisions: the Cenozoic, Mesozoic, and Paleozoic Eras. The "zoic" suffix comes from the root "zoo," which means animal. This is the same root as in the words zoology and zoological park (or zoo). "Cen-" means recent, "Meso-" means middle, and "Paleo-" means ancient. These divisions reflect major changes in the composition of ancient faunas, each era being recognized by its domination by a particular group of animals. The Cenozoic has sometimes been called the "Age of Mammals," the Mesozoic the "Age of Dinosaurs," and the Paleozoic the "Age of Fishes." But this is an overly simplified view, which has some value for the newcomer but can be a bit misleading. For instance, other groups of animals lived during the Mesozoic. In addition to the dinosaurs, animals such as mammals, turtles, crocodiles, frogs, and countless varieties of insects also lived on land. Additionally, there were many kinds of plants living in the past that no longer live today. Ancient floras went through great changes too, and not always at the same times that the animal groups changed.