Instruct: Given a question, retrieve relevant documents that can help answer the question.
Query: How many geologic eons are there?
The primary defined divisions of time are eons, in sequence the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three of these can be referred to collectively as the Precambrian supereon. Eons are divided into eras, which are in turn divided into periods, epochs and ages.
The early Earth (sometimes referred to as Gaia) is loosely defined as Earth in its first one billion years, or gigayear. On the geologic time scale, this comprises all of the Hadean eon (starting with the formation of the Earth about 4.6 billion years ago), as well as the Eoarchean (starting 4 billion years ago) and part of the Paleoarchean (starting 3.6 billion years ago) eras of the Archean eon.
The geologic record of the Proterozoic Eon is more complete than that for the preceding Archean Eon. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of those rocks are less metamorphosed than are Archean ones, and many are unaltered. Studies of these rocks have shown that the eon continued the massive continental accretion that had begun late in the Archean Eon. The Proterozoic Eon also featured the first definitive supercontinent cycles and wholly modern mountain building activity (orogeny).
Since few geological traces of this eon remain on Earth, there is no official subdivision. However, the Lunar geologic timescale embraces several major divisions relating to the Hadean, so these are sometimes used in an informal sense to refer to the same periods of time on Earth.
The term "Precambrian" is recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time; it is so called because it includes the Hadean (~4.6—4 billion), Archean (4—2.5 billion), and Proterozoic (2.5 billion—541 million) eons. (There is only one other eon: the Phanerozoic, 541 million-present.) "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names. , the United States Geological Survey considers the term informal, lacking a stratigraphic rank.
A geologic era is a subdivision of geologic time that divides an eon into smaller units of time. The Phanerozoic Eon is divided into three such time frames: the Paleozoic, Mesozoic, and Cenozoic (meaning "old life", "middle life" and "recent life") that represent the major stages in the macroscopic fossil record. These eras are separated by catastrophic extinction boundaries, the P-T boundary between the Paleozoic and the Mesozoic and the K-Pg boundary between the Mesozoic and the Cenozoic. There is evidence that catastrophic meteorite impacts played a role in demarcating the differences between the eras.
The Phanerozoic Eon[3] is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present,[4] and began with the Cambrian Period when diverse hard-shelled animals first appeared. Its name was derived from the Ancient Greek words φανερός (phanerós) and ζωή (zōḗ), meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953).[5][6] The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.
The jiffy is the amount of time light takes to travel one fermi (about the size of a nucleon) in a vacuum. Planck time is the time light takes to travel one Planck length. Theoretically, this is the smallest time measurement that will ever be possible. Smaller time units have no use in physics as we understand it today. The TU (for Time Unit) is a unit of time defined as 1024µs for use in engineering. The Svedberg is a time unit used for sedimentation rates (usually of proteins). It is defined as 10−13 seconds (100 fs). The galactic year, based on the rotation of the galaxy, and usually measured in million years.[2] The geological time scale relates stratigraphy to time. The deep time of Earth’s past is divided into units according to events which took place in each period. For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event. The largest unit is the supereon, composed of eons. Eons are divided into eras, which are in turn divided into periods, epochs and ages. It is not a true mathematical unit, as all ages, epochs, periods, eras or eons don't have the same length; instead, their length is determined by the geological and historical events that define them individually.
The Archean Eon (, also spelled Archaean or Archæan) is one of the four geologic eons of Earth history, occurring (4 to 2.5 billion years ago). During the Archean, the Earth's crust had cooled enough to allow the formation of continents and life started to form.
The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline.
The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution. The succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land; the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale.
Corresponding to eons, eras, periods, epochs and ages, the terms "eonothem", "erathem", "system", "series", "stage" are used to refer to the layers of rock that belong to these stretches of geologic time in Earth's history.
In a letter to "Nature", Veizer et al.(2000), compared the reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past ~550 Myr) with the variable galactic cosmic rays and concluded that their results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.
In large-scale natural science, there is need for another time perspective, independent from human activity, and indeed spanning a far longer period (mainly prehistoric), where "geologic era" refers to well-defined time spans. 
The next-larger division of geologic time is the eon. The Phanerozoic Eon is subdivided into eras. There are currently three eras defined in the Phanerozoic; the following table lists them from youngest to oldest (BP is an abbreviation for "before present").
The Phanerozoic Eon is the current eon in the geologic timescale. It covers roughly 541 million years. During this period continents drifted about, eventually collected into a single landmass known as Pangea and then split up into the current continental landmasses.
Nevertheless, at least one notable scientific work has advocated using the lunar geological time scale to subdivide the Hadean eon of Earth's geologic time scale. In particular, it is sometimes found that the Hadean is subdivided into the Cryptic, Basin Groups 1-9, Nectarian, and Early Imbrian. This notation is not entirely consistent with the above lunar geologic time scale in that the Cryptic and Basin Groups 1-9 (both of which are only informal terms that are not used in geologic maps) comprise the Pre-Nectarian period.Cited references
Chaotian (geology)– Proposed era of, or eon preceding, the Hadean eon Formation and evolution of the Solar System– Formation of the Solar System by gravitational collapse of a molecular cloud and subsequent geological history Hadean zircon– The oldest-surviving crustal material from the Earth's earliest geological time period History of Earth– The development of planet Earth from its formation to the present day– the first sections describe the formation of the Earth Oldest dated rocks– Includes rocks over 4 billion years old from the Hadean Eon Precambrian– The earliest part of Earth's history Timeline of natural history
The geologic record of the Proterozoic () is more complete than that for the preceding Archean. In contrast to the deep-water deposits of the Archean, the Proterozoic features many strata that were laid down in extensive shallow epicontinental seas; furthermore, many of these rocks are less metamorphosed than Archean-age ones, and plenty are unaltered. Study of these rocks show that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and wholly modern orogenic activity. Roughly , the earliest-known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 Ma.
This is a list of official and unofficial names for time spans in the geologic timescale and units of chronostratigraphy. Since many of the smallest subdivisions of the geologic timescale were in the past defined on regional lithostratigraphic units, there are many alternative names that overlap. The body concerned with standardizing the names of geochronologic units is the International Commission on Stratigraphy (ICS). In 2008 however, even though the Phanerozoic eon is almost completely divided into internationally recognizable units, local subdivisions are often still preferred over the international ones.
The Neoarchean (; also spelled Neoarchaean) is a geologic era within the Archaean Eon. 
Archean subduction is a contentious topic involving the possible existence and nature of subduction in the Archean, a geologic eon extending from 4.0-2.5 billion years ago. Until recently there was little evidence unequivocally supporting one side over the other, and in the past many scientists either believed in shallow subduction or its complete non-existence. However, the past decade has witnessed the potential beginning of a change in geologic understanding as new evidence is increasingly indicative of episodic, non-shallow subduction.
The Precambrian (or Pre-Cambrian, sometimes abbreviated pЄ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.
Digital mapping technology can be applied to traditional geologic mapping, reconnaissance mapping, and surveying of geologic features. At international digital field data capture (DFDC) meetings, major geological surveys (e.g., British Geological Survey and Geological Survey of Canada) discuss how to harness and develop the technology. Many other geological surveys and private companies are also designing systems to conduct scientific and applied geological mapping of, for example, geothermal springs and mine sites.
It turns out that Young-eon had been hearing the voices of her teacher and of the ghost, Hyo-jung, the whole time. Young-eon wanted the teacher dead to get rid of Hyo-jung's voice. Hyo-jung was a student and top singer at the school who fell in love with the music teacher. She has the same voice as Young-eon and they are both recorded in a song. Hyo-jung is revealed to have shot the music sheet at Young-eon`s throat in the opening scene out of anger at losing her voice. Seon-min thinks Young-eon should move on but Young-eon is angered by her statement, since she wants nothing more than to live again. Young-eon kills Cho-ah and takes over Seon-min's body. She then walks alongside Seon-min's mother, talking about how, as soon as she is old enough, she will learn to drive.
Its Earth equivalents are the Neoproterozoic era of the Proterozoic eon and the whole of the Phanerozoic eon. So, while animal life bloomed on Earth, the Moon's geologic activity was coming to an end.