In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age but shorter than a period. The current epoch is the Holocene Epoch of the Quaternary Period. Rock layers deposited during an epoch are called a series. Series are subdivisions of the stratigraphic column that, like epochs, are subdivisions of the geologic timescale. Like other geochronological divisions, epochs are normally separated by significant changes in the rock layers to which they correspond.
Epochs are most commonly used for the younger Cenozoic Era, where a greater collection of fossils has been found and paleontologists have more detailed knowledge of the events that occurred during those times. They are less commonly referred to for the other eras and eons, since less fossil evidence exists that allows us to form a clearer view of those time periods.
Geochronology: From largest to smallest:
A climate oscillation or climate cycle is any recurring cyclical oscillation within global or regional climate, and is a type of climate pattern. These fluctuations in atmospheric temperature, sea surface temperature, precipitation or other parameters can be quasi-periodic, often occurring on inter-annual, multi-annual, decadal, multidecadal, century-wide, millennial or longer timescales. They are not perfectly periodic and a Fourier analysis of the data does not give a sharp spectrum.
A prominent example is the El Niño Southern Oscillation, involving sea surface temperatures along a stretch of the equatorial Central and East Pacific Ocean and the western coast of tropical South America, but which affects climate worldwide.
Records of past climate conditions are recovered through geological examination of proxies, found in glacier ice, sea bed sediment, tree ring studies or otherwise.Epoch
An epoch, for the purposes of chronology and periodization, is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by congruity (makes simple sense), or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.Geology of Ethiopia
The geology of Ethiopia includes rocks of the Neoproterozoic East African Orogeny, Jurassic marine sediments and Quaternary rift-related volcanism. Events that greatly shaped Ethiopian geology is the assembly and break-up of Gondwana and the present-day rifting of Africa.
Rocks formed by the East African Orogeny 880 to 550 million years ago make up the oldest geological units in Ethiopia. The orogeny caused the closure of the ancient Mozambique Ocean. Rocks of Ethiopia formed concurrently with the Mozambique Belt and the Arabian-Nubian Shield forming a large north-south (predent-day coordinates) mountain chain called the Transgondwanan Supermountain. Erosion of this mountain may have played a role in triggering the Cambrian explosion. Erosion of the orogen and mountain was such that by the early Paleozoic a planation surface extended across Ethiopia. Sedimentary rocks of Ordovician age cover this surface making it largely an unconformity. At parts the unconformity of the Precambrian basement has glacial striations, rôche moutonnées and chatter marks formed likely during the Karoo Ice Age. The Paleozoic sedimentary cover above the unconformity is of fluvial and glacial origin.Early Jurassic marine sediments cover much of the older sediments including a planation surface of Triassic age. These sediments deposited as result of a regional marine transgression swept over the Horn of Africa during the initial break-up of Gondwana. Resultant rocks include sandstone, limestone, shale, marls and evaporites. A third major plantion surface and unformity formed in the following a tectonic event in the Early Cretaceous that tilted carbonates in the Tigray and Dire Dawa-Harar areas. On top of this surface lies a series a fluvial sediments. Depositions of marine sediments continued in eastern Ethiopia's Ogaden basin until the Eocene.The Ethiopia-Yemen Continental Flood Basalts or Ethiopian traps that cover much of Ethiopia flowed over both irregular surfaces and peneplains preserving laterite soil beneath. The flood basalts covered initially a much larger area (>750,000 km2) just after eruption about 30 million years ago in the Oligocene with volumes reaching 350,000 km3. Since then erosion has reduced areal extent and volumes. The modern pattern of volcanism concentrated to the Afar Depression and the Main Ethiopian Rift begun in Late Miocene time. The area of modern volcanism contains the bulk of Ethiopias geothermal energy resources.Snowball Earth
The Snowball Earth hypothesis proposes that during one or more of Earth's icehouse climates, Earth's surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago). Proponents of the hypothesis argue that it best explains sedimentary deposits generally regarded as of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the implications of the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean and emphasize the difficulty of escaping an all-frozen condition. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water.
The snowball-Earth episodes are proposed to have occurred before the sudden radiation of multicellular bioforms, known as the Cambrian explosion. The most recent snowball episode may have triggered the evolution of multicellularity. Another, much earlier and longer snowball episode, the Huronian glaciation, which would have occurred 2400 to 2100 Mya, may have been triggered by the first appearance of oxygen in the atmosphere, the "Great Oxygenation Event".TEX86
TEX86 is an organic paleothermometer based upon the membrane lipids of mesophilic marine Thaumarchaeota (formerly Marine Group 1 Crenarchaeota).Timeline of glaciation
There have been five or six major ice ages in the history of Earth over the past 3 billion years.
The Late Cenozoic Ice Age began 34 million years ago, its latest phase being the Quaternary glaciation, in progress since 2.58 million years ago.
Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively. The Earth is currently in such an interglacial period of the Quaternary glaciation, with the last glacial period of the Quaternary having ended approximately 11,700 years ago, the current interglacial being known as the Holocene epoch.
Based on climate proxies, paleoclimatologists study the different climate states originating from glaciation.
(541.0 Mya–2.5 Gya)
|Archean eon (2.5–4 Gya)|
|Hadean eon (4–4.6 Gya)|