Geological history of Earth
[6] Major volcanic events altering Earth's environment and causing extinctions may have occurred 10 times in the past 3 billion years.
[8] The Hadean Eon is not formally recognized, but it essentially marks the era before we have adequate record of significant solid rocks.
Eventually, the outer layer of the planet cooled to form a solid crust when water began accumulating in the atmosphere.
[12][13] More recent potassium isotopic studies suggest that the Moon was formed by a smaller, high-energy, high-angular-momentum giant impact cleaving off a significant portion of Earth.
[1][2][3] During the Hadean the Late Heavy Bombardment occurred (approximately 4,100 to 3,800 million years ago) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well.
However, some scientists argue against this hypothetical Late Heavy Bombardment, pointing out that the conclusion has been drawn from data which are not fully representative (only a few crater hotspots on the Moon have been analyzed).
Some scientists think because Earth was hotter in the past,[28][29] plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material.
Others argue that the subcontinental lithospheric mantle is too buoyant to subduct and that the lack of Archean rocks is a function of erosion and subsequent tectonic events.
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.
[34] Study of these rocks shows that the eon featured massive, rapid continental accretion (unique to the Proterozoic), supercontinent cycles, and wholly modern orogenic activity.
The Paleozoic era spanned roughly 539 to 251 million years ago (Ma)[39] and is subdivided into six geologic periods: from oldest to youngest, they are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
The most-commonly accepted theory is that these events were triggered by the onset of an ice age, in the Hirnantian faunal stage that ended the long, stable greenhouse conditions typical of the Ordovician.
The ice age was probably not as long-lasting as once thought; study of oxygen isotopes in fossil brachiopods shows that it was probably no longer than 0.5 to 1.5 million years.
Evidence of these ice caps has been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time.
The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the Tropic of Capricorn.
There was also a drop in south polar temperatures; southern Gondwana was glaciated throughout the period, though it is uncertain if the ice sheets were a holdover from the Devonian or not.
[10] During the Cretaceous, the late Paleozoic-early Mesozoic supercontinent of Pangaea completed its breakup into present day continents, although their positions were substantially different at the time.
Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide.
Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds.
In the area that is now India, massive lava beds called the Deccan Traps were laid down in the very late Cretaceous and early Paleocene.
The Paleogene (alternatively Palaeogene) Period is a unit of geologic time that began 66 and ended 23.03 Ma[10] and comprises the first part of the Cenozoic Era.
[53] The Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, which ended in the succeeding epoch.
Africa was heading north toward Europe, slowly closing the Tethys Ocean, and India began its migration to Asia that would lead to a tectonic collision and the formation of the Himalayas.
At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents mixed with colder Antarctic waters, distributing the heat around the world and keeping global temperatures high.
In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts.
Near the end of the Pliocene, about 2.58 million years ago (the start of the Quaternary Period), the current ice age began.
[10] The modern continents were essentially at their present positions during the Pleistocene, the plates upon which they sit probably having moved no more than 100 kilometres (62 mi) relative to each other since the beginning of the period.
Other than higher latitude temporary marine incursions associated with glacial depression, Holocene fossils are found primarily in lakebed, floodplain and cave deposits.
Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any likely upthrusting of non-glacial origin.
The equivalent event in North America was the rebound of Hudson Bay, as it shrank from its larger, immediate post-glacial Tyrrell Sea phase, to near its present boundaries.
