VII. LISTS
Glacials
There have been five major glacials in prehistory, each lasting millions or even hundreds of millions of years! There is not a cyclic component to the glacials - it might be speculated that they are caused by reduced solar input owing to transitions through arms of galactic dust. Over the last 3 billion years, despite their extended duration, glacials have occupied only 25% of earth's existence. The other 75% of earth's prehistory has been free of surface ice.
Glacials of Earth’s History Pongola ~2700-2100 mya Huronian ~1800-1600 mya Cryogenian ~1250-750 mya Karoo-Andean ~600-300 mya Quaternary ~260-0 mya
Supercontinents
Supercontinents are an aggregation of most of the landmasses on the earth's surface. Landmasses are the exposed surface of sections or plates of earth's crust, also called cratons. Over millions of years, these change position, sometimes coming together, sometimes separating. These movements are thought to be impelled by conditions hundreds of miles below the surface, where the molten magma though under tremendous pressure may circulate within itself from thermal and mass variations, and from earth's rotation.
Paleogeologists have traced the movements of cratons throughout prehistory. Various names for various cratonic collections have been ascribed, but it is generally agreed that there have been six supercontinents in earth's geologic past. Supercontinents (and corresponding vast suboceanic formations) obviously have great effects on all aspects of earth life: atmospheric and oceanic circulation and chemistry, albedo (and retention of solar energy), humidity, rainfall, erosion, and flora and fauna.
Supercontinents of Earth’s History Ur (Vaalbara) ~3600-2900 mya Kenorland ~2700-2100 mya Columbia (Nuna) ~1800-1500 mya Rodinia ~1250-750 mya Pannotia (Vendian) ~600-300 mya Pangaea/Gondwana ~300-200 mya
Extinctions
Mass extinctions of 50% to 96% of species have occurred from time to time since the beginnings of life in the ocean. They are all alarming to review, and each recovery seems miraculous. Extinction of individual species occurs continually, but mass extinctions apply to a large percentage of all species on earth; they occur when there is a large change or event in earth's natural environment. Typically, scientists look to events such as large-scale plate movements or magma flows, loss of atmospheric oxygen, temperature drops, or meteor strikes.
During the Ordovician, most animal life was aquatic. During the last prehistoric extinction most land animals were reptilian, including dinosaurs. The dinosaurs' extinction is most well-known, and its cause was clearly owing to a meteor strike. The causes of other extinctions are less clearly identifiable, and have been ascribed to various circumstances, cited below.
Extinctions occur over a relatively short time, but not in just a few decades or even centuries — even when a triggering geologic event happens very quickly, extinction still usually takes ages or longer. On a positive note, the disappearance of species makes room in the environment for the emergence or development of others. Humans probably owe their own evolution to the extinction of ground-based dinosaurs, and small mammals themselves appeared in number only after this extinction.
The current extinction began with the expansion of human population and activity. In recent centuries, hundreds of plant and animal species have vanished owing to habitat loss, extreme hunting, or new chemical environments. New threats are DNA manipulation and biowarfare. Developments in these areas are advanced and far-reaching and not under the direct control of any broad multinational group of citizenry. Because of the ages it takes to fully vanish, however, humankind will have the time to manage their own extinction, should such ever to appear.
Mass Extinctions in Earth’s History Ordovician ~440- mya Devonian ~375- mya Permian ~250- mya Triassic-Jurassic ~200- mya Cretaceous-Tertiary ~65 mya Anthropodine ~0 mya
