Paleogeography

Paleogeografie, ook wel gespeld paleogeografie, de oude geografie van het aardoppervlak. De geografie van de aarde verandert voortdurend: continenten bewegen als gevolg van platentektonische interacties; bergketens worden omhoog geduwd en eroderen; en de zeespiegel stijgt en daalt naarmate het volume van de oceaanbekkens verandert. Deze geografische veranderingen kunnen worden opgespoord door de studie van het rots- en fossielenbestand en gegevens kunnen worden gebruikt om paleogeografische kaarten te maken, die illustreren hoe de continenten zijn verplaatst en hoe de locaties in het verleden van bergen, laaglanden, ondiepe zeeën en diepe oceaanbekkens zijn veranderd.

De studie van paleogeografie heeft twee hoofddoelen. De eerste is om de vroegere posities van de continenten en oceaanbekkens in kaart te brengen, en de tweede is om de veranderende geografische kenmerken van de aarde door de tijd te illustreren.

Het in kaart brengen van continenten en oceanen

De eerdere posities van de continenten kunnen worden bepaald door gebruik te maken van zes belangrijke bewijslijnen: paleomagnetisme, lineaire magnetische anomalieën, hot-spot tracks, paleobiogeografie, paleoklimatologie en geologische en tektonische geschiedenis.

Paleomagnetisme

Door het remanente magnetische veld te meten dat vaak wordt bewaard in gesteenten die ijzerhoudende mineralen bevatten, kan paleomagnetische analyse bepalen of een gesteente is gemagnetiseerd nabij een van de aardpolen of nabij de evenaar. IJzerhoudende mineralen die zich vormen in stollingsgesteente, sluiten zich aan bij het magnetische veld van de aarde terwijl het gesmolten gesteente afkoelt. Deze mineralen komen ook op één lijn wanneer ze worden afgezet in sedimenten en ze behouden hun oriëntatie terwijl ze verharden in sedimentair gesteente. Krachtlijnen in het magnetische veld van de aarde zijn evenwijdig aan het oppervlak van de planeet aan de evenaar en staan ​​verticaal aan de polen. Daarom zullen ijzerhoudende mineralen die op lage breedtegraden zijn gevormd of afgezet bijna parallel lopen met het aardoppervlak, terwijl die op hoge breedtegraden steil zullen dalen. Als de rotsen later worden getransporteerd door tektonische processen,hun oorspronkelijke breedtegraad van afzetting kan worden bepaald door hun oriëntatie. Paleomagnetisme is een direct bewijs van de noord-zuid (breedtegraad) positie van een continent, maar beperkt de oost-west (longitudinale) positie niet.

Linear magnetic anomalies

Earth’s magnetic field has another important property. Like the Sun’s magnetic field, Earth’s magnetic field periodically “flips,” or reverses polarity—that is, the North and South poles switch places. Fluctuations, or anomalies in the intensity of the magnetic field, occur at the boundaries between normally magnetized sea floor and sea floor magnetized in the reversed direction. The age of these magnetic anomalies can be established by using fossil evidence and radiometric age determinations. Because these magnetic anomalies form at oceanic ridges, they tend to be long, linear features (hence the name linear magnetic anomalies) that are symmetrically disposed about ridge axes. The past positions of the continents during the last 150 million years (the maximum age of most of the ocean floor) can be directly reconstructed by superimposing linear magnetic anomalies of the same age, in effect “undoing” the results of sea-floor spreading since that time.

Hot-spot tracks

Some of the world’s volcanoes are formed by jets of molten rock that arise at the boundary between Earth’s core and mantle (at a depth of about 2,900 km, or 1,800 miles). These rising plumes, or hot spots, puncture the lithosphere, and, as a tectonic plate moves across the hot spot, a line of islands is generated. The island directly above the hot spot is the youngest, and islands become progressively older with distance from the hot spot. There are more than a dozen well-documented hot-spot tracks. Perhaps the most obvious is the Hawaiian Islands, which trace an east-west arc across the central Pacific Ocean. Hot-spot tracks accurately record plate motions and can be used to determine the past latitudinal and longitudinal position of the continents.

Paleobiogeography

The past distribution of plants and animals can give important clues about the latitudinal position of the continents as well as their relative positions. Cold-water faunas can often be distinguished from warm-water faunas, and ancient floras reflect both paleotemperature and paleorainfall. The diversity of plants and animals tends to increase toward the Equator, and the adaptations of plants (such as smooth-edged leaves in the tropics and serrated-edged leaves in the temperate belts) are often good indicators of the amount of ancient rainfall.

The similarity or dissimilarity of faunas and floras on different continents can also be used to estimate their geographic proximity. In addition, the evolutionary history of groups of plants and animals on different continents can reveal when these continents were connected or isolated from each other. For example, Australia’s unique marsupial fauna is the result of its isolation from the other continents at the time when placental mammals were evolving on the other continents during the early Paleogene Period.

Paleoclimatology

Earth’s climate is primarily a result of the redistribution of the Sun’s energy across the surface of the globe. It is warm near the Equator and cool near the poles. Wetness or rainfall also varies systematically from the Equator to the pole in alternating bands. It is wet near the Equator, dry in the subtropics, wet in the temperate belts, and dry near the poles. Certain kinds of rocks form under specific climatic conditions. For example, coals occur where wet climates once supported lush vegetation; bauxite (the principal ore of aluminum) is formed in warm and wet conditions, evaporites and calcretes require warmth and aridity to form; and tillites are deposited during the movement of glacial ice. The ancient distribution of these and other rock types can indicate how the global climate has changed through time and how the continents have traveled across the climatic belts.

Geologic and tectonic history

In order to reconstruct the past positions of the continents, it is necessary to understand the evolution of plate tectonic boundaries. Only by understanding the regional geologic and tectonic history of an area can the location and timing of rifting events, subduction activity, continental collision, and other major plate tectonic events be determined.