The Paleogeographic Method
The study of paleogeography has two principle goals. The first goal is to map the past positions of the continents. The second goal is to illustrate the changing distribution of mountains, lowlands, shallow seas, and deep ocean basins through time.
Mapping the Past Positions of the Continents
The past positions of the continents can be determined using the following five lines of evidence: paleomagnetism, linear magnetic anomalies, paleobiogeography, paleoclimatology, and geologic history.
Paleomagnetism. By measuring the remanent magnetic field often preserved in iron-bearing rock formations, paleomagnetic analysis can determine whether a rock was magnetized near the Pole or near the Equator. Paleomagnetism provides direct evidence of a continent's N-S (latitudinal) position, but does not constrain its E-W (longitudinal) position.
Linear Magnetic Anomalies. The Earth's magnetic field has another important property. Like the Sun's magnetic field, the Earth's magnetic field "flips" or reverses polarity. 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 "reverse" direction. The age of these linear magnetic anomalies can be determined using fossil evidence and radiometric age determinations. Because these magnetic anomalies form at the mid-ocean ridges, they tend to be long, linear features (hence the name "linear magnetic anomalies") that are symmetrically disposed about the ridges axes. The past positions of the continents during the last 150 million years can be directly reconstructed by superimposing linear magnetic anomalies of the same age.
Paleobiogeography. The past distribution of plants and animals can give important clues concerning the latitudinal position of the continents as well as their relative positions. Cold-water faunas can aften be distinguished from warm-water faunas, and ancient floras both reflect paleo-temperature and paleo-rainfall. The similarity or dissimilarity of faunas and floras on different continents can 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.
Paleoclimatology. The 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. 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 it is wet, bauxite occurs where it is warm and wet, evaporites and calcretes occur where it is warm and dry, and tillites occur where it is wet and cool. The ancient distribution of these, and other, rock types can tell us how the global climate has changed through time and how the continents have travelled across climatic belts.
Geologic and Tectonic History. In order to reconstruct the past positions of the continents it is necessary to understand the development of the plate tectonic boundaries that separate continents and bring them back together again. Only by understanding the regional geological and tectonic evolution of an area can you determine the location and timing of rifting, subduction, continental collision and other major plate tectonic events.
Mapping the Changing Distribution of Mountains, Lowlands, Shallow Seas and Deep Ocean Basins
Some paleogeographic features change very slowly and are easy to map. Other paleogeographic features change very rapidly and, therefore, any map, at best, is an approximation. In this regard, the Earth, since the early Precambrian, has been divided into deep ocean basins (average depth 3.5 km) and high-standing continents (average elevation about 800 meters). Continental lithosphere, because it is less dense, is more bouyant and is not easily subducted, or recycled back into the Earth's interior. As a result, continents are made-up of very old rocks, some dating back 3.8 billion years. The amount of continental lithosphere has probably changed very little during the last 2.6 billion years (possibly increasing 10-15%). What has changed, is the shape and the distribution of continents across the globe. The ocean basins, on the other hand, are all less than 150 million years old. Oceanic lithosphere, because it is more dense is continually recylced back into the interior of the Earth.
In contrast to the continents and ocean basins, which are permanent geographic features, the height and location of mountain belts and the shape of the Earth's shorelines constantly change. Mountain belts either form where oceanic lithosphere is subducted beneath the margin of a continent, giving rise to a linear range of mountains, like the Andes mountains of western South America, or where continents collide forming, high mountains and broad plateaus like the Himalayan mountains and Tibetan Plateau of central Asia. Less extensive mountains can also form when continents rift apart (e.g. East African Rift), or where hot spots form volcanic uplifts.
In most cases mountain ranges take 10's of millions of years to form, and depending on the climate, may last for 100's of millions of years. Though the Appalachian mountains of the eastern United States were formed over 300 million years ago, due to the collision of North America and western Africa, remnants of this collisional mountain belt still reach heights of over than 2000 meters. The Himalayan Mountains, the world's tallest mountain range, began to rise from the sea nearly 50 million years ago when northern India collided with Eurasia. On the paleogeographic maps shown in this atlas, the extent of the mountain ranges increases during the collisional phase and is slowly reduced, by erosion, in subsequent maps.
In comparison to topographic features such as mountain ranges, the Earth's shorelines are ephemeral. The familiar shapes that characterize today's shorelines such as Hudson's Bay, the Florida peninsula, or the numerous fiords of Norway, are all less than 12,000 years old. The shape of the modern coastlines is the result of a 70 meter rise in sea level that took place in the last 12,000 years after the last great ice sheet that covered much of North America and Europe had melted.
It is important to note that the shoreline, though the edge of land, is not the edge of the continent. In most cases, the continent extends seaward 100's of kilometers beyond the shoreline. The actual edge of the continent is marked by the transition from the continental slope to the continental rise . This steep bathymetric gradient marks the boundary between continental lithosphere and oceanic lithosphere and is marked by the transtion from light blue (shallow shelf) to dark blue (deep ocean).
The position of the shoreline is a function of both continental topography and sea level. Though topography changes slowly (10's of millions of years), global sea level can change rapidly (10's of thousand of years). Several factors can affect sea-level change. As we have seen, one of this factors in the amount of ice on the continents. At times when the continents were covered by great ice sheets, sea level was low and the continents were exposed. The last glacial maximum was 18,000 years ago. For the last 20 million years, the continents have been largely high and dry because there has been extensive mountain building in Asia and there has been significant ice on Antarctica. Other important global episodes of glaciation occurred 300, 450 and 650 million yeares ago. The oldest known glaical episode occurred in the Precambrian, approximately 2.2 billion years ago.
Sea level also changes more slowly (10's of millions of years) due to changes in the
volume of the ocean basins. Water from the interior of the Earth, erupted as gas in
volcanic eruptions, condensed on the cooling surface of the Earth to form the world's
oceans. However, there has been no significant addition to the volume of water on the
Earth since early Precambrian times.
Changes in sea level,therefore, are not due to changes in the amount of water on the Earth, but rather are due to changes in the shape and size of the ocean basins.