The present arrangement of the continents with similarities in outline of opposite seaboards and coasts and, much more reliably, geological data show that the Earth’s continents that are now on the opposite sides of the oceans were once joined together. Today, scientists believe that about 200 million years ago the continents were joined together to form one supercontinent Pangaea. As the plates that the continents sit on moved, the supercontinent broke up and began to move apart. This continental drift is continuing.
However, the ideas of continental drift and especially seafloor spreading remained a minority view until 1950s, when seismic techniques made possible surveying of the crust under the oceans. Then, in the 1950s, it was discovered that the oceanic crust is relatively thin, 5 to 7 kilometres in some places, and the ocean floor hides oceanic ridges, thousands of kilometres in length and several kilometres above the ocean floor. The best-studied example is Mid-Atlantic Ridge, approximately halfway between Europe and America, running north-south up the North Atlantic Ocean. Along the centre of this ridge there is an active rift valley.
In 1960 Harry Hess, of Princeton University tried to explain the newly discovered structure of the ocean floor and other previously unknown features with the first model of so-called sea-floor spreading, in accordance to which the ocean ridges are formed by upwelling convection currents in the fluid material of the mantle. As these currents bring material up to the surface at the oceanic ridge, where it spreads outward. As new ocean basins are formed, the continents are pushed further apart. To balance the formation of new crust at the ocean ridge, some oceanic crust is dragged back down under the edge of continents.
That takes place at the deep trench systems, mainly in the western Pacific as well as at the edge of some other oceans. Not surprisingly, than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific “Ring of Fire. ” According to Hess’ model, The Atlantic Ocean becomes wider, at a rate of about 2 centimetres per year, the Pacific shrinks as North America slowly drifting westwards, towards Asia. In 200 million years, people in America probably won’t need to cross the Pacific to reach what now is Far East.
So according to the “plate-tectonics” theory, Earth’s surface is broken into about a dozen of rigid shifting slabs or plates, which average about 80 kilometres in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates of a few inches per year. There are three common types of boundaries between these moving plates: Divergent or spreading. Adjacent plates pull apart, which causes sea-floor spreading, as described above for the Mid-Atlantic Ridge, which separates the North and South American Plates from the Eurasian and African Plates. Convergent.
Plates moving in opposite directions meet and one is dragged down (or subducted) beneath the other. Convergent plate boundaries are also called subduction zones and are typified by the Aleutian Trench, where the Pacific Plate is being subducted under the North American Plate. Transform fault. One plate slides horizontally past another, as in the San Andreas fault zone of California, which marks the boundary between the Pacific and North American Plates. The history of Earth’s plates since Pangaea’s break up until present time is relatively well studied. However, the motion of the plates is less clear in pre-Pangaea times.
Oceanic crust has an average age of only 55 million years, as the age of continental crust averages about 2. 3 billion years, with the oldest known rocks dating back 3. 96 billion years. Probably our planet has had several supercontinents like Pangaea throughout time. These supercontinents all went through a cycle similar to Pangaea’s. Geological data show that more than 600 millions years ago most of the land that now forms South America, Africa, India, Antarctica and Australia was grouped together in one supercontinent located roughly across the equator, called Gondwanaland.
Other continents were also assembling together. North America and Greenland had, by that time, been attached to each other for hundreds of millions of years. By about 400 millions years ago, this chunk of continental material collided with what is now part of Europe, and the pieces welded together to form so-called Old Red Sandstone. By then Gondwanaland crossed the South Pole and was moving northward. A little more than 250 million years ago, Gondwanaland and the Old Red Sandstone continent collided and struck together.
Then the last remaining independent plate, present-day Asia, collided with the northern part of this supercontinent and was welded on to Europe. That’s how all modern continents were joined together in Pangaea, which was stretching from the South Pole to high northern latitudes. In the foreseeable future, the Atlantic Ocean will be expanding, pushing North America westwards, while the Pacific Ocean will be shrinking. The Mediterranean Sea will eventually disappear, connecting Africa with Europe. India will be continuing to push into the southern Asian subcontinent, pushing the Himalayas even higher.
Short description of the web sites: Plate Tectonics, the Cause of Earthquakes including the chapter “Earthquakes are caused by plate movement”, http://www. seismo. unr. edu/ftp/pub/louie/class/100/plate-tectonics. html. The site explains and illustrates the links between the plates and earthquakes, featuring a few highly informative satellite pictures and drawings. It is shown that the plates consist of an outer layer of the Earth, the lithosphere. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot.
Only lithosphere has the strength and the brittle behavior to fracture in an earthquake. The location of earthquakes around the globe is shown. The site demonstrates that the boundaries between the plates grind against each other, producing most earthquakes, thus the lines of earthquakes help define the plates. Earthquake occurrence in different plate tectonic settings is shown with figures and pictures. Plate Tectonics, http://www. ucmp. berkeley. edu/geology/tectonics. html. This site explains the history of human understanding of the Earth and provides a brief overview of the theories behind it.
13 wonderful animations of Plate Tectonics movement in different epochs of Earth’s history are available on the site in the following formats: *. gif (these load rather slowly) and *. avi, *. mov (for faster Internet connection). Rates of Plate Movement During the Phanerozoic, www. geocities. com/earthhistory/plate2. htm. According to various forms of the Noah`s Flood model, rates of plate motion during the `Phanerozoic` were on the order of several thousand meters per day, and all or most Phanerozoic crustal displacement is considered to have occurred during a brief catastrophe occurring about 2500BCE and lasting only `weeks or months.
` There are a variety of methods which can be used to estimate rates of plate movement for given times in the past. Today, the movement of tectonic plates can be directly measured by a variety of geodetic technologies, including satellite laser ranging (SLR), Very Long Baseline Interferometry (VLBI), and Global Positioning Systems (GPS). Magnetic Island Formation, www. rzg. mpg. de/~sip/thesis/node58. html. Magnetic islands may form upon the flux surfaces for which the field lines are orthogonal to the wave vector of the perturbation since no energy is required to bend the magnetic field lines.
The sites show how an expression for the width of these islands is derived. The analytical calculation using this expression is found to be in good agreement with real space data. Island Formation, http://www. hawaii. edu/environment/ainakumuwai/html/ainakumuwaiislandformation. htm. The formation and evolution of Kaua’i, the oldest of the eight major Hawaiian Islands and a younger member of the Hawaiian-Emperor Volcanic Chain, are thoroughly studied. The Tethyan Himalayas, http://www. geoahead. com/strati/india/index. cfm? page=himalayas_tethyan. The site is devoted to the geology of The Tethyan Himalayas. The belt extending from Kashmir to Nepal can be best studied in two areas – Spiti valley in Himachal Pradesh and Kashmir – where we can see a continuous succession from Precambrian to Mesozoic ages. The Phanerozoic rocks have yielded rich fossils of trilobites, graptolites, brachiopods, cephalopods, gastropods, etc. Facts About Mountains, http://www. woodlands-junior. kent. sch. uk/Homework/mountains. htm.
This is the wide illustrated collection of basic facts about mountains, which includes the chapters as various as “What are Mountains? “, “Use of Mountains”, “How are Mountains formed? “, “Climate and Mountains”, “Types of mountains”, “Nature and Mountains”, “Tallest Mountains”, “People, Mountains and Tourism”, “Mountains around the World”, “Volcanoes”, “Mountain Ranges”. “Quick Facts about Mountains” are also included. Moon Has a Small Core Says LP Scientists,
http://www. spacedaily. com/news/water-99l. html. The site cites the data from NASA’s $63 million Lunar Prospector, which supports mounting evidence that the moon may still retain a small molten core, and was formed in a way unique within our solar system, after a Mars-sized planet smashed into a proto-Earth. It is noted that the new data agrees with Apollo mission seismic and sample-return evidence that suggests the moon is partly made of the same stuff as the Earth’s upper crust, or mantle.
And their findings agree with results released earlier this year by NASA Jet Propulsion Lab scientists who used Lunar Prospector to make a gravity map of the moon and who also conclude that the moon has a small, partially molten core. Works used J. Gribbin. Almost Everyone’s Guide to Science: The Universe, Life and Everything. Weidenfeld & Nicolson. London, 1998. Tilling, Heliker, and Wright. Eruptions of Hawaiian Volcanoes: Past, Present, and Future: Department of the Interior/U. S. Geological Survey Publication, Washington, D. C. , 1987. The web sites described above.