question: If the Earth used to be covered by ocean how did the continents rise up above the water, or did the water levels lower and expose the continents?
comment: "While it is uncertain why continental crust made its first appearance, the leading theory links the emergence of continents to the onset of plate tectonics. As Earth's land plates began to shift and move, less dense rock may have been forced upward, shaping the continents we see today on Earth's crust." URL_0
passages: The surface area of the oceans can change when continents rift (stretching the continents decreases ocean area and raises sea level) or as a result of continental collision (compressing the continents increases ocean area and lowers sea level). Increasing sea level will flood the continents, while decreasing sea level will expose continental shelves.
  
  Because the continental shelf has a very low slope, a small increase in sea level will result in a large change in the percent of continents flooded.
   The major geographical changes during this time period included the emergence of the Strait of Bosphorus and Skagerrak during glacial epochs, which respectively turned the Black Sea and Baltic Sea into fresh water, followed by their flooding (and return to salt water) by rising sea level; the periodic filling of the English Channel, forming a land bridge between Britain and the European mainland; the periodic closing of the Bering Strait, forming the land bridge between Asia and North America; and the periodic flash flooding of Scablands of the American Northwest by glacial water.
   If the world ocean on average is young, the seafloor will be relatively shallow, and sea level will be high: more of the continents are flooded. If the world ocean is on average old, seafloor will be relatively deep, and sea level will be low: more of the continents will be exposed.
  
  There is thus a relatively simple relationship between the supercontinent cycle and the mean age of the seafloor.
  
  BULLET::::- Supercontinent = lots of old seafloor = low sea level
  
  BULLET::::- Dispersed continents = lots of young seafloor = high sea level
   At times during Earth's long history, the configuration of the continents and sea floor has changed due to plate tectonics. This affects global sea level by altering the depths of various ocean basins and also by altering glacier distribution with resulting changes in glacial-interglacial cycles. Changes in glacial-interglacial cycles are at least partially affected by changes in glacier distributions across the a Earth.
   The depth of the ocean basins is a function of the age of oceanic lithosphere (the tectonic plates beneath the floors of the world's oceans). As older plates age, they become denser and sink, allowing newer plates to rise and take their place. Therefore, a configuration with many small oceanic plates that rapidly recycle the oceanic lithosphere would produce shallower ocean basins and (all other things being equal) higher sea levels. A configuration with fewer plates and more cold, dense oceanic lithosphere, on the other hand, would result in deeper ocean basins and lower sea levels.
   During colder climatic periods, more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age, glaciers covered almost one-third of Earth's land mass with the result being that the oceans were about 122 m (400 ft) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 5.5 m (18 ft) higher than they are now. About three million years ago the oceans could have been up to 50 m (165 ft) higher.
   At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia, Baltica, Siberia and Gondwana. During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods. This causes the sea level to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.
   Because of the large size of Panthalassa a hundred million years could separate the accretion of different groups of fusulines. Assuming a minimum accretion rate of , the seamount chains on which these groups evolved would be separated by at least —these groups apparently evolved in completely different environments.
  
  A significant sea-level drop at the end of the Permian led to the end-Capitanian extinction event. The cause for this extinction is disputed, but a likely candidate is an episode of global cooling which transformed large amount of sea-water into continental ice.
   As the surface continually reshaped itself over hundreds of millions of years, continents formed and broke apart. They migrated across the surface, occasionally combining to form a supercontinent. Roughly , the earliest-known supercontinent Rodinia, began to break apart. The continents later recombined to form Pannotia, , then finally Pangaea, which broke apart .
  
  The present pattern of ice ages began about , then intensified at the end of the Pliocene. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years. The last glacial period of the current ice age ended about 10,000 years ago.
  
  Section::::Precambrian.
   This appeared to be confirmed by the exploration of the deep sea beds conducted by the Challenger expedition, 1872-6, which showed that contrary to expectation, land debris brought down by rivers to the ocean is deposited comparatively close to the shore on what is now known as the continental shelf. This suggested that the oceans were a permanent feature of the Earth's surface, and did not change places with the continents.
  
  Section::::History.:Wegener and his predecessors.
   BULLET::::- c.620 Ma – Timanide Orogeny (to c.550 Ma) affects northern Baltic Shield: gneiss province divided into several north-south trending segments experiences numerous metasedimentary and metavolcanic deposits - last major orogenic event of Precambrian
  
  BULLET::::- c.600 Ma – Pan-African Orogeny begins: Arabian-Nubian Shield formed between plates separating supercontinent fragments Gondwana and Pannotia - Supercontinent Pannotia (to c.500 Ma) completed, bordered by Iapetus and Panthalassa oceans. Accumulation of atmospheric oxygen allows for the formation of ozone layer: prior to this, land-based life would probably have required other chemicals to attenuate ultraviolet radiation enough to permit colonization of the land
   Past hydrological changes on our planet have had enormous effects on the environment. Over most of geologic time, the long-term mean sea level has been higher than today. Only at the Permian-Triassic boundary ~250 million years ago was the long-term mean sea level lower than today. Long term changes in the mean sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. Twenty thousand years ago, the sea level was lower than at present with vast amounts of water being locked up as ice; in the northern hemisphere, there was twenty times more ice than the present volume. Before the abrupt desertification of North Africa about five thousand years ago, the Sahara was the home of Neolithic men and supported verdant vegetation and diverse wildlife.
   This order of sea-level change has yet to be fully explained. It was originally thought that glaciers controlled these sea-level changes. But glaciers form and retreat far too rapidly, only tens of thousands of years instead of over a million years. Instead, short-term changes in earth's surface due to volcanics and tectonic events could change global sea levels over a million years. This change to earth's shape could produce "bulges" or "sags" that contribute to ocean level fluctuations.
  
  Section::::Divisions.:Fourth order cycles.
   It is known that sea level is generally low when the continents are together and high when they are apart. For example, sea level was low at the time of formation of Pangaea (Permian) and Pannotia (latest Neoproterozoic), and rose rapidly to maxima during Ordovician and Cretaceous times, when the continents were dispersed. This is because the age of the oceanic lithosphere provides a major control on the depth of the ocean basins, and therefore on global sea level. Oceanic lithosphere forms at mid-ocean ridges and moves outwards, conductively cooling and shrinking, which decreases the thickness and increases the density of the oceanic lithosphere, and lowers the seafloor away from mid-ocean ridges. For oceanic lithosphere that is less than about 75 million years old, a simple cooling half-space model of conductive cooling works, in which the depth of the ocean basins "d" in areas in which there is no nearby subduction is a function of the age of the oceanic lithosphere "t". In general,
   Since the new oceanic basins are shallower than the old oceanic basins, the total capacity of the world's ocean basins decreases during times of active sea floor spreading. During the opening of the Atlantic Ocean, sea level was so high that a Western Interior Seaway formed across North America from the Gulf of Mexico to the Arctic Ocean.
  
  Section::::Debate and search for mechanism.
   The northern supercontinent of Laurasia began to break up, as Europe, Greenland and North America drifted apart. In western North America, mountain building started in the Eocene, and huge lakes formed in the high flat basins among uplifts. In Europe, the Tethys Sea finally vanished, while the uplift of the Alps isolated its final remnant, the Mediterranean, and created another shallow sea with island archipelagos to the north. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. India continued its journey away from Africa and began its collision with Asia, creating the Himalayan orogeny.
   The position of the continents determines the geometry of the oceans and therefore influences patterns of ocean circulation. The locations of the seas are important in controlling the transfer of heat and moisture across the globe, and therefore, in determining global climate. A recent example of tectonic control on ocean circulation is the formation of the Isthmus of Panama about 5 million years ago, which shut off direct mixing between the Atlantic and Pacific Oceans. This strongly affected the ocean dynamics of what is now the Gulf Stream and may have led to Northern Hemisphere ice cover. During the Carboniferous period, about 300 to 360 million years ago, plate tectonics may have triggered large-scale storage of carbon and increased glaciation. Geologic evidence points to a "megamonsoonal" circulation pattern during the time of the supercontinent Pangaea, and climate modeling suggests that the existence of the supercontinent was conducive to the establishment of monsoons.
   where "d" is in meters and "t" is in millions of years, so that just-formed crust at the mid-ocean ridges lies at about 2,500 m depth, whereas 50-million-year-old seafloor lies at a depth of about 5,000 m.
  
  As the mean level of the sea floor decreases, the volume of the ocean basins increases, and if other factors that can control sea level remain constant, sea level falls. The converse is also true: younger oceanic lithosphere leads to shallower oceans and higher sea levels if other factors remain constant.
   To reveal Eurasia's origins, Prof Iain Stewart climbs up to the "eternal flames" of Mount Chimaera and explains that where the South of Eurasia is today, there was once a ninety-million-square-kilometre Ocean known as the Tethys. Destruction of the Tethys Ocean led to Eurasia's formation. The freshwater fish called Karimeen, from the backwaters of Kerala in Southern India is a clue that India was once four thousand kilometres south of its current position on the other side of the Tethys, joined to Madagascar.
  
  Section::::Merchandise.
  
  A two-disc DVD of the series was released by the BBC on 1 July 2013.
   A type of basin formed by the moving apart of two pieces of a continent is called a rift basin. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is stretched and thinned, so that the hot asthenosphere rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of volcanic deposits. When the basin grows due to continued stretching of the lithosphere, the rift grows and the sea can enter, forming marine deposits.
   The total surface area of Earth is about . Of this, 70.8%, or , is below sea level and covered by ocean water. Below the ocean's surface are much of the continental shelf, mountains, volcanoes, oceanic trenches, submarine canyons, oceanic plateaus, abyssal plains, and a globe-spanning mid-ocean ridge system. The remaining 29.2%, or , not covered by water has terrain that varies greatly from place to place and consists of mountains, deserts, plains, plateaus, and other landforms. Tectonics and erosion, volcanic eruptions, flooding, weathering, glaciation, the growth of coral reefs, and meteorite impacts are among the processes that constantly reshape the Earth's surface over geological time.
   Modern geological knowledge rules out "lost continents" of any significant size. According to the theory of plate tectonics, which has been extensively confirmed since the 1970s, the Earth's crust consists of lighter "sial" rocks (continental crust rich in aluminium silicates) that float on heavier "sima" rocks (oceanic crust richer in magnesium silicates). The sial is generally absent in the ocean floor where the crust is a few kilometers thick, while the continents are huge solid blocks tens of kilometers thick. Since continents float on the sima much like icebergs float on water, a continent cannot simply "sink" under the ocean.
   The continental shelves are covered by terrigenous sediments; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers; some 60–70% of the sediment on the world's shelves is "relict sediment", deposited during the last ice age, when sea level was 100–120 m lower than it is now.
  
  Sediments usually become increasingly fine with distance from the coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore. These accumulate 15–40 cm every millennium, much faster than deep-sea pelagic sediments.
  
  Section::::Biota.
   Older references (e.g., Littlehales 1930) consider the oceanic basins to be the complement to the continents, with erosion dominating the latter, and the sediments so derived ending up in the ocean basins. More modern sources (e.g., Floyd 1991) regard the ocean basins more as basaltic plains, than as sedimentary depositories, since most sedimentation occurs on the continental shelves and not in the geologically-defined ocean basins.
   It is true that continental drift and seafloor spreading can change the shape and position of continents and occasionally break a continent into two or more pieces (as happened to Pangaea). However, these are very slow processes that occur in geological time scales (hundreds of millions of years). Over the scale of history (tens of thousands of years), the sima under the continental crust can be considered solid, and the continents are basically anchored on it. It is almost certain that the continents and ocean floors have retained their present position and shape for the whole span of human existence.
  