The view of the world that includes most of the land, still contains a large share of the oceans. In this part of the globe, live over 90% of all people. It is also where most environmental problems are felt. In the centre one sees the Atlantic Ocean and east of Africa the Indian Ocean, looking much smaller due to the curvature of the globe.

The view of the water world, contains very little land and few people. The main continents of North and South America and Asia, are just visible around the edges. New Zealand is the continent, that lies closest to the centre of the water world, the Pacific Ocean. West of Australia the Indian Ocean.

Because most of the land is found in the Northern Hemisphere, the two hemispheres behave very differently in response to annual variation in solar radiation. The stabilising effect of the oceans makes that the Southern Hemisphere has smaller temperature differences between winter and summer, resulting in less wind and more temperate winters and summers (See Temperature, below). It may also explain why the large Pacific Ocean is more tranquil than the smaller Atlantic Ocean.
 
 

Distribution of surface area Most maps of the planet show height contours. These are hypsographic maps (Gk hypsos = height). When all height contours of both sea and land are put together, the hypsographic curve results as shown here. It gives an immediate overview of the distribution of height and depth over the planet. Horizontally is the surface area of the planet as a percentage of the total (5.2 million km2). The seas occupy just over 70% and the land just under 30%. The amount of land above sea level (brown) is very much less than the volume of sea (blue). If the land was spread into the sea, the oceans would still be 3000m deep! The highest point on land is Mount Everest (8848m). The deepest point in the ocean is in the Mariana Trench (11034m)

The diagram gives us an idea of the general shape of the ocean basin. Only a very small part goes deeper than 6000m and the deepest troughs of 10km are hardly visible here, likewise for the land and its tallest mountains. The most important bits are almost invisible on this diagram: the land up to 500m where most of the people live and the sea down to 200m where most of the ocean's productivity is found. From this small margin of about 12 + 5 = 17%, the world population feeds itself. Click here for a larger diagram.
 
 

In the lefthand margin of the hypsographic curve above, a histogram is shown of the distribution of height of Earth's surface, and this curve is shown in more detail here. The zero axis (red line) is sea level but not average height. Since the Magellan space probe mapped the landscape of Venus accurately in 1990, the two surface areas can be compared. The red curve is that of Venus, centred around its average height because Venus has no oceans. It is a bell shaped curve as can be expected from any planet, but it differs remarkably from Earth's. Either the oceans have carved a deep dent into the smooth bell of Earth's crust, or we must assume that the crust consists of two separate parts, the continents and the ocean basins. As we shall see later, this is the case.

 

But all the time that the earth formed, it was too hot for an ocean. In fact, the oldest rocks found are about 3800 million years (My) old (Rocks from the moon 4550 My), whereas the age of this planet is estimated at 4600 million years. By the time the earth had cooled enough, most of the cosmic impact that formed it, had also ceased. Why is it that the only planet with life is also the only planet with an ocean? And why is it that other planets, that were formed in the same way as ours, do not have oceans?
 
 

Co-evolution of climate, oceans and life Independent scientist James Lovelock, as part of his Gaia hypothesis, made us look at the world as if it were a single organism. Lovelock observed that life always changed the conditions of its own existence. Life evolved to make life better and its environment more suitable for life itself. Organisms steering the other way would simply not survive. As a result of evolution, the planet acquired the atmosphere we know now, and an ocean. As part of its ability to change its environment, life has also acquired primitive means to regulate the temperature of this planet (green sawtooth line). Otherwise, due to increasing solar radiation, the temperature would at least have been 50-60ºC (orange dotted line).

The idea is that a small amount of water formed after cooling (the archean sea, perhaps 100-300 m deep in small seas here and there, the amount of water necessary for changing CH4 to C to CO2 and NH4 to N2 and SO4 to S, and an atmosphere consisting largely of nitrogen, carbon dioxide, methane and hydrogen - a mixture lethal to organisms alive today. Imagine this ocean as a shallow, warm, acidic, black cesspool full of stinking bacteria. The water for this ocean came from the earth's crust, and from sunlight dissociating CO2 in the upper atmosphere. The formed oxygen could then bind with the escaping hydrogen to form water. The warm ocean evaporated much water vapour and it rained torrentially. Because no living organisms were found on the land, erosion must have been very intense, resulting in enormous amounts of run-off, containing sediment and nutrients.

For more water to form, the hydrogen, escaping from the planet, had to be bound with more oxygen. But oxygen was available only as carbon compounds in carbon dioxide. Only life, in the act of photosynthesis, is able to split carbon and oxygen from carbon dioxide in large quantities. The carbon that formed, was buried underground and for each atom of carbon, two molecules of water were formed. The cyanobacteria capable of doing this, formed about 3600 My ago. By 2500 My ago, the oceans would have formed and the atmosphere made suitable for higher organisms than bacteria, the prokaryotes. The light blue band in the diagram is perhaps all it took to create the bulk of our oceans. By 570 My ago, the Cambrian period started, with plants and moving animals in recognisable forms. By that time, the atmosphere had evolved much to what we have today.

As scientists are analysing ratios of elements in ancient rocks, they are piecing together evidence for the composition of Earth's ancient atmosphere and its temperature, and with it the answer on how our atmosphere, climate and oceans formed.

Blue-green algae, also called cyano-bacteria, invented photo-synthesis some 3600 million years ago. They were also the first living things to make oxygen, and were the first to learn to breathe it and not get damaged by oxygen's free radicals. This cyanobacterium is Microcoleus chthonoloplastes, living on arid shorelines around the world.

Eoastrion (the dawn star), was found in the chert (silica mineral) of the Gunflint formation. Now extinct, this fossil is nearly 2000 million years old. It is amazing that such fragile life forms have been embedded into hard crystals that took a very long time to grow, and that their forms have been preserved with such detail.

References, available from the Seafriends Library:
Allaby, Michael: Guide to Gaia. 1989.
Gribbin, John: Hothouse Earth, the Greenhouse Effect & Gaia.1990.
Joseph, Lawrence E: GAIA, the growth of an idea.1990.
Lovelock, James E: GAIA, a new look at life on earth. 1987.
Lovelock, James E: The ages of GAIA.1988.
Schneider, Stephen H and Randi Londer: The Co-Evolution of Climate and Life.1984.

Tectonic plates As recently as 40 years ago, continental drift became an accepted theory. Since then, scientists in many disciplines have found overwhelming evidence of the way continents move. One of the puzzles was that on the ocean floor, the rocks never seemed to grow older than about 100 million years, whereas on land they have been dated back to 3800 million years. Mysterious ridges exist in the centres of the Atlantic Ocean and the Pacific Ocean. Along 'rims of fire', mountain ridges and active volcanoes are found. Similar rocks, fossils and organisms are found on opposite sides of oceans. It wasn't until all these facts were brought together that scientists realised that the seafloor is gradually disappearing and that the continents are drifting.

The map shows the tectonic plates relative to the positions of the continents. The red dotted lines are mountain ranges and rims of active volcanoes. Here the plates collide, causing mountains to rise and volcanoes to spew liquid lava. By contrast, along the mid-ocean ridges, the seafloor is spreading, creating new oceanic crust.
 
 

The diagram shows how oceanic crust is produced in areas where the seafloor is spreading, while it is pushed underneath continents (subducting) elsewhere. On a continuous basis new ocean crust is created while thousands of kilometres further, the crust is subducted and absorbed by the hot mantle. The Earth's soft mantle consists of hot, semi-liquid stone, which circulates very slowly by way of convection currents. Where these currents move upward, undersea volcanic activity occurs and lava pours out, immediately solidifying in the deep sea. As the crust spreads, it cools further, shrinks somewhat, breaks and slumps into ridges that run parallel with the spreading zone. The speed of plate movement is about equal to a growing fingernail  but may differ from place to place (3-15 mm/year). On the ocean floor no rocks have been found older than 180 million years.

The rocks start to weather and erode, depositing ocean sediment on top of the ocean crust. Light particles from the land, such as clay are often blown far out into sea and settle on the sea floor. Sea organisms with hard skeletons rain down from the plankton-rich layers above. Metals such as manganese, form nodules on the sea bottom. The ocean sediments are a mix of many of such ingredients.
In the subduction zone, the ocean crust is pushed underneath the continent, gradually melting back into the mantle. In the process, sediments from the land are drawn under too. Due to heat from compression, some of the material may melt a path through the continental crust, causing volcanic activity. New continental crust is formed.

The rightmost diagram in above picture shows how both continents and ocean crust float in balance on the mantle. The continental crust, which was formed before the oceans, consists of granite, a silica-alumina (SiAl) rock and is 30-40 km thick. It is lighter (density 2.84) than the mantle (density 3.27). The weight of the continent equals the displaced weight of the mantle. The ocean crust, about 11 km thick, is made of  silica-magnesia rock (SiMa) and is heavier (density 3.00) than the continental crust but lighter than the mantle. It floats also in balance on top of the mantle: water (density 1.03) + sediment (density 2.30) + crust (density 3.00) = displaced mantle (density 3.27).
 
 
 

A new view of Earth's interior In order to explain very deep earthquakes 300-600 km down, scientists have begun to think that the whole mantle is solid. Movements within it occur due to the rock changing its structure under the influence of heat and pressure. (The pressure at 100km depth exceeds 30,000 times atmospheric pressure). The rocky material which comprises the lithosphere of the earth is special, because the rocks contain water, which is trapped within the crystal structure of the rocks themselves. These special 'hydrated minerals' have the ability to slide against each other. On other planets, where the minerals of the lithosphere do not contain water, the lithosphere does not flow and hence these planets do not have plates moving on the surface.  As the crust is pushed down into the mantle, the rock collapses into different crystal structures, producing earthquakes in the process. Likewise, in convection zones, the rock can expand and 'flow' in a similar way. The drawing shows the various layers now thought to exist within our planet. The heavy iron core is thought to consist of a solid part in the very centre, surrounded by a molten outer core. The lower mantle, consisting of oxides and silicates, may also be less liquid than originally thought. The upper mantle which is more finely layered as shown on right, consists mainly of magnesium-iron silicates (Mg, Fe)2 SiO4 in various crystal forms. How is it possible that the inner part of the Earth, thought to be 4800 degrees C, is solid? At this temperature, iron is well and truly liquid. The answer must be sought in the two opposing forces that increase with depth: temperature and pressure: whereas temperature makes stuff more liquid, pressure does the opposite. The heat in the core is produced by radioactive heavy elements. For instance, Uranium-238 has a half-life of 4.5 thousand million years (Gy), so about half of it is still left over since Earth formed. Thorium-232 has a half life of 14 Gy, so the furnace deep inside the planet will be producing heat for a very long time to come. Heat radiates out, while cooling in the process. Whereas heat makes matter liquid, pressure, which increases formidably, tends to make matter solid. These two opposing forces cause alternating zones of solids and liquids. A pressure of 3.5 million bar as it exists in the core, is very hard to imagine. A car tyre contains 2 bar pressure (200 KPa); a dive cylinder 200 (20MPa); most concrete cracks at 500 bar (50 MPa). (Source: H W Green II: Solving the paradox of deep earthquakes. SciAm Sep 1994)

Vertical movements of continents due to ice ages During the ice ages, the ice was in places over one km thick, causing the continents to sink further into the mantle. After the ice ages, the continents slowly bobbed up again, a process that takes thousands of years, and that is still happening today in some places.The map shows clearly how Canada, Greenland and Scandinavia are still rising from the loss of the heavy glaciers some 20,000 years ago. In the process of continents tipping, some places are moving down. For example, Holland is sinking, which the Dutch are very well aware of.  (Source: W R Peltier & A M Tuschingham, Univ Toronto in David Schneider: The rising seas, SciAm Mar 1997)

It is known that sound travels faster in a denser medium. From measurements of the speed of sound, the densities of the various layers could be calculated. Underneath the continental crust, the Mohorovicic Discontinuity or Moho (Yugoslav geologist Andrija Mohorovicic, 1906) was discovered, marking the boundary between the solid crust and the upper mantle at about 40km depth.
In 1926, the German scientist Beno Gutenberg, discovered another discontinuity, about 150 km deeper down under continents and 70 km under the sea, marking the boundary with the deeper plastic mantle. It is now accepted that the tectonic plates extend 70 km deep, the lithosphere (Gk lithos = stone),  which float on a softer mantle, the asthenosphere (Gk asthenes = weak).
 
 

How did the continents drift? Why the continents suddenly started to drift about 300 million years ago (The age of Earth is about 4500 million years), is a puzzle (also see box). Until that time, the continents formed one land mass, named Pangea (Greek pan, pas = the whole; geo = earth). The first split between the northern half, now named Laurasia (Europe, North America and Asia), and the southern half, named Gondwanaland (South America, Africa, India, Antarctica, Australia and New Zealand), began 300 million years ago (300Mya). It is possible that the gap between the two continents was sufficiently large to form the first circumglobal sea, allowing ocean currents to travel around the world, resulting in a warm equitable climate everywhere. In any case, the seas that formed between the splitting continents, were warm and rich in nutrients and marine life, laying down all the mineral oil we now mine. These changes must have been accompanied with severe climate changes and changes in vegatation, accompanied by massive erosion. Click here for a larger version of this map.

On the map one can see how the continents travelled, propelled by ocean plates that have changed shape enormously. The red curves are areas of collision, whereas the blue lines are those where the seafloor was spreading. Where the continents collided, high mountain ridges were formed, such as in Europe (colliding with Africa): The Pyrenees, the Alps and more. An interesting case is India which travelled all the way north to collide with Asia, forming the Himalayan mountains. Australia broke away from Antarctica at a later stage, pulling New Zealand with it. But how did scientists puzzle this story together?

The secret lies in a weak property of rock: its magnetic field. Inside many rocks is found the element iron, a very common element on this planet. Iron and some of its oxides can be magnetised by the magnetic field of Earth. This won't happen as long as the rock is liquid, but by the time it cools sufficiently to become rock, the Earth's magnetic field is 'frozen' in place. Geologists drill deep sampling cores and analyse the magnetic field orientation. To make matters worse and easier at the same time, the magnetic poles have reversed several times and they have been wandering around somewhat. So the data needs to be corrected by what is known about the oscillations in the Earth's magnetic field. But at the same time, the field reversals are also convenient time markers to age the layers in the core samples.
 
 
 

The continents may have been drifting for much longer New evidence with respect to tectonic plate movements, suggests that the continents may always have been drifting apart, then together again. When continents collide, they form folded mountain ranges. A number of such ranges could be explained only by assuming that the continents have collided once or twice before. A difficulty in studying old mountain ranges is that they have all but completely eroded away. But as scientists are drilling more holes, their knowledge pieces together the continental crust movements dating back to 500 million years. In the drawing of the world map with all continents joined together, the red mountain ranges could be explained by subduction of ocean plates, uplifting the continents at their margins. Yet some of these ranges are folded extensively. The purple ranges are all folded and are thought to have occured by the continents colliding towards the Pangea configuration. On left the mountain range from Ouachita belt, through Appalachian belt and the Caledonian belt over Iceland, Scandinavia and Greenland. The Hercynian belt runs through  the Pyrenees and the Alps. The Uralian belts criss-cross through Asia and Siberia. This map shows continental movement back to 500 Mya, where the history of Pangea began, when continents were dispersed around the Iapetus Ocean. About 400 Mya, Laurentia (America) collided with Baltica (Europe) to form the super continent Laurasia. In the process the Apalachians, Iceland, Scandinavia and Greenland mountain belts were formed. The Iapetus Ocean vanished when Laurasia fused with Gondwana (all the other continents), creating Pangea (300 Mya). The mountain belts of the Atlas were formed and the Urals between Siberia and Europe and the many mountain belts in Asia. The plate motions changed as Pangea dispersed. North America moved north, then west away from Eurasia (180 Mya). In a cycle of 500-600 My, the continents may have been dancing to and fro for over 1500 million years. As scientists gather more data, this puzzle may be pieced together more accurately. (See also the Geologic Time Table)  (Source: J Brendan Murphy and R Damian Nance: Mountain belts and the supercontinent cycle, Sci Am Apr 1992 p34-41

A possible mechanism But what could the mechanism for such movement be? A simple explanation is shown in the four drawings on right. It is generally accepted that the crustal movement is caused by convection currents, originating from differences in temperature between spots in the liquid mantle. The Earth cools its interior by leaking heat to the outside. Where the crust is thin, such as underneath oceans, heat is lost more easily, resulting in a cooler area. But since continents are four times thicker, they also insulate better. A hotspot could appear underneath a large continental slab. As the solid mantle heats up, the coninent is bulged up and cracks. It allows a convection zone to form, pushing the broken halves apart and ending up as a spreading mid-ocean ridge. At some other place on the globe, continents ae pushed together again, and a new hot spot is formed, while the old hot spot shrinks. Continents break up again and reverse their travel.

Growth and formation of continental crust One of the remaining problems is that land erosion happens so fast, that all the continents should have disappeared below sea level, a very long time ago. Particularly in the azoic (no-life) era (till 2.5 eons ago) without any cover on the land, erosion would have been very high. By measuring the very low concentrations of the rare-earth elements (EER or Lantanides, see the Periodic Table of Elements), geologists discovered that the sedimentary rocks, originating from mud washing into the oceans, represent a very good average of the composition of the continental crust. The oldest mineral (zircon) is found in sedimentary rock in Australia and is 4.2 eons old. In north-west Canada, the Acnasta Gneiss formation yielded the oldest rock (granite) that was not formed from sediment.  Now that a very large number of measurements have been made on sedimentary rocks of various ages, the actual volume of the continental crust can be plotted. In the figure on right, the bottom graph shows how the volume of crust has been growing, sometimes slowly, sometimes more rapidly. The time scale is in eons (thousand million years). Almost immediately after Earth accreted from particles and meteors, a small amount of crust was formed and during the first eon, this amount did not increase very much (erosion was very high). Only when the ocean plates started moving, was continental crust formed, first fast because the interior of Earth was hotter, then more slowly. The crust that formed in the first episode of fast growth (granite?), is different from later rocks, perhaps due to  the possibility that oceanic crust was recycled much faster than today. (due to the presence of a deep ocean??). The top diagram shows a slab of oceanic crust subducting under a continent. By pushing sediments up, the continent grows from accretion. It also acquires more volume from volcanoes. It is now thought that sedimentary rock melts, not only from friction but mainly from the amount of water it contains, which acts as a flux additive in a foundry, inducing first the sedimentary rock to melt and then the continental rock as it moves upward to the surface. Magma chambers do not always reach the surface but can convert to granite, which is included as part of the continental crust. Current oceanic crust forms mainly by the eruption of basaltic lava along a globe-encircling network of mid-ocean ridges. More than 18 million cubic km of rock are produced this way, each year. The simple concept of continents made from granite, formed when the mantle was liquid, while staying the same size, needs to be changed in the light of these new findings. Note that the first rapid growth of continental crust suggests that oceans were formed just before 3 eons ago. Source: S R Taylor & S M McLennan: The evolution of continental crust. Sci Am Jan 1996. p76-81

Naming the features of the ocean basins In this drawing of a typical ocean profile, the vertical scale has been exaggerated, to better illustrate the ocean's shape.Where the land meets the water, continents start with a gradual slope down to about 200m depth. It is called the continental shelf. For various reasons, this is the most productive part of the ocean. Beyond the continental shelf, the seafloor dips down more steeply (the continental slope) until it becomes more gradual again (the continental rise). Some continents are flanked by deep trenches. Where the continental rise ends (a vague boudary), the abyssal plane starts (Gk a = no; bussos = depth; bottomless). Where the ocean's floor is spreading, an ocean ridge is found, flanked by large fracture zones.

Main ocean trenches a Aleutian b Kuril c Japan d Mariana e Philippine f Java g Tonga, Kermadec h Peru-Chile

Main mountain ridges 1 Andes 2 Atlas 3 Alps, Apenines, Carpathians 4 Caucasus 5 Turkey, Iran, Afganistan 6 Himalayas 7 Tienshan, Sayan, Stanovoy 8 Rocky Mountains

Oceans and seas of the world

Oceans A Arctic Ocean B Atlantic Ocean C Indian Ocean D Pacific Ocean E Southern Ocean * Enclosed or almost enclosed seas a Baltic Sea b Mediterranean Sea c Black Sea d Red Sea e Persian Gulf f Gulf of Oman Large coastal seas 1  Gulf of Mexico 2  Caribbean Sea 3  Greenland Sea 4  Norwegian Sea 5  Barents Sea 6  Kara Sea 7  Laptev Sea 8  East Siberian Sea 9  Arabian Sea 10 Bay of Bengal 11 Bering Sea 12 Sea of Okhotsk 13 Sea of Japan 14 East China Sea 15 South China Sea 16 Philippine Sea 17 Indonesian Archipelago 18 Coral Sea 19 Tasman Sea 20 North Sea Click on the image for a larger map  (*) The boundaries of the Southern Ocean are not well defined