A light granitic crust that is about 40 km thick underlies the continents. A dense basaltic crust about 7 km thick underlies the oceans (fig. 4.2). These continental and oceanic crusts are in turn both underlain themselves by a mantle of denser material called the lithosphere. This lithosphere is about 70 to 150 km thick. It rests above a weak and very hot layer, the asthenosphere that becomes increasingly more viscous with depth.
Figure 4.2. The lithosphere plate of solidified rock floats on a molten or semi molten asthenosphere. In this schematic view, the lithosphere is rafting a continent carrying a continental crust towards a subduction zone. The depression will be terminated once collision occurs between two unsinkable continental crusts. At the fold, melting occurs. Volcanic chains add new strips of continental crust. Ridges occur in oceans.
4.4.1 Plate tectonics
At certain locations within the oceans, the basaltic crust supporting the oceans is not very thick. There, ridges exist where the earth has opened up. The inner plasma extrudes and slowly forms a new surface area. This expansion pushes the different oceanic and continental lithospheric plates away and apart. These rigid plates are thus in relative motion on top of a spherical earth. Several such plates exist, varying in size from the plate carrying the entire Pacific Ocean to the small plate that is coextensive with Turkey. Some of these plates are growing, as the entire African plate. This means that somewhere else, some surface area is destroyed at about the same rate at which new surface is created. The lithosphere, somewhere, sinks into the hot asthenosphere with the appearance of earthquakes at the upper bending. However, it is not possible for a plate under a continental crust to sink, because this light granitic crust is thick (about 40 km, versus only about 7 km for the dense oceanic basaltic crust) and buoyant. As a result, a continental collision terminates subduction and a major depression is in this way eliminated. New depressions form as a consequence somewhere else in the oceans.
The super continent Rodinia, made of all the continents, existed between 1200 and 600 million years ago. The Atlantic Ocean initially opened during the late Precambrian period, 600 million years ago. This was the time when chordates separated from insects (see fig. 5.1). North America and Africa drifted apart and the first Atlantic was inserted within the pit by the process of floor spreading. Glacial deposits indicate that a southern polar ice cap at one time covered the Sahara, while Eastern North America lay near the equator (fig. 4.3). This indicates that an ocean about 10,000 kilometers wide separated Africa and North America. Erosion began to act on the margins of these continents and sediments settled on the continental plains as well as in the seas.
Figure 4.3. The ancient universal landmass Pangea that existed 200 million years ago is reconstructed by fitting together the major continental landmasses. Mountain belts formed more than 260 million years ago, such as the Urals, China, Australian and Caledonian Mountains indicate lines of collision between continental fragments antedating Pangea. Such collisions explain how the Equator and South Pole of 440 million years ago were brought close together after the formation of Pangea. The reconstitution here drawn does not take into account the possibility that the earth was much less voluminous in earlier times. If the diameter of the earth was smaller in those days, then Australia and Southeast Asia should lie very close to each other. Such an hypothesis is sustained by paleontological evidence: primitive fauna and flora are found in these regions, that could not possibly have colonized these parts of the earth if they had not lain very close to each other.
Thereafter, the Atlantic closed again and Africa collided against America. The result of this is that crushed sea sediments formed the Appalachian Mountains on the American side, with the presence of volcanoes. Older than 200 million years are also the Caledonian, the Ural and the Australian Mountains. Africa and America were completely joined about 350 to 225 million years ago, forming a vast continent called Pangea. Wegener (German geologist 1880-1930) coined the name. His theory of continental drift, now universally accepted, met with hostility and was bitterly and brutally opposed during twenty years by his colleagues.
However, about 180 million years ago, at the end of the Trias period, the Atlantic opened again along the old suture line. This opening is proceeding further today at the rate of 3 to 10 cm per year (fig. 4.4). In the meantime, new sedimentary deposits are formed. These become so heavy in the long run that they depress the ocean crust and cause it to subside under the load. The depression then allows further sedimentation, until an overall thickness of about 15 kilometers is reached.
Figure 4.4. Pangea was almost completely broken up at the end of the Cretaceous period, 65 million years ago. North America was still linked to Europe and Australia was linked to Antarctica. A link may also have existed between Antarctica and South America.
Plate tectonics thus account for the collapse of the long prisms of sedimentary rocks that are laid down in the course of the ages along the edges of continents. These sedimentary rocks, containing many fossils, are crushed against the continents during a collision. In other words, the eroded continental shelves produce detritus that are lost to the oceans but these detritus are returned to the continents later on.
Plate tectonics provide in addition a means whereby the total volume of the continental crust increases with time. The moving tectonic plates have a leading edge. On this edge, the descending plate will undergo partial melting that will yield liquids, once the hottest regions of the asthenosphere are reached. These liquids will erupt and build volcanic chains. These volcanic rocks have, however, the same composition as the continental crust. Volcanic chains are thus the sites where strips of continental crust are generated. Yet, they lie on the leading edges of plates. Their destiny is thus to collide sooner or later with continental margins or perhaps other volcanic chains. In this way, new strips of light continental crust will be added to light continental margins. Yet, there are no means to destroy this light continental crust.
The complementary processes of accumulation of sedimentary deposits, on one hand, and the incorporation of igneous rock from volcanic origin on the other hand, mean that a complete inundation of the erosionally leveled continents will never come to pass. On the contrary, the total volume of continental crust has been increasing for the last 2 billion years. Since it increased by 2% during the last 200 million years, assuming a regular process, one may calculate that it increased by 20% during the last 2 billion years.
Mountain belts older than 600 million years are however different in composition than that of younger mountains, indicating that the sea-floor spreading happening 600 million years ago originated a different crust and mantle. Plates may indeed have been getting thicker and plate boundaries have become more narrowly localized with time. Mountain building older than 2 billion years is absent, so that one must assume another mechanism than plate tectonics for the evolution of the earth’s crust in earlier periods. Some regions of the continents contain rocks older than 2.4 billion years. These rocks are distributed in swirling patterns over areas so wide that processes arising at the boundaries of rigid plates can hardly explain them.
It appears thus that some areas of the earth’s crust were stabilized about 2.4 billion years ago. Four hundred million years later, a lithosphere developed that was endowed with sufficient rigidity to crack up into a plate mosaic. The driving mechanism for plate motion is itself obscure. It may be the thermal convection in the upper mantle. Another possible cause is the retarding effect of earth tides raised by the gravitational attraction of the moon. It may also be due simply to an increase in the volume of the earth that is assumed to have been, initially, 50% smaller than it now is.
The question is why Earth developed plate tectonics at all, because Mars and Venus do not exhibit this behavior. They display a rigid lid. Continents cover nearly one third of Earth’s surface and consist of buoyant material about 300 km thick that remains at the earth’s surface for billions of years. Buoyancy alone is not enough to stabilize 300 km thick continents for billions of years. A great viscosity is also necessary. The earth evolved into a layered body early in its history. It is generally believed that the core began forming soon after Earth accreted from the solar nebula. The heat produced by impacts during accretion was probably sufficient to cause large-scale melting of the planet. Molten iron would have separated from the surrounding mixture of oxides and silicates to form a dense liquid that sank to the center. A sizable inner core was formed about 2 billion years ago. This separation represents a global change of a colossal scale.
The descent of the liquid iron into the center of the Earth released enough gravitational energy to warm the planet by several thousand degrees. The density of the molten liquid center is 10% lower than the density of pure iron, indicating that the iron dissolved lighter alloying elements. Molten metal, mainly iron, descended to form the core while silicates and oxides formed the mantle. The exclusion to a large degree of light elements from the inner core provides an important source of buoyancy for convection. Planetary rotation promotes flows that generate a magnetic field. There is no evidence of a magnetic field on Venus. Mars once had a field, early in its history. The persistence of the magnetic field over geological time requires continuous regeneration because resistance losses can dissipate the field in about 10,000 years. Planetary rotation causes convective flows sufficient to overcome these losses.
The formation of a lithosphere and the subsequent initiation of the process of plate tectonics quite fundamentally allowed the emergence of living forms out of the primeval waters. The formation of isolated plates and isolated seas was in addition an occasion for mutated animal and plant groups to survive and fix their mutated traits. The extruding magma released also sometimes toxic substances such as sulfur, cadmium and other metals that could have had a tremendous influence on the survival possibilities of several groups and cause a mass extinction of some of the least resistant ones. Conversely, it may have been, as was the case with eukaryotic algae resistant to acids, the occasion for some otherwise repressed organisms to establish themselves more firmly.
During the past 500 million years (the Phanerozoic eon), there have been very significant changes in seawater. Ever since the 1977 discovery of hot springs along the crest of the volcanic midocean ridges, oceanographers have presumed that chemical reactions between seawater and hot ridge rocks would influence seawater composition. The more ridges there are, the faster they produce hot ocean crust, and the more altered brine will be produced. Plates drifting on the mantle have repeatedly merged into a single super continent that eventually breaks up. When there is a super continent, the total length of ridges is at a minimum; when the continents are dispersed, total ridge length peaks. And indeed the ratio of magnesium molecules versus calcium molecules has been rising and falling during the past half-billion years. The seawater ratio Mg/Ca was high (about 4) 550 million years ago, just before the Cambrian explosion of shelled animals. It dropped during the Paleozoic era to 1, which allowed the blossoming of the animals forming calcareous shells in the Cambrian explosion of life. It rose again 275 million years ago, during the Permian, when the ratio reached 4. It fell again to 1 during the Cretaceous of 120 million years ago. Today it is a high 5. These swings in the Mg/Ca ratio act as evolutionary gatekeepers. Corals and mollusks that build massive reefs came and went during the Phanerozoic depending on whether they were equipped to deal with a new Mg/Ca ratio. Likewise the carbonate-producing nanoplankton produced the massive White Cliffs of Dover only when the low Mg/Ca ratio favored them, 60 to 100 million years ago.
The initial relative thinness of the continental plates presented another selective pressure of momentous importance: the plates were quite easily submerged with water, be it either because the whole plate totally subsided or else tilted in one direction or another. This is very plausible, since the rivers of the Amazonian basin have been found to flow at one time in one direction and at another time in the opposite direction, indicating a tilt. Also frequent were rises in the sea level. Until a critical thickness was reached, floodings must have been frequent. As late as the Cretaceous period, a large part of North America was flooded and this is still by no means impossible now for such lands as Florida, which is flat country only a few feet above sea level. No doubt that such floodings were a bonus to sea-dwelling living-forms and a veritable catastrophe to all land animals, since it drastically reduced their vital space.
4.4.2 Flood volcanic events
There exist discontinuities of thickness and properties in the lithosphere, which allow the mantle to melt and rise buoyantly. This results in flood volcanism, which is an episodic process wherewith amounts of lava of the order of 106 cubic kilometers/ year are transferred from the earth’s interior to its surface during a period of 1 to 3 million years. Such events occurred about a dozen times during the last several hundred million years. The largest known continental flood basalt province, with a total volume of magma of 3x 106 cubic kilometers extending over an area of about 4 x 106 km2 now buried beneath 2 km of sediments appeared in Siberia at the end of the Permian, 250 million years ago. The Central Atlantic magma province appeared at the end of the Triassic, 200 million years ago and is now scattered across eastern North America, South America and Western Africa. The third biggest flood event is the Deccan trap, at the end of the Cretaceous.
The more voluminous a magma system is, the more likely it is to generate climate-modifying gases such as CO2 and SO2. The synchrony between flood volcanic events and mass extinctions has been noted for years. This connection is all the more intriguing in light of hints of evidence of large meteor impacts coincident with these events. The need for a geophysically plausible unifying theory linking all three phenomena is clear.
During the last 13.3% of the total lifetime of our planet, 3 major glaciations took place. These glaciations define 3 eras. The latest Ice Age, i.e. the Pleistocene glaciation, took place during the last 1 to 2 million years and may not yet be finished. Another series of cold waves occurred between 300 to 250 million years ago, during the Permian and Upper Carboniferous periods. This Permio-Carboniferous glaciation seems to have spread over some tens of millions of years. A third glaciation took place about 600 million years ago, just before the beginning of the Cambrian period. This Infra-Cambrian Ice Age may well have lasted as long as the Permio-Carboniferous one and surpassed it in geographical extent. Apparently, the whole earth froze down. This glaciation lasting from 750 million years ago to 580 million years ago, was interrupted by four short but intense heat waves due to the carbon dioxide accumulated by the volcanic activity. During the last 10 million years of this Ice Age, the earth was a ball of ice at a surface temperature of –50°C with only the deep oceans freed of ice due to the heat of the nucleus. Little is known about earlier glaciations; it seems that very probably at least two additional glaciations took place in earlier times.
18.104.22.168 The Great Glaciations
The great glaciations appear every 200 million years and last a few dozen million years. This is a cosmic phenomenon due to the rotation of the sun in the galaxy. The sun is located on the exterior of a spiral arm of the galaxy. It turns around the center of the galaxy with a speed of 470 km per second and makes a full turn in 250 million years. At a given moment, the sun travels through a cloud of interstellar dust. The attraction of the sun precipitates large amounts of dust on the solar surface and a transient decrease in the intensity of the solar radiations may ensue, that will provoke a cooling of the Earth. When it enters this region every 200 million years, the rays it emits are received in an attenuated way by the earth and a glaciary period will set in, for about 50 million years. We entered the cloud six million years ago, for the fifth time since glaciation records were begun, and this glaciation may last several million years more.
At least eleven great paleontological periods are recognized within the last three era defined by the great glaciations. Their appearances are also attributed to cosmic events. The sun lies in the galactic disc at a distance of about 30,000 light-years from the center of the galaxy. At that distance from the central bulb, the galactic disc is about 800 light-years thick. The sun does not maintain a constant place within this saucer. It travels periodically the thickness of the disc, up and down, on a width of about 600 light-years. Each period lasts 74 million years, which means that the sun crosses the central part of the disc every 37 million years.
Matter is much denser in the central plane of the galactic disc than in the outer layers and is not uniformly spread. When the sun crosses the central plane, it runs the risk of meeting destabilizing gravitational conditions resulting in a shower of metal-bearing comets (iridium, vanadium or others). One estimates that, only during the last billion years, the diameter of the earth increased by 12 km due to meteoritic contributions. The phenomenon is thus by no means negligible. The passage from the Trias to the Jurassic and from the Cretaceous to the Tertiary period is coincident with multiple meteoritic impacts18. The Earth also runs the risk of traveling through dense clouds of graphite, sulfuric acid, water, CO2 and other molecules, or a mixture of them. If it does, the induced destabilization is recognized as a new geological period. If it does not, the geological period continues unhampered for the next 37 million years or multiples thereof and this is also what the record shows to occur.
The beginning of the latest deposit of an ice cap in the Arctic occurred about 7 to 6 million years ago. Yet, seven million years ago, the African continent was pushing against Europe. The Mediterranean Sea that originally was a waterway connecting the Atlantic with the Indian Ocean dried up (figure 4.5).
Figure 4.5. Seven million years ago, most of Northeastern Europe was covered by a large lake that extended from the vicinity of Vienna to beyond the Aral Sea. This "Lac Mer" drained into the Mediterranean area and supplied water to several lakes there. The subsequent rise of the Carpathians shut off this supply and transformed the whole area into a hot desert. When the Gibraltar straits opened up, the Atlantic rushed in, filled the Mediterranean and the latest glaciation initiated.
It became a great depression filled with several lakes fed by European rivers. There also existed a great lake, the Lac Mer, in Northeastern Europe. By six million years ago, the formation of the Carpathians shut off the supply of fresh water from the Lac Mer and the Mediterranean became a hot desert. The water withdrawn from the pit piled up everywhere else and raised the level of all oceans by about 10 meters. About 5.5 million years ago, the Gibraltar straits opened up and the Atlantic rushed in. This sudden intrusion of water drastically changed the climate of Europe that turned from warm to cold and wet. It is then that the latest Ice Age began.
The disappearance and reappearance of the Mediterranean is a showcase for the demonstration of geographical isolation as well as evolutionary pressures. Marine animals had to adapt to high salinity, were isolated from the Atlantic and Indian oceans and had to withstand heat, all within 2 million years. The climate of Europe and North Africa also changed drastically while Europe became a prison for land animals.
22.214.171.124 Secondary glaciations
Besides the great glaciations, there exist secondary glaciations. For 800,000 years, five of them have occurred, with amplitude serious enough to cause trouble (the glaciations of Donau, of Günz, of Mindel, of Riss and of Würm). In addition to these five major glaciations, there exist a dozen smaller ones. The last Würm glaciation initiated 25,000 years ago and attained a maximum about 13,000 years ago. Suddenly, 12,000 years ago, there occurred an important thawing. The glaciary ice cap, which extended as far as England and South of Germany, regressed beyond Sweden. In the mean time, the level of the seas increased by 100 meters and an inundation of the Piedmont occurred. The minimum of glaciation was reached 6,000 years ago. This heralded the beginning of agricultural civilization. Since then, it seems that glaciations are on their way again. These five secondary glaciations are evidently a submission to a cyclic phenomenon. The influencing phenomena have been recognized.
The first is the movement of the earth around the sun. This movement passes from an almost perfectly circular orbit to an accentuated ellipse in 95,000 years. The second variation is the precession of the equinox, which is due to the joint action of the moon and sun on the equator of the earth. Through this joint action, the North-South axis of the earth is not fixed but oscillates upon itself in a cone of 47° of aperture. The induced movement is complex, with a calculated periodicity of 25,800 years. Finally, the axis North-South does not only oscillate in a cone but oscillates also with an angle of 23° vis-à-vis the equator. The earth is balancing slowly upon itself between a position that exposes preferentially the Northern Hemisphere to the sun’s rays towards a position where this hemisphere is less exposed. The balancing from one extreme position to the other lasts 41,000 years. For 800,000 years, the variations of temperature of the earth have very exactly followed the global fluctuations of insulation, as these were calculated following the above outlined variation parameters. According to these, we will witness a secondary glaciation period during the next 60,000 years.
Tectonic plates move at velocities of up to several centimeters per year. The melting of the major ice sheets during the past 15,000 years perturbs the current plate motion for the Antarctic, North American and Eurasian plates.
126.96.36.199 Transient cold waves
There exists a type of temperature variation that lasts only a few human generations.
A severe cooling event occurred 8200 years ago. It lasted a mere 37 years and hit Greenland, Europe, North America and Venezuela. The rapid commencement of the cold wave suggests a single catastrophic triggering event, which was the draining of the 4.7 x 1014 cubic meters of freshwater lakes Agassiz and Ojibway through the St. Lawrence into the Labrador Sea and the Atlantic Ocean, over 20 years.
In Antiquity and during the Middle Ages, the alpine glaciers were almost nonexistent. As a result, the invasion of Italy by Hannibal from the North was possible, as was possible also the penetration by Germanic, Celtic, Magyar or Hunnic tribes. Around the years 1600 of this era, the Alps were on the contrary almost uncrossable. The glaciers formed innumerable lakes, which did not fail to inundate, sooner or later, the valleys under them.
During the Carolus Magnus period (800 AD) the weather was cold. This may explain in part the appearance of the Vikings. Thereafter, reasonably good weather settled in Europe for the next 700 years, until the epoch of Charles the Fifth (about 1500). Suddenly, within 150 years, the weather became cold, to attain a minimum under Louis XIV, around 1650. A very short warm wave occurred 80 years later (1730) to fall off again very sharply during Napoleon’s Russian Retreat (1812). By 1880, the weather in Europe was mild again. This was manifest in French Literature and Painting, in the straw hats described by Maupassant and painted by Renoir (fig. 4.6).
Figure 4.6. P.A. Renoir. ‘The windmill of the Galette", fragment, 1876. Oil. Museum of Orsay, Paris.
Nowadays, it is cold again but heating rapidly up. These variations may be restricted to a region or to a continent and may find their roots in local or else global motives, or both (continental drift, solar wind, volcanic explosive eruptions, carbon dioxide, etc.).
The glaciers profoundly affect the evolution of life. Until about 20 years ago, the normal pace of a progressing glacier was about 100 meters a year. The progression of some of them could be much faster. The Variegated glacier in Alaska was known to progress at a speed of 65 meters a day during several months and the Alaskan Hubbard glacier was progressing at a speed of 34 meters a day. Similar surges were found in Pamir, Alaska, Spitzberg and the Andes and these phenomena may profoundly influence the ecological aspect of the surroundings. The Hubbard glacier, by advancing at such a speed into the Disenchantment Bay, had closed the Nautak and the Russell fjords. These were filled with rainwater and the level of the fjords had risen by 10 meters. Two hundred years ago, the level of the rainwater in the same fjords reached 30 meters above sea level. Entrapped algae and marine animals such as seals, dolphins, sharks, salmons, cephalopods, crabs and other were subject to a formidable adaptation challenge.
188.8.131.52 Sunspots and weather
Helium nuclei are negative particles that constitute a form of solar radiation, which profoundly affects the lower atmosphere of the earth. These particles shower the earth in the form of a solar wind that affects the ozone concentration and water vapor. The emission of these energetic particles by the sun occurs in sunspots and this activity has been observed to undergo cyclic changes that last for eleven years. A strong correlation exists between this solar cycle and the weather as I have shown in figure 1.10. Droughts and unusually long "growing seasons" i.e. number of days in the year on which the averaged temperature exceeds 5.6°C in central England (5° N; 3° W) are dependent on the solar cycle. Also, reduced winter rainfall occurs over parts of North Africa and North India, following periods of high solar activity.
Even the slightest temperature variations have the most dramatic effects on the evolution of Life. A few tenths of degrees of variation in the mean temperature signify gains or losses of dozens of millions of quintals of grain for a geographic region. All civilizations that emerged about 4,000 to 2,000 years ago were based on agriculture (Mesopotamia, Nile Valley, Memphis and Milwaukee, Mayas, Incas and Mexico, Ganges River, Yang-Tsi-Kiang River). For their emergence, very regular harvests were needed. A few days more or a few days less of rain or sunshine have a dramatic effect upon plant germination and fruit maturation. The agricultural civilization could emerge only where the stability of the meteorological conditions was sufficient and propitious for earnings that outdid the immediate needs of the farmer. It is only once it is firmly established that the newborn culture becomes able to undergo and withstand the stresses (droughts, famines) that endanger its life. For example, the Aztecs and the Incas could face drought but the Mayas were too fragile and disappeared. What is true for human cultures is also true at lower evolutive levels. The great reptiles of the Cretaceous period were probably wiped out because of an unbearable temperature stress.
In 1972, a slide into the next glacial seemed imminent. Based on geological records then available, the last two interglacials were believed to have lasted about 10,000 years each. This is the length of the current warm interval, the Holocene, to date, and it was concluded that the present-day warm epoch would terminate relatively soon. Today, a different future is depicted. Even without human perturbation, future climate may not develop as in past interglacials. A long interglacial is now foreseen, that may last another 50,000 years.
18. J. Spray et al.: Evidence for a late Triassic multiple impact event on Earth. Nature 392, 171-173, 1998, and Alvarez et al.: Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208, 1095-1108, 1980