Visions of the Cosmos

Life Among the Stars

Plate Tectonics

Alfred Wegener (1880-1930)

 The Father of Plate Tectonics

Some truly revolutionary scientific theories may take years or decades to win general acceptance among scientists. This is certainly true of plate tectonics one of the most important and far-ranging geological theories of all time. When 'Continental Drift' the forerunner of the now well established science of plate tectonics was first proposed, it was ridiculed, but steadily accumulating evidence finally prompted its acceptance, with immense consequences for geology, geophysics, oceanography, and paleontology. Although it had been suggested by Taylor and other earlier scientists, the man who should be given the title of 'The Father of Plate Tectonics' was a brilliant interdisciplinary scientist, Alfred Wegener.

Born on November 1, 1880, Alfred Lothar Wegener earned a PhD in astronomy from the University of Berlin in 1904. However, he had always been interested in geophysics, and also became fascinated with the developing fields of meteorology and climatology. During his life, Wegener made several key contributions to meteorology: he pioneered the use of balloons to track air circulation, and in 1911, wrote a textbook that became standard throughout Germany. In 1906 Wegener joined an expedition to Greenland to study polar air circulation. Returning, he accepted a post as tutor at the University of Marburg, taking time to visit Greenland again in 1912-1913. (The photograph of Wegener on the right was taken during this expedition). In 1914 he was drafted into the German army, but was released from combat duty after being wounded, and served out the war in the Army weather forecasting service. After the war, Wegener returned as a lecturer to Marburg, but became frustrated with the obstacles to advancement placed in his way.  He was finding it very difficult to obtain a professorship in any German University because of his theory of continental drift which was at the time considered to be 'scientific heresy'.

While at Marburg, in the autumn of 1911, Wegener was browsing in the University Library when he came across a scientific paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic. Intrigued by this information, Wegener began to look for, and find, more cases of similar organisms separated by great oceans. Orthodox science at the time explained such cases by postulating that land bridges, now sunken, had once connected far-flung continents. But Wegener noticed the close fit between the coastlines of Africa and South America. Might the similarities among organisms be due, not to land bridges, but to the continents having been joined together at one time?

Such an insight, to be accepted, would require large amounts of supporting evidence. Wegener found that large-scale geological features on separated continents often matched very closely when the continents were brought together. Wegener also found that the fossils discovered in a certain place often indicated a climate utterly different from the climate of today: for example, fossils of tropical plants, such as ferns and cycads, were found on the Arctic island of Svalbad or Spitsbergen. All of these facts supported Wegener's theory  which he called continental Drift In 1915 the first edition of The Origin of Continents and Oceans, a book outlining Wegener's theory, was published; expanded editions were published in 1920, 1922, and 1929. About 300 million years ago, claimed Wegener, the continents had formed a single mass, called Pangaea (from the Greek for "all the Earth"). Pangaea had rifted, or split, and its pieces had been moving away from each other ever since. Wegener was not the first to suggest that the continents had once been connected, but he was the first to present extensive evidence from several fields.

Modern reconstruction of Pangaea, ca. 255 million years ago.

Reaction to Wegener's theory was almost uniformly hostile, and often exceptionally harsh and scathing.  Many speakers were sarcastic to the point of insult.   Part of the problem was that Wegener had no convincing mechanism for how the continents might move. Arthur Holmes another famous maverick geologist supported Wegener and in fact suggested mechanisms similar to sea-floor spreading which is accepted to-day as part of the plate tectonics theory.  There were other scientists who supported Wegener: the South African geologist Alexander Du Toit supported it as an explanation for the close similarity of strata and fossils between Africa and South America, and the Swiss geologist Émile Argand saw continental collisions as the best explanation for the folded and buckled strata that he observed in the Swiss Alps. Wegener's theory found more scattered support after his death, but the majority of geologists continued to believe in static continents and land bridges.

What prompted the revival of continental drift? In large part it was increased exploration of the Earth's crust, notably the ocean floor, beginning in the 1950s and continuing on to the present day. By the late 1960s, plate tectonics was well supported and accepted by almost all geologists.)

In 1930 Wegener mounted another trip to Greenland.  The expedition reached a place which they named Eismitte at an elevation of about 3000 meters (about 10,000 feet).  There on November 1 they celebrated Wegener's fiftieth birthday.  Because supplies were low Wegener and his team member  Rasmus Villlumsen set out for the coast.  Their friends never saw them again.  On 12 May 1931 Wegener's frozen body was found fully dressed in his tent.  His eyes were open and the expression on his face was calm and peaceful.  The picture on the right courtesy Wegener Institute of Wegener with an Inuit companion was taken on the fatal expedition to Greenland where Wegener met his death. (Photo - Courtesy Wegener Institute.)

By 1930 Wegener's theory had been almost universally rejected and sank into obscurity.  It was not until the 1960s that his ideas began to receive acclaim.  Despite some differences in detail Wegener was proved right in all his major concepts. We now know that Wegener's theory was wrong in one point: continents do not plow through the ocean floor. Instead, both continents and ocean floor form solid plates, which "float" on the asthenosphere, the underlying rock that is under such tremendous heat and pressure that it behaves as an extremely viscous liquid. This is why the older term "continental drift" is not quite accurate -- both continents and oceanic crust move.

Plate Tectonics was born and Alfred Wegener's work was acclaimed as one of the greatest advances in the Earth Sciences.  It is a vindication of his life that an important Institute is named after him.  In my opinion Wegener's name should rank among the greatest along with Galileo, Newton, Darwin and Einstein for his contribution to science.

Since Wegener's day, scientists have mapped and explored the great system of oceanic ridges, the sites of frequent earthquakes, where molten rock rises from below the crust and hardens into new crust. We now know that the farther away you travel from a ridge, the oolder the crust is, and the older the sediments on top of the crust are. The clear implication is that the ridges are the sites where plates are moving apart. Where plates collide, great mountain ranges may be pushed up, such as the Himalayas; or if one plate sinks below another, deep oceanic trenches and chains of volcanoes are formed. Earthquakes are by far most common along plate boundaries and rift zones: plotting the location of earthquakes allows seismologists to map plate boundaries and depths. Paleomagnetic data have allowed us to map past plate movements much more precisely than before. It is even possible to measure the speed of movement of continental plates extremely accurately, using satellite technology. Nevertheless, Wegener's basic insights remain sound, and the lines of evidence that he used to support his theory are still actively being researched and expanded.

Although Wegener's great achievement was not fully appreciated in his lifetime he was a appreciated for his work in meteorology and has been honoured in his own country and throughout the world by having an Institute named after him.  The photograph on the left is by courtesy of the Wegener Institute

Stiftung Alfred-Wegener-Institut fuer Polar- und Meeresforschung

The Alfred Wegener Institute

Germany's leading institute for polar and marine research  

It was established as a public foundation in 1980  The Institute conducts research in the Arctic, the Antarctic and at temperate latitudes. It coordinates Polar research in Germany  Given the major role played by these regions within the Earth's climate system, global change is the central focus of the research effort of the Institute.  The Institute collaborates closely with international research programmes and maintains close contacts with many universities and institutes in Europe and farther afield. It sends scientists to other institutes throughout the world, to other research ships and stations, and invites scientists from other nations to cruises aboard the research ship Polarstern (Pole Star)  About a quarter of quarter of those participating in Polarstern expeditions are scientists from abroad. 

 Plate Tectonics - Modern Aspects

Plate tectonics deals with the dynamics of the Earth's outer shell which is called the CRUST or LITHOSPHERE.  The lithosphere consists of about a dozen large plates and several smaller ones.  The continents sit on the plates.  The plates rest on the semi-molten asthenosphere of magma and move slowly across the asthenosphere relative to each other. 

The lithosphere is the rigid outer layer of the Earth. It differs from the underlying asthenosphere in terms of its mechanical (or rheological, ie, 'flow') properties rather than its chemical composition. Under the influence of the low-intensity, long-term stresses that drive plate tectonic motions, the lithosphere responds essentially as a rigid shell whilst the asthenosphere behaves as a highly viscous liquid.   The weaker mechanical properties of the asthenosphere are attributable to the fact that, within this part of the upper mantle, the temperature lies close to the melting temperature. The base of the lithosphere is conventionally defined as the 1,300  degrees Celsius  isotherm since mantle rocks below this temperature are sufficiently cool to behave in a rigid manner

 Plates can drift apart (sea floor spreading), push into each other or slide past each other.

Sea Floor Spreading

The ocean plates diverge from each other and produce a ridge of under ocean volcanoes, such as the mid-Atlantic Ridge. This produces sub-marine volcanic mountains the tops of which form islands such as the Canaries, Iceland the Azores etc.   This phenomenon is called sea floor spreading   Along the mid Atlantic ridge, there are diverging plates (North American and Eurasian). There are many earthquakes along the ridge. They are moving apart and therefore causing North America and Eurasia AND South America and Africa  to drift further and further apart. Picture two giant conveyor belts, facing each other but slowly moving in opposite directions as they transport newly formed oceanic crust away from the ridge crest. There are many earthquakes along the ridge.  A huge earth quake occurred in 1755 in which Lisbon was destroyed partly by the earthquake and partly by the tsunamis and fires which followed it.
The Mid-Atlantic Ridge is known as a divergent boundary.  There is a submerged mountain range, extending from the Arctic Ocean to beyond the southern tip of Africa. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 cm’s per year, or 25 km in a million years. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that it is today.

Where plates collide pushing into each other earthquakes and volcanoes occur.  The 'ring of fire' is very active in the Pacific particularly in Indonesia and Japan.  In some cases plate collisions result in the building of mountain ranges.  The building of a mountain range is called an OROGENY.    The description below together with the map is also shown in another page of this web-site

 

 The Himalayas: Two continents collide

Plates were very much on the move during the Cainozoic. The subcontinent of India was on its way to Asia. Australia and Antarctica parted and cruised away from each other. South America was on its own.

About 225 million years ago, India was a large island still situated off the Australian coast.

A vast ocean (called the Tethys) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. By studying the history -- and ultimately the closing-- of the Tethys, scientists have reconstructed India's northward journey.

About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India began to ram into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas.  The final collision of the land masses took place in the Miocene epoch.

Among the most dramatic and visible creations of plate-tectonic forces are the lofty Himalayas, which stretch 2,900 km along the border between India and Tibet. This immense mountain range began to form between 40 and 50 million years ago, when two large landmasses, India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only be relieved by thrusting skyward, contorting the collision zone, and forming the jagged Himalayan peaks.

India was once situated well south of the Equator, near the continent of Australia.

The progress of the last 6,000-km part of the journey of the India landmass (Indian Plate) before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago is shown in the illustration.   Map Courtesy United States Geological Service

An excellent example of plate migration leading eventually to an orogeny is the formation of the Himalayas

Sometimes plates grind up against one another.  Volcanoes are not formed but severe earthquakes may occur an example being the San Andreas Fault in California. The December 2005 Asian Tsunami was caused by two plates pushing against one another causing a large earthquake and a devastating tsunami. 

Summarising

plate_colour_diagram2.gif (21726 bytes)

 

The map which is shown above illustrates how the lithosphere is segmented into plates like a massive jig-saw.  Credit due to Moorland School web-site  Dr Paul Smith.

The concepts of Plate Tectonic activity are admirably illustrated in the diagram which is reproduced below by kind permission of Professor L.S.Fichter of James Madison University in Harrisonburg VA USA.

                                         

With the exception of hot spots all volcanic activity takes place at convergent and divergent plate boundaries. A convergent plate boundary is where two plate are pushing against one another. Fractional melting takes place at convergent plate boundaries.  These are called subduction zones.  A volcano is formed.  Most volcanoes on land are of this type.

At divergent plate boundaries, as in sea floor spreading, magma is brought up from deep in the mantle toward the surface via convection cells. This material is very hot, and under enormous pressure.  There are massive amounts of volcanic activity but most of it is not obvious because it takes place under the sea

The Lisbon Earthquake

The Lisbon Earthquake struck on the morning of the first of November 1755.    Contemporary reports state that the earthquake lasted between three-and-a-half and six minutes, causing gigantic fissures five metres (16 ft) wide to appear in the city centre. The survivors rushed to the open space of the docks for safety and watched as the water receded, revealing a sea floor littered by lost cargo and old shipwrecks. Several tens of minutes after the earthquake, an enormous tsunami engulfed the harbour and downtown, It was followed by two more waves. In the areas unaffected by the tsunami, fire quickly broke out, and flames raged for five days.

 The shockwaves of the earthquake were felt throughout the whole of Europe as far away as Finland.  Tsunamis up to 20 metres (66 ft) in height swept the coast of North Africa and even hit Martinique and Barbados. Even the coast of southern England and the west coast of Ireland were hit by three metre high tsunamis.

Of a Lisbon population of 275,000, up to 90,000 were killed. Eighty-five percent of Lisbon's buildings were destroyed.  Another 10,000 people were killed in Morocco.  It is said that many animals sensed danger and fled to higher ground before the tsunamis struck  The Lisbon quake is the first documented reporting of such a phenomenon in Europe.

This is all very reminiscent of the disaster which struck South East Asia in December 2005.

 Hot Spots

A third major category of volcanic activity has little to do with plate tectonics. Hot spot volcanoes often (but not always) occur within plates. They are generated by isolated, stationary, plumes of magma that arise deep in the mantle. Common examples are the shield volcanoes of the Hawaiian islands, the Galapagos Islands and Yellowstone Park in Wyoming.

 

Volcanic Activity

There are essentially three types of volcanoes:-

1) Where two or three tectonic plates converge.  An example of this is found in the Pacific Ring of Fire.  One of the most volcanic parts of the world is the Indonesian Archipelago.

2)  Where tectonic Plates diverge.  An example of this is the Mid-Atlantic Ridge.

3)  Hotspots.  These are caused by the upwelling of magma from deep in the mantle.  These so called mantle plumes are found in places far from plate boundaries and can produce very high mountains.  Mauna Kea in the Hawaiian islands is higher than Mount Everest if you measure it from its base deep in the Ocean and not from sea level.  As a plate moves across a hot spot it can create over millions of years whole chains of islands as found in Hawaii and in the Galapagos Islands.  Hotspots are found elsewhere in the Solar System - examples being Mars and Venus.

 

Acknowledgement USGS (United States Department of the Interior - United States Geological Survey

Throughout the history of the Earth there have been particularly violent periods of volcanic activity.  One of these was in Permian Times in the Siberia.  Another was in the Deccan Traps in the Indian Plates.

The existence of massive volcanic activity was one of the theories advanced as the cause of the dinosaur extinction.  This suggests that huge volcanic activity involving the formation of massive volcanic plumes and immense basaltic lava flows could have brought about the Cretaceous Extinction. The volcanic activity that produced the huge lava flows that formed the Deccan traps in India was sited as a possible culprit.  There are still many earth scientists who consider that, as well as the impact, the volcanic activity contributed very considerably to the demise of the dinosaurs.  

There can be little doubt than considerable volcano activity did take place at the end of the Cretaceous /beginning of the Tertiary. A huge outpouring of the earth's interior that occurred over much of present-day India 65 million years ago came from the boundary between the earth's lower mantle and its molten iron core some 1,800 miles beneath the surface, scientists have determined.  The same lower mantle signature also typifies several "hot spots," volcanic areas beneath earth's plates responsible for forming chains of volcanoes as the plate gradually slides above. Reunion Island in the Indian Ocean is thought to be the hot spot that formed the Deccan Traps.   the volcanic activity occurred as the Indian Plate was making its way northward and at the time 'India' was migrating from the southern to the northern hemisphere in its 'mad rush' geologically speaking towards its eventual collision in the Miocene Epoch when it collided with Asia to form the Himalayas.

Toba the Terrible

Sometime long ago the tropical island of Sumatra lay bathed in the eternal summer and the birds and animals pursued their lives as usual.  Hundreds and even thousands of kilometres away one of natures latest creations had emerged.  The men and women of those times may have led a simple life  but they used tools, had discovered fire and could talk to each other -they were human beings of the species Homo sapiens and were our direct ancestors.   They had no knowledge that deep beneath the surface a huge mantle plume was about to erupt.  At a place we now call Toba the largest super-volcano for 28 million years was about to explode.   To-day a beautiful lake, which has become a tourist resort, marks the place where the terrible event occurred together with genetic evidence scientists have proved that all  6 billion of us of every ethnic group including the Australian Aborigines (who arrived in Australia ten thousand years after the disaster) owe our existence to the few survivors of the super-volcano.   The evil myths of racism have been scientifically proved TO BE TOTALLY AND UTTERLY WRONG. 

Some scientists estimate that many humans were killed by the event as a direct result at the time - volcanic ash several meters deep was scattered for hundreds of kilometres from the explosion.  Many of those that survived the explosion died as a result of the appalling weather conditions and famines that followed. 

It is estimated by a number of authorities in the field that only 10 to 20 thousand people over the whole world survived the disaster.  That means that every man and every woman alive to-day are the descendants of the few survivors.  The evil myths of racism have been scientifically proved TO BE TOTALLY AND UTTERLY WRONG.

Lake Toba to-day  Credit Tri Jay Tour and Travel

E-mail address:trijaya@attglobal.net

Tri Jaya/ Tour Travel
Hotel Deli River
Jl. Raya Namorambe 129 pasar IV
Medan 20356

 

According to a recent hypothesis our species came very close to extinction because of the eruption of a super-volcano in Sumatra about 73,000 years ago.  Evidence is emerging from a number of different fields - geological, anthropological, climatological and genetic that this event not only caused enormous damage over the whole area of Indonesia, South East Asia, India, the Middle East and parts of Africa but led to a 'volcanic winter" that lasted for about six years and caused a 1000 years of the coldest ice age on record.   It has been estimated that the eruption lasted around 9-14 days (ref. Ledbetter M. et al, 1979) and took place in the northern summer.  The pyroclastic ash fall is estimated at 800 cubic kilometres which gives an average thickness of 10 cm over the whole Earth.  As far away from the epicentre of the eruption as Vansadhara and other places in India there is evidence of ash falls between 1.5 and 6 metres 5-20 ft.) deep occurred.   Using the estimated eruption time of 9-14 days (ref. Rose W.I., 1990) and an eruption volume of 2,800 cu. km, an average eruption rate of  8 million metric tons of material per second has been calculated.  The areas directly affected by ash fall from the Toba explosion must be speculative but one site in central India, for example, today has a thickness of no less than 6 m (20 ft) (ref. Acharya S.K. et al., 1993). It is possible that this thickest of deposits found so far represents an accumulation of wind-blown or water-driven ash, but even if it does, most of Southeast Asia, parts of Sunda, the Andaman and Nicobar islands, along with all of the Indian subcontinent and Sri Lanka were covered in deep ash. Such a heavy fall would have exterminated most plant and animal life in the affected areas. The deepness of the ash reduces gradually towards the west but can still be expected to have been substantial in the Middle East and parts of East Africa.

In addition, it is thought that the staggering amount of 1010 metric tons of H2SO4 (sulphuric acid) was blown into the atmosphere by the Toba event (Huang et al. 2001).

Prior to the eruption of 73,000 years ago, Toba had produced at least two earlier major eruptions 800,000 and 500,000 years ago. The earlier outbreaks were not as large as the colossal eruption of 73,000 years ago which is the only eruption of class VEI 8 to have taken place since primates have appeared on planet earth.   It was of an order of magnitude vastly more than Laki in 1783, Tambora in 1815 and Krakatao in 1883 three of the greatest Holocene eruptions. The Toba eruption may have caused about 3 to 4 degree C cooling at the surface (Sigurdsson, 1990).

The Toba Eruption is what volcanologists call a VEI-8 volcanic event.  The last four such super-volcanoes were

Lake Taupo North Island New Zealand       26,500 years ago     Volume of erupted material 1,170 cubic kilometres

Lake Toba Sumatra Indonesia                    75,000 years ago      Volume of erupted material 2,800 cubic kilometres

Lake Toba                                              500,000 years ago

Lake Toba                                              800,000 years ago

Yellowstone Caldera Wyoming USA         640,000 years ago     Volume of erupted material 1,000 cubic kilometres

Yellowstone Caldera Wyoming USA      2,200,000 years ago      Volume of erupted material 2,500 cubic kilometres

La Garita Caldera Colorado USA         27,000,000 years ago      Volume of erupted material 5,000 cubic kilometres

Very Mild by Comparison!

The Laki eruption in Iceland  lasted eight months during which time about 14 cubic km of basaltic lava and some tephra were erupted. Haze from the eruption was reported from Iceland to Syria. In Iceland, the haze lead to the loss of most of the island's livestock (by eating fluorine contaminated grass), crop failure (by acid rain), and the death of one-quarter of the human residents (by famine). The climatic and atmospheric effects of the Laki eruption are impressive. In the eastern United States, the winter average temperature was 4.8 degrees C below the 225 year average. The estimate for the temperature decrease of the entire Northern Hemisphere was about 1 degree C.

Note after this page of the web-site was written there was a most interesting and informative programme on BBC 2 television on Timewatch 19 January 2007about the Laki eruption and its effects.  Apart from the 10,000 people in Iceland who died, a very large number of people in Western Europe perished from the toxic cloud of sulphur dioxide that swept in an arc across Norway then down to Poland, Czechlands, Germany France and finally to the British Isles.  Crop failure and famine and a terrible winter ensued. The famous book the Natural History of Selbourne was mentioned in ther programme including the national hero of Iceland who led his people through the time of their tribulation     The site of the eruptions has recently been investigated by Thor Thordardson see  hyperlink at end of this chapter of the web-site.

Tambora erupted in 1815 killing 92 000 people. 1816 became the year without a summer as the global climate effects were felt. Aerosols from the Tambora eruption blocked out sunlight and reduced global temperatures by 3 deg C. Europe missed a summer, and India had crop failures following the Tambora eruption. 100 cubic km of magma was erupted.    Tambora is a stratovolcano, forming the Sanggar peninsula of Sumbawa Island. The diameter of the volcano at sea-level is about 38 miles (60 km). Prior to the 1815 eruption, the volcano may have been as tall as 13,000 feet (4,000 m). The 1815 eruption formed a caldera about 4 miles (6 km) in diameter. The caldera is 3,640 feet (1,110 m) deep.

The paroxysmal explosion at Tambora was heard 1700 km away, and Madura Island (500 km away) was in darkness for 3 days. Rock fragments 15 cm in diameter were ejected 40 km from the volcano

Krakatao is situated between the islands of Java and Sumatra in Indonesia.   The 1883 eruption ejected more that six cubic miles of rock and ash and generated the loudest sound ever reported. Atmospheric shock waves reverberated around the world seven times and were felt for five days. Near Krakatoa, according to records, villages and towns were destroyed and many seriously damaged, more than 36,000 people died, and many thousands were injured by the eruption, mostly in the tsunamis, which followed the explosion.

Credit  Lonely Islands The Andaman and Nicabar Islands George Weber and  Simron Jet Singh

Map of Central Sumatra

Lake Toba

Credit  Lonely Islands The Andaman and Nicabar Islands George Weber and  Simron Jet Singh

Earth - The Genesis of a Living World

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