The Isthmus of Panama is a land bridge whose appearance 3 million years ago allowed the Great American Interchange.

A land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonise new lands. Land bridges can be created by marine regression, in which sea levels fall, exposing shallow, previously submerged sections of continental shelf; or when new land is created by plate tectonics; or occasionally when the sea floor rises due to post-glacial rebound after an ice age.

Prominent examples[link]

• The Bering land bridge, which intermittently connected Asia with North America as sea levels rose and fell under the effect of ice ages;
• Doggerland, a former landmass in the southern North Sea which connected the island of Great Britain to mainland Europe during the last ice age.
• The Isthmus of Panama, whose appearance three million years ago allowed the Great American Interchange;
• The Sinai Peninsula, linking Africa and Eurasia;
• The Adam's Bridge (known as Rama Setu), connecting India and Sri Lanka.

Land bridge theory[link]

In the 19th century a number of scientists noted puzzling geological and zoological similarities between widely separated areas. To solve these problems, "…whenever geologists and paleontologists were at a loss to explain the obvious transoceanic similarities of life that they deduced from the fossil records, they sharpened their pencils and sketched land bridges between appropriate continents."^[1] The concept was first proposed by Jules Marcou in Lettres sur les roches du Jura.^[2]

These hypothetical land bridges included:^[3]

• Archatlantis from the West Indies to North Africa
• Archhelenis from Brazil to South Africa
• Archiboreis in the North Atlantic
• Archigalenis from Central America through Hawaii to Northeast Asia
• Archinotis from South America to Antarctica
• Lemuria in the Indian Ocean

All of these became obsolete with the gradual acceptance of continental drift and the development of plate tectonics by the mid-20th century.

See also[link]

• Habitat fragmentation
• Sea level rise

References[link]

Alfred Wegener
Alfred Wegener, around 1925
Born	(1880-11-01)November 1, 1880 Berlin, German Empire
Died	November 1930 (aged 50) Clarinetania, Greenland
Residence	Germany
Citizenship	German
Nationality	German
Fields	Meteorology, Geology, Astronomy
Alma mater	University of Berlin
Doctoral advisor	Julius Bauschinger
Known for	Continental drift theory
Influenced	Johannes Letzmann
Signature

Alfred Lothar Wegener (November 1, 1880 – November 1930) was a German polar researcher, geophysicist and meteorologist.

During his lifetime he was primarily known for his achievements in meteorology and as a pioneer of polar research, but today he is most remembered for first proposing the theory of continental drift (Kontinentalverschiebung) in 1912, which hypothesized that the continents were slowly drifting around the Earth. His hypothesis was controversial and not widely accepted until the 1950s, when numerous discoveries such as palaeomagnetism provided strong support for continental drift, and thereby a substantial basis for today's model of Plate tectonics.^[1][2] Wegener was involved in several expeditions to Greenland to study polar air circulation before the existence of the jet stream was accepted. Expedition participants made many meteorological observations and achieved the first-ever overwintering on the inland Greenland ice sheet as well as the first-ever boring of ice cores on a moving Arctic glacier.

Biography[link]

Early life and education[link]

On November 1, 1880, Alfred Wegener was born in Berlin as the youngest of five children in a clergyman's family. His father, Richard Wegener, was a theologian and teacher of classical languages at the Berlinisches Gymnasium zum Grauen Kloster. In 1886 his family purchased a former manor house near Rheinsberg, which they used as a vacation home. Today there is an Alfred Wegener Memorial site and tourist information office in a nearby building that was once the local schoolhouse^[3].

Commemorative plaque on Wegener's former school in Wallstrasse

Wegener attended school at the Köllnische Gymnasium on Wallstrasse in Berlin (a fact which is memorialized on a plaque on this protected building, now a school of music), graduating as the best in his class. Afterward he studied Physics, meteorology and Astronomy in Berlin, Heidelberg and Innsbruck. From 1902 to 1903 during his studies he was an assistant at the Urania astronomical observatory. He obtained a doctorate in astronomy in 1905 based on a dissertation written under the supervision of Julius Bauschinger at Friedrich Wilhelms University (today Humboldt University), Berlin. Wegener had always maintained a strong interest in the developing fields of meteorology and climatology and his studies afterwards focused on these disciplines.

In 1905 Wegener became an assistant at the Aeronautischen Observatorium Lindenberg near Beeskow. He worked there with his brother Kurt, two years his senior, who was likewise a scientist with an interest in meteorology and polar research. The two pioneered the use of weather balloons to track air masses. On a balloon ascent undertaken to carry out meteorological investigations and to test a celestial navigation method using a particular type of quadrant (“Libellenquadrant”), the Wegener brothers set a new record for a continuous balloon flight, remaining aloft 52.5 hours from April 5-7, 1906.^[4]

First Greenland expedition and years in Marburg[link]

In that same year 1906, Wegener participated in the first of his three Greenland expeditions, later regarding this experience as marking a decisive turning point in his life. The expedition was led by the Dane Ludvig Mylius-Erichsen and charged with studying the last unknown portion of the northeastern coast of Greenland. During the expedition Wegener constructed the first meteorological station in Greenland near Danmarkshavn, where he launched kites and tethered balloons to make meteorological measurements in an Arctic climatic zone. Here Wegener also made his first acquaintance with death in a wilderness of ice when the expedition leader and two of his colleagues died on an exploratory trip undertaken with sled dogs.

After his return in 1908 and until World War I, Wegener was a lecturer in meteorology, applied astronomy and cosmic physics at the University of Marburg. His students and colleagues in Marburg particularly valued his ability to clearly and understandably explain even complex topics and current research findings without sacrificing precision. His lectures formed the basis of what was to become a standard textbook in meteorology, first written In 1909/1910: Thermodynamik der Atmosphäre (Thermodynamics of the Atmosphere), in which he incorporated many of the results of the Greenland expedition.

On 6 January 1912 he publicized his first thoughts about continental drift in a lecture at a session of the Geologischen Vereinigung at the Senckenberg-Museum, Frankfurt/M and in three articles in the journal Petermanns Geographischen Mitteilungen^[5]

Second Greenland expedition[link]

After a stopover in Iceland to purchase and test ponys as pack animals, the expedition arrived in Danmarkshavn. Even before the trip to the inland ice began the expedition was almost annihilated by a calving glacier. The Danish expedition leader, Johan Peter Koch, broke his leg when he fell into a glacier crevasse and spent months recovering in a sickbed. Wegener and Koch were the first to winter on the inland ice in northeast Greenland.^[6] Inside their hut they drilled to a depth of 25 m with an auger. In summer 1913 the team crossed the inland ice, the four expedition participants covering a distance twice as long as Fridtjof Nansen's southern Greenland crossing in 1888. Only a few kilometers from the western Greenland setlement of Kangersuatsiaq the small team ran out of food while struggling to find their way through difficult glacial breakup terrain. But at the last moment, after the last pony and dog had been eaten, they were picked up at a fjord by the clergyman of Upernavik, who just happened to be visiting a remote congregation at the time.

Later in 1913 after his return Wegener married Else Köppen, the daughter of his former teacher and mentor, the meteorologist Wladimir Köppen. The young pair lived in Marburg, where Wegner resumed his university lectureship.

World War I[link]

As an infantry reserve officer Wegener was immediately conscripted when war began in 1914. On the war front in Belgium he experienced fierce fighting but his term lasted only a few months: after being wounded twice he was declared unfit for active service and assigned to the army weather service. This activity required him to travel constantly between various weather stations in Germany, on the Balkans, on the Western Front and in the Baltic region.

Nevertheless he was able in 1915 to complete the first version of his major work, Die Entstehung der Kontinente und Ozeane (“The Origin of Continents and Oceans”). His brother Kurt remarked that Alfred Wegener’s motivation was to “reestablish the connection between geophysics on the one hand and geography and geology on the other, which had become completely ruptured because of the specialized development of these branches of science.”

Interest in this small publication was however low, also because of wartime chaos. By the end of the war Wegener had published almost 20 additional meteorological and geophysical papers in which he repeatedly embarked for new scientific frontiers. In 1917 he undertook a scientific investigation of the Treysa meteorite.

Postwar period and third expedition[link]

Wegener obtained a position as a meteorologist at the German Naval Observatory (Deutsche Seewarte) and moved to Hamburg with his wife and their two daughters. In 1921 he was appointed senior lecturer at the new University of Hamburg. From 1919 to 1923 Wegener worked on Die Klimate der geologischen Vorzeit (“The Climates of the Geological Past”), published together with his father-in-law, Wladimar Köppen. In 1922 the third, fully revised edition of “The Origin of Continents and Oceans” appeared, and discussion began on his theory of continental drift, first in the German language area and later internationally. Withering criticism was the response of most experts.

In 1924 Wegener was appointed to a professorship in meteorology and geophysics in Graz, which finally provided him with a secure position for himself and his family. He concentrated on physics and the optics of the atmosphere as well as the study of tornados. Scientific assessment of his second Greenland expedition (ice measurements, atmospheric optics, etc.) continued to the end of the 1920s.

In November 1926 Wegener presented his continental drift theory at a symposium of the American Association of Petroleum Geologists in New York City, again earning rejection from everyone but the chairman. Three years later the fourth and final expanded edition of “The Origin of Continents and Oceans” appeared.

In 1929 Wegener embarked on his third trip to Greenland, which laid the groundwork for a later main expedition and included a test of an innovative, propeller-driven snowmobile.

Fourth and last expedition[link]

Wegener (left) and Villumsen (right) in Greenland; November 1st, 1930.

Wegener's last Greenland expedition was in 1930. The 14 participants under his leadership were to establish three permanent stations from which the thickness of the Greenland ice sheet could be measured and year-round Arctic weather observations made. Wegener felt personally responsible for the expedition's success, as the German government had contributed $120,000 ($1.5 million in 2007 dollars) at a time when Germans were starving to death owing to post-war shortages. Success depended on enough provisions being transferred from West camp to Eismitte ("mid-ice") for two men to winter there, and this was a factor in the decision that led to his death. Owing to a late thaw, the expedition was six weeks behind schedule and, as summer ended, the men at Eismitte sent a message that they had insufficient fuel and so would return on October 20.

Vehicles used by the 1930 expedition (stored).

On September 24, although the route markers were by now largely buried under snow, Wegener set out with thirteen Greenlanders and his meteorologist Fritz Loewe to supply the camp by dog sled. During the journey the temperature reached −60 °C (−76 °F) and Loewe's toes became so frostbitten they had to be amputated with a penknife without anesthetic. Twelve of the Greenlanders returned to West camp. On October 19, the remaining three members of the expedition reached Eismitte. There being only enough supplies for three at Eismitte, Wegener and Rasmus Villumsen took two dog sleds and made for West camp. They took no food for the dogs and killed them one by one to feed the rest until they could run only one sled. While Villumsen rode the sled, Wegener had to use skis. They never reached the camp. The expedition was completed by his brother, Kurt Wegener.

Death[link]

Six months later, on May 12, 1931, Wegener's body was found halfway between Eismitte and West camp. It had been buried (by Villumsen) with great care and a pair of skis marked the grave site. Wegener had been fifty years of age and a heavy smoker and it was believed that he had died of heart failure brought on by overexertion. His body was reburied in the same spot by the team that found him and the grave was marked with a large cross. After burying Wegener, Villumsen had resumed his journey to West camp but was never seen again. He was twenty three when he died and it is estimated that his body, and Wegener's diary, now lie under more than 100 metres (330 ft) of accumulated ice and snow.

Continental drift theory[link]

Alfred Wegener first thought of this idea by noticing that the different large landmasses of the Earth almost fit together like a jigsaw. The Continental shelf of the Americas fit closely to Africa and Europe, and Antarctica, Australia, India and Madagascar fit next to the tip of Southern Africa. But Wegener only took action after reading a paper in Autumn 1911 and seeing that a flooded land-bridge contradicts isostasy.^[7] Wegener's main interest was meteorology, and he wanted to join the Denmark-Greenland expedition scheduled for mid 1912. So he hurried up to present his Continental Drift hypothesis on January 6, 1912. He analyzed either side of the Atlantic Ocean for rock type, geological structures and fossils. He noticed that there was a significant similarity between matching sides of the continents, especially in fossil plants. His hypothesis was thus strongly supported by the physical evidence, and was a pioneering attempt at a rational explanation.

Fossil patterns across continents (Gondwanaland).

From 1912, Wegener publicly advocated the theory of "continental drift", arguing that all the continents were once joined together in a single landmass and have drifted apart. He supposed the cause might be the centrifugal force of the Earth's rotation ("Polflucht") or the astronomical precession. Wegener also speculated on sea-floor spreading and the role of the mid-ocean ridges, stating: the Mid-Atlantic Ridge ... zone in which the floor of the Atlantic, as it keeps spreading, is continuously tearing open and making space for fresh, relatively fluid and hot sima [rising] from depth.^[8] However, he did not pursue these ideas in his later works.

In 1915, in The Origin of Continents and Oceans (Die Entstehung der Kontinente und Ozeane), Wegener published the theory that there had once been a giant continent, he named "Urkontinent" (German word meaning "origin of the continents",^[9] in a way equivalent to the Greek "Pangaea",^[10] meaning "All-Lands" or "All-Earth") and drew together evidence from various fields. Expanded editions during the 1920s presented the accumulating evidence. The last edition, just before his untimely death, revealed the significant observation that shallower oceans were geologically younger.

Wegener during the J.P. Koch's Expedition 1912 - 1913 in the winter base "Borg".

Reaction[link]

In his work, Wegener presented a large amount of very strong evidence in support of continental drift, but the mechanism remained elusive. While his ideas attracted a few early supporters such as Alexander Du Toit from South Africa and Arthur Holmes in England, the hypothesis was generally met with skepticism from largely conservative scientists, who were resistant to any change in the status quo. The one American edition of Wegener's work, published in 1925, was received so poorly that the American Association of Petroleum Geologists organized a symposium specifically in opposition to the continental drift hypothesis. Its opponents could argue, as did the Leipziger geologist Franz Kossmat, that the oceanic crust was too "firm" for the continents to "simply plough through", a suggestion which ignored the plasticity of all rocks at depth and at high temperatures and pressures. The comment also ignored the vast time-scale over which continental drift has occurred, effectively the total age of the earth of about 4.5 billion years.

In 1943 George Gaylord Simpson wrote a vehement attack on the theory (as well as the rival theory of sunken land bridges) and put forward his own permanentist views.^[11] Alexander du Toit wrote a rejoinder in the following year,^[12] but G.G.Simpson's influence was so powerful that even in countries previously sympathetic towards continental drift, like Australia, Wegener's hypothesis fell out of favour.

Modern developments[link]

The tectonic plates of the world were mapped in the second half of the 20th century.

In the early 1950s, the new science of paleomagnetism pioneered at Cambridge University by S. K. Runcorn and at Imperial College by P.M.S. Blackett was soon producing data in favour of Wegener's theory. By early 1953 samples taken from India showed that the country had previously been in the Southern hemisphere as predicted by Wegener. By 1959, the theory had enough supporting data that minds were starting to change, particularly in the United Kingdom where, in 1964, the Royal Society held a symposium on the subject.^[13]

Additionally, the 1960s saw several developments in geology, notably the discoveries of seafloor spreading and Wadati-Benioff zones, led to the rapid resurrection of the continental drift hypothesis and its direct descendant, the theory of plate tectonics. Alfred Wegener was quickly recognized as the founding father of one of the major scientific revolutions of the 20th century.

With the advent of the Global Positioning System (GPS), it became possible to measure continental drift directly.^[14]

Awards and honors[link]

The Alfred Wegener Institute for Polar and Marine Research in Bremerhaven, Germany, was established in 1980 on his centenary. It awards the Wegener Medal in his name.^[15] The crater Wegener on the Moon and the crater Wegener on Mars, as well as the asteroid 29227 Wegener and the peninsula where he died in Greenland (Wegener Peninsula near Ummannaq, 71°12′N 51°50′W / 71.2°N 51.833°W / 71.2; -51.833), are named after him.^[16]

The European Geosciences Union sponsors an Alfred Wegener Medal & Honorary Membership "for scientists who have achieved exceptional international standing in atmospheric, hydrological or ocean sciences, defined in their widest senses, for their merit and their scientific achievements."^[17]

See also[link]

• Hair ice

References[link]

This article incorporates information from the equivalent article on the German Wikipedia.

Selected works[link]

• Wegener, Alfred (1911) (in German). Thermodynamik der Atmosphäre [Thermodynamics of the Atmosphere]. Leipzig: Verlag Von Johann Ambrosius Barth. http://books.google.com/?id=slxDAAAAIAAJ&pg=PR1&dq=alfred+wegener. 
• Wegener, Alfred (1912). "Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane), auf geophysikalischer Grundlage" (in German). Petermanns Geographische Mitteilungen 63: 185–195, 253–256, 305–309. Presented at the annual meeting of the German Geological Society, Frankfurt am Main (January 6, 1912). 
• Wegener, Alfred (July 1912). "Die Entstehung der Kontinente" (in German). Geologische Rundschau 3 (4): 276–292. Bibcode 1912GeoRu...3..276W. DOI:10.1007/BF02202896. 
• Wegener, Alfred (1922) (in German). Die Entstehung der Kontinente und Ozeane [The Origin of Continents and Oceans]. ISBN 3-443-01056-3. LCCN unk83068007. 
• Wegener, Alfred (1929) (in German). Die Entstehung der Kontinente und Ozeane [The Origin of Continents and Oceans] (4 ed.). Braunschweig: Friedrich Vieweg & Sohn Akt. Ges. ISBN 3-443-01056-3. 
• Wegener, Elsie; Loewe, Fritz, eds. (1939). Greenland Journey, The Story of Wegener’s German Expedition to Greenland in 1930-31 as told by Members of the Expedition and the Leader’s Diary. London: Blackie & Son Ltd.. Translated from the seventh German edition by Winifred M. Deans. 
• Wegener, Alfred (1966). The Origin of Continents and Oceans. New York: Dover. ISBN 0-486-61708-4. Translated from the fourth revised German edition by John Biram. 
• Wegener, Alfred (1968). The Origin of Continents and Oceans. London: Methuen. ISBN 0-486-61708-4. Translated from the fourth German edition by John Biram with an introduction by B.C. King. 

External links[link]

Wikimedia Commons has media related to: Alfred Wegener

• Wegener Institute website
• USGS biography of Wegener
• Wegener biography at Pangaea.org
• Alfred Wegener (1880-1930) - Biographical material