Wegener had attended the University of Berlin, where he earned a PhD in astronomy in 1904. As is often the case in science, he was soon distracted by pursuits in other areas. A long-time fascination with climate soon led him to the developing fields of meteorology, climatology, and geophysics. He made several key contributions to meteorology and wrote a textbook that became standard throughout Germany. An avid and experienced balloonist, Wegener pioneered the use of balloons to study the atmosphere. He also became a noted Arctic explorer, joining a 1906 expedition to Greenland to study polar air circulation. It was his passion for the Arctic that would eventually lead to his untimely death.
In 1911, Wegener became a lecturer at the University of Marburg. While teaching there, he pursued his own research into geophysics and climate. He noticed, as so many others before him, that the continents on the globe fit together like the pieces of a jigsaw puzzle. But Wegener looked deeper into the matter, comparing the geological strata found at the matching continental edges. He was struck by the presence of identical fossils in rock strata that were on opposite sides of the ocean. At the time, the accepted explanation for the similarities in fossil records was that land bridges during the Ice Age allowed creatures to migrate among continents. Wegener did not find this explanation credible.
He came to believe that the modern-day continents had all broken away from a single massive supercontinent some time in the prehistoric past. Judging from the fossil evidence, this supercontinent must have broken apart more than 200 million years ago. Using geological features, fossil records, and climate as evidence, he built an overwhelming case to support his continental drift hypothesis. Among the evidence: coal fields in North America and Europe matched, mountain ranges in Africa and South America lined up, and fossils from the same species of dinosaurs were found in places that are now separated by oceans. Finding it implausible that these beasts could have swum across oceans, Wegener was convinced that these reptiles had once lived on a single continent.
Wegener introduced his theory at a meeting of the German Geological Association in 1912. His paper was published later that year and he published a more detailed explanation of his theory in a book in 1915. His ideas about moving continents proved quite controversial.
At the outbreak of World War I, in 1914, he was drafted into the German army. After being wounded in combat, he served out the war as a weatherman in the Army weather forecasting service. Wegener returned to Marburg after the Armistice, but soon became frustrated with his slow advancement. In 1924 he accepted a professorship in meteorology and geophysics at the University of Graz, in Austria.
Between the World Wars, the idea of continental drift caused sharp disagreement among geologists. In 1921, the Berlin Geological Society held a symposium on the theory. By 1922, Wegener's book had been translated into English, introducing his ideas to a wider audience. In 1923, the theory was discussed at conferences by the Geological Society of France, the Geological Section of the British Association for the Advancement of Science, and the Royal Geological Society. In 1926, the American Association of Petroleum Geologists (AAPG) held a symposium at which the continental drift hypothesis was vigorously debated. The resulting papers were published in 1928, under the title Theory of Continental Drift, with Wegener contributing a paper to the volume.
Even though he had amassed copious volumes of supporting data, geologists mostly ignored Wegener's theory. The theory's main problem was the lack of a satisfactory explanation of how continents moved through the solid rock of the ocean basins. Most geologists did not believe that this could be possible. Instead of investigating possible sources for Wegener's anomalies, the geological community rejected the theory out of hand, preferring their erroneous but familiar existing beliefs. As Wegener himself said in his “The Origins of Continents and Oceans:”
“Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence... It is only by combing the information furnished by all the earth sciences that we can hope to determine 'truth' here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw.”
Wegener made what was to be his last expedition to Greenland in November, 1930. On that ill-fated journey, Alfred Wegener and a companion went missing. Wegener's body was eventually found in May, 1931. The suspected cause of death was heart failure through overexertion. His theory of continental drift had not found acceptance during his life.
Continental Movement in the Past
Four major scientific developments spurred the formulation of the plate-tectonics theory: discovery of the youth of the ocean floor; confirmation of repeated reversals of Earth magnetic field in the geologic past; proof of seafloor-spreading and the recycling of oceanic crust; and the realization that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.241
Illustration 67: Ocean floor Magnetic striping, Source: USGS.
Harry Hammond Hess, a Professor of Geology at Princeton University, helped set the stage for the emerging Plate Tectonics theory. While serving in the US Navy during World War II, Hess was able to conduct echo-sounding surveys in the Pacific while cruising from one battle to the next. From these data, he formulated a theory called seafloor spreading, stating the ideas that new crust was added at mid-ocean ridges and consumed in deep trenches. In 1962, his ideas were published in a paper titled “History of Ocean Basins.”242 The paper would prove to be one of the most important contributions in the development of plate tectonics.
In the 1950s, scientists using magnetometers, instruments that detect changes in magnetic fields, began noticing odd magnetic variations across the ocean floor. These unexpected variations occur because the iron-rich, volcanic basalt rock that makes up the ocean floor contains magnetite. Magnetite is a strongly magnetic mineral that can locally distort compass readings.
As molten rock emerging at an ocean ridge solidifies, it takes on the same magnetic orientation as Earth's magnetic field. From time to time, Earth's magnetic field reverses its orientation, swapping north and south poles. This leads to a striped magnetic pattern in the rock expanding away from ocean ridges: a record of both seafloor spreading and the reversal of Earth's magnetic field over time. This phenomenon showed that the seafloor becomes older with increasing distance from ocean ridge crests, helping confirm Hess' theory. Unlike Wegener, Hess, who died in 1969, lived to see his seafloor-spreading hypothesis become accepted during his lifetime.243
Over geologic time, the continents have shifted radically. By comparing the layers of rock, geologists try to piece the parts of the puzzle back together. Unfortunately, over long periods of geologic time, the rock layers become jumbled and new rock is created, confusing the matter. It has taken years of scientific investigation to produce a detailed history of Earth's changing continents. Geologists' records now extend back more than a thousand million years.
Illustration 68 The drifting continents since the Permian Era. Source NOAA.
Illustration 68 shows a series of views of Earth since the end of the Permian Era, 250 million years ago. Notice how most of the land was concentrated in a single continent around the time of the Permian-Triassic extinction, 250 mya. Here is Wegener's single supercontinent, where dinosaurs once roamed freely.
Plate Tectonics
It was not until the 1960s that geologists came to accept Earth's continents did move. This realization came after geologists began to figure out the structure of Earth's interior. Earth's surface solidified permanently after the Great Bombardment ended over 4 billion years ago (page 47). The top layer of Earth's surface is called the lithosphere, and it consists o
f two parts, the crust and the upper mantle. The lithosphere is only a thin layer on top of many that make up Earth's interior, as can be seen in Illustration 69.
Illustration 69: Layers of Earth's interior. Source USGS.
The crust is Earth's outermost layer, forming the familiar continents and ocean floor. The crust varies in thickness from 3 to 60 miles (5 to 100 km) and though it is solid, it is not a single, continuous surface.
Earth's crust is broken into a number of pieces, called plates, that ride on top of the more pliable layers of the mantle. Oceanic crust is a thin layer of dense, basaltic rock averaging about 3 miles (5 km) thick. The continental crust is less dense, consisting of lighter-colored granitic rock, and varies from 18 to 60 miles (30 to 100 km) in thickness. Most of Earth's seismic activity takes place at the boundaries where plates interact. Like the shell of an egg, Earth's crust is brittle and can break. Oceanic trenches, mountain ranges, volcanoes and earthquakes are all the result of collisions and partings among these plates.
The continents ride around on top of the crustal plates, which extend under the world's oceans. The various different types of plates interaction are depicted in Illustration 70. If two plates are sliding past each other, heading in different directions, a transform plate boundary occurs. An example of this type of boundary is the San Andreas Fault that runs the length of California.
Where plates are pulling apart from each other, a rift is formed. At rifts, new crust is formed by upwelling magma. When this happens under the ocean, an oceanic ridge is formed, like the mid-Atlantic ridge. But rifts can also occur on land, as along the African Rift Valley.
Illustration 70: Types of tectonic plates, by José F. Vigil from This Dynamic Planet, U.S. Geological Survey.
When two plates collide, moving toward each other, a subduction zone is formed. Subduction zones are places where two lithospheric plates come together, one riding over the other. Usually, when two oceanic plates collide, the younger of the two plates will ride over the edge of the older plate. This is because oceanic plates grow more dense as they cool and move further away from the Mid-Ocean Ridge. This leads to the younger plates being lighter than older ones. Subduction zones are often highly active seismically, meaning they are prone to earthquakes.
Because the less dense continental material cannot sink, a descending oceanic plate carrying a continent will dive into a trench behind the leading oceanic crust until it collides with the overriding plate. This causes the continent's leading edge to fold up, forming a mountain range. This can also result in some of the oceanic crust of the overriding plate to be deposited on top of the continent. Pressure builds up until the trench flips, and the previously overriding oceanic plate dives under the continental crust. This explains why most ocean trenches are found along the edges of continents.244
Illustration 71: Colliding plates raising the Himalayas. Source USGS.
If two plates collide at subduction zone, and both plates carry a continent, a collision of land masses is unavoidable. When this happens, subduction stops along the collision zone and the trench disappears. The continents pile up, resulting in the birth of a new mountain range. A good example of this is the collision between the Indian plate and the Eurasian plate, shown in Illustration 71. Their ongoing engagement continues to raise the Himalayas and the Tibetan Plateau to new heights.
Colliding plates create the bands of volcanoes first noticed by Humboldt. A map showing earthquake locations around the world, for the decade starting in 1990, can be seen in Illustration 72. These areas also show high levels of volcanic activity. Today, the geologically active area around the rim of the Pacific Ocean are called the Ring of Fire.
Illustration 72: Earthquakes from 1990-2000. Source NOAA.
Plates have been in motion since Earth's crust formed, but a plate may not exist forever. If a plate's leading edge is being consumed in a subduction trench faster than new crust is being added to its trailing edge, the entire plate can disappear. When this happens, the ridge slowly moves toward the trench and the whole plate is eventually drawn down into the mantle, swallowed by Earth. The disappearance of an entire plate causes a global realignment of the remaining plates and their borders.
One of the dire predictions made by global warming proponents is that the level of the ocean will rise, flooding coastlines and low-lying areas. What is usually not mentioned is that geology has as much to do with local sea levels changing, as do melting glaciers. Continental elevations are due to contributions from compositional buoyancy, thermal and geological forces.
Researchers have calculated elevation adjustments for parts of North America ranging from -3600 feet (−1100 m) for the southern Rocky Mountains to +7500 feet (+2300 m) for the Gulf of California.245 Adjusted elevations show clear trends, with an average 1.86 mile (3 km) difference between hot and cold crustal regions. The internal forces of the planet cause significant, ongoing shifts in elevation. When the shifted land is on a coast, sea levels change.
Illustration 73: Tectonic plates circa 2000. Source USGS.
Such tectonic effects can be seen on San Clemente Island, off the coast of California, which has been rising since the early Pleistocene at a rate of 8-16 inches (20-40 cm) per thousand years. Similar rising coasts can be found in places like New Guinea, Barbados, and the Palos Verdes Hills on the Californian mainland.246
Earth's Plates Today
Today, Earth's outermost layer is broken into seven large plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist, including the Arabian, Indian, Nazca, and Philippines plates.
The current configuration of Earth's tectonic plates is shown in Illustration 73. Areas where the arrowheads point in opposite directions are ocean rifts where new crust is being created. Prominent among these rift zones are the Mid-Atlantic Ridge and the East Pacific Rise. The Mid-Atlantic Ridge extends from 200 miles (330 km) south of the North Pole, to Bouvet Island in the far south Atlantic and the East Pacific Rise runs from near Antarctica north through the Gulf of California. Ocean ridges run around the world like the stitching of a baseball, forming a nearly continuous system 25,000 miles (40,000 km) long.
South America and Africa are moving apart at an average of 2.25 inches (5.7 cm) per year, about as fast as a human fingernail grows. This is due to sea floor spreading along the Mid-Atlantic Ridge, which has been happening since Pangaea broke apart during the CAMP (page 98). The fastest recorded sea floor spreading is along the East Pacific Rise at 6.7 in (17.2 cm) per year.
Illustration 74: The Puerto Rico Trench, the arc of the Greater and Lesser Antilles, and surrounding ocean basins. USGS.
Trenches formed where plates are colliding include some of the deepest parts of the ocean. The Puerto Rico Trench, with maximum depth 28,232 ft (8,605 m), is the deepest point in the Atlantic. The Mariana Trench, with a depth of 35,798 ft (10,911 m) holds that distinction for the Pacific. Trenches are generally found parallel to volcanic island chains, with the trench about 120 miles (200 km) from the arc of land. An undersea view of the Puerto Rico Trench and islands of the Greater and Lesser Antilles demonstrates this arrangement in Illustration 74.
Trenches can also be found near rift boundaries. The South Sandwich Trench in the South Atlantic Ocean, 60 miles (100 km) to the east of the Sandwich Islands, is formed by the small Sandwich Plate being subducted under the South American plate. This is an anomaly, since the Sandwich Plate is one of the youngest plates on Earth. Geologists think that some force is actively pulling the Sandwich Plate down, even though it is the younger plate.247
The Great Ocean Conveyor Belt
Worldwide circulation of ocean water helps to redistribute heat around the globe. This causes some areas to be warmer than would be expected, given their latitude, and others to be colder. The great Humboldt Current lowers the temperature of the western coast of South America while the Gulf Stream helps to keep Western Europe temperate. Without the Gulf Stream, Europe woul
d have the same climate as Siberia.
Over the ages, ocean circulation patterns changed as the continents shifted. We have already mentioned South America and Australia moving north, forming the circumpolar current that thermally isolates Antarctica today. Equatorial currents linked the Pacific and Atlantic oceans, until North and South America became joined by the Isthmus of Panama. Over time, other changes will occur in the world's circulatory systems, as the continents continue their slow but inexorable movement.
One of the major consequences of the way continental land masses partition the oceans is the meridional overturning circulation (MOC), more commonly called the great ocean conveyor belt. The stability of Earth's climate largely depends on oceanic and atmospheric currents transporting heat energy from low latitudes to high latitudes. Up to four petawatts248 of thermal energy is redistributed by these currents.
The thermally driven MOC carries one-third of this heat, making it a major factor in Earth's climate system. This circulation pattern is also referred to as the thermohaline circulation because it is driven by differences in water density which, in turn, are determined by both temperature and salinity. Warm, less dense water flows to higher latitudes in surface currents, like the Gulf Stream. This moderates the climate in higher latitudes, but the flow of warm water must be balanced by a similar return volume of water.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 15
