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5.5: Continental Drift - Geosciences

5.5: Continental Drift - Geosciences


INTRODUCTION

The continental drift hypothesis was developed in the early part of the 20th century, mostly by Alfred Wegener. While Wegener was alive, scientists did not believe that the continents could move.

THE CONTINENTAL DRIFT IDEA

Find a map of the continents and cut each one out. Better yet, use a map where the edges of the continents show the continental shelf. That’s the true size and shape of a continent. Can you fit the pieces together? The easiest link is between the eastern Americas and western Africa and Europe, but the rest can fit together too (figure 1).


Figure 1. The continents fit together like pieces of a puzzle. This is how they looked 250 million years ago.

Alfred Wegener proposed that the continents were once united into a single supercontinent named Pangaea, meaning all earth in ancient Greek. He suggested that Pangaea broke up long ago and that the continents then moved to their current positions. He called his hypothesis continental drift.

EVIDENCE FOR CONTINENTAL DRIFT

Besides the way the continents fit together, Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis.

  • Identical rocks, of the same type and age, are found on both sides of the Atlantic Ocean. Wegener said the rocks had formed side-by-side and that the land had since moved apart.
  • Mountain ranges with the same rock types, structures, and ages are now on opposite sides of the Atlantic Ocean. The Appalachians of the eastern United States and Canada, for example, are just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway (figure 2). Wegener concluded that they formed as a single mountain range that was separated as the continents drifted.


Figure 2. The similarities between the Appalachian and the eastern Greenland mountain ranges are evidences for the continental drift hypothesis.

  • Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but are on continents that are now widely separated (figure 3). Wegener proposed that the organisms had lived side by side, but that the lands had moved apart after they were dead and fossilized. He suggested that the organisms would not have been able to travel across the oceans.
    • Fossils of the seed fern Glossopteris were too heavy to be carried so far by wind.
    • Mesosaurus was a swimming reptile but could only swim in fresh water.
    • Cynognathus and Lystrosaurus were land reptiles and were unable to swim


Figure 3. Wegener used fossil evidence to support his continental drift hypothesis. The fossils of these organisms are found on lands that are now far apart.

  • Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. This would indicate that the glaciers either formed in the middle of the ocean and/or covered most of the Earth. Today glaciers only form on land and nearer the poles. Wegener thought that the glaciers were centered over the southern land mass close to the South Pole and the continents moved to their present positions later on.
  • Coral reefs and coal-forming swamps are found in tropical and subtropical environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. Wegener suggested that these creatures were alive in warm climate zones and that the fossils and coal later had drifted to new locations on the continents.

Take a look at this animation showing that Earth’s climate belts remain in roughly the same position while the continents move and this animation showing how the continents split up.

Although Wegener’s evidence was sound, most geologists at the time rejected his hypothesis of continental drift. Why do you think they did not accept continental drift?

Scientists argued that there was no way to explain how solid continents could plow through solid oceanic crust. Wegener’s idea was nearly forgotten until technological advances presented even more evidence that the continents moved and gave scientists the tools to develop a mechanism for Wegener’s drifting continents.

MAGNETIC POLARITY EVIDENCE

Puzzling new evidence came in the 1950s from studies on the Earth’s magnetic history (figure 4). Scientists used magnetometers, devices capable of measuring the magnetic field intensity, to look at the magnetic properties of rocks in many locations.


Figure 4. Earth’s magnetic field is like a magnet with its north pole near the geographic North Pole and the south pole near the geographic South Pole.

Magnetite crystals are like tiny magnets that point to the north magnetic pole as they crystallize from magma. The crystals record both the direction and strength of the magnetic field at the time. The direction is known as the field’s magnetic polarity.

Magnetic Polarity on the Same Continent with Rocks of Different Ages

Geologists noted important things about the magnetic polarity of different aged rocks on the same continent:

  • Magnetite crystals in fresh volcanic rocks point to the current magnetic north pole (figure 5) no matter what continent or where on the continent the rocks are located.


Figure 5. Earth’s current north magnetic pole is in northern Canada.

  • Older rocks that are the same age and are located on the same continent point to the same location, but that location is not the current north magnetic pole.
  • Older rock that are of different ages do not point to the same locations or to the current magnetic north pole.

In other words, although the magnetite crystals were pointing to the magnetic north pole, the location of the pole seemed to wander. Scientists were amazed to find that the north magnetic pole changed location through time (figure 6).


Figure 6. The location of the north magnetic north pole 80 million years before present (mybp), then 60, 40, 20, and now.

There are three possible explanations for this:

  1. The continents remained fixed and the north magnetic pole moved.
  2. The north magnetic pole stood still and the continents moved.
  3. Both the continents and the north pole moved.

Magnetic Polarity on Different Continents with Rocks of the Same Age

Geologists noted that for rocks of the same age but on different continents, the little magnets pointed to different magnetic north poles.

  • 400-million-year-old magnetite in Europe pointed to a different north magnetic pole than the same-aged magnetite in North America.
  • 250 million years ago, the north poles were also different for the two continents.

The scientists looked again at the three possible explanations. Only one can be correct. If the continents had remained fixed while the north magnetic pole moved, there must have been two separate north poles. Since there is only one north pole today, the only reasonable explanation is that the north magnetic pole has remained fixed but that the continents have moved.

To test this, geologists fitted the continents together as Wegener had done. It worked! There has only been one magnetic north pole and the continents have drifted (figure 7). They named the phenomenon of the magnetic pole that seemed to move but actually did not apparent polar wander.



Figure 7. On the left: The apparent north pole for Europe and North America if the continents were always in their current locations. The two paths merge into one if the continents are allowed to drift.

This evidence for continental drift gave geologists renewed interest in understanding how continents could move about on the planet’s surface.

LESSON SUMMARY

  • In the early part of the 20th century, scientists began to put together evidence that the continents could move around on Earth’s surface.
  • The evidence for continental drift included the fit of the continents; the distribution of ancient fossils, rocks, and mountain ranges; and the locations of ancient climatic zones.
  • Although the evidence for continental drift was extremely strong, scientists rejected the idea because no mechanism for how solid continents could move around on the solid earth was developed.
  • The discovery of apparent polar wander renewed scientists interest in continental drift.

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Continental drift

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Continental drift, large-scale horizontal movements of continents relative to one another and to the ocean basins during one or more episodes of geologic time. This concept was an important precursor to the development of the theory of plate tectonics, which incorporates it.

The idea of a large-scale displacement of continents has a long history. Noting the apparent fit of the bulge of eastern South America into the bight of Africa, the German naturalist Alexander von Humboldt theorized about 1800 that the lands bordering the Atlantic Ocean had once been joined. Some 50 years later, Antonio Snider-Pellegrini, a French scientist, argued that the presence of identical fossil plants in both North American and European coal deposits could be explained if the two continents had formerly been connected, a relationship otherwise difficult to account for. In 1908 Frank B. Taylor of the United States invoked the notion of continental collision to explain the formation of some of the world’s mountain ranges.

The first truly detailed and comprehensive theory of continental drift was proposed in 1912 by Alfred Wegener, a German meteorologist. Bringing together a large mass of geologic and paleontological data, Wegener postulated that throughout most of geologic time there was only one continent, which he called Pangea. Late in the Triassic Period (which lasted from approximately 251 million to 199.6 million years ago), Pangea fragmented, and the parts began to move away from one another. Westward drift of the Americas opened the Atlantic Ocean, and the Indian block drifted across the Equator to merge with Asia. In 1937 Alexander L. Du Toit, a South African geologist, modified Wegener’s hypothesis by suggesting two primordial continents: Laurasia in the north and Gondwana in the south.

Aside from the congruency of continental shelf margins across the Atlantic, modern proponents of continental drift have amassed impressive geologic evidence to support their views. Indications of widespread glaciation from 380 to 250 million years ago are evident in Antarctica, southern South America, southern Africa, India, and Australia. If these continents were once united around the south polar region, this glaciation would become explicable as a unified sequence of events in time and space. Also, fitting the Americas with the continents across the Atlantic brings together similar kinds of rocks, fossils, and geologic structures. A belt of ancient rocks along the Brazilian coast, for example, matches one in West Africa. Moreover, the earliest marine deposits along the Atlantic coastlines of either South America or Africa are Jurassic in age (approximately 199.6 million to 145.5 million years old), which suggests that the ocean did not exist before that time.

Interest in continental drift increased in the 1950s as knowledge of Earth’s geomagnetic field during the geologic past developed from the studies of the British geophysicists Stanley K. Runcorn, Patrick M.S. Blackett, and others. Ferromagnetic minerals such as magnetite acquire a permanent magnetization when they crystallize as constituents of igneous rock. The direction of their magnetization is the same as the direction of Earth’s magnetic field at the time and place of crystallization. Particles of magnetized minerals released from their parent igneous rocks by weathering may later realign themselves with the existing magnetic field at the time these particles are incorporated into sedimentary deposits. Studies by Runcorn of the remanent magnetism in suitable rocks of different ages from Europe produced a “polar wandering curve” indicating that the magnetic poles were in different places at different times. This could be explained either by the migration of the magnetic pole itself (that is, polar wandering) or by the migration of Europe relative to a fixed pole (that is, continental drift).

However, further work showed that the polar wandering curves are different for the various continents. The possibility that they might reflect true wander of the poles was discarded, because it implies separate wanderings of many magnetic poles over the same period. However, these different paths are reconciled by joining the continents in the manner proposed by Wegener. The curves for Europe and North America, for example, are reconciled by the assumption that the latter has drifted about 30° westward relative to Europe since the Triassic Period.

Increased knowledge about the configuration of the ocean floor and the subsequent formulation of the concepts of seafloor spreading and plate tectonics provided further support for continental drift. During the early 1960s, the American geophysicist Harry H. Hess proposed that new oceanic crust is continually generated by igneous activity at the crests of oceanic ridges—submarine mountains that follow a sinuous course of about 65,000 km (40,000 miles) along the bottom of the major ocean basins. Molten rock material from Earth’s mantle rises upward to the crests, cools, and is later pushed aside by new intrusions. The ocean floor is thus pushed at right angles and in opposite directions away from the crests.

By the late 1960s, several American investigators, among them Jack E. Oliver and Bryan L. Isacks, had integrated this notion of seafloor spreading with that of drifting continents and formulated the basis of plate tectonic theory. According to the latter hypothesis, Earth’s surface, or lithosphere, is composed of a number of large, rigid plates that float on a soft (presumably partially molten) layer of the mantle known as the asthenosphere. Oceanic ridges occur along some of the plate margins. Where this is the case, the lithospheric plates separate, and the upwelling mantle material forms new ocean floor along the trailing edges. As the plates move away from the flanks of the ridges, they carry the continents with them.

On the basis of all these factors, it may be assumed that the Americas were joined with Europe and Africa until approximately 190 million years ago, when a rift split them apart along what is now the crest of the Mid-Atlantic Ridge. Subsequent plate movements averaging about 2 cm (0.8 inch) per year have taken the continents to their present position. It seems likely, though it is still unproven, that this breakup of a single landmass and the drifting of its fragments is merely the latest in a series of similar occurrences throughout geologic time.

The Editors of Encyclopaedia Britannica This article was most recently revised and updated by Adam Augustyn, Managing Editor, Reference Content.


Fossils

THIS DYNAMIC PLANET - USGS
Dynamic Planet: A World Map of Volcanoes, Earthquakes, and continental drift&mdashhad its inception as early as the late 16th century, continental lithosphere because it is composed of heavier minerals. . Access Document

Continental Shelf - Wikipedia
The continental shelf is an underwater landmass which extends from a continent, resulting in an area of relatively shallow water known as a shelf sea. . Read Article

Continental Drift Activity Packet
Continental Drift Activity Packet Name_____ Instructions: You will be piecing together a puzzle of the supercontinent Pangea based on . Visit Document

Fossil Evidence For Continental Drift
Fossil Evidence for Continental Drift Mesosaurus Fossils of Mesosaurus, a freshwater, crocodile-like reptile that lived during the early Triassic time . Fetch Here

Continental Drift - Imaginique Bengals
Continental Drift Looking at a map of the Earth, of the continental slope &mdash the actual extent of the continental crust. Evidence for Continental Drift: Fossil Evidence Wegener found that identical fossils were located directly opposite on widely separated continents. . Read Full Source

Continental drift And Plate Tectonics - Misssimpson.com
320 Science Alive for VELS Level 5 Continental drift and plate tectonics f you look closely at a world map, you might notice something about the . Access Content

Continental Drift And Plate Tectonics - University Of Iceland
Of plant and animal fossils found on the matching coastlines of South &ndash Continental mergers triggered extinctions &ndash Separation of believe that so much continental drift could have occurred in so short time. . Retrieve Full Source

Science - Chaparral Middle School
STUDY GUIDE FOR: FOSSILS / CONTINENTAL DRIFT. What were the four different types of evidence that Wegener used to support his Continental Drift theory? - Make sure you can explain how each one is important to Wegener's theory. 20) . Retrieve Doc

LAB: WEGENER&rsquoS PUZZLING CONTINENTS
LAB: WEGENER&rsquoS PUZZLING CONTINENTS his presentation of carefully compiled evidence for continental drift inspired decades of scientific debate. Wegener's evidence, &bull Then look for fossils that extend beyond the plate boundary such as the fern Glossopteris and the . Retrieve Doc

Continental Drift Theory - 5 Evidences And 4 Major Processes
The theory of continental drift propounded by Alfred Wegner is explained in this video. Dr. Manishika Jain starts with the concept of the theory, the need for origin of this theory, 5 evidences in support of this theory which are: 1. Jig-saw fit 2. Fossils Similarities 3 . View Video

31. Fr Om Continental Drift To Plate T Ectonics: The Evidence
Continental drift consists of the shapes of the continents. continental shelves (which, one of the keys to continental drift was similarities in fossils, particularly in the five southern landmasses: . Document Viewer

Continental drift - Wikipedia, The Free Encyclopedia
From Wikipedia, the free encyclopedia Continental drift is the movement of the Earth's discovery of fossils of the aquatic reptile Lystrosaurus from rocks of Continental drift - Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Continental_drift . Fetch Content

Speciation - Evolution Definition
Speciation is the branching off of individuals from the species they originally were. transitional forms between the species were only around for such a short period of time that none were preserved as fossils and may be a "missing link" forever. . Read Article

Ice Age: Continental Drift - Geography SAC
A collaboration between KIDS DISCOVER and ICE AGE: CONTINENTAL DRIFT Ice Age: Continental Drift The Science Behind The Drift . View Full Source

Eocene - Wikipedia
The main hypothesis for such a radical transition was due to the continental drift and collision of the India continent with the Asia continent A few of these proxies include the presence of fossils native to warm climates and by the end of the Eocene continental interiors had . Read Article


Animated Life: Pangea, Wegener, and Continental Drift &mdash HHMI BioInteractive Video drift explained why continents' shapes fit together like pieces of a puzzle and why distant continents had the same fossils . Continental Drift and Plate Tectonics Theory for . View Video

SECTION 1 Continental Drift Main Idea - Young Scholar's Academy
From continental drift, seafloor spreading, and many other discoveries 8 When two continental plates move toward each other, hypothesis of continental drift. 3. Summarize the ways that fossils helped support the . View Doc

Continental Drift Theory - Mr. Pezzuto's Science Classes - Home
FOSSILS Wegener also CONTINENTAL DRIFT THEORY Continental drift is the hypothesis that all the continents had once been joined together in a single landmass. . Get Document

Geol 101: Physical Geology PAST EXAM QUESTIONS
Physical Geology PAST EXAM QUESTIONS LECTURE 3: PLATE TECTONICS I C. continental drift D. paleomagnetic reversals E. jigsaw geology 3. The first line of evidence used to promote continental drift was that B. there are similar fossils of plants and animals on different continents . Access Full Source


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1. Compare and contrast Virgin Atlantic Airway 's strategic development with any other (non-virgin) airline. According to (1980), firms are under great pressure to have modern plans to be competitive and should have adequate capability to achieve their target market share. Virgin Atlantic Airways is among the most successful business ventures of the Virgin empire owned by Richard Branson. Its strategic development is rooted on the need to surpass competitors while balancing financial resources


Facts about Continental Drift 5: Wegener’s theory

The theory of Wegener was more complete even though he made it independently. There were several past authors that he cited had the similar ideas like William Henry Pickering and Roberto Mantovani.

Facts about Continental Drift 6: Frank Bursley Taylor

Frank Bursley Taylor proposed the idea about continental drift without expansion. He believed that continental creep process occurred where the continents moved from its current position.

Facts about Continental Drift


Contents

By convention, "continents are understood to be large, continuous, discrete masses of land, ideally separated by expanses of water." [3] In modern schemes with five or more recognised continents, at least one pair of continents is joined by land in some way. The criterion "large" leads to arbitrary classification: Greenland, with a surface area of 2,166,086 square kilometres (836,330 sq mi), is considered the world's largest island, while Australia, at 7,617,930 square kilometres (2,941,300 sq mi), is deemed the smallest continent.

Earth's major landmasses all have coasts on a single, continuous World Ocean, which is divided into a number of principal oceanic components by the continents and various geographic criteria. [4] [5]

Extent Edit

The most restricted meaning of continent is that of a continuous [6] area of land or mainland, with the coastline and any land boundaries forming the edge of the continent. In this sense, the term continental Europe (sometimes referred to in Britain as "the Continent") is used to refer to mainland Europe, excluding islands such as Great Britain, Iceland, Ireland, and Malta while the term continent of Australia may refer to the mainland of Australia, excluding New Guinea, Tasmania, and other nearby islands. Similarly, the continental United States refers to the 48 contiguous states and the District of Columbia and may include Alaska in the northwest of the continent (the two being separated by Canada), while excluding Hawaii in the Pacific Ocean.

From the perspective of geology or physical geography, continent may be extended beyond the confines of continuous dry land to include the shallow, submerged adjacent area (the continental shelf) [7] and the islands on the shelf (continental islands), as they are structurally part of the continent. [8]

From this perspective, the edge of the continental shelf is the true edge of the continent, as shorelines vary with changes in sea level. [9] In this sense the islands of Great Britain and Ireland are part of Europe, while Australia and the island of New Guinea together form a continent.

As a cultural construct, the concept of a continent may go beyond the continental shelf to include oceanic islands and continental fragments. In this way, Iceland is considered part of Europe and Madagascar part of Africa. Extrapolating the concept to its extreme, some geographers group the Australian continental plate with other islands in the Pacific into one "quasi-continent" called Oceania. This divides the entire land surface of Earth into continents or quasi-continents. [10]

Separation Edit

The ideal criterion that each continent is a discrete landmass is commonly relaxed due to historical conventions. Of the seven most globally recognized continents, only Antarctica and Australia are completely separated from other continents by the ocean. Several continents are defined not as absolutely distinct bodies but as "more or less discrete masses of land". [11] Asia and Africa are joined by the Isthmus of Suez, and North and South America by the Isthmus of Panama. In both cases, there is no complete separation of these landmasses by water (disregarding the Suez Canal and Panama Canal, which are both narrow and shallow, as well as man-made). Both of these isthmuses are very narrow compared to the bulk of the landmasses they unite.

North America and South America are treated as separate continents in the seven-continent model. However, they may also be viewed as a single continent known as America. This viewpoint was common in the United States until World War II, and remains prevalent in some Asian six-continent models. [12] The single American continent model remains the more common view in France, Germany, Greece, Hungary, Italy, Malta, Portugal, Spain, and Latin American countries.

The criterion of a discrete landmass is completely disregarded if the continuous landmass of Eurasia is classified as two separate continents: Europe and Asia. Physiographically, Europe and South Asia are peninsulas of the Eurasian landmass. However, Europe is widely considered a continent with its comparatively large land area of 10,180,000 square kilometres (3,930,000 sq mi), while South Asia, with less than half that area, is considered a subcontinent. The alternative view—in geology and geography—that Eurasia is a single continent results in a six-continent view of the world. Some view separation of Eurasia into Asia and Europe as a residue of Eurocentrism: "In physical, cultural and historical diversity, China and India are comparable to the entire European landmass, not to a single European country. [. ]." [13] However, for historical and cultural reasons, the view of Europe as a separate continent continues in several categorizations.

If continents are defined strictly as discrete landmasses, embracing all the contiguous land of a body, then Africa, Asia, and Europe form a single continent which may be referred to as Afro-Eurasia. [14] Combined with the consolidation of the Americas, this would produce a four-continent model consisting of Afro-Eurasia, America, Antarctica and Australia.

When sea levels were lower during the Pleistocene ice ages, greater areas of continental shelf were exposed as dry land, forming land bridges between Tasmania and Australian mainland. At those times Australia–New Guinea was a single, continuous continent. [15] Likewise, the Americas and Afro-Eurasia were joined by the Bering Land Bridge. Other islands such as Great Britain were joined to the mainlands of their continents. At that time there were just three discrete continents: Afro-Eurasia-America, Antarctica, and Australia-New Guinea.

Number Edit

There are several ways of distinguishing the continents:

  • The seven-continent model is usually taught in most English-speaking countries including the United States, United Kingdom[36] and Australia, [37] and also in China, India, Pakistan, the Philippines, and parts of Western Europe.
  • The six-continent combined-Eurasia model is mostly used in Russia, Eastern Europe, and Japan.
  • The six-continent combined-America model is often used in Latin America, [38]Greece, [28] and countries that speak Romance languages.
  • The Olympic flag's [39] five rings represent the five inhabited continents of the combined-America model, excluding Antarctica.

As previously mentioned, some geographers use the name Oceania for a region including most of the island countries and territories in the Pacific Ocean as well as the continent of Australia.

The following table summarizes the area and population of the continental regions used by the United Nations. [40] These regions differ from the physical continents in various ways that are explained in the notes.

Continent Area (km 2 )
[ failed verification ]
Area (sq mi)
[ failed verification ]
Percent
total
landmass
[ failed verification ]
Population Percent
total pop.
Most populous
city (proper)
Africa [note 1] 30,370,000 11,730,000 20.4% 1,287,920,000 16.9% Lagos, Nigeria
Antarctica [note 2] 14,000,000 5,400,000 9.2% 4,490 [41] <0.1% McMurdo Station
Asia [note 3] 44,579,000 17,212,000 29.5% 4,545,133,000 59.5% Shanghai, China
Europe [note 4] 10,180,000 3,930,000 6.8% 742,648,000 9.7% Moscow, Russia [42]
North America [note 5] 24,709,000 9,540,000 16.5% 587,615,000 7.7% Mexico City, Mexico
South America 17,840,000 6,890,000 12.0% 428,240,000 5.6% São Paulo, Brazil
Australia [note 6] 8,600,000 3,300,000 5.9% 41,261,000 0.5% Sydney, Australia

  1. ^ Includes the Sinai Peninsula in Egypt.
  2. ^ Population is non-permanent and varies.
  3. ^ Includes East Thrace (Turkey) and Western New Guinea (Indonesia), excludes European Russia and Egypt.
  4. ^ Excludes Asiatic Russia, excludes Turkey.
  5. ^ Includes Central America and the Caribbean.
  6. ^ Excludes Indonesia.

The total land area of all continents is 148,647,000 square kilometres (57,393,000 sq mi), or 29.1% of earth's surface (510,065,600 km 2 or 196,937,400 sq mi).

Supercontinents Edit

Apart from the current continents, the scope and meaning of the term continent includes past geological ones. Supercontinents, largely in evidence earlier in the geological record, are landmasses that comprise more than one craton or continental core. These have included Laurasia, Gondwana, Vaalbara, Kenorland, Columbia, Rodinia, and Pangaea. Over time, these supercontinents broke apart into large land masses which formed the present continents.

Subcontinents Edit

Certain parts of continents are recognized as subcontinents, especially the large peninsulas separated from the main continental landmass by geographical features. The most notable examples are the Indian subcontinent and the Arabian Peninsula. [43] The Southern Cone of South America and the Alaska Peninsula of North America are other examples. [43]

In many of these cases, the "subcontinents" concerned are on different tectonic plates from the rest of the continent, providing a geological justification for the terminology. [44] Greenland, generally reckoned as the world's largest island on the northeastern periphery of the North American Plate, is sometimes referred to as a subcontinent. [45] [46] This is a significant departure from the more conventional view of a subcontinent as comprising a very large peninsula on the fringe of a continent.

Where the Americas are viewed as a single continent (America), it is divided into two subcontinents (North America and South America) [47] [48] [49] or three (with Central America being the third). [50] [51] When Eurasia is regarded as a single continent, Europe is treated as a subcontinent. [43]

Submerged continents Edit

Some areas of continental crust are largely covered by the sea and may be considered submerged continents. Notable examples are Zealandia, emerging from the sea primarily in New Zealand and New Caledonia, [52] and the almost completely submerged Kerguelen Plateau in the southern Indian Ocean. [53]

Microcontinents Edit

Some islands lie on sections of continental crust that have rifted and drifted apart from a main continental landmass. While not considered continents because of their relatively small size, they may be considered microcontinents. Madagascar, the largest example, is usually considered an island of Africa, but its divergent evolution has caused it to be referred to as "the eighth continent" from a biological perspective. [54]

Early concepts of the Old World continents Edit

The term "continent" translates Greek ἤπειρος , properly "landmass, terra firma", the proper name of Epirus and later especially used of Asia (i.e. Asia Minor), [55] The first distinction between continents was made by ancient Greek mariners who gave the names Europe and Asia to the lands on either side of the waterways of the Aegean Sea, the Dardanelles strait, the Sea of Marmara, the Bosporus strait and the Black Sea. [56] The names were first applied just to lands near the coast and only later extended to include the hinterlands. [57] But the division was only carried through to the end of navigable waterways and ". beyond that point the Hellenic geographers never succeeded in laying their finger on any inland feature in the physical landscape that could offer any convincing line for partitioning an indivisible Eurasia . " [56]

Ancient Greek thinkers subsequently debated whether Africa (then called Libya) should be considered part of Asia or a third part of the world. Division into three parts eventually came to predominate. [58] From the Greek viewpoint, the Aegean Sea was the center of the world Asia lay to the east, Europe to the north and west, and Africa to the south. [59] The boundaries between the continents were not fixed. Early on, the Europe–Asia boundary was taken to run from the Black Sea along the Rioni River (known then as the Phasis) in Georgia. Later it was viewed as running from the Black Sea through Kerch Strait, the Sea of Azov and along the Don River (known then as the Tanais) in Russia. [60] The boundary between Asia and Africa was generally taken to be the Nile River. Herodotus [61] in the 5th century BC objected to the whole of Egypt being split between Asia and Africa ("Libya") and took the boundary to lie along the western border of Egypt, regarding Egypt as part of Asia. He also questioned the division into three of what is really a single landmass, [62] a debate that continues nearly two and a half millennia later.

Eratosthenes, in the 3rd century BC, noted that some geographers divided the continents by rivers (the Nile and the Don), thus considering them "islands". Others divided the continents by isthmuses, calling the continents "peninsulas". These latter geographers set the border between Europe and Asia at the isthmus between the Black Sea and the Caspian Sea, and the border between Asia and Africa at the isthmus between the Red Sea and the mouth of Lake Bardawil on the Mediterranean Sea. [63]

Through the Roman period and the Middle Ages, a few writers took the Isthmus of Suez as the boundary between Asia and Africa, but most writers continued to consider it the Nile or the western border of Egypt (Gibbon). [ citation needed ] In the Middle Ages, the world was usually portrayed on T and O maps, with the T representing the waters dividing the three continents. By the middle of the 18th century, "the fashion of dividing Asia and Africa at the Nile, or at the Great Catabathmus [the boundary between Egypt and Libya] farther west, had even then scarcely passed away". [64]

European arrival in the Americas Edit

Christopher Columbus sailed across the Atlantic Ocean to the West Indies in 1492, sparking a period of European exploration of the Americas. But despite four voyages to the Americas, Columbus never believed he had reached a new continent—he always thought it was part of Asia.

In 1501, Amerigo Vespucci and Gonçalo Coelho attempted to sail around what they considered the southern end of the Asian mainland into the Indian Ocean, passing through Fernando de Noronha. After reaching the coast of Brazil, they sailed a long way farther south along the coast of South America, confirming that this was a land of continental proportions and that it also extended much farther south than Asia was known to. [65] On return to Europe, an account of the voyage, called Mundus Novus ("New World"), was published under Vespucci's name in 1502 or 1503, [66] although it seems that it had additions or alterations by another writer. [67] Regardless of who penned the words, Mundus Novus credited Vespucci with saying, "I have discovered a continent in those southern regions that is inhabited by more numerous people and animals than our Europe, or Asia or Africa", [68] the first known explicit identification of part of the Americas as a continent like the other three.

Within a few years, the name "New World" began appearing as a name for South America on world maps, such as the Oliveriana (Pesaro) map of around 1504–1505. Maps of this time though, still showed North America connected to Asia and showed South America as a separate land. [67]

In 1507 Martin Waldseemüller published a world map, Universalis Cosmographia, which was the first to show North and South America as separate from Asia and surrounded by water. A small inset map above the main map explicitly showed for the first time the Americas being east of Asia and separated from Asia by an ocean, as opposed to just placing the Americas on the left end of the map and Asia on the right end. In the accompanying book Cosmographiae Introductio, Waldseemüller noted that the earth is divided into four parts, Europe, Asia, Africa and the fourth part, which he named "America" after Amerigo Vespucci's first name. [69] On the map, the word "America" was placed on part of South America.

The word continent Edit

From the 16th century the English noun continent was derived from the term continent land, meaning continuous or connected land [70] and translated from the Latin terra continens. [71] The noun was used to mean "a connected or continuous tract of land" or mainland. [70] It was not applied only to very large areas of land—in the 17th century, references were made to the continents (or mainlands) of Isle of Man, Ireland and Wales and in 1745 to Sumatra. [70] The word continent was used in translating Greek and Latin writings about the three "parts" of the world, although in the original languages no word of exactly the same meaning as continent was used. [72]

While continent was used on the one hand for relatively small areas of continuous land, on the other hand geographers again raised Herodotus's query about why a single large landmass should be divided into separate continents. In the mid-17th century, Peter Heylin wrote in his Cosmographie that "A Continent is a great quantity of Land, not separated by any Sea from the rest of the World, as the whole Continent of Europe, Asia, Africa." In 1727, Ephraim Chambers wrote in his Cyclopædia, "The world is ordinarily divided into two grand continents: the old and the new." And in his 1752 atlas, Emanuel Bowen defined a continent as "a large space of dry land comprehending many countries all joined together, without any separation by water. Thus Europe, Asia, and Africa is one great continent, as America is another." [73] However, the old idea of Europe, Asia and Africa as "parts" of the world ultimately persisted with these being regarded as separate continents.

Beyond four continents Edit

From the late 18th century, some geographers started to regard North America and South America as two parts of the world, making five parts in total. Overall though, the fourfold division prevailed well into the 19th century. [74]

Europeans discovered Australia in 1606, but for some time it was taken as part of Asia. By the late 18th century, some geographers considered it a continent in its own right, making it the sixth (or fifth for those still taking America as a single continent). [74] In 1813, Samuel Butler wrote of Australia as "New Holland, an immense island, which some geographers dignify with the appellation of another continent" and the Oxford English Dictionary was just as equivocal some decades later. [75] It was in the 1950s that the concept of Oceania as a "great division" of the world was replaced by the concept of Australia as a continent. [76]

Antarctica was sighted in 1820 during the First Russian Antarctic Expedition and described as a continent by Charles Wilkes on the United States Exploring Expedition in 1838, the last continent identified, although a great "Antarctic" (antipodean) landmass had been anticipated for millennia. An 1849 atlas labelled Antarctica as a continent but few atlases did so until after World War II. [77]

From the mid-19th century, atlases published in the United States more commonly treated North and South America as separate continents, while atlases published in Europe usually considered them one continent. However, it was still not uncommon for American atlases to treat them as one continent up until World War II. [78] From the 1950s, most U.S. geographers divided the Americas into two continents. [78] With the addition of Antarctica, this made the seven-continent model. However, this division of the Americas never appealed to Latin Americans, who saw their region spanning an América as a single landmass, and there the conception of six continents remains dominant, as it does in scattered other countries. [ citation needed ]

Some geographers regard Europe and Asia together as a single continent, dubbed Eurasia. [79] In this model, the world is divided into six continents, with North America and South America considered separate continents.

Geologists use the term continent in a different manner from geographers. In geology, a continent is defined by continental crust, which is a platform of metamorphic and igneous rock, largely of granitic composition. Continental crust is less dense and much thicker than oceanic crust, which causes it to "float" higher than oceanic crust on the dense underlying mantle. This explains why the continents form high platforms surrounded by deep ocean basins. [80]

Some geologists restrict the term 'continent' to portions of the crust built around stable regions called cratons. Cratons have largely been unaffected by mountain-building events (orogenies) since the Precambrian. A craton typically consists of a continental shield surrounded by a continental platform. The shield is a region where ancient crystalline basement rock (typically 1.5 to 3.8 billion years old) is widely exposed at the surface. The platform surrounding the shield is also composed of ancient basement rock, but with a cover of younger sedimentary rock. [81] The continents are accretionary crustal "rafts" that, unlike the denser basaltic crust of the ocean basins, are not subjected to destruction through the plate tectonic process of subduction. This accounts for the great age of the rocks comprising the continental cratons. [82]

The margins of geologic continents are characterized by currently active or relatively recently active mobile belts and deep troughs of accumulated marine or deltaic sediments. Beyond the margin, there is either a continental shelf and drop off to the basaltic ocean basin or the margin of another continent, depending on the current plate-tectonic setting of the continent. A continental boundary does not have to be a body of water. [83]

By this definition, Eastern Europe, India and some other regions could be regarded as continental masses distinct from the rest of Eurasia because they have separate ancient shield areas (i.e. East European craton and Indian craton). Younger mobile belts (such as the Ural Mountains and Himalayas) mark the boundaries between these regions and the rest of Eurasia. [ citation needed ]

Plate tectonics provides yet another way of defining continents. Today, Europe and most of Asia constitute the unified Eurasian Plate, which is approximately coincident with the geographic Eurasian continent excluding India, Arabia, and far eastern Russia. India contains a central shield, and the geologically recent Himalaya mobile belt forms its northern margin. North America and South America are separate continents, the connecting isthmus being largely the result of volcanism from relatively recent subduction tectonics. North American continental rocks extend to Greenland (a portion of the Canadian Shield), and in terms of plate boundaries, the North American plate includes the easternmost portion of the Asian landmass. Geologists do not use these facts to suggest that eastern Asia is part of the North American continent, even though the plate boundary extends there the word continent is usually used in its geographic sense and additional definitions ("continental rocks," "plate boundaries") are used as appropriate. [ citation needed ]

Over geologic time, continents are periodically submerged under large epicontinental seas, and continental collisions result in a continent becoming attached to another continent. The current geologic era is relatively anomalous in that so much of the continental areas are "high and dry" that is, many parts of the continents that were once below sea level are now elevated well above it due to changes in sea levels and the subsequent uplifting of those continental areas from tectonic activity. [84]

There are many microcontinents, or continental fragments, that are built of continental crust but do not contain a craton. Some of these are fragments of Gondwana or other ancient cratonic continents: Zealandia, [85] which includes New Zealand and New Caledonia Madagascar the northern Mascarene Plateau, which includes the Seychelles. Other islands, such as several in the Caribbean Sea, are composed largely of granitic rock as well, but all continents contain both granitic and basaltic crust, and there is no clear boundary as to which islands would be considered microcontinents under such a definition. The Kerguelen Plateau, for example, is largely volcanic, but is associated with the break-up of Gondwanaland and is considered a microcontinent, [86] [87] whereas volcanic Iceland and Hawaii are not. The British Isles, Sri Lanka, Borneo, and Newfoundland are margins of the Laurasian continent—only separated by inland seas flooding its margins.

The movement of plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent that contains most or all of the continents. The supercontinent Columbia or Nuna formed during a period of 2.0–1.8 billion years ago and broke up about 1.5–1.3 billion years ago. [88] [89] The supercontinent Rodinia is thought to have formed about 1 billion years ago and to have embodied most or all of Earth's continents, and broken up into eight continents around 600 million years ago. The eight continents later re-assembled into another supercontinent called Pangaea Pangaea broke up into Laurasia (which became North America and Eurasia) and Gondwana (which became the remaining continents).

The following table lists the seven continents with their highest and lowest points on land, sorted in decreasing highest points.

Continent Highest point Elevation Country or territory containing highest point Lowest point Elevation Country or territory containing lowest point
(metres) (feet) (metres) (feet)
Asia Mount Everest 8,848 29,029 China, Nepal Dead Sea −427 −1,401 Israel, Jordan, and Palestine
South America Aconcagua 6,960 22,830 Argentina Laguna del Carbón −105 −344 Argentina
North America Denali 6,198 20,335 United States Death Valley † −86 −282 United States
Africa Mount Kilimanjaro 5,895 19,341 Tanzania Lake Assal −155 −509 Djibouti
Europe Mount Elbrus 5,642 18,510 Russia Caspian Sea −28 −92 Kazakhstan, Russia, and Azerbaijan
Antarctica Vinson Massif 4,892 16,050 None Deep Lake, Vestfold Hills † −50 −160 None ††
Australia Puncak Jaya 4,884 16,024 Indonesia (Papua) Lake Eyre −15 −49 Australia

† The lowest exposed points are given for North America and Antarctica. The lowest non-submarine bedrock elevations in these continents are the trough beneath Jakobshavn Glacier, at −1,512 metres (−4,961 ft) [90] and Bentley Subglacial Trench, at −2,540 metres (−8,330 ft), but these are covered by kilometres of ice.

Some sources list the Kuma–Manych Depression (a remnant of the Paratethys) as the geological border between Europe and Asia. [91] [92] This would place the Caucasus outside of Europe, thus making Mont Blanc (elevation 4810 m) in the Graian Alps the highest point in Europe – the lowest point would still be the shore of the Caspian Sea.


January 6, 1912: Continental Drift!

January 6, 1912 the German meteorologist Alfred Wegener presented in a lecture entitled "Die Heraushebung der Großformen der Erdrinde (Kontinente und Ozeane) auf geophysikalischer Grundlage" (The uprising of large features of earth's crust (Continents and Oceans) on geophysical basis) for the first time his hypothesis of the ancient supercontinent Pangaea, from which all modern continents split apart.

Three years later he will publish his book "Entstehung der Kontinente und Ozeane", translated in the third edition and published in 1922 as "The origin of continents and oceans."

Wegener didn't propose something completely new as he based his idea on earlier observations and suggestions, but in his work he had collected a broad array of evidence and his lectures initiated a fierce discussion in the scientific community.

In 1889 and 1909 the Italian violinist and scientist Roberto Mantovani published a hypothesis based on his observations on the volcanic island of Réunion: cracks forming during volcanic eruptions could separate even large parts of an island, could it then be possible that entire continents split apart? Mantovani collected various evidence and published maps to show the shape of the hypothetical former continents (Wegener will use these maps to support his idea), however he explained the driving force behind the breakup of former large continents by the slow expansion of the earth.

In 1908 the self-educated geologist Frank B. Taylor proposed that the crust of earth was influenced by tidal forces of the moon and the continents were pulled apart in some regions and pushed together in other regions, forming folds like a carpet. However the involved forces were to weak and his explanation wasn't deemed plausible. The Austrian geologist Otto Ampferer speculated in 1906 that the Alps were formed by folding of the upper crust, as driving force he proposed magma sinking into the mantle and pulling pieces of crust downwards (!). This "Unterströmungstheorie (also Subfluenztheorie)" lacked however a convincing source of energy and couldn't explain all aspects of the genesis of the Alps, as it implied only pulling and not compressive forces needed to form folds and overthrust faults.

Wegener became interested in the idea of a single continent in 1910, by observing an atlas and noting the coasts of the Africa and South American. Some time later he read a paleontological paper discussing the similarities of terrestrial fossil life forms between separated continents.

Wegener collected various published evidence to support his theory of a single continent:

- Like a puzzle also the outlines of continents (especially the continental shelves) seem to fit together.

- There are various geomorphologic and geological similarities along the coasts of South America-Africa and Europe-North America.

- Fossil of land vertebrates and plants can be found on different continents, separated today by large oceans.

- Fossil evidence of ancient climates, today without a recognizable pattern, will form climate zones when the continents are put together.

Wegner considered the prevailing explanation for the patterns in the fossil record as impossible: ancient land bridges that connected continents and habitats (like the Isthmus of Panama today) were composed of light continental granitic crust, such pieces of less dense rocks couldn't simply sink into the much denser oceanic basalts and disappear without trace.

He will explain in 1911 his idea in a letter to his father-in-law, Professor Wladimir Peter Köppen:

"You consider my primordial continent to be a figment of my imagination, but it is only a question of the interpretation of observations. I came to the idea on the grounds of the matching coastlines, but the proof must come from the geological observations.

These compel us to infer, for example, a land connection between South America and Africa. This can be explained in two ways: the sinking of a connecting continent or separation. Previously, because of the unproven concept of permanence, people have considered only the former and have ignored the latter possibility. But the modern teaching of isostasy and more generally our current geophysical ideas oppose the sinking of a continent because it is lighter than the material on which it rests. Thus we are forced to consider the alternative interpretation. And if we now find many surprising simplifications and can begin at last to make real sense of an entire mass of geological data, why should we delay in throwing the old concept overboard?"

Wegener hypothesis of continental drift (a catchy phrase adopted mainly by the critics, as Wegener talks more general of "displacement theory") was received with mixed feelings. Most geologists regarded it as cherry-picking of anecdotes from the literature. However some geologist with field experience, especially in Africa and South-America, became soon convinced of this possibility.

Like Taylor also Wegener could not explain the forces necessary to move the continents trough the crust. Wegener imagined the continents like gigantic ice floes swimming on and surrounded by the much denser oceanic crust. He proposed gravitational pull, tidal and centrifugal forces, but the English geophysicist Harold Jeffreys demonstrated that these forces are much too weak or if strong enough, had to stop earth's rotation.

Wegner himself reacted to the critics and tried to respond to them in various editions of his books, however with moderate success. The greatest problem remained the lack of direct evidence for the movements of continents and the needed explanation for the mechanism and the immense energy supply. Most importantly Wegener considered his work as starting point and stimulus for other or even future scientists, a message that wasn't fully understand at his time.

Fig.1. - 3. "Eppur si muove!" Reconstruction of the former supercontinent of Pangaea and the subsequent breakup in various smaller continents from the Carboniferous to the Quaternary. From WEGENER, A. (1929): Die Entstehung der Kontinente und Ozeane. 4th ed. (images in public domain).

Wegener will die in 1930. His continental drift hypothesis is in many aspects erroneous: not the single continents move but entire plates of the crust and the driving force comes from within the planet, not from outside. However his most important legacy is to have introduced the idea of moving continents to the scientific community and the public (even Lovecraft will became inspired by Wegener's writings) - decades later this legacy will influence a new kind of theory: Plate Tectonics.

MILLER, R. & ATWATER, T. (1983): Continents in Collision. Time-life books, Amsterdam: 176

SCALERA, G. (2003): Roberto Mantovani an Italian defender of the continental drift and planetary expansion. From Scalera, G. and Jacob, K.-H. (eds.): Why expanding Earth? - A book in honour of O.C. Hilgenberg. INGV, Rome: 71-74

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

ABOUT THE AUTHOR(S)

My name is David Bressan and I'm a freelance geologist working mainly in the Austroalpine crystalline rocks and the South Alpine Palaeozoic and Mesozoic cover-sediments in the Eastern Alps. I graduated with a project on Rock Glaciers dynamics and hydrology, this phase left a special interest for quaternary deposits and modern glacial environments. During my research on glaciers, studying old maps, photography and reports on the former extent of these features, I became interested in history, especially the development of geomorphologic and geological concepts by naturalists and geologists. Living in one of the key area for the history of geology, I combine field trips with the historic research done in these regions, accompanied by historic maps and depictions. I discuss broadly also general geological concepts, especially in glaciology, seismology, volcanology, palaeontology and the relationship of society and geology.


Continental Drift: The Top 25 Club Tracks of 2020

Gabe Meier meditates on the everlasting presence of Baltimore club, footwork, and litefeet culture in his end of the year reflection.

In the first Continental Drift column of 2020, we laid out a mission to address and contextualize “newly manifesting forms on their merits” and to match them with an “adaptable, energetic criticism.” Nine months later, the material foundation of the dance music industry has eroded, while its constituents are resigned to embodying their digital selves more than ever before. Still, that mission remains salient as is the necessity to move away from the commodification of music and towards a holistic understanding of the craft, which includes some of the ordinary, quotidian functions of creative life itself.

At a glance, the street dance rituals of Baltimore club, footwork, and litefeet are an odd fit with Raymond Williams’ 1958 essay “Culture is Ordinary,” but the Welsh theorist’s clarion call to understand culture as a “whole way of life” could not be more crucial to interpreting a feverish year for the dance music massive. In March, producer CalvoMusic crafted a cypher of early pandemic angst in track “Bitch We From Baltimore,” adding a Bmore classic to the canon in the process. The video tributes, and later music video, immediately gave the song, in the words of Williams, its “own shape, its own purposes, its own meanings.” In short, it retained a cultural presence beyond the phenomenon of attention deficiency and depression that Italian autonomist philosopher and media activist Franco “Bifo” Berardi referred to in his 2009 treatise on semiocapitalism The Soul at Work as the failure of “the individual brain’s limited capacities of elaboration” against an endless scroll of information or “the infinite vastness of the Infosphere.” Koppi Mizrahi’s track “Wash Your Hands Wash Your Mouth” played a similar role, immediately translating the pain, confusion, and excessive noise of early spring into an anthemic, satirical ballroom vernacular.

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Presence is also an element of what German Jewish cultural critic Walter Benjamin referred to as the “aura” of an art object in his seminal 1935 essay “The Work of Art in the Age of Mechanical Reproduction.” Aura, he claimed, depreciates as the artwork is wrenched from its ritual and historical context. Kodwo Eshun further developed Benjamin’s idea of aura about half a century later in his book on Afrofuturism More Brilliant Than The Sun: Adventures in Sonic Fiction for the age of the dubplate, arguing that the reproduction of a riddim on acetate discs destines it to be “reborn in the middle of the industrial reproduction” as a one-off remix or “third copy”—not exactly one-of-a-kind, but certainly not a mere reproduction. Yet the key to sustaining this aura is the specificity of the culture surrounding soundsystems and clashes. These are ritual operations that are present today in the sounds of artists like HANN, Shanique Marie and UNIIQU3, as well as in the appearance of a footwork dancer on the roof of a cop car during June’s uprisings in Chicago. Through engagement with the deep traditions and contemporary practices of sounds like litefeet, dancehall and Jersey club, these artists have produced sounds that transcend DJ livestreams and the collapse of industry infrastructure.

Due to stay-at-home orders and the homogenization of music consumption, it was easier than ever to engage with niche, geographically-dispersed sounds this year. Yet how much of that, unmoored from any conceivable way of life, will inevitably go down a black hole of memory? The sonic output of international clubbing brands could not hold less relevance to a dance music culture that ties the everyday life forms of artists, from DJ Manny’s transcendent performance of the Chicago canon (Teklife isn’t a misnomer) to BADSISTA’s kaleidoscopic funk carioca-rave sounds, together in brilliant symbiosis. These are discrete cultures that are heterogeneous, yet based in parallel ways of life. In essence, they retain their presence.

Aura is not inherently lost in digital reproduction, but it is lost when cultural forms are denuded of the ritual building blocks that contextualize them in history and within contemporary social life. The rave, especially in its most commercial configurations in cities like Berlin, London, and New York, has been proven to be ephemeral in 2020, but ballroom’s family structure, Chicago’s lineage of sound and movement, and the street-level dance culture of litefeet and flex dance music, evince a timeless resilience. This is not down to any proven sonic hierarchy, but to their basis in language, history, and dance traditions. In “Culture is Ordinary,” Williams understood that the challenge of the era was to forge a “good common culture.” As the industry itself desiccates and falls away, this simple message cannot be understated.

The Top 25 Club Tracks of 2020

  1. Amazondotcom & Siete Catorce – Shells
  2. BADSISTA – XERECARIA (ft. TAYA)
  3. CalvoMusic – Bitch We From Baltimore (feat. Charles)
  4. DJAaron – 6 MILLION WAYS
  5. Dj Deekie West – THE PERFECT BEAT
  6. DJ MACK – AS PIRANHA DE BH MR BIM
  7. DJ Manny – U Got Me Dancing
  8. DJ Sliink & DJ Jayhood – Jersey
  9. DJ SWISHA & Kush Jones – Torcida
  10. DJ Wawa- What It Means (DJ Delish’s West Philly Dub)
  11. Dominowe – The Prayer feat. Mr Alhab (Core Tribe)
  12. FITNESSS – DÆ1R (ft. DÆMON & SAJDAH)
  13. Fridge, Organ Tapes & Yayoyanoh – Stunna
  14. HANN – Zilla
  15. Jlin – Lotus
  16. Kamixlo – Sick
  17. Koppi Mizrahi – Wash Your Hands Wash Your Mouth
  18. Leonce x Neana – White Tee (Summer Walker)
  19. Loraine James – Mmm
  20. NÍDIA – CHEF
  21. Shanique Marie – Ring The Alarm
  22. slikback – WAXFIGURE
  23. TAYHANA – Amarte Azul
  24. UNIIQU3 – 7 DAY WEEKENDS
  25. Zebra Katz – LICK IT N SPLIT (ft. Shygirl)

Gabe Meier is based in Chicago and has been writing about the latest evolutions in dancefloor music since 2012. The Astral Plane is his primary outlet, spanning a blog, record label and radio show. Follow him on Twitter here.


In structuralist literature (Balzer et al. 1987, 167–177), the notion of a theory-net is defined by establishing two conditions: 1) that there should exist a finite non-empty set of theory-elements T E and a specialization relation σ 2) that the specialization relation be restricted to the set T E . Given that the definition of a theory-net presupposes that of specialisation, the principle defining features of this should be mentioned, even if in passing. They are as follows: a) equality between the classes of potential models and partially potential models of the respective related theory-elements and, b) the inclusion of the current class of models, the class of constraints, the class of links and that of intended applications of the (resulting) specialized theory, respectively, in the class of actual models, the class of constraints, the class of links, and that of the intended applications of the theory that is specialized. Expressed in other terms, two theory-elements that are related by means of a specialization relation will share their conceptual apparatus, while they will diverge with regard to the scope of their laws and, consequently, with regard to the extension of their classes of intended applications, since the theory-element that specializes restricts the laws and the empirical scope of the specialized theory-element.

One of the few places (if not the only one) where Kuhn elaborates on this matter is 1962/1970, 95–97.

The reader should note that the symbol ‘ (hat< in >) ’ expresses a relation between a component of a structure and the structure itself, and so it is not to be confused with the element-of symbol ‘∈’ used later, which expresses a relation between an element and a set.

Here we are assuming the notion of substructure that is standard in model theory. Under this characterization, a structure S is a substructure of other S’ when the domains of S are proper or improper subsets of the domains of S’, and, therefore, the relations of S are restrictions over the relations of S’.

Function ‘r’ assigns to every actual model ‘m’ the corresponding partial potential model ‘y’ that results from cutting the T-theoretical concepts from ‘m’.

Furthermore, in Kuhn’s later proposal (1983, 670–1), he explicitly characterizes the notion of incommensurability as a local or partial relation between theories.

By stressing the importance of evidence quality as the driving force in the career of geological theories, she therefore also opposes the popular view put forward by Stephen Jay Gould (1977), which implied that the initial rejection of drift theory was due to a lack of an adequate mechanism to move continents through a static ocean floor.

Kinetic friction is thus one of the parameters common to both DRIFT and TEC. To be more faithful to the historical background, it would be necessary to distinguish between viscous friction and rigid friction. Wegener postulated a viscous friction that would operate between continents and sea floors, since he conjectured that there was a process of plastic yielding of the oceanic "sima" and likewise a process of plastic folding (mountain building) of the continental "sial" upon lateral compression. On the other hand, by adding the concept of rigid plate, TEC also contains viscous friction both at the plates' bottom (convection as a plate driving force) and between continents as well as rigid friction at the plates' margins (as the cause of earthquakes). However, for reasons of parsimony, the above distinctions have not been included in the formal reconstruction of both theories.

In ‘xDi’ ‘i’ is a sub-index of a sub-index (‘T’). The same goes for ‘xTk’, ‘yDi’, yTk’.


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This four-volume treatise on the continental drift controversy is the first complete history of the revolutionary theory of plate tectonics. Based on extensive interviews, archival papers and original works, Frankel weaves together the lives and work of the scientists involved, producing an accessible narrative for scientists and non-scientists alike. Volume 1 : Wegener and the Early Debate Volume 2 : Paleomagnetism and Confirmation of Drift Volume 3 : Introduction of Seafloor Spreading Volume 4 : Evolution into Plate Tectonics.

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Resolution of the sixty-year debate over continental drift, culminating in the triumph of plate tectonics, changed the very fabric of Earth science. This four-volume treatise on the continental drift controversy is the first complete history of the origin, debate and gradual acceptance of this revolutionary theory. Based on extensive interviews, archival papers and original works, Frankel weaves together the lives and work of the scientists involved, producing an accessible narrative for scientists and non-scientists alike. Volume 1 covers the early 1900s when Wegener first proposed that the continents had once been a single landmass. Volume 2 describes the growing paleomagnetic case for continental drift in the 1950s and development of Apparent Polar Wander Paths. Volume 3 describes the expansion of the land-based paleomagnetic case for drifting continents, and Volume 4 recounts the discovery of geomagnetic reversals leading to the rapid acceptance of seafloor spreading and the birth of plate tectonics.


Watch the video: Earth Science: Lecture 3 - Minerals