WOW: 2012

Sunday, 21 October 2012

Computational inference of aesthetics

Since about 2005, computer scientists have attempted to develop automated methods to infer aesthetic quality of images.^[48]^[49]^[50] Typically, these approaches follow a machine learning approach, where large numbers of manually rated photographs are used to "teach" a computer about what visual properties are of relevance to aesthetic quality. The Acquine engine, developed at Penn State University, rates natural photographs uploaded by users.^[51]
Notable in this area is Michael Leyton, professor of psychology at Rutgers University. Leyton is the president of the International Society for Mathematical and Computational Aesthetics and the International Society for Group Theory in Cognitive Science and has developed a generative theory of shape.
There have also been relatively successful attempts with regard to chess and music

Aesthetic ethics

Aesthetic ethics refers to the idea that human conduct and behaviour ought to be governed by that which is beautiful and attractive. John Dewey^[43] has pointed out that the unity of aesthetics and ethics is in fact reflected in our understanding of behaviour being "fair" - the word having a double meaning of attractive and morally acceptable. More recently, James Page^[44] has suggested that aesthetic ethics might be taken to form a philosophical rationale for peace education

Aesthetics and information

In the 1970s, Abraham Moles and Frieder Nake were among the first to analyze links between aesthetics, information processing, and information theory.^[29]^[30]
In the 1990s, Jürgen Schmidhuber described an algorithmic theory of beauty which takes the subjectivity of the observer into account and postulates: among several observations classified as comparable by a given subjective observer, the aesthetically most pleasing one is the one with the shortest description, given the observer’s previous knowledge and his particular method for encoding the data.^[31]^[32] This is closely related to the principles of algorithmic information theory and minimum description length. One of his examples: mathematicians enjoy simple proofs with a short description in their formal language. Another very concrete example describes an aesthetically pleasing human face whose proportions can be described by very few bits of information,^[33]^[34] drawing inspiration from less detailed 15th century proportion studies by Leonardo da Vinci and Albrecht Dürer. Schmidhuber's theory explicitly distinguishes between what's beautiful and what's interesting, stating that interestingness corresponds to the first derivative of subjectively perceived beauty. Here the premise is that any observer continually tries to improve the predictability and compressibility of the observations by discovering regularities such as repetitions and symmetries and fractal self-similarity. Whenever the observer's learning process (which may be a predictive neural network - see also Neuroesthetics) leads to improved data compression such that the observation sequence can be described by fewer bits than before, the temporary interestingness of the data corresponds to the number of saved bits. This compression progress is proportional to the observer's internal reward, also called curiosity reward. A reinforcement learning algorithm is used to maximize future expected reward by learning to execute action sequences that cause additional interesting input data with yet unknown but learnable predictability or regularity. The principles can be implemented on artificial agents which then exhibit a form of artificial curiosity.

Evolutionary aesthetics

Evolutionary aesthetics refers to evolutionary psychology theories in which the basic aesthetic preferences of Homo sapiens are argued to have evolved in order to enhance survival and reproductive success. One example being that humans are argued to find beautiful and prefer landscapes which were good habitats in the ancestral environment. Another example is that body symmetry is an important aspect of physical attractiveness which may be due to this indicating good health during body growth. Evolutionary explanations for aesthetical preferences are important parts of evolutionary musicology, Darwinian literary studies, and the study of the evolution of emotion

Post-modern aesthetics and psychoanalysis

Early-twentieth-century artists, poets and composers challenged existing notions of beauty, broadening the scope of art and aesthetics. In 1941, Eli Siegel, American philosopher and poet, founded Aesthetic Realism, the philosophy that reality itself is aesthetic, and that "The world, art, and self explain each other: each is the aesthetic oneness of opposites."^[14]^[15]
Various attempts have been made to define Post-modern aesthetics. The challenge to the assumption that beauty was central to art and aesthetics, thought to be original, is actually continuous with older aesthetic theory; Aristotle was the first in the Western tradition to classify "beauty" into types as in his theory of drama, and Kant made a distinction between beauty and the sublime. What was new was a refusal to credit the higher status of certain types, where the taxonomy implied a preference for tragedy and the sublime to comedy and the Rococo.
Croce suggested that “expression” is central in the way that beauty was once thought to be central. George Dickie suggested that the sociological institutions of the art world were the glue binding art and sensibility into unities.^[16] Marshall McLuhan suggested that art always functions as a "counter-environment" designed to make visible what is usually invisible about a society.^{[page needed]} Theodor Adorno felt that aesthetics could not proceed without confronting the role of the culture industry in the commodification of art and aesthetic experience. Hal Foster attempted to portray the reaction against beauty and Modernist art in The Anti-Aesthetic: Essays on Postmodern Culture. Arthur Danto has described this reaction as "kalliphobia" (after the Greek word for beauty - 'kalos').^[17] André Malraux explains that the notion of beauty was connected to a particular conception of art that arose with the Renaissance and was still dominant in the eighteenth century (but was supplanted later). The discipline of aesthetics, which originated in the eighteenth century, mistook this transient state of affairs for a revelation of the permanent nature of art.^[18] Brian Massumi suggests to reconsider beauty following the aesthetical thought in the philosophy of Deleuze and Guattari.^[19]
Daniel Berlyne created the field of experimental aesthetics in the 1970s, for which he is still the most cited individual decades after his death.^[20]
Pneumaist aestheticism is a theory of art and a highly experimental approach to art negating historical preconceptions of the aesthetic.
Jean-François Lyotard re-invokes the Kantian distinction between taste and the sublime. Sublime painting, unlike kitsch realism, "...will enable us to see only by making it impossible to see; it will please only by causing pain."^[21]^[22]
Sigmund Freud inaugurated aesthetical thinking in Psychoanalysis mainly via the "Uncanny" as aesthetical affect.^[23] Following Freud and Merleau-Ponty,^[24] Jacques Lacan theorized aesthetics in terms of sublimation and the Thing^[25]
Guy Sircello pioneered efforts in analytic philosophy to develop a rigorous theory of aesthetics, focusing on the concepts of beauty,^[26] love^[27] and sublimity.^[28] In contrast to romantic theorists Sircello argued for the objectivity of beauty and formulated a theory of love on that basis.

Modern aesthetics

From the late 17th to the early 20th century Western aesthetics underwent a slow revolution into what is often called modernism. German and British thinkers emphasised beauty as the key component of art and of the aesthetic experience, and saw art as necessarily aiming at absolute beauty.
For Alexander Gottlieb Baumgarten aesthetics is the science of the sense experiences, a younger sister of logic, and beauty is thus the most perfect kind of knowledge that sense experience can have. For Immanuel Kant the aesthetic experience of beauty is a judgment of a subjective but similar human truth, since all people should agree that “this rose is beautiful” if it in fact is. However, beauty cannot be reduced to any more basic set of features. For Friedrich Schiller aesthetic appreciation of beauty is the most perfect reconciliation of the sensual and rational parts of human nature.
For Friedrich Wilhelm Joseph Schelling, the philosophy of art is the "organon" of philosophy concerning the relation between man and nature. So aesthetics began now to be the name for the philosophy of art. Friedrich von Schlegel, August Wilhelm Schlegel, Friedrich Schleiermacher and Georg Wilhelm Friedrich Hegel have also given lectures on aesthetics as philosophy of art after 1800.
For Hegel all culture is a matter of "absolute spirit" coming to be manifest to itself, stage by stage, changing to a perfection that only philosophy can approach. Art is the first stage in which the absolute spirit is manifest immediately to sense-perception, and is thus an objective rather than subjective revelation of beauty.
For Arthur Schopenhauer aesthetic contemplation of beauty is the most free that the pure intellect can be from the dictates of will; here we contemplate perfection of form without any kind of worldly agenda, and thus any intrusion of utility or politics would ruin the point of the beauty. It is thus for Schopenhauer one way to fight the suffering.
The British were largely divided into intuitionist and analytic camps. The intuitionists believed that aesthetic experience was disclosed by a single mental faculty of some kind. For Anthony Ashley-Cooper, 3rd Earl of Shaftesbury this was identical to the moral sense, beauty just is the sensory version of moral goodness. For Ludwig Wittgenstein aesthetics consisted in the description of a whole culture which is a linguistic impossibility. That which constitutes aesthetics lies out side the realm of the language game.
For Oscar Wilde the contemplation of beauty for beauty's sake was not only the foundation for much of his literary career but was quoted as saying "Aestheticism is a search after the signs of the beautiful. It is the science of the beautiful through which men seek the correlation of the arts. It is, to speak more exactly, the search after the secret of life.".^[12]
Wilde famously toured the United States in 1882. He travelled across the United States spreading the idea of Aesthetics in a speech called "The English Renaissance." In his speech he proposed that Beauty and Aesthetics was "not languid but energetic. By beautifying the outward aspects of life, one would beautify the inner ones." The English Renaissance was, he said, "like the Italian Renaissance before it, a sort of rebirth of the spirit of man

Western medieval aesthetics

Surviving medieval art is primarily religious in focus and funded largely by the State, Roman Catholic or Orthodox church, powerful ecclesiastical individuals, or wealthy secular patrons. These art pieces often served a liturgical function, whether as chalices or even as church buildings themselves. Objects of fine art from this period were frequently made from rare and valuable materials, such as gold and lapis, the cost of which commonly exceeded the wages of the artist.
Medieval aesthetics in the realm of philosophy built upon Classical thought, continuing the practice of Plotinus by employing theological terminology in its explications. St. Bonaventure’s “Retracing the Arts to Theology”, a primary example of this method, discusses the skills of the artisan as gifts given by God for the purpose of disclosing God to mankind, which purpose is achieved through four lights: the light of skill in mechanical arts which discloses the world of artifacts; which light is guided by the light of sense perception which discloses the world of natural forms; which light, consequently, is guided by the light of philosophy which discloses the world of intellectual truth; finally, this light is guided by the light of divine wisdom which discloses the world of saving truth.
Saint Thomas Aquinas's aesthetic is probably the most famous and influential theory among medieval authors, having been the subject of much scrutiny in the wake of the neo-Scholastic revival of the late 19th and early 20th centuries and even having received the approbation of the celebrated Modernist writer, James Joyce. Thomas, like many other medievals, never gives a systematic account of beauty itself, but several scholars have conventionally arranged his thought—though not always with uniform conclusions—using relevant observations spanning the entire corpus of his work. While Aquinas's theory follows generally the model of Aristotle, he develops a singular aesthetics which incorporates elements unique to his thought. Umberto Eco's The Aesthetics of Thomas Aquinas identifies the three main characteristics of beauty in Aquinas's philosophy: integritas sive perfectio, consonantia sive debita proportio, and claritas sive splendor formae. While Aristotle likewise identifies the first two characteristics, St. Thomas conceives of the third as an appropriation from principles developed by neo-Platonic and Augustinian thinkers.

Lorsch Gospels 778–820. Charlemagne's Court School.

With the shift from the Middle Ages to the Renaissance, art likewise changed its focus, as much in its content as in its mode of expression.

African aesthetics

The Great Mosque's signature trio of minarets overlooks the central market of Djenné. Unique Malian aesthetic

African art existed in many forms and styles, and with fairly little influence from outside Africa. Most of it followed traditional forms and the aesthetic norms were handed down orally as well as written. Sculpture and performance art are prominent, and abstract and partially abstracted forms are valued, and were valued long before influence from the Western tradition began in earnest. The Nok culture is testimony to this. The mosque of Timbuktu shows that specific areas of Africa developed unique aesthetics

Indian aesthetics

Indian art evolved with an emphasis on inducing special spiritual or philosophical states in the audience, or with representing them symbolically. According to Kapila Vatsyayan, "Classical Indian architecture, sculpture, painting, literature (kāvya), music, and dancing evolved their own rules conditioned by their respective media, but they shared with one another not only the underlying spiritual beliefs of the Indian religio-philosophic mind, but also the procedures by which the relationships of the symbol and the spiritual states were worked out in detail."
In the Pan Indian philosophic thought the term 'Satyam Shivam Sundaram' is another name for the concept of the Supreme. 'Sat' is the truth value, 'Shiv' is the good value & 'Sundaram' is the beauty value. Man through his 'Srabana' or education, 'Manana' or experience and conceptualization and 'Sadhana' or practice, through different stages of life (Asramas) comes to form and realize the idea of these three values to develop a value system. This Value-system helps us to develop two basic ideas 1) that of 'Daksha' or the adept/expert and 2) of Mahana/Parama or the Absolute and thus to judge anything in this universe in the light of these two measures, known as 'Adarsha'. A person who has mastered great amounts of knowledge of the grammars, rules, & language of an art-form are adepts (Daksha), whereas those who have worked through the whole system and journeyed ahead of these to become a law unto themselves is called a Mahana. Individuals idea of 'Daksha' and 'Mahana' is relative to one's development of the concept of 'Satyam-Shivam-Sundaram.' For example, Tagore's idea of these two concepts should be way above any common man's and many perceive Tagore as a 'Mahana' Artist in the realm of literature. This concept of Satyam-Shivam-Sundaram, a kind of Value Theory is the cornerstone of Indian Aesthetics.
Of particular concern to Indian drama and literature are the term 'Bhava' or the state of mind and rasa referring generally to the emotional flavors/essence crafted into the work by the writer and relished by a 'sensitive spectator' or sahṛdaya or one with positive taste and mind. Poets like Kālidāsa were attentive to rasa, which blossomed into a fully developed aesthetic system. Even in contemporary India the term rasa denoting "flavor" or "essence" is used colloquially to describe the aesthetic experiences in films; "māsala mix" describes popular Hindi cinema films which serve a so-called balanced emotional meal for the masses, savored as rasa by these spectators.
Rasa theory blossoms beginning with the Sanskrit text Nātyashāstra (nātya meaning "drama" and shāstra meaning "science of"), a work attributed to Bharata Muni where the Gods declare that drama is the 'Fifth Veda' because it is suitable for the degenerate age as the best form of religious instruction. While the date of composition varies wildly among scholars, ranging from the era of Plato and Aristotle to the seventh century CE. The Nātyashāstra presents the aesthetic concepts of rasas and their associated bhāvas in Chapters Six and Seven respectively, which appear to be independent of the work as a whole. Eight rasas and associated bhāvas are named and their enjoyment is likened to savoring a meal: rasa is the enjoyment of flavors that arise from the proper preparation of ingredients and the quality of ingredients. What rasa actually is, in a theoretical sense, is not discussed and given the Nātyashāstra's pithy wording it is unlikely the exact understanding of the original author(s) will be known.
The theory of the rasas develops significantly with the Kashmiri aesthetician Ãndandavardhana's classic on poetics, the Dhvanyāloka which introduces the ninth rasa, shānta-rasa as a specifically religious feeling of peace (śānta) which arises from its bhāva, weariness of the pleasures of the world. The primary purpose of this text is to refine the literary concept dhvani or poetic suggestion, by arguing for the existence of rasa-dhvani, primarily in forms of Sanskrit including a word, sentence or whole work "suggests" a real-world emotional state or bhāva, but thanks to aesthetic distance, the sensitive spectator relishes the rasa, the aesthetic flavor of tragedy, heroism or romance.
The 9th - 10th century master of the religious system known as "the nondual Shaivism of Kashmir" (or "Kashmir Shaivism") and aesthetician, Abhinavagupta brought rasa theory to its pinnacle in his separate commentaries on the Dhvanyāloka, the Dhvanyāloka-locana (translated by Ingalls, Masson and Patwardhan, 1992) and the Abhinavabharati, his commentary on the Nātyashāstra, portions of which are translated by Gnoli and Masson and Patwardhan. Abhinavagupta offers for the first time a technical definition of rasa which is the universal bliss of the Self or Atman colored by the emotional tone of a drama. Shānta-rasa functions as an equal member of the set of rasas but is simultaneously distinct being the most clear form of aesthetic bliss. Abhinavagupta likens it to the string of a jeweled necklace; while it may not be the most appealing for most people, it is the string that gives form to the necklace, allowing the jewels of the other eight rasas to be relished. Relishing the rasas and particularly shānta-rasa is hinted as being as-good-as but never-equal-to the bliss of Self-realization experienced by yogis.

Islamic aesthetics

Islamic art does not pertain to religion only. The term "Islamic" refers not only to the religion, but to any form of art created in an Islamic culture or in an Islamic context. Not all Muslims are in agreement on the use of art in religious observance, the proper place of art in society, or the relation between secular art and the demands placed on the secular world to conform to religious precepts. Islamic art frequently adopts secular elements and elements that are frowned upon, if not forbidden, by some Islamic theologians.^[9]
According to Islam, human works of art are inherently flawed compared to the work of God; thus, it is believed by many that attempting to realistically depict the form of an animal or person is insolence to God. This tendency effected the narrowing field of artistic possibility to such forms of art as Arabesque, mosaic, Islamic calligraphy, and Islamic architecture, as well as any form of abstraction that can claim the status of non-representational art.
Limited possibilities have been explored by artists as an outlet to artistic expression, and has been cultivated to become a positive style and tradition, emphasizing the decorative function of art, or its religious functions via non-representational forms such as Geometric patterns, floral patterns, and arabesques.
Human or animal depiction is generally forbidden. Muslims believe these depictions lead to sculptural pieces, which then leads to worship of that sculpture or "idol". Human portrayals can be found in early Islamic cultures with varying degrees of acceptance by religious authorities. Human representation for the purpose of worship is uniformly considered idolatry as forbidden in Sharia law.^[10]^[11]
The calligraphic arts grew out of an effort to devote oneself to the study of the Quran. By patiently transcribing each word of the text, the writer was made to contemplate the meaning of it. As time passed, these calligraphic works began to be prized as works of art, growing increasingly elaborate in the illumination and stylizing of the text. These illuminations were applied to other works besides the Quran, and it became a respected art form in and of itself.

Ancient aesthetics

Examples of pre-historic art are rare. The context of their production and use is unclear. Aesthetic doctrines that guided their production and interpretation are mostly unknown.
Ancient art was largely, but not entirely, based on the nine great ancient civilizations: Egypt, Mesopotamia, Persia, Greece, China, Rome, India, the Celtic peoples, and Maya. Each of these centers of early civilization developed a unique and characteristic style in its art. Greece had the most influence on the development of aesthetics in the West. This period of Greek art saw a veneration of the human physical form and the development of corresponding skills to show musculature, poise, beauty and anatomically correct proportions. Furthermore, in many Western and Eastern cultures alike, traits such as body hair are rarely depicted in art that addresses physical beauty.^{[citation needed]} In contrast with this Greek-Western aesthetic taste is the genre of the grotesque.^[7]
Greek philosophers initially felt that aesthetically appealing objects were beautiful in and of themselves. Plato believed that for us to have a perception of beauty there must be a transcendent form for beauty in which beautiful objects partake and which causes them to be beautiful also. He felt that beautiful objects incorporated proportion, harmony, and unity among their parts. Similarly, in the Metaphysics, Aristotle found that the universal elements of beauty were order, symmetry, and definiteness. An example of ancient aesthetics in Greece through poetry is Plato's quote: "For the authors of those great poems which we admire, do not attain to excellence through the rules of any art; but they utter their beautiful melodies of verse in a state of inspiration, and, as it were, possessed by a spirit not their own.

BEAUTY

David Lance Goines

Version of November 15, 1995(Begun on September 23, 1989)

"Beauty is truth, truth beauty;"
that is all Ye
know on earth, and all ye need to know.

- John Keats (1795-1821), Ode on a Grecian Urn (29)

And the true order of going, or being led by another, to the things of love, is to begin from the beauties of earth and mount upwards for the sake of that other beauty, using these steps only, and from one going on to two, and from two to all fair forms to fair practices, and from fair practices to fair notions, until from fair notions he arrives at the notion of absolute beauty, and at last knows what the essence of beauty is. - Plato (c 428-c 348 BC) Dialogues, Symposium 211 (30)

MAYBE THIS IS JUST A BAD TRANSLATION. Though the impenetrable load of double talk that Plato has here shoveled out sounds nice (especially if you don't really think about it but just let the words wash over you like warm honey, and don't actually have to go out and do something with them), it isn't good for much. When you try to figure out how you can actually use Plato's ideas to make something beautiful, or evaluate something to discover whether it is beautiful or not, you find that this sort of philosophical lumber lets you down rather badly.

So to begin with, let's just forget about totalitarian anti-art Plato and his incomprehensible ideal forms and the other-worldly mystic Saint Thomas Aquinas, nasty lunatic John Ruskin and that sausage-gobbling Kraut Hegel and all those old frauds and their transcendent hogwash.

Beauty is real. Beauty is the expression through art of wealth and power. The vehicle by which beauty comes into the world is art; anticipating the ideals of wealth and power, art gives form to the standards by which society judges itself.

Art creates beauty. Art is the vanguard of taste, trumpeting fashion before it actually exists.

Art, like science, goes where the money is. If you follow the history of art, you also follow the history of political power. Where is the nexus of culture? Why, it is always where the most impressive military and economic society of the day holds sway. Babylonia, Egypt, Athens, Rome, Florence, London, Paris, New York, Los Angeles, Tokyo. These are, or have been, centers of beauty, taste and art. Not coincidentally, these also are or were centers of political and financial power. Artists are paid to tell everybody what beauty is, and to display that beauty for the glorification of their patrons.

Beauty is an index of leisure, which is itself an index of wealth, which is an index of power. Flower arranging, for example, takes a long time to learn and a long time to learn to appreciate. Poor people do not acquire these refined tastes. For the poor, these tastes do not exist. The subtleties of such things are totally lost on the poor. The poor do not much like the art of Rauschenberg, Oldenberg, Klee, Arp or Pollock. If they think of them at all, they think they're silly. Let's face it: only the rich can afford aesthetics. When the poor want to become like the rich they emulate the tastes of the rich.

About the only thing that is constant in beauty is that it is the opposite of ugly. (31) Ugly, too is a constant. It is whatever the rich, healthy, youthful, strong and powerful are not doing.

Beauty is constantly changing, and culture-bound. What one person at one time finds beautiful, another person from another culture will often finds ludicrous, incomprehensible and ugly. So few of our young women wear brass hoops that stretch the neck, plates in their lips and heavy facial or body tattooing. Hardly any men on the streets of New York sport a penis sheath. We do not dye ourselves blue.

"Clothes maketh the man." - Mark Twain (attributed)

As an example of beauty in small, let us examine the infinitely fascinating arena of clothing fashions.

First, what was fashionable (beautiful) yesterday is absurdly unfashionable (ugly) today. We can tolerate outdated fashion in specific contexts, such as period costume in a play or film (though indeed it is usually heavily modified to suit the modern aesthetic), but in real life outdated fashion is not attractive. The more outdated it gets, the more ugly it becomes.

Eighteenth and nineteenth century fashion makes much of the conspicuous, even lavishly wasteful, use of fabric. Fabric, especially fancy fabric, was expensive, and since everything was made by hand, clothing was even more so. Common people had few clothes. Rich people had many clothes of relatively sumptuous make. Rich people kept up with fashion, and poor people mostly didn't. What rich people wore was, by definition, beautiful. What poor people wore was, by definition, not. Rich people had window curtains, and the poor who emulated them, such as my "lace-curtain Irish," forebears, strove to work the sympathetic magic and get rich by copying the rich.

In the latter part of our own dangerous century, we see little in the way of obvious contrast between the clothing of rich and poor. We have adopted as our models the class of performing artists (rock stars, movie stars) whose clothing is more a product of the imagination (Flashdance, Saturday Night Fever) than a concession to either the elements or outward signs of wealth. We have put most of our effort into the body itself, neglecting the outer integument. The poor have as little ability to be "body fashionable" now as they did to be "clothing fashionable" in the 19th century. Rich people jog and have a membership in a gym; they watch their diets and are concerned with cholesterol; they do not smoke; they do not drink to excess ("Just Perrier, please"); they do not take drugs; they practice safe sex; they wear their seatbelts. Poor people don't do any of this stuff.

Throughout most of the world's history, fatness was admired as a sign of wealth, health and fertility; thinness was a sign of poverty, disease and barrenness. In the case of fatness in a time of general food shortage, the wealthy person is beautiful because he doesn't look poor. In the case of thinness in a time of plenty, the wealthy person is slender and athletic by way of contrast to those who have little leisure for sport and health maintenance.

Continental drift

For more details on this topic, see Continental drift.

In the late 19th and early 20th centuries, geologists assumed that the Earth's major features were fixed, and that most geologic features such as basin development and mountain ranges could be explained by vertical crustal movement, described in what is called the geosynclinal theory. Generally, this was placed in the context of a contracting planet Earth due to heat loss in the course of a relatively short geological time.

Alfred Wegener in Greenland in the winter of 1912-13.

It was observed as early as 1596 that the opposite coasts of the Atlantic Ocean—or, more precisely, the edges of the continental shelves—have similar shapes and seem to have once fitted together.^[34]
Since that time many theories were proposed to explain this apparent complementarity, but the assumption of a solid Earth made these various proposals difficult to accept.^[35]
The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of the Earth.^[36] since this had previously been estimated by its cooling rate and assumption the Earth's surface radiated like a black body.^[37] Those calculations had implied that, even if it started at red heat, the Earth would have dropped to its present temperature in a few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that the Earth would be much older, and that its core was still sufficiently hot to be liquid.
By 1915, after having published a first article in 1912,^[38] Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans.^[27] In that book (re-issued in four successive editions up to the final one in 1936), he noted how the east coast of South America and the west coast of Africa looked as if they were once attached. Wegener was not the first to note this (Abraham Ortelius, Snider-Pellegrini, Eduard Suess, Roberto Mantovani and Frank Bursley Taylor preceded him just to mention a few), but he was the first to marshal significant fossil and paleo-topographical and climatological evidence to support this simple observation (and was supported in this by researchers such as Alex du Toit). Furthermore, when the rock strata of the margins of separate continents are very similar it suggests that these rocks were formed in the same way, implying that they were joined initially. For instance, parts of Scotland and Ireland contain rocks very similar to those found in Newfoundland and New Brunswick. Furthermore, the Caledonian Mountains of Europe and parts of the Appalachian Mountains of North America are very similar in structure and lithology.
However, his ideas were not taken seriously by many geologists, who pointed out that there was no apparent mechanism for continental drift. Specifically, they did not see how continental rock could plow through the much denser rock that makes up oceanic crust. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930.

Summary

Detailed map showing the tectonic plates with their movement vectors.

In line with other previous and contemporaneous proposals, in 1912 the meteorologist Alfred Wegener amply described what he called continental drift, expanded in his 1915 book The Origin of Continents and Oceans^[27] and the scientific debate started that would end up fifty years later in the theory of plate tectonics.^[28] Starting from the idea (also expressed by his forerunners) that the present continents once formed a single land mass (which was called Pangea later on) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt.^[29] Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast, and from the matching of the rock formations along these edges. Confirmation of their previous contiguous nature also came from the fossil plants Glossopteris and Gangamopteris, and the therapsid or mammal-like reptile Lystrosaurus, all widely distributed over South America, Africa, Antarctica, India and Australia. The evidence for such an erstwhile joining of these continents was patent to field geologists working in the southern hemisphere. The South African Alex du Toit put together a mass of such information in his 1937 publication Our Wandering Continents, and went further than Wegener in recognising the strong links between the Gondwana fragments.
But without detailed evidence and a force sufficient to drive the movement, the theory was not generally accepted: the Earth might have a solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Distinguished scientists, such as Harold Jeffreys and Charles Schuchert, were outspoken critics of continental drift.
Despite much opposition, the view of continental drift gained support and a lively debate started between "drifters" or "mobilists" (proponents of the theory) and "fixists" (opponents). During the 1920s, 1930s and 1940s, the former reached important milestones proposing that convection currents might have driven the plate movements, and that spreading may have occurred below the sea within the oceanic crust. Concepts close to the elements now incorporated in plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove.
One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a variable magnetic field direction, evidenced by studies since the mid–nineteenth century. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time. Initially, during the first half of the twentieth century, the latter phenomenon was explained by introducing what was called "polar wander" (see apparent polar wander), i.e., it was assumed that the north pole location had been shifting through time. An alternative explanation, though, was that the continents had moved (shifted and rotated) relative to the north pole, and each continent, in fact, shows its own "polar wander path". During the late 1950s it was successfully shown on two occasions that these data could show the validity of continental drift: by Keith Runcorn in a paper in 1956,^[30] and by Warren Carey in a symposium held in March 195

Relative significance of each driving force mechanism

The actual vector of a plate's motion must necessarily be a function of all the forces acting upon the plate. However, therein remains the problem regarding what degree each process contributes to the motion of each tectonic plate.
The diversity of geodynamic settings and properties of each plate must clearly result in differences in the degree to which such processes are actively driving the plates. One method of dealing with this problem is to consider the relative rate at which each plate is moving and to consider the available evidence of each driving force upon the plate as far as possible.
One of the most significant correlations found is that lithospheric plates attached to downgoing (subducting) plates move much faster than plates not attached to subducting plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than the plates of the Atlantic basin, which are attached (perhaps one could say 'welded') to adjacent continents instead of subducting plates. It is thus thought that forces associated with the downgoing plate (slab pull and slab suction) are the driving forces which determine the motion of plates, except for those plates which are not being subducted.^[18] The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics

Driving forces related to Earth rotation

Alfred Wegener, being a meteorologist, had proposed tidal forces and pole flight force as main driving mechanisms for continental drift. However, these forces were considered far too small to cause continental motion as the concept then was of continents plowing through oceanic crust.^[20] Therefore, Wegener converted to convection currents as the main driving force in the last edition of his book in 1929.
In the plate tectonics context (accepted since the seafloor spreading proposals of Heezen, Hess, Dietz, Morley, Vine and Matthews (see below) during the early 1960s) though, oceanic crust is in motion with the continents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are:

Tidal drag due to the gravitational force the Moon (and the Sun) exerts on the crust of the Earth^[21]
Shear strain of the Earth globe due to N-S compression related to the rotation and modulations of it;
Pole flight force: equatorial drift due to rotation and centrifugal effects: tendency of the plates to move from the poles to the equator ("Polflucht");
Coriolis effect acting on plates when they move around the globe;
Global deformation of the geoid due to small displacements of rotational pole with respect to the Earth crust;
Other smaller deformation effects of the crust due to wobbles and spin movements of the Earth rotation on a smaller time scale.

For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation, and the geographical latitudinal and longitudinal grid of the Earth itself. Ironically, these systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century do underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to the Earth equator and axis, and that gravitational driving forces were generally acting vertically and caused only locally horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page) therefore invoked many of the relationships recognized during this pre-plate tectonics period, to support their theories (see the anticipations and reviews in the work of van Dijk and collaborators).^[22]
Of the many forces discussed in this paragraph, tidal force is still highly debated and defended as a possible principle driving force, whereas the other forces are used or in global geodynamic models not using the plate tectonics concepts (therefore beyond the discussions treated in this section), or proposed as minor modulations within the overall plate tectonics model.

Driving forces related to gravity

Gravity related forces are usually invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above.
Gravitational sliding away from a spreading ridge: According to many authors, plate motion is driven by the higher elevation of plates at ocean ridges.^[17] As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus distance from the ridge). Cool oceanic lithosphere is significantly denser than the hot mantle material from which it is derived and so with increasing thickness it gradually subsides into the mantle to compensate the greater load. The result is a slight lateral incline with distance from the ridge axis.
This force is regarded as a secondary force often referred to as "ridge push". This is a misnomer as nothing is "pushing" horizontally and tensional features are dominant along ridges. It is more accurate to refer to this mechanism as gravitational sliding as variable topography across the totality of the plate can vary considerably and the topography of spreading ridges is only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of the lithosphere before it dives underneath an adjacent plate, which produces a clear topographical feature that can offset or at least affect the influence of topographical ocean ridges, and mantle plumes and hot spots, which are postulated to impinge on the underside of tectonic plates.
Slab-pull: Current scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along the base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this current understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches.^[18] Recent models indicate that trench suction plays an important role as well. However, as the North American Plate is nowhere being subducted, yet it is in motion presents a problem. The same holds for the African, Eurasian, and Antarctic plates.
Gravitational sliding away from mantle doming: According to older theories one of the driving mechanisms of the plates is the existence of large scale asthenosphere/mantle domes, which cause the gravitational sliding of lithosphere plates away from them. This gravitational sliding represents a secondary phenomenon of this, basically vertically oriented mechanism. This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin.

Driving forces related to mantle dynamics

Main article: Mantle convection

For a considerable period of around 25 years (last quarter of the twentieth century) the leading theory envisaged large scale convection currents in the upper mantle which are transmitted through the asthenosphere as the main driving force of the tectonic plates. This theory was launched by Arthur Holmes and some forerunners in the 1930s and was immediately recognized as the solution for the acceptance of the theory discussed since its occurrence in the papers of Alfred Wegener in the early years of the century. It was, though, long debated because the leading ("fixist") theory was still envisaging a static Earth without moving continents, up until the major break–throughs in the early sixties.
Two– and three–dimensional imaging of the Earth's interior (seismic tomography) shows that there is a laterally varying density distribution throughout the mantle. Such density variations can be material (from rock chemistry), mineral (from variations in mineral structures), or thermal (through thermal expansion and contraction from heat energy). The manifestation of this varying lateral density is mantle convection from buoyancy forces.^[15]
How mantle convection relates directly and indirectly to the motion of the plates is a matter of ongoing study and discussion in geodynamics. Somehow, this energy must be transferred to the lithosphere for tectonic plates to move. There are essentially two types of forces that are thought to influence plate motion: friction and gravity.

Basal drag (friction): The plate motion is in this way driven by friction between the convection currents in the asthenosphere and the more rigid overlying floating lithosphere.
Slab suction (gravity): Local convection currents exert a downward frictional pull on plates in subduction zones at ocean trenches. Slab suction may occur in a geodynamic setting wherein basal tractions continue to act on the plate as it dives into the mantle (although perhaps to a greater extent acting on both the under and upper side of the slab).

Lately, the convection theory is much debated as modern techniques based on 3D seismic tomography of imaging the internal structure of the Earth's mantle still fail to recognize these predicted large scale convection cells. Therefore, alternative views have been proposed:
In the theory of plume tectonics developed during the 1990s, a modified concept of mantle convection currents is used, related to super plumes rising from the deeper mantle which would be the drivers or the substitutes of the major convection cells. These ideas, which find their roots in the early 1930s with the so-called "fixistic" ideas of the European and Russian Earth Science Schools, find resonance in the modern theories which envisage hot spots/mantle plumes in the mantle which remain fixed and are overridden by oceanic and continental lithosphere plates during time, and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as a modulator). The modern theories that continue building on the older mantle doming concepts and see the movements of the plates a secondary phenomena, are beyond the scope of this page and are discussed elsewhere for example on the plume tectonics page.
Another suggestion is that the mantle flows neither in cells nor large plumes, but rather as a series of channels just below the Earth's crust which then provide basal friction to the lithosphere. This theory is called "surge tectonics" and became quite popular in geophysics and geodynamics during the 1980s and 1990s.

Driving forces of plate motion

Plate tectonics is basically a kinematic phenomenon: Earth scientists agree upon the observation and deduction that the plates have moved one with respect to the other, and debate and find agreements on how and when. But still a major question remains on what the motor behind this movement is; the geodynamic mechanism, and here science diverges in different theories.
Generally, it is accepted that tectonic plates are able to move because of the relative density of oceanic lithosphere and the relative weakness of the asthenosphere. Dissipation of heat from the mantle is acknowledged to be the original source of energy driving plate tectonics, through convection or large scale upwelling and doming. As a consequence, in the current view, although it is still a matter of some debate, because of the excess density of the oceanic lithosphere sinking in subduction zones a powerful source of plate motion is generated. When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone.^[13] Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted. The same is true for the enormous Eurasian Plate. The sources of plate motion are a matter of intensive research and discussion among earth scientists. One of the main points is that the kinematic pattern of the movements itself should be separated clearly from the possible geodynamic mechanism that is invoked as the driving force of the observed movements, as some patterns may be explained by more than one mechanism.^[14] Basically, the driving forces that are advocated at the moment, can be divided in three categories: mantle dynamics related, gravity related (mostly secondary forces), and Earth rotation related.

Types of plate boundaries

Main article: List of tectonic plate interactions

Three types of plate boundaries exist,^[10] with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:^[11]^[12]

Transform boundaries (Conservative) occur where plates slide or, perhaps more accurately, grind past each other along transform faults. The relative motion of the two plates is either sinistral (left side toward the observer) or dextral (right side toward the observer). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
Divergent boundaries (Constructive) occur where two plates slide apart from each other. Mid-ocean ridges (e.g., Mid-Atlantic Ridge) and active zones of rifting (such as Africa's East African Rift) are both examples of divergent boundaries.
Convergent boundaries (Destructive) (or active margins) occur where two plates slide towards each other commonly forming either a subduction zone (if one plate moves underneath the other) or a continental collision (if the two plates contain continental crust). Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called "foreland basins". The subducting slab contains many hydrous minerals, which release their water on heating; this water then causes the mantle to melt, producing volcanism. Examples of this are the Andes mountain range in South America and the Japanese island arc.
Plate boundary zones occur where the effects of the interactions are unclear and the boundaries, usually occurring along a broad belt, are not well defined, and may show various types of movements in different episode

Key principles

The outer layers of the Earth are divided into lithosphere and asthenosphere. This is based on differences in mechanical properties and in the method for the transfer of heat. Mechanically, the lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. In terms of heat transfer, the lithosphere loses heat by conduction whereas the asthenosphere also transfers heat by convection and has a nearly adiabatic temperature gradient. This division should not be confused with the chemical subdivision of these same layers into the mantle (comprising both the asthenosphere and the mantle portion of the lithosphere) and the crust: a given piece of mantle may be part of the lithosphere or the asthenosphere at different times, depending on its temperature and pressure.
The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like (visco-elastic solid) asthenosphere. Plate motions range up to a typical 10–40 mm/a (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/a (Nazca Plate; about as fast as hair grows).^[4] The driving mechanism behind this movement is described separately below.
Tectonic lithosphere plates consist of lithospheric mantle overlain by either or both of two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). Average oceanic lithosphere is typically 100 km (62 mi) thick;^[5] its thickness is a function of its age: as time passes, it conductively cools and becomes thicker. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones; for shorter or longer distances, the subduction zone (and therefore also the mean) thickness becomes smaller or larger, respectively.^[6] Continental lithosphere is typically ~200 km thick, though this also varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic (35 km vs. 6 km).^[7]
The location where two plates meet is called a plate boundary, and plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being most active and most widely known. These boundaries are discussed in further detail below. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation^[8] and to mantle plumes.
As explained above, tectonic plates can include continental crust or oceanic crust, and many plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes; though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust, these are considered part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic").^[9] As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while the continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).

Plate tectonics

Plate tectonics (from the Late Latin tectonicus, from the Greek: τεκτονικός "pertaining to building")^[1] is a scientific theory that describes the large-scale motions of Earth's lithosphere. The theory builds on the concepts of continental drift, developed during the first decades of the 20th century. It was accepted by the geoscientific community after the concepts of seafloor spreading were developed in the late 1950s and early 1960s.
The lithosphere is broken up into tectonic plates. On Earth, there are seven or eight major plates (depending on how they are defined) and many minor plates. Where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries. The lateral relative movement of the plates typically varies from zero to 100 mm annually.^[2]
Tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading. In this way, the total surface of the globe remains the same. This prediction of plate tectonics is also referred to as the conveyor belt principle. Earlier theories (that still have some supporters) proposed gradual shrinking (contraction) or gradual expansion of the globe.^[3]
Tectonic plates are able to move because the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Lateral density variations in the mantle result in convection. Their movement is thought to be driven by a combination of the motion of seafloor away from the spreading ridge (due to variations in topography and density of the crust that result in differences in gravitational forces) and drag, downward suction, at the subduction zones. A different explanation lies in different forces generated by the rotation of the globe and tidal forces of the Sun and the Moon. The relative importance of each of these factors is unclear, and is still subject to debate (see also below)