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117-624: Terra Sirenum is named after the Sirens , who were birds with the heads of girls. In the Odyssey these girls captured passing seamen and killed them. Evidence of deposits of chloride based minerals in Terra Sirenum was discovered by the 2001 Mars Odyssey orbiter's Thermal Emission Imaging System in March 2008. The deposits are approximately 3.5 to 3.9 billion years old. This suggests that near-surface water

234-531: A Sophocles fragment makes Phorcys their father, when sirens are named, they are usually as daughters of the river god Achelous , either by the Muse Terpsichore , Melpomene or Calliope or lastly by Sterope , daughter of King Porthaon of Calydon . In Euripides 's play Helen (167), Helen in her anguish calls upon "Winged maidens, daughters of the Earth ( Chthon )." Although they lured mariners,

351-675: A brittle layer, such as a rock thrown through a glass window. These particular fractures were probably created by something emerging from below the brittle Martian surface. Ice may have accumulated under the surface in a lens shape; thus making these cracked mounds. Ice being less dense than rock, pushed upwards on the surface and generated these spider web-like patterns. A similar process creates similar sized mounds in arctic tundra on Earth. Such features are called pingos . Pingos would contain pure water ice; thus they could be sources of water for future colonists of Mars. Concentric crater fill , like lobate debris aprons and lineated valley fill ,

468-439: A consequence, a powerful source generating plate motion is the excess density of the oceanic lithosphere sinking in subduction zones. 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

585-450: A few tens of millions of years. Armed with the knowledge of a new heat source, scientists realized that 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, Alfred Wegener was making serious arguments for the idea of continental drift in the first edition of The Origin of Continents and Oceans . In that book (re-issued in four successive editions up to

702-529: A funerary context are attested since the classical era, in mainland Greece , as well as Asia Minor and Magna Graecia . The so-called "Siren of Canosa"— Canosa di Puglia is a site in Apulia that was part of Magna Graecia —was said to accompany the dead among grave goods in a burial. She appeared to have some psychopomp characteristics, guiding the dead on the afterlife journey. The cast terracotta figure bears traces of its original white pigment. The woman bears

819-571: A layer of basalt (sial) underlies the continental rocks. However, based on abnormalities in plumb line deflection by the Andes in Peru, Pierre Bouguer had deduced that less-dense mountains must have a downward projection into the denser layer underneath. The concept that mountains had "roots" was confirmed by George B. Airy a hundred years later, during study of Himalayan gravitation, and seismic studies detected corresponding density variations. Therefore, by

936-439: A middle crustal spreading zone. Because the area containing the magnetic stripes is about 4 billion years old, it is believed that the global magnetic field probably lasted for only the first few hundred million years of Mars' life, when the temperature of the molten iron in the planet's core might have been high enough to mix it into a magnetic dynamo. There are no magnetic fields near large impact basins like Hellas. The shock of

1053-400: A misnomer as there is no force "pushing" horizontally, indeed tensional features are dominant along ridges. It is more accurate to refer to this mechanism as "gravitational sliding", since the topography across the whole plate can vary considerably and spreading ridges are only the most prominent feature. Other mechanisms generating this gravitational secondary force include flexural bulging of

1170-575: A scene in the Odyssey in which Odysseus saves his crew's lives. Roman poets place them on some small islands called Sirenum scopuli . In some later, rationalized traditions, the literal geography of the "flowery" island of Anthemoessa , or Anthemusa, is fixed: sometimes on Cape Pelorum and at others in the islands known as the Sirenuse , near Paestum , or in Capreae . All such locations were surrounded by cliffs and rocks. Sirens continued to be used as

1287-551: A secondary phenomenon of this basically vertically oriented mechanism. It finds its roots in the Undation Model of van Bemmelen . This can act on various scales, from the small scale of one island arc up to the larger scale of an entire ocean basin. Alfred Wegener , being a meteorologist , had proposed tidal forces and centrifugal forces as the main driving mechanisms behind continental drift ; however, these forces were considered far too small to cause continental motion as

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1404-662: A siren in Canto 19 of Purgatorio , the second canticle of the Divine Comedy . Here, the pilgrim dreams of a female who is described as "stuttering, cross-eyed, and crooked on her feet, with stunted hands, and pallid in color." It is not until the pilgrim "gazes" upon her that she is turned desirable and is revealed by herself to be a siren. This siren then claims that she "turned Ulysses from his course, desirous of my / song, and whoever becomes used to me rarely / leaves me, so wholly do I satisfy him!" Given that Dante did not have access to

1521-407: A solid crust and mantle and a liquid core, but there seemed to be no way that portions of the crust could move around. Many distinguished scientists of the time, 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

1638-478: A static Earth without moving continents up until the major breakthroughs of the early sixties. Two- and three-dimensional imaging of Earth's interior ( seismic tomography ) shows a varying lateral 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

1755-425: A symbol of the dangerous temptation embodied by women regularly throughout Christian art of the medieval era. "Siren" can also be used as a slang term for a woman considered both very attractive and dangerous. The etymology of the name is contested. Robert S. P. Beekes has suggested a Pre-Greek origin. Others connect the name to σειρά ( seirá , "rope, cord") and εἴρω ( eírō , "to tie, join, fasten"), resulting in

1872-406: A variety of musical instruments, especially the lyre , kithara , and aulos . The tenth-century Byzantine dictionary Suda stated that sirens ( Ancient Greek : Σειρῆνας ) had the form of sparrows from their chests up, and below they were women or that they were little birds with women's faces. Originally, sirens were shown as male or female, but the male siren disappeared from art around

1989-475: A word for " owls " in the Book of Jeremiah 50:39. The siren is allegorically described as a beautiful courtesan or prostitute, who sings pleasant melodies to men, and is the symbolic vice of Pleasure in the preaching of Clement of Alexandria (2nd century). Later writers such as Ambrose (4th century) reiterated the notion that the siren stood as a symbol or allegory for worldly temptations. and not an endorsement of

2106-438: Is mantle convection from buoyancy forces. How mantle convection directly and indirectly relates to plate motion 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 main types of mechanisms that are thought to exist related to the dynamics of the mantle that influence plate motion which are primary (through

2223-527: Is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers. Continental crust is formed through arc volcanism and accretion of terranes through plate tectonic processes. Oceanic crust is denser than continental crust because it has less silicon and more of the heavier elements than continental crust . As a result of this density difference, oceanic crust generally lies below sea level , while continental crust buoyantly projects above sea level. Average oceanic lithosphere

2340-401: Is believed to be ice-rich. Based on accurate topography measures of height at different points in these craters and calculations of how deep the craters should be based on their diameters, it is thought that the craters are 80% filled with mostly ice. That is, they hold hundreds of meters of material that probably consists of ice with a few tens of meters of surface debris. The ice accumulated in

2457-456: Is called a plate boundary . Plate boundaries are where geological events occur, such as earthquakes and the creation of topographic features such as mountains , volcanoes , mid-ocean ridges , and oceanic trenches . The vast majority of the world's active volcanoes occur along plate boundaries, with the Pacific plate's Ring of Fire being the most active and widely known. Some volcanoes occur in

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2574-533: 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. 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. Since that time many theories were proposed to explain this apparent complementarity, but

2691-452: Is continually calling on Persephone. The term " siren song " refers to an appeal that is hard to resist but that, if heeded, will lead to a bad conclusion. Later writers have implied that the sirens ate humans , based on Circe 's description of them "lolling there in their meadow, round them heaps of corpses rotting away, rags of skin shriveling on their bones." As linguist Jane Ellen Harrison (1850–1928) notes of " The Ker as siren": "It

2808-487: 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 (see the paragraph on Mantle Mechanisms). This gravitational sliding represents

2925-408: Is invoked as the major driving force, through slab pull along subduction zones. Gravitational sliding away from a spreading ridge is one of the proposed driving forces, it proposes plate motion is driven by the higher elevation of plates at ocean ridges. As oceanic lithosphere is formed at spreading ridges from hot mantle material, it gradually cools and thickens with age (and thus adds distance from

3042-415: Is still advocated to explain the break-up of supercontinents during specific geological epochs. It has followers amongst the scientists involved in the theory of Earth expansion . Another theory is that the mantle flows neither in cells nor large plumes but rather as a series of channels just below Earth's crust, which then provide basal friction to the lithosphere. This theory, called "surge tectonics",

3159-488: Is strange and beautiful that Homer should make the sirens appeal to the spirit, not to the flesh." The siren song is a promise to Odysseus of mantic truths; with a false promise that he will live to tell them, they sing, Once he hears to his heart's content, sails on, a wiser man. We know all the pains that the Greeks and Trojans once endured on the spreading plain of Troy when the gods willed it so— all that comes to pass on

3276-576: Is the scientific theory that Earth 's lithosphere comprises a number of large tectonic plates , which have been slowly moving since 3–4 billion years ago. The model builds on the concept of continental drift , an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s. The processes that result in plates and shape Earth's crust are called tectonics . Tectonic plates also occur in other planets and moons. Earth's lithosphere,

3393-488: Is to consider the relative rate at which each plate is moving as well as the evidence related to the significance of each process to the overall driving force on the plate. One of the most significant correlations discovered to date is that lithospheric plates attached to downgoing (subducting) plates move much faster than other types of plates. The Pacific plate, for instance, is essentially surrounded by zones of subduction (the so-called Ring of Fire) and moves much faster than

3510-407: Is typically 100 km (62 mi) thick. Its thickness is a function of its age. As time passes, it cools by conducting heat from below, and releasing it raditively into space. The adjacent mantle below is cooled by this process and added to its base. 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

3627-435: Is used. It asserts that super plumes rise from the deeper mantle and are the drivers or substitutes of the major convection cells. These ideas find their roots in the early 1930s in the works of Beloussov and van Bemmelen , which were initially opposed to plate tectonics and placed the mechanism in a fixed frame of vertical movements. Van Bemmelen later modified the concept in his "Undation Models" and used "Mantle Blisters" as

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3744-452: The Argonautica (third century BC), Jason had been warned by Chiron that Orpheus would be necessary in his journey. When Orpheus heard their voices, he drew out his lyre and played his music more beautifully than they, drowning out their voices. One of the crew, however, the sharp-eared hero Butes , heard the song and leapt into the sea, but he was caught up and carried safely away by

3861-494: The Odyssey , the siren's claim that she turned Ulysses from his course is inherently false because the sirens in the Odyssey do not manage to turn Ulysses from his path. Ulysses and his men were warned by Circe and prepared for their encounter by stuffing their ears full of wax, except for Ulysses, who wishes to be bound to the ship's mast as he wants to hear the siren's song. Scholars claim that Dante may have "misinterpreted"

3978-558: 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. As it

4095-562: The Renaissance , female court musicians known as courtesans filled the role of an unmarried companion, and musical performances by unmarried women could be seen as immoral. Seen as a creature who could control a man's reason, female singers became associated with the mythological figure of the siren, who usually took a half-human, half-animal form somewhere on the cusp between nature and culture. Leonardo da Vinci wrote of them in his notebooks, stating "The siren sings so sweetly that she lulls

4212-610: The Septuagint translation of the aforementioned Isaiah 13:21–22, and 34:14. They also appear together in some Latin bestiaries of the First Family subgroup called B-Isidore ("B-Is"). The siren's bird-like description from classical sources was retained in the Latin version of the Physiologus (6th century) and several subsequent bestiaries into the 13th century, but at some time during

4329-422: 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

4446-724: The fluid-like solid the asthenosphere . Plate motions range from 10 to 40 millimetres per year (0.4 to 1.6 in/year) at the Mid-Atlantic Ridge (about as fast as fingernails grow), to about 160 millimetres per year (6.3 in/year) for the Nazca plate (about as fast as hair grows). Tectonic lithosphere plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust (in older texts called sima from silicon and magnesium ) and continental crust ( sial from silicon and aluminium ). The distinction between oceanic crust and continental crust

4563-473: The lithosphere and asthenosphere . The division is based on differences in mechanical properties and in the method for the transfer of heat . 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

4680-546: The 1960s, they were taken as evidence of plate tectonics . Researchers believe these magnetic stripes on Mars are evidence for a short, early period of plate tectonic activity. When the rocks became solid they retained the magnetism that existed at the time. A magnetic field of a planet is believed to be caused by fluid motions under the surface. However, there are some differences, between the magnetic stripes on Earth and those on Mars. The Martian stripes are wider, much more strongly magnetized, and do not appear to spread out from

4797-590: The Earth's rotation and the Moon as main driving forces for the plates. The vector of a plate's motion is a function of all the forces acting on the plate; however, therein lies the problem regarding the degree to which each process contributes to the overall motion of each tectonic plate. The diversity of geodynamic settings and the properties of each plate result from the impact of the various processes actively driving each individual plate. One method of dealing with this problem

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4914-572: The Greek myth. The early Christian euhemerist interpretation of mythologized human beings received a long-lasting boost from the Etymologiae by Isidore of Seville (c. 560–636): They [the Greeks] imagine that "there were three sirens, part virgins, part birds," with wings and claws. "One of them sang, another played the flute, the third the lyre. They drew sailors, decoyed by song, to shipwreck. According to

5031-852: The Greeks portrayed the sirens in their "meadow starred with flowers" and not as sea deities. Epimenides claimed that the sirens were children of Oceanus and Ge . Sirens are found in many Greek stories, notably in Homer's Odyssey . Their number is variously reported as from two to eight. In the Odyssey , Homer says nothing of their origin or names, but gives the number of the sirens as two. Later writers mention both their names and number: some state that there were three, Peisinoe , Aglaope and Thelxiepeia or Aglaonoe , Aglaopheme and Thelxiepeia; Parthenope , Ligeia , and Leucosia ; Apollonius followed Hesiod gives their names as Thelxinoe , Molpe , and Aglaophonos ; Suidas gives their names as Thelxiepeia, Peisinoe, and Ligeia; Hyginus gives

5148-612: The Phoenician, implies a songstress. Hence it is probable, that in ancient times there may have been excellent singers, but of corrupt morals, on the coast of Sicily, who by seducing voyagers, gave rise to this fable." John Lemprière in his Classical Dictionary (1827) wrote, "Some suppose that the sirens were several lascivious women in Sicily, who prostituted themselves to strangers, and made them forget their pursuits while drowned in unlawful pleasures. The etymology of Bochart , who deduces

5265-533: The actual motions of the Pacific plate and other plates associated with the East Pacific Rise do not correlate mainly with either slab pull or slab push, but rather with a mantle convection upwelling whose horizontal spreading along the bases of the various plates drives them along via viscosity-related traction forces. The driving forces of plate motion continue to be active subjects of on-going research within geophysics and tectonophysics . The development of

5382-494: The ancient Hebrew Book of Enoch , the women who were led astray by the fallen angels will be turned into sirens. By the fourth century, when pagan beliefs were overtaken by Christianity , the belief in literal sirens was discouraged Saint Jerome , who produced the Latin Vulgate version of the bible, used the word sirens to translate Hebrew tannīm (" jackals ") in the Book of Isaiah 13:22, and also to translate

5499-478: The assumption of a solid Earth made these various proposals difficult to accept. The discovery of radioactivity and its associated heating properties in 1895 prompted a re-examination of the apparent age of Earth . This had previously been estimated by its cooling rate under the assumption that Earth's surface radiated like a black body . Those calculations had implied that, even if it started at red heat , Earth would have dropped to its present temperature in

5616-399: The asthenosphere. 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 as originally discussed in the papers of Alfred Wegener in the early years of the 20th century. However, despite its acceptance, it was long debated in the scientific community because the leading theory still envisaged

5733-469: The base of the lithosphere. Slab pull is therefore most widely thought to be the greatest force acting on the plates. In this understanding, plate motion is mostly driven by the weight of cold, dense plates sinking into the mantle at trenches. Recent models indicate that trench suction plays an important role as well. However, the fact that the North American plate is nowhere being subducted, although it

5850-495: The bathymetry of the deep ocean floors and the nature of the oceanic crust such as magnetic properties and, more generally, with the development of marine geology which gave evidence for the association of seafloor spreading along the mid-oceanic ridges and magnetic field reversals , published between 1959 and 1963 by Heezen, Dietz, Hess, Mason, Vine & Matthews, and Morley. Simultaneous advances in early seismic imaging techniques in and around Wadati–Benioff zones along

5967-463: The body of a maiden, but have scaly fishes' tails". The siren appeared in several illustrated manuscripts of the Physiologus and its successors called the bestiaries . The siren was depicted as a half-woman and half-fish mermaid in the 9th century Berne Physiologus , as an early example, but continued to be illustrated with both bird-like parts (wings, clawed feet) and fish-like tail. Although

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6084-456: The commingling involve the treatment of sirens in the medieval Physiologus and bestiaries , both iconographically, as well as textually in translations from Latin to vulgar languages, as described below. The sirens of Greek mythology first appeared in Homer 's Odyssey , where Homer did not provide any physical descriptions, and their visual appearance was left to the readers' imagination. It

6201-413: The concept was of continents plowing through oceanic crust. Therefore, Wegener later changed his position and asserted that convection currents are the main driving force of plate tectonics in the last edition of his book in 1929. However, 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),

6318-485: The crater from snowfall in previous climates. Recent modeling suggests that concentric crater fill develops over many cycles in which snow is deposited, then moves into the crater. Once inside the crater shade and dust preserve the snow. The snow changes to ice. The many concentric lines are created by the many cycles of snow accumulation. Generally snow accumulates whenever the axial tilt reaches 35 degrees. The Mars Global Surveyor (MGS) discovered magnetic stripes in

6435-502: The crust of Mars, especially in the Phaethontis and Eridania quadrangles ( Terra Cimmeria and Terra Sirenum). The magnetometer on MGS discovered 100 km wide stripes of magnetized crust running roughly parallel for up to 2000 km. These stripes alternate in polarity with the north magnetic pole of one pointing up from the surface and the north magnetic pole of the next pointing down. When similar stripes were discovered on Earth in

6552-415: The deep mantle at subduction zones, providing most of the driving force for plate movement. The weakness of the asthenosphere allows the tectonic plates to move easily towards a subduction zone. For much of the first quarter of the 20th century, the leading theory of the driving force behind tectonic plate motions envisaged large scale convection currents in the upper mantle, which can be transmitted through

6669-530: The discussions treated in this section) or proposed as minor modulations within the overall plate tectonics model. In 1973, George W. Moore of the USGS and R. C. Bostrom presented evidence for a general westward drift of Earth's lithosphere with respect to the mantle, based on the steepness of the subduction zones (shallow dipping towards the east, steeply dipping towards the west). They concluded that tidal forces (the tidal lag or "friction") caused by Earth's rotation and

6786-466: The driving force for horizontal movements, invoking gravitational forces away from the regional crustal doming. The theories find resonance in the modern theories which envisage hot spots or mantle plumes which remain fixed and are overridden by oceanic and continental lithosphere plates over time and leave their traces in the geological record (though these phenomena are not invoked as real driving mechanisms, but rather as modulators). The mechanism

6903-511: The ending of the encounter between the pilgrim and the siren. In Geoffrey of Monmouth 's Historia Regum Britanniae ( c.  1136 ), Brutus of Troy encounters sirens at the Pillars of Hercules on his way to Britain to fulfil a prophecy that he will establish an empire there. The sirens surround and nearly overturn his ships until Brutus escapes to the Tyrrhenian Sea . By the time of

7020-580: The feet, wings and tail of a bird. The sculpture is conserved in the National Archaeological Museum of Spain , in Madrid. The sirens were called the Muses of the lower world. Classical scholar Walter Copland Perry (1814–1911) observed: "Their song, though irresistibly sweet, was no less sad than sweet, and lapped both body and soul in a fatal lethargy, the forerunner of death and corruption." Their song

7137-586: The fertile earth, we know it all! "They are mantic creatures like the Sphinx with whom they have much in common, knowing both the past and the future", Harrison observed. "Their song takes effect at midday, in a windless calm. The end of that song is death ." That the sailors' flesh is rotting away, suggests it has not been eaten. It has been suggested that, with their feathers stolen, their divine nature kept them alive, but unable to provide food for their visitors, who starved to death by refusing to leave. According to

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7254-665: The fifth century BC. Some surviving Classical period examples had already depicted the siren as mermaid-like. The sirens are described as mermaids or "tritonesses" in examples dating to the 3rd century BC, including an earthenware bowl found in Athens and a terracotta oil lamp possibly from the Roman period. The first known literary attestation of siren as a "mermaid" appeared in the Anglo-Latin catalogue Liber Monstrorum (early 8th century AD), where it says that sirens were "sea-girls... with

7371-473: 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 , Antonio 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

7488-691: The forces acting upon it by the Moon are a driving force for plate tectonics. As Earth spins eastward beneath the Moon, the Moon's gravity ever so slightly pulls Earth's surface layer back westward, just as proposed by Alfred Wegener (see above). Since 1990 this theory is mainly advocated by Doglioni and co-workers ( Doglioni 1990 ), such as in a more recent 2006 study, where scientists reviewed and advocated these ideas. It has been suggested in Lovett (2006) that this observation may also explain why Venus and Mars have no plate tectonics, as Venus has no moon and Mars' moons are too small to have significant tidal effects on

7605-588: The geographical latitudinal and longitudinal grid of Earth itself. These systematic relations studies in the second half of the nineteenth century and the first half of the twentieth century underline exactly the opposite: that the plates had not moved in time, that the deformation grid was fixed with respect to Earth's equator and axis, and that gravitational driving forces were generally acting vertically and caused only local horizontal movements (the so-called pre-plate tectonic, "fixist theories"). Later studies (discussed below on this page), therefore, invoked many of

7722-521: The goddess Aphrodite . Odysseus was curious as to what the sirens sang to him, and so, on the advice of Circe , he had all of his sailors plug their ears with beeswax and tie him to the mast. He ordered his men to leave him tied tightly to the mast, no matter how much he might beg. When he heard their beautiful song, he ordered the sailors to untie him but they bound him tighter. When they had passed out of earshot, Odysseus demonstrated with his frowns to be released. Some post-Homeric authors state that

7839-547: The gods, persuaded the Sirens to enter a singing contest with the Muses . The Muses won the competition and then plucked out all of the sirens' feathers and made crowns out of them. Out of their anguish from losing the competition, writes Stephanus of Byzantium , the sirens turned white and fell into the sea at Aptera ("featherless"), where they formed the islands in the bay that were called Leukai ("the white ones", modern Souda ). In

7956-412: The impact may have erased the remnant magnetization in the rock. So, magnetism produced by early fluid motion in the core would not have existed after the impacts. When molten rock containing magnetic material, such as hematite (Fe 2 O 3 ), cools and solidifies in the presence of a magnetic field, it becomes magnetized and takes on the polarity of the background field. This magnetism is lost only if

8073-460: The interim, the mermaid shape was introduced to this body of works. The siren was illustrated as a woman-fish (mermaid) in the Bern Physiologus dated to the mid-9th century, even though this contradicted the accompanying text which described it as avian. An English-made Latin bestiary dated 1220–1250 also depicted a group of sirens as mermaids with fishtails swimming in the sea, even though

8190-708: The interiors of plates, and these have been variously attributed to internal plate deformation and to mantle plumes. Tectonic plates may include continental crust or oceanic crust, or both. For example, the African plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. Some pieces of oceanic crust, known as ophiolites , failed to be subducted under continental crust at destructive plate boundaries; instead these oceanic crustal fragments were pushed upward and were preserved within continental crust. Three types of plate boundaries exist, characterized by

8307-412: The large scale convection cells) or secondary. The secondary mechanisms view plate motion driven by friction between the convection currents in the asthenosphere and the more rigid overlying lithosphere. This is due to the inflow of mantle material related to the downward pull on plates in subduction zones at ocean trenches. Slab pull may occur in a geodynamic setting where basal tractions continue to act on

8424-472: The late 12th century. A counterexample is also given where the illustrated sirens (group of three) are bird-like, conforming to the text. The siren was sometimes drawn as a hybrid with a human torso, a fish-like lower body, and bird-like wings and feet. While in the Harley 3244 (cf. fig. top right) the wings sprout from around the shoulders, in other hybrid types, the style places the siren's wings "hanging at

8541-421: The lithosphere before it dives underneath an adjacent plate, producing a clear topographical feature that can offset, or at least affect, the influence of topographical ocean ridges. Mantle plumes and hot spots are also postulated to impinge on the underside of tectonic plates. Slab pull : Scientific opinion is that the asthenosphere is insufficiently competent or rigid to directly cause motion by friction along

8658-403: The lower mantle, there is a slight westward component in the motions of all the plates. They demonstrated though that the westward drift, seen only for the past 30 Ma, is attributed to the increased dominance of the steadily growing and accelerating Pacific plate. The debate is still open, and a recent paper by Hofmeister et al. (2022) revived the idea advocating again the interaction between

8775-405: The many geographical, geological, and biological continuities between continents. In 1912, the meteorologist Alfred Wegener described what he called continental drift, an idea that culminated fifty years later in the modern theory of plate tectonics. Wegener expanded his theory in his 1915 book The Origin of Continents and Oceans . Starting from the idea (also expressed by his forerunners) that

8892-716: The mariners to sleep; then she climbs upon the ships and kills the sleeping mariners." However, in the 17th century, some Jesuit writers began to assert their actual existence, including Cornelius a Lapide , who said of woman, "her glance is that of the fabled basilisk , her voice a siren's voice—with her voice she enchants, with her beauty she deprives of reason—voice and sight alike deal destruction and death." Antonio de Lorea also argued for their existence, and Athanasius Kircher argued that compartments must have been built for them aboard Noah's Ark . Charles Burney expounded c.  1789 , in A General History of Music : "The name, according to Bochart , who derives it from

9009-429: 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

9126-405: The meaning "binder, entangler", i.e. one who binds or entangles through magic song. This could be connected to the famous scene of Odysseus being bound to the mast of his ship, to resist their song. Sirens were later often used as a synonym for mermaids and portrayed with upper human bodies and fish tails. This combination became iconic in the medieval period. The circumstances leading to

9243-563: The motion picture of the Atlantic region", processes that anticipated seafloor spreading and subduction . 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,

9360-438: The motion. At a subduction zone the relatively cold, dense oceanic crust sinks down into the mantle, forming the downward convecting limb of a mantle cell , which is the strongest driver of plate motion. The relative importance and interaction of other proposed factors such as active convection, upwelling inside the mantle, and tidal drag of the Moon is still the subject of debate. The outer layers of Earth are divided into

9477-462: The name from a Phoenician term denoting a songstress, favours the explanation given of the fable by Damm . This distinguished critic makes the sirens to have been excellent singers, and divesting the fables respecting them of all their terrific features, he supposes that by the charms of music and song, they detained travellers, and made them altogether forgetful of their native land." The French impressionist composer, Claude Debussy , composed

9594-418: 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, and by Warren Carey in a symposium held in March 1956. The second piece of evidence in support of continental drift came during the late 1950s and early 60s from data on

9711-487: The number of the sirens as four: Teles, Raidne , Molpe, and Thelxiope ; Eustathius states that they were two, Aglaopheme and Thelxiepeia; an ancient vase painting attests the two names as Himerope and Thelxiepeia. Their names are variously rendered in the later sources as Thelxiepeia/Thelxiope/Thelxinoe, Molpe, Himerope, Aglaophonos/Aglaope/Aglaopheme, Pisinoe/Peisinoë/ Peisithoe , Parthenope, Ligeia, Leucosia, Raidne, and Teles. According to Ovid (43 BC–17 AD),

9828-407: The oceanic crust is suggested to be in motion with the continents which caused the proposals related to Earth rotation to be reconsidered. In more recent literature, these driving forces are: Forces that are small and generally negligible are: For these mechanisms to be overall valid, systematic relationships should exist all over the globe between the orientation and kinematics of deformation and

9945-437: The oceanic lithosphere and the thicker continental lithosphere, each topped by its own kind of crust. Along convergent plate boundaries , the process of subduction carries the edge of one plate down under the other plate and into the mantle . This process reduces the total surface area (crust) of the Earth. The lost surface is balanced by the formation of new oceanic crust along divergent margins by seafloor spreading, keeping

10062-603: The orchestral work Nocturnes in which the third movement, " Sirènes ", depicts sirens. According to Debussy, "'Sirènes' depicts the sea and its countless rhythms and presently, amongst the waves silvered by the moonlight, is heard the mysterious song of the Sirens as they laugh and pass on" . In 1911, French composer Lili Boulanger composed " Les sirènes " for mezzo-soprano soloist, choir, and piano. Plate tectonics Plate tectonics (from Latin tectonicus , from Ancient Greek τεκτονικός ( tektonikós )  'pertaining to building')

10179-463: The planet. In a paper by it was suggested that, on the other hand, it can easily be observed that many plates are moving north and eastward, and that the dominantly westward motion of the Pacific Ocean basins derives simply from the eastward bias of the Pacific spreading center (which is not a predicted manifestation of such lunar forces). In the same paper the authors admit, however, that relative to

10296-399: 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). Furthermore, slabs that are broken off and sink into the mantle can cause viscous mantle forces driving plates through slab suction. In the theory of plume tectonics followed by numerous researchers during the 1990s, a modified concept of mantle convection currents

10413-426: 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. This view however has been contradicted by a recent study which found that

10530-408: The present continents once formed a single land mass (later called Pangaea ), Wegener suggested that these separated and drifted apart, likening them to "icebergs" of low density sial floating on a sea of denser sima . Supporting evidence for the idea came from the dove-tailing outlines of South America's east coast and Africa's west coast Antonio Snider-Pellegrini had drawn on his maps, and from

10647-459: The relationships recognized during this pre-plate tectonics period to support their theories (see reviews of these various mechanisms related to Earth rotation the work of van Dijk and collaborators). Of the many forces discussed above, tidal force is still highly debated and defended as a possible principal driving force of plate tectonics. The other forces are only used in global geodynamic models not using plate tectonics concepts (therefore beyond

10764-428: 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

10881-399: 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 increased distance from the ridge axis. This force is regarded as a secondary force and is often referred to as " ridge push ". This is

10998-614: The rigid outer shell of the planet including the crust and upper mantle , is fractured into seven or eight major plates (depending on how they are defined) and many minor plates or "platelets". Where the plates meet, their relative motion determines the type of plate boundary (or fault ): convergent , divergent , or transform . The relative movement of the plates typically ranges from zero to 10 cm annually. Faults tend to be geologically active, experiencing earthquakes , volcanic activity , mountain-building , and oceanic trench formation. Tectonic plates are composed of

11115-471: The rock is subsequently heated above a particular temperature (the Curie point which is 770 °C for iron). The magnetism left in rocks is a record of the magnetic field when the rock solidified. Siren (mythology) In Greek mythology , sirens ( Ancient Greek : singular: Σειρήν , Seirḗn ; plural: Σειρῆνες , Seirênes ) are female humanlike beings with alluring voices; they appear in

11232-731: The shape of streams are further evidence of water flowing on the Martian surface in past times. Terra Sirenum is the location of many Martian gullies that may be due to recent flowing water. Some are found in the Gorgonum Chaos and in many craters near the large craters Copernicus and Newton . Gullies occur on steep slopes, especially on the walls of craters. Gullies are believed to be relatively young because they have few, if any craters. Moreover, they lie on top of sand dunes which themselves are considered to be quite young. The radial and concentric cracks visible here are common when forces penetrate

11349-454: The siren's claim from an episode in Cicero 's De finibus . The pilgrim's dream comes to an end when a lady "holy and quick" who had not yet been present before suddenly appears and says, "O Virgil, Virgil, who is this?" Virgil , the pilgrim's guide, then steps forward and tears the clothes from the siren's belly which, "awakened me [the pilgrim] with the stench that issued from it." This marks

11466-582: The sirens were fated to die if someone heard their singing and escaped them, and that after Odysseus passed by they therefore flung themselves into the water and perished. The first-century Roman historian Pliny the Elder discounted sirens as a pure fable, "although Dinon, the father of Clearchus, a celebrated writer, asserts that they exist in India , and that they charm men by their song, and, having first lulled them to sleep, tear them to pieces." Statues of sirens in

11583-482: The sirens were the companions of young Persephone . Demeter gave them wings to search for Persephone when she was abducted by Hades . However, the Fabulae of Hyginus (64 BC–17 AD) has Demeter cursing the sirens for failing to intervene in the abduction of Persephone. According to Hyginus , sirens were fated to live only until the mortals who heard their songs could pass by them. One legend says that Hera , queen of

11700-486: 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. Wegener's work was initially not widely accepted, in part due to a lack of detailed evidence but mostly because of the lack of a reasonable physically supported mechanism. Earth might have

11817-492: The surface layers, but left behind the more resistant deposits. Other ways of making inverted relief might be lava flowing down a stream bed or materials being cemented by minerals dissolved in water. On Earth, materials cemented by silica are highly resistant to all kinds of erosional forces. Examples of inverted channels on Earth are found in the Cedar Mountain Formation near Green River, Utah . Inverted relief in

11934-433: The text stated they resembled winged fowl ( volatilis habet figuram ) down to their feet. Illustrating the siren as a pure mermaid became commonplace in the "second family" bestiaries, and she was shown holding a musical instrument in the classical tradition, but also sometimes holding apparently an eel -fish. An example of the siren-mermaid holding such a fish is found in one of the earlier codices in this group, dated

12051-470: The theory of plate tectonics was the scientific and cultural change which occurred during a period of 50 years of scientific debate. The event of the acceptance itself was a paradigm shift and can therefore be classified as a scientific revolution, now described as the Plate Tectonics Revolution . Around the start of the twentieth century, various theorists unsuccessfully attempted to explain

12168-502: 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 of plate tectonics were proposed by geophysicists and geologists (both fixists and mobilists) like Vening-Meinesz, Holmes, and Umbgrove. In 1941, Otto Ampferer described, in his publication "Thoughts on

12285-476: The total surface area constant in a tectonic "conveyor belt". Tectonic plates are relatively rigid and float across the ductile asthenosphere beneath. Lateral density variations in the mantle result in convection currents, the slow creeping motion of Earth's solid mantle. At a seafloor spreading ridge , plates move away from the ridge, which is a topographic high, and the newly formed crust cools as it moves away, increasing its density and contributing to

12402-429: The trenches bounding many continental margins, together with many other geophysical (e.g., gravimetric) and geological observations, showed how the oceanic crust could disappear into the mantle, providing the mechanism to balance the extension of the ocean basins with shortening along its margins. All this evidence, both from the ocean floor and from the continental margins, made it clear around 1965 that continental drift

12519-399: The truth, however, they were prostitutes who led travelers down to poverty and were said to impose shipwreck on them." They had wings and claws because Love flies and wounds. They are said to have stayed in the waves because a wave created Venus . The siren and the onocentaur , two hybrid creatures, appear as the subject of a single chapter in the Physiologus , as they appear together in

12636-788: The waist". Also, a siren may be holding a comb, or a mirror. Thus the comb and mirror, which are now emblematic of mermaids across Europe, derive from the bestiaries that describe the siren as a vain creature requiring those accoutrements. Later, bestiary texts appeared which were modified to accommodate the artistic conventions. It is explained that the siren's "other part" may be "like fish or like bird" in Guillaume le clerc 's Old French verse bestiary (1210 or 1211), as well as Philippe de Thaun 's Anglo-Norman verse bestiary (c. 1121–1139). There also appeared medieval works that conflated sirens with mermaids while citing Physiologus as their source. Italian poet Dante Alighieri depicts

12753-467: 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: 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 the original source of the energy required to drive plate tectonics through convection or large scale upwelling and doming. As

12870-539: Was Apollonius of Rhodes in Argonautica (3rd century BC) who described the sirens in writing as part woman and part bird. By the 7th century BC, sirens were regularly depicted in art as human-headed birds. They may have been influenced by the ba-bird of Egyptian religion. In early Greek art, the sirens were generally represented as large birds with women's heads, bird feathers and scaly feet. Later depictions shifted to show sirens with human upper bodies and bird legs, with or without wings. They were often shown playing

12987-478: Was 200 meters deep. Other evidence that supports this lake are normal and inverted channels like ones found in the Atacama Desert . Some areas of Mars show inverted relief , where features that were once depressions, like streams, are now above the surface. It is believed that materials like large rocks were deposited in low-lying areas. Later, erosion (perhaps wind which can't move large rocks) removed much of

13104-531: Was feasible. The theory of plate tectonics was defined in a series of papers between 1965 and 1967. The theory revolutionized the Earth sciences, explaining a diverse range of geological phenomena and their implications in other studies such as paleogeography and paleobiology . In the late 19th and early 20th centuries, geologists assumed that 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

13221-599: Was formed. For a typical distance that 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. Continental lithosphere is typically about 200 km (120 mi) thick, though this varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The location where two plates meet

13338-424: Was observed early that although granite existed on continents, seafloor seemed to be composed of denser basalt , the prevailing concept during the first half of the twentieth century was that there were two types of crust, named "sial" (continental type crust) and "sima" (oceanic type crust). Furthermore, it was supposed that a static shell of strata was present under the continents. It therefore looked apparent that

13455-443: Was popularized during the 1980s and 1990s. Recent research, based on three-dimensional computer modelling, suggests that plate geometry is governed by a feedback between mantle convection patterns and the strength of the lithosphere. Forces related to gravity are invoked as secondary phenomena within the framework of a more general driving mechanism such as the various forms of mantle dynamics described above. In modern views, gravity

13572-560: 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

13689-402: Was widespread in early Martian history, which has implications for the possible existence of Martian life . Besides finding chlorides, MRO discovered iron/magnesium smectites which are formed from long exposure in water. Based on chloride deposits and hydrated phyllosilicates, Alfonso Davila and others believe there is an ancient lakebed in Terra Sirenum that had an area of 30,000 km and

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